![Focused Ion Beam Based Three-Dimensional Nano-Machining 11
and quantum current standards (Bylander, 2005). In long arrays of junctions, Ec is enhanced
by electron-electron interactions (Likharev, 1989, 1995) by a factor [C/C0]1/2, where C is the
junction capacitance and C0 is the stray capacitance to ground. The large ratio C/C0 ∼ 106 for
intrinsic junctions makes them particularly suited to the applications involving Coulomb
blockade effects. The features of the single Cooper-pair tunneling effect from the layered
structure of Bi-family as well as for YBCO will also be discussed in detail.
Superconductivity is a phenomenon when the resistance of the material becomes zero and it
expels all the magnetic field below a certain temperature usually at very low temperature.
The phenomenon of superconductivity was discovered in 1911 by the Dutch physicist H.
Kamerlingh Onnes. The quantum application of superconductivity was introduced in 1962.
B. D. Josephson discovered a tunnel junction consists of two strips of superconductors
separated by an insulator where the insulator is so thin that electrons can tunnel through it
known as Josephson junction.
The schematic of different types of Josephson junctions are shown below in Fig.7. S stands
for superconductor, S’ for a superconductor above Tc, N for normal metal, Se for
semiconductor, and I for an insulator.
Fig. 7. The schematics of different types of superconducting devices.
The term high-temperature superconductor was first introduced in 1986 to designate the
new family of cuprate-perovskite ceramic materials discovered by Johannes George Bednorz
and Karl Alexander Müller [J. G. Bednorz, K. A. Mueller (1986) "Possible high TC
superconductivity in the Ba-La-Cu-O system", Zeitschrift für Physik B 64 (2) 189–193
doi:10.1007/BF01303701] for which they won the Nobel Prize in Physics in the following
year. Their discovery of the first high-temperature superconductor, LaBaCuO, with a
transition temperature of 30 K, generated great excitement. In 1988, BSCCO (Bi2Sr2Can-
1CunO2n+4+x, with n=2 being the most commonly studied compound, though n=1 and n=3
have also received significant attention) as a new class of superconductor was discovered by
Maeda and coworkers [H. Maeda, Y. Tanaka, M. Fukutumi, and T. Asano (1988) "A New
High-Tc Oxide Superconductor without a Rare Earth Element" Jpn. J. Appl. Phys. 27 (2) L209–
L210. doi:10.1143/JJAP.27.L209.] at the National Research Institute for Metals in Japan,
though at the time they were unable to determine its precise composition and structure. The
discovery of these high temperature superconductors gave a path for the application of the
superconductivity at higher temperature.](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-23-320.jpg)
![Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 23
room temperature (19 °C) to 520 °C is measured by a thermocouple (TES-1310, Type K, TES
Electrical Electronic Corp.). The spectrum and temperature were recorded when both of
them are stable for several minutes (Kou et al., 2010b).
The temperature sensitivity ST is defined as the interference wavelength shift divided by the
corresponding temperature change. ST depends on temperature through the thermal
expansion and/or thermo-optics effect (Choi et al., 2008; Kou et al., 2010b):
1 2 2
2 1 2 1 2
2 2
( ) (2 )
( ) ( )
[ ]
T T c
T T
d d d
d
S L
dT dT dT dT
d n n n n
dz
dT n r
(4)
where σT (1.1×10-5 /°C) is the thermo-optics coefficient and αT (5.5×10-7 /°C) is the thermal
expansion coefficient. There are two contributions from temperature change: the
temperature-induced length variation in the cavity, and the temperature-induced index
variation and taper volume variation in taper transition. The first one is less than 1 pm/°C
and ignorable, it agrees with the fact that those previous micro-cavity FP interferences in
SMF by femtosecond laser machining are temperature-insensitive; the second one is about
10 ~ 20 pm/°C and dominates in temperature sensing (Kou et al., 2010b).
Figure 7 displays the measured interferometer wavelength shifts (Δλ) and error on
temperature (T). As the temperature increases, the interferometer wavelength shifts to
longer wavelength. A third-order polynomial was used to fit the wavelength shifts across
the entire calibration range. The average sensitivity of the device is ~ 17 pm/°C, which is
very close to the theoretical result. Higher sensitivity can be obtained by optimizing the
profile of the SMF-TT or using special fiber taper with higher thermo-optics coefficient (Kou
et al., 2010b).
Fig. 7. Dependence of the measured wavelength shift on temperature. The asterisk
represents the measured results while the solid line is the fitting result. The inset shows the
dependence of error on temperature (Kou et al., 2010b). Reprinted with permission.
Copyright 2010 Optical Society of America
0 200 400 600
0
2
4
6
8
10
Temperature (C)
Wavelength
shift
(nm)
experimental data
poly fitting
=-7.42810
-8
T
3
+5.63610
-5
T
2
+6.52110
-3
T+9.83210
-2
200 400 600
-10
0
10
20
Thermocouple (C)
Error
(%)](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-35-320.jpg)
![Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 31
are not. The different valleys and peaks correspond to the coupling between these different
forward and backward propagating modes, with different response properties for the outer
environment.
The reflection resonant condition for the grating is:
2 2
[ ]
f b
g
n n
(7)
where nf and nb are the effective indices of the forward and backward modes, respectively.
For simplicity, we assume a theoretical model to explain our experimental results which is
simple and not perfectly matched with the experiment but can give the fundamental
mechanism of the device. Within the model, the microfiber is 6 μm in diameter with uniform
metal cladding (20 nm in thickness). However, the real device is much more complicated,
with nonuniform metal cladding and diameter. And if an asymmetrical mode field lies
mainly near the grating, leading to a larger modal overlap with the grating, it may result in
a higher sensitivity. Figure 3 shows the calculation on the effective index of one cladding
mode and one core mode as a function of outer liquid refractive index nl. Due to the
existence of the metal layer, the cladding mode has a larger effective index (corresponding
to long resonant wavelength) than that of the core mode (corresponding to short resonant
wavelength) and has a larger overlap with the taper surface and the outside environment,
leading to a higher sensitivity to the surrounding medium which is in coincidence with the
spectra of Fig. 2.
1540 1560 1580 1600
-50
-45
-40
Wavelength(nm)
Reflection
(dB)
Acetone
Isopropanol
a
b
c
d
Fig. 17. Measured reflection spectra of the FTG when immersed in acetone and isopropanol
(Kou et al., 2011b).
The performance of resonant refractive index sensors can be evaluated by using sensitivity S,
which is defined as the magnitude in shift of the resonant wavelength divided by the change
in refractive index of the analyte. In our experiment, the sensitivity is measured by inserting
the sensor in a beaker containing mixtures of isopropanol and acetone, where the isopropanol
component has the following ratios: 0, 1/7, 2/7, 3/7, 4/7 5/7, 6/7, and 1 (Kou et al., 2011b).
Figure 18 displays measured resonant wavelength shifts of several peaks and valleys and
fitting of this FTG on the liquid refractive index (a, b, c, d as marked in Fig. 2, a and c are](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-43-320.jpg)
![Fundamentals of Laser Ablation of the Materials Used in Microfluiducs 41
for the beam to reach the substrate (Eloy, 1987). This luminous plasma is what is usually
responsible for the “sparkles” that mark the travel of the laser beam across the substrate.
Achieving beam positioning and pulse timing to avoid the plasma and plume occlusion of
the beam is a central part of tuning the recipe for any laser micromachining. This problem is
generally severe in continuous wave (CW) laser micromachining, but greatly reduced for
pulsed lasers which are Q-switched. While the complete plume of vaporized material
usually does not have time to fully disperse in between Q-switched pulses, the more
optically opaque and higher density plasma does, and laser ablation can continue onward
with usually only minor attenuation. If the beam positioning is not well designed, however,
the plasma and plume can become trapped into the confined spaces of the kerf, and greater
time will be required for their dispersal. The most common symptom of this effect is a
reduced depth of ablation for a given beam transversal rate.
2.1 Ablation process models
Laser ablation involves a complex interaction between optical, thermal, and chemical
processes, but some simplifications can lead to models which can be useful for
characterization, optimization, and troubleshooting of the process. Most such models start
with the optics of a Gaussian beam and compute the conductive flow of heat from this
source to find the temperature distribution, adding in the thermal effects which are needed
to account for melting and vaporization of the substrate (Engin & Kirby, 1996; Kaplan, 1996;
Olson & Swope, 1992). An idealized geometry is illustrated in Fig. 1 in which a circularly
symmetric Gaussian laser beam is moved across the substrate at a constant speed v in the +x
direction. The beam has an average power of P0 = πrB
2I0, where I0 is the peak intensity and rB
is the 1/e beam radius. The beam propagates in the +z direction and meets the substrate
surface in the x-y plane. The situation is more easily described by using the relative
coordinate ξ = x − vt which moves along with the laser beam.
The interaction of the laser beam with the substrate first involves absorption of the optical
radiation and its conversion into heat for thermal (non-photo-chemical) ablation. Shorter
wavelengths are absorbed more strongly at the surface with a higher absorption coefficient
, and since this is usually ~104 cm−1 or greater, the heating is effectively concentrated at the
surface of the substrate. Volumetric heating effects have been considered by Zhang, et al.
(2006). The surface heating density is then
2 2
2
0 2
B
y
q( ,y) (1 R)I exp [W / m ],
r
where R is the reflectivity loss from the surface of the substrate.
The heat transfer within the substrate is entirely by conduction, so the resulting temperature
field is given by a solution to the heat conduction equation (Carslaw & Jaeger, 1959)
2
T
D T 0,
t
where D = κ/ρC is the thermal diffusivity, κ is the thermal conductivity, ρ is the mass density,
and C is the specific heat capacity. The surface heating density provides a source boundary
condition for the solution of the heat conduction equation. Ashby and Easterling (1984) have
shown that a close analytical approximation to the solution of this problem is given by](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-53-320.jpg)
![embarrassments, that after a very careful consideration, Mr. Scott
determined to dissolve the concern." He adds: "This became a matter
of less difficulty, because time had in a great measure worn away the
differences between Mr. Scott and Mr. Constable, and Mr. Hunter was
now out of Constable's concern.[23] A peace, therefore, was speedily
made up, and the old habits of intercourse were restored."
How reluctantly Scott had made up his mind to open such a
negotiation with Constable, as involved a complete exposure of the
mismanagement of John Ballantyne's business as a publisher, will
appear from a letter dated about the Christmas of 1812, in which he
says to James, who had proposed asking Constable to take a share
both in Rokeby and in the Annual Register, "You must be aware, that
in stating the objections which occur to me to taking in Constable, I
think they ought to give way either to absolute necessity or to very
strong grounds of advantage. But I am persuaded nothing ultimately
good can be expected from any connection with that house, unless
for those who have a mind to be hewers of wood and drawers of
water. We will talk the matter coolly over, and, in the mean while,
perhaps you could see W. Erskine, and learn what impression this
odd union is like to make among your friends. Erskine is sound-
headed, and quite to be trusted with your whole story. I must own I
can hardly think the purchase of the Register is equal to the loss of
credit and character which your surrender will be conceived to infer."
At the time when he wrote this, Scott no doubt anticipated that
Rokeby would have success not less decisive than The Lady of the
Lake; but in this expectation—though 10,000 copies in three months
would have seemed to any other author a triumphant sale—he had
been disappointed. And meanwhile the difficulties of the firm,
accumulating from week to week, had reached, by the middle of May,
a point which rendered it absolutely necessary for him to conquer all
his scruples.
Mr. Cadell, then Constable's partner, says in his Memoranda,—"Prior
to this time the reputation of John Ballantyne and Co. had been
decidedly on the decline. It was notorious in the trade that their](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-57-320.jpg)
![had gone a week or two earlier to London in quest of some similar
assistance there:—
TO MR. JOHN BALLANTYNE, CARE OF MESSRS. LONGMAN & CO.,
LONDON.
Printing-Office, May 18, 1813.
Dear John,—After many offs and ons, and as many projets and
contre-projets as the treaty of Amiens, I have at length concluded
a treaty with Constable, in which I am sensible he has gained a
great advantage;[24] but what could I do amidst the disorder and
pressure of so many demands? The arrival of your long-dated bills
decided my giving in, for what could James or I do with them? I
trust this sacrifice has cleared our way, but many rubs remain;
nor am I, after these hard skirmishes, so able to meet them by
my proper credit. Constable, however, will be a zealous ally; and
for the first time these many weeks I shall lay my head on a quiet
pillow, for now I do think that, by our joint exertions, we shall get
well through the storm, save Beaumont from depreciation, get a
partner in our heavy concerns, reef our topsails, and move on
securely under an easy sail. And if, on the one hand, I have sold
my gold too cheap, I have, on the other, turned my lead to gold.
Brewster[25] and Singers[26] are the only heavy things to which I
have not given a blue eye. Had your news of Cadell's sale[27]
reached us here, I could not have harpooned my grampus so
deeply as I have done, as nothing but Rokeby would have barbed
the hook.
Adieu, my dear John. I have the most sincere regard for you,
and you may depend on my considering your interest with quite
as much attention as my own. If I have ever expressed myself
with irritation in speaking of this business, you must impute it to
the sudden, extensive, and unexpected embarrassments in which
I found myself involved all at once. If to your real goodness of
heart and integrity, and to the quickness and acuteness of your](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-59-320.jpg)
![would soon finish me, and yet I am sensible I live too genially in
Edinburgh as it is. Yours very truly,
W. Scott.
Among Scott's early pets at Abbotsford there was a huge raven,
whose powers of speech were remarkable, far beyond any parrot's
that he had ever met with; and who died in consequence of an
excess of the kind to which James Ballantyne was addicted.
Thenceforth, Scott often repeated to his old friend, and occasionally
scribbled by way of postscript to his notes on business—
"When you are craving,
Remember the Raven."
Sometimes the formula is varied to—
"When you've dined half,
Think on poor Ralph!"
His preachments of regularity in book-keeping to John, and of
abstinence from good cheer to James Ballantyne, were equally vain;
but on the other hand it must be allowed that they had some reason
for displeasure—(the more felt, because they durst not, like him,
express their feelings)[28]—when they found that scarcely had these
"hard skirmishes" terminated in the bargain of May 18, before Scott
was preparing fresh embarrassments for himself, by commencing a
negotiation for a considerable addition to his property at Abbotsford.
As early as the 20th of June he writes to Constable as being already
aware of this matter, and alleges his anxiety "to close at once with a
very capricious person," as the only reason that could have induced
him to make up his mind to sell the whole copyright of an as yet
unwritten poem, to be entitled The Nameless Glen. This copyright he
then offered to dispose of to Constable for £5000; adding, "this is
considerably less in proportion than I have already made on the
share of Rokeby sold to yourself, and surely that is no unfair
admeasurement." A long correspondence ensued, in the course of](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-62-320.jpg)
![which Scott mentions The Lord of the Isles, as a title which had
suggested itself to him in place of The Nameless Glen; but as the
negotiation did not succeed, I may pass its details. The new property
which Scott was so eager to acquire was that hilly tract stretching
from the old Roman road near Turn-again towards the Cauldshiels
Loch: a then desolate and naked mountain-mere, which he likens, in
a letter of this summer (to Lady Louisa Stuart), to the Lake of the
Genie and the Fisherman in the Arabian Tale. To obtain this lake at
one extremity of his estate, as a contrast to the Tweed at the other,
was a prospect for which hardly any sacrifice would have appeared
too much; and he contrived to gratify his wishes in the course of that
July, to which he had spoken of himself in May as looking forward
"with the deepest anxiety."
Nor was he, I must add, more able to control some of his minor
tastes. I find him writing to Mr. Terry, on the 20th of June, about
"that splendid lot of ancient armor, advertised by Winstanley," a
celebrated auctioneer in London, of which he had the strongest fancy
to make his spoil, though he was at a loss to know where it should be
placed when it reached Abbotsford; and on the 2d of July, this
acquisition also having been settled, he says to the same
correspondent: "I have written to Mr. Winstanley. My bargain with
Constable was otherwise arranged, but Little John is to find the
needful article, and I shall take care of Mr. Winstanley's interest, who
has behaved too handsomely in this matter to be trusted to the
mercy of our little friend the Picaroon, who is, notwithstanding his
many excellent qualities, a little on the score of old Gobbo—doth
somewhat smack—somewhat grow to.[29] We shall be at Abbotsford
on the 12th, and hope soon to see you there. I am fitting up a small
room above Peter-House, where an unceremonious bachelor may
consent to do penance, though the place is a cock-loft, and the
access that which leads many a bold fellow to his last nap—a ladder."
[30] And a few weeks later, he says, in the same sort, to his sister-in-
law, Mrs. Thomas Scott: "In despite of these hard times, which affect
my patrons the booksellers very much, I am buying old books and old
armor as usual, and adding to what your old friend Burns[31] calls—](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-63-320.jpg)
![who carries this to Dumfries, has promised that the guard of the
mail-coach shall deliver it by five to-morrow. I was less anxious,
as your note said you could clear this month. It is a cruel thing
that no State you furnish excludes the arising of such unexpected
claims as this for the taxes on the printing-office. What unhappy
management, to suffer them to run ahead in such a manner!—but
it is in vain to complain. Were it not for your strange
concealments, I should anticipate no difficulty in winding up these
matters. But who can reckon upon a State where claims are kept
out of view until they are in the hands of a writer? If you have no
time to say that this comes safe to hand, I suppose James may
favor me so far. Yours truly,
W. S.
Let the guard be rewarded.
Let me know exactly what you can do and hope to do for next
month; for it signifies nothing raising money for you, unless I see
it is to be of real service. Observe, I make you responsible for
nothing but a fair statement.[32] The guard is known to the
Marquis, who has good-naturedly promised to give him this letter
with his own hand; so it must reach you in time, though probably
past five on Saturday.
Another similar application reached Scott the day after the guard
delivered his packet. He writes thus, in reply:](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-68-320.jpg)
![Clerkship, £1300
Sheriffdom, 300
Mrs. Scott, 200
Interest, 100
Somers, (say) 200
———
£2100
Abbotsford, 24th August, 1813.
Dear James,—Mr. Constable's advice is, as I have always found
it, sound, sensible, and friendly,—and I shall be guided by it. But
I have no wealthy friend who would join in security with me to
such an extent; and to apply in quarters where I might be
refused would insure disclosure. I conclude John has shown Mr.
C. the state of the affairs; if not, I would wish him to do so
directly. If the proposed accommodation could be granted to the
firm on my personally joining in the security, the whole matter
would be quite safe, for I have to receive in the course of the
winter some large sums from my father's estate.[33] Besides
which, I shall certainly be able to go to press in November with a
new poem; or, if Mr. Constable's additional security would please
the bankers better, I could insure Mr. C. against the possibility of
loss, by assigning the copyrights, together with that of the new
poem, or even my library, in his relief. In fact, if he looks into the
affairs, he will I think see that there is no prospect of any
eventual loss to the creditors, though I may be a loser myself. My
property here is unincumbered; so is my house in Castle Street;
and I have no debts out of my own family, excepting a part of
the price of Abbotsford, which I am to retain for four years. So
that, literally, I have no claims upon me unless those arising out
of this business; and when it is considered that my income is
above £2000 a year, even if the printing-office pays nothing, I
should hope no one can possibly be a loser by me.
I am sure I would strip myself to my shirt
rather than it should be the case; and my only
reason for wishing to stop the concern was to
do open justice to all persons. It must have
been a bitter pill to me. I can more confidently
expect some aid from Mr. Constable, or from
Longman's house, because they can look into
the concern and satisfy themselves how little chance there is of
their being losers, which others cannot do. Perhaps between](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-73-320.jpg)
![The letter which first announced the Prince Regent's proposal was
from his Royal Highness's librarian, Dr. James Stanier Clarke; but
before Scott answered it he had received a more formal notification
from the late Marquis of Hertford, then Lord Chamberlain. I shall
transcribe both these documents.
TO WALTER SCOTT, ESQ., EDINBURGH.
Pavilion, Brighton, August 18, 1813.
My dear Sir,—Though I have never had the honor of being
introduced to you, you have frequently been pleased to convey
to me very kind and flattering messages,[34] and I trust,
therefore, you will allow me, without any further ceremony, to
say—That I took an early opportunity this morning of seeing the
Prince Regent, who arrived here late yesterday; and I then
delivered to his Royal Highness my earnest wish and anxious
desire that the vacant situation of poet laureate might be
conferred on you. The Prince replied, "that you had already been
written to, and that if you wished it, everything would be settled
as I could desire."
I hope, therefore, I may be allowed to congratulate you on this
event. You are the man to whom it ought first to have been
offered, and it gave me sincere pleasure to find that those
sentiments of high approbation which my Royal Master had so
often expressed towards you in private, were now so openly and
honorably displayed in public. Have the goodness, dear sir, to
receive this intrusive letter with your accustomed courtesy, and
believe me, yours very sincerely,
J. S. Clarke.
Librarian to H. R. H., the Prince Regent.](https://image.slidesharecdn.com/2046252-250529101027-822ca03f/85/Micromachining-Techs-For-Fab-Of-Micro-Nano-Structs-M-Kahrizi-78-320.jpg)
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- 5. Contents Preface IX Chapter 1 Focused Ion Beam Based Three-Dimensional Nano-Machining 1 Gunasekaran Venugopal, Shrikant Saini and Sang-Jae Kim Chapter 2 Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 17 Fei Xu, Jun-long Kou, Yan-qing Lu and Wei Hu Chapter 3 Fundamentals of Laser Ablation of the Materials Used in Microfluiducs 35 Tai-Chang Chen and Robert Bruce Darling Chapter 4 Microwave Meta-Material Absorbers Utilizing Laser Micro-Machining Technology 61 Hongmin Lee Chapter 5 Laser Micromachining and Micro-Patterning with a Nanosecond UV Laser 85 Xianghua Wang, Giuseppe Yickhong Mak and Hoi Wai Choi Chapter 6 Laser Ablation for Polymer Waveguide Fabrication 109 Shefiu S. Zakariyah Chapter 7 Micro Eletro Discharge Milling for Microfabrication 131 Mohammad Yeakub Ali, Reyad Mehfuz, Ahsan Ali Khan and Ahmad Faris Ismail Chapter 8 Mechanical Micromachining by Drilling, Milling and Slotting 159 T. Gietzelt and L. Eichhorn Chapter 9 Release Optimization of Suspended Membranes in MEMS 183 Salvador Mendoza-Acevedo, Mario Alfredo Reyes-Barranca, Edgar Norman Vázquez-Acosta, José Antonio Moreno-Cadenas and José Luis González-Vidal
- 6. VI Contents Chapter 10 Micro Abrasive-Waterjet Technology 205 H.-T. Liu and E. Schubert Chapter 11 Electrochemical Spark Micromachining Process 235 Anjali Vishwas Kulkarni Chapter 12 Integrated MEMS: Opportunities & Challenges 253 P.J. French and P.M. Sarro Chapter 13 Modeling and Simulation of MEMS Components: Challenges and Possible Solutions 277 Idris Ahmed Ali
- 9. Preface Making microsystems at a scale level of few microns is called micromachining. Micromachining is used to fabricate three-dimensional microstructures. It is the foundation of a technology called Micro-Electro-Mechanical-Systems (MEMS). MEMS usually consist of three major parts: sensors, actuators, and an associate electronic circuitry that acts as the brain and controller of the whole system. There are two types of micromachining. Bulk micromachining starts with a silicon wafer or other substrate, which is selectively etched using dry or wet etching techniques, laser ablation, or focused ion beams. The most common substrate in this technology is single crystal silicon. Variation in the strength of bonds along various planes in this periodic structure makes it susceptible to etching with various rates along different crystal orientations. The wet anisotropic etching of silicon in hydroxide solutions, like potassium hydroxide (KOH) or tetra methyl ammonium hydroxide (TMAH), is performed to etch silicon selectively along a specific orientation. Due to the high selective ratio, the etch rate varies along various orientations in this semiconductor, making it possible to design and fabricate many 3-D microstructures. This type of etching is inexpensive and is generally used in early, low-budget research. Although the wet etching is the most common practice in micromachining, dry etching techniques like laser ablation and focused ion beams, are also often used to produce microstructures. This technique is not only used to produce micro devices; it has now been extended to fabricate many devices at the level of nano scales. Another micromachining technique is surface micromachining, which involves fabrication of layers (usually using standard CMOS technology) on the surface of a substrate, followed by etching of the sacrificial layers. The purpose of this book is to introduce advances in micromachining technology. For this, we have gathered review articles related to various techniques and methods of micro/nano fabrications from esteemed researchers and scientists. The book consists of 13 chapters. The first two chapters demonstrate fabrication of several micro and nano devices using Focused Ion Beams techniques. The next five chapters are related to the application of lasers and laser ablation techniques used in bulk micromachining. Several other specialized methods and technologies are presented in the subsequent chapters. Throughout the book, each chapter gives a complete description of a specific
- 10. X Preface micromachining method, design, associate analytical works, experimental set-up, and the final fabricated devices, followed by many references related to this field of research available in other literature. Due to the multidisciplinary nature of MEMS and nanotechnology, this collection of articles can be used by scientists and researchers in the disciplines of engineering, material sciences, physics and chemistry Mojtaba Kahrizi, Professor ECE Department, Concordia University, Montreal, Quebec, Canada
- 13. 1 Focused Ion Beam Based Three-Dimensional Nano-Machining Gunasekaran Venugopal1,2, Shrikant Saini1 and Sang-Jae Kim1,3 1Jeju National University, Department of Mechanical Engineering, Jeju, 2Karunya University, Department of Nanosciences and Technology, Tamil Nadu, 3Jeju National University, Department of Mechatronics Engineering, Jeju, 1,3South Korea 2India 1. Introduction In recent days, the micro/nano machining becomes an important process to fabricate micro/nano scale dimensional patterns or devices for many applications, especially in electrical and electronic devices. There are two kinds micro-machining in use. i) bulk micro- machining, ii) surface micro-maching. In the case of bulk micromaching, the structures can be made by etching inside a substrate selectively, however, in the case of surface micromachining; the patterns can be made on the top a desired substrate. FIB machining is considered as a one of famous bulk micro-machining processes. Many fabrication methods have been applied to fabricate the devices with smaller sizes (Kim, 1999; Latyshev, 1997). However, conventional until now the size of the smallest pattern was only 2×2 μm2 was achieved with a lithography technique (Odagawa et al., 1998). Three dimensional as an alternative approach, focused-ion-beam (FIB) etching technique is the best choice for the micro/nano scale patterning. FIB 3-D etching technology is now emerged as an attractive tool for precision lithography. And it is a well recognized technique for making nanoscale stacked-junction devices, nano-ribbons and graphene based 3-D Single Electron Transistor (SET) devices. FIB micro/nano machining is a direct etching process without the use of masking and process chemicals, and demonstrates sub-micrometer resolution. FIB etching equipments have shown potential for a variety of new applications, in the area of imaging and precision micromachining (Langford, 2001; Seliger, 1979). As a result, the FIB has recently become a popular candidate for fabricating high-quality micro-devices or high-precision microstructures (Melnagilis et al., 1998). For example, in a micro-electro-mechanical system (MEMS), this processing technique produces an ultra microscale structure from a simple sensor device, such as, the Josephson junction to micro-motors (Daniel et al., 1997). Also, the FIB processing enables precise cuts to be made with great flexibility for micro- and nano- technology. Also, the method of fabricating three-dimensional (3-D) micro- and nano- structures on thin films and single crystals by FIB etching have been developed in order to fabricate the 3-D sensor structures (Kim, 2008, 1999).
- 14. Micromachining Techniques for Fabrication of Micro and Nano Structures 2 In this chapter, the focused ion beam (FIB) based three-dimensional nano-machining will be discussed in detail in which the nano-machining procedures are focused with fabricating nanoscale stacked junctions of layered-structured materials such as graphite, Bi2Sr2Can- 1CunO2n+4+x (BSCCO) family superconductor (Bi-2212, Bi-2223, etc.,) and YBa2Cu3O7 (YBCO) single crystals, etc. This work could show a potential future in further development of nano- quantum mechanical electron devices and their applications. 2. Classification of machining Micromachining is the basic technology for fabrication of micro-components of size in the range of 1 to 500 micrometers. Their need arises from miniaturization of various devices in science and engineering, calling for ultra-precision manufacturing and micro-fabrication. Micromachining is used for fabricating micro-channels and micro-grooves in micro-fluidics applications, micro-filters, drug delivery systems, micro-needles, and micro-probes in biotechnology applications. Micro-machined components are crucial for practical advancement in Micro-electromechanical systems (MEMS), Micro-electronics (semiconductor devices and integrated circuit technology) and Nanotechnology. This kind of machining can be applicable for the bulk materials in which the unwanted portions of the materials can be removed while patterning. In the bulk machining, the materials with the dimensions of more than in the range of micrometer or above centimetre scale are being used for the machining process. A best example for the bulk machining process is that the thread forming process on a screw or bolt, formation of metal components. Also this process can be applicable to produce 3D MEMS structures, which is now being treated as one of older techniques. This also uses anisotropic etching of single crystal silicon. For example, silicon cantilever beam for atomic force microscope (AFM). Surface micro-machining is another new technique/process for producing MEMS structures. This uses etching techniques to pattern micro-scale structures from polycrystalline (poly) silicon, or metal alloys. Example: accelerometers, pressure sensors, micro gears and transmission, and micro mirrors etc. Micromachining has evolved greatly in the past few decades, to include various techniques, broadly classified into mask-based and tool-based, as depicted in the diagram below.
- 15. Focused Ion Beam Based Three-Dimensional Nano-Machining 3 While mask-based processes can generate 2-D/2.5-D features on substrates like semiconductor chips, tools-based processes have the distinct advantage of being able to adapt to metallic and non-metallic surfaces alike, and also generate 3-D features and/or free-form sculpted surfaces. However, the challenges of achieving accuracy, precision and resolution persist. Internationally, the race to fabricate the smallest possible component has lead to realization of sizes ever below 10 µm, even though the peak industrial requirement has been recognized at 100s of µm. Thus, the present situation is particularly advantageous for the industry that develops/fabricates nano/micron scale components. 2.1 Various techniques of micromachining Micromachining can be done by following various techniques. a. Photolithography b. Etching c. LIGA d. Laser Ablation e. Mechanical micromachining Photolithography This technique is being used in microelectronics fabrication and also used to pattern oxide/nitride/polysilicon films on silicon substrate. In this process, the basic steps involved are, photoresist development, etching, and resist removal. Photolithographic process can be described as follows: The wafers are chemically cleaned to remove particulate matter, organic, ionic, and metallic impurities. High-speed centrifugal whirling of silicon wafers known as "Spin Coating" produces a thin uniform layer of photoresist (a light sensitive polymer) on the wafers. Photoresist is exposed to a set of lights through a mask often made of quartz. Wavelength of light ranges from 300-500 nm (UV) and X-rays (wavelengths 4-50 Angstroms). Two types of photoresist are used: (a) Positive: whatever shows, goes (b) Negative: whatever shows, stays. The photo resist characteristics after UV exposure are shown below in Fig. 1 Fig. 1. Photoresist characteristics in UV exposure Etching Normally etching process can be classified in to two kinds. (a) Wet etching (b) Dry etching. The wet etching process involves transport of reactants to the surface, surface reaction and transport of products from surfaces. The key ingredients are the oxidizer (e.g. H2O2, HNO3),
- 16. Micromachining Techniques for Fabrication of Micro and Nano Structures 4 the acid or base to dissolve the oxidized surface (e.g. H2SO4, NH4OH) and dilutent media to transport the products through (e.g. H2O). Dry etching process involves two kinds. (a) plasma based and (b) non plasma based. LIGA The LIGA is a German term which means LIthographie (Lithography) Galvanoformung (Electroforming) Abforming (Molding). The exact English meaning of LIGA is given in parenthesis. This process involves X-ray irradiation, resist development, electroforming and resist removal. The detailed LIGA process description is discussed below: Deep X-ray lithography and mask technology - Deep X-ray (0.01 – 1nm wavelength) lithography can produce high aspect ratios (1 mm high and a lateral resolution of 0.2 μm). - X-rays break chemical bonds in the resist; exposed resist is dissolved using wet- etching process. Electroforming - The spaces generated by the removal of the irradiated plastic material are filled with metal (e.g. Ni) using electro-deposition process. - Precision grinding with diamond slurry-based metal plate used to remove substrate layer/metal layer. Resist Removal - PMMA resist exposed to X-ray and removed by exposure to oxygen plasma or through wet-etching. Plastic Molding - Metal mold from LIGA used for injection molding of MEMS. LIGA Process Capability High aspect ratio structures: 10-50 μm with Max. height of 1-500 μm Surface roughness < 50 nm High accuracy < 1μm Laser ablation High-power laser pulses are used to evaporate matter from a target surface. In this process, a supersonic jet of particles (plume) is ejected normal to the target surface which condenses on substrate opposite to target. The ablation process takes place in a vacuum chamber - either in vacuum or in the presence of some background gas. The graphical process scheme is given below in Fig.2. Fig. 2. Laser ablation experiment.
- 17. Focused Ion Beam Based Three-Dimensional Nano-Machining 5 Mechanical micromachining Lithography or etching methods are not capable of making true 3D structures e.g. free form surfaces and also limited in range of materials. Mechanical machining is capable of making free form surfaces in wide range of materials. Can we scale conventional/non-traditional machining processes down to the micron level? Yes! There are two approaches used to machine micron and sub-micron scale features. 1. Design ultra precision (nanometer positioning resolution) machine tools and cutting tools. For this, ultra precision diamond turning machines can be used. 2. Design miniature but precise machine tools Example: Micro-lathe, micro-mill, micro-EDM, etc Mechanical micromachining process descriptions are given below: Can produce extremely smooth, precise, high resolution true 3D structures Expensive, non-parallel, but handles much larger substrates Precision cutting on lathes produces miniature screws, etc with 12 μm accuracy Relative tolerances are typically 1/10 to 1/1000 of feature Absolute tolerances are typically similar to those for conventional precision machining (Micrometer to sub-micrometer) 2.2 Focused-ion-beam (FIB) technique for nanofabrication The focused ion beam based nanofabrication method can be followed for the fabricating the nanoscale devices on materials based on metal and non-metallic elements, particularly the layered structure materials like graphite, Bi-2212 and YBCO which are recently attracted the world scientific community due to their interesting electrical and electronic properties reported in recent reports (Venugopal, 2011; Kim, 2001). Graphite is considered as a well known layered-structured material in which carbon sheets are arranged in a stacked-manner with interlayer distance of 0.34 nm. Each single graphite sheet is known as a graphene layer which is now becoming as one of hot topic in the world scientific community. In the recent reports (Venugopal, 2011a, 2011b, 2011c), the fabrication of submicron and below submicron stacked junctions were carved from the bulk graphite materials using FIB 3-D etching. The interesting results were obtained in those observations that the graphite stacked-junction with in-plane area A of 0.25 μm2 showed nonlinear concave-like I–V characteristics even at 300 K; however the stack with A ≥ 1 μm2 were shown an ohmic-like I–V characteristic at 300 K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. These results may open road to develop further graphite based nonlinear electronic devices. Further researches are being carried out to find unexplored properties of graphite nano devices fabricated using FIB micro/nano machining technology. The focused ion beam (FIB) machining to make micro-devices and microstructures has gained more and more attention recently (Tseng, 2004). FIB can be used as a direct milling method to make microstructures without involving complicated masks and pattern transfer processes. FIB machining has advantages of high feature resolution, and imposes no limitations on fabrication materials and geometry. Focused ion beams operate in the range of 10-200 keV. As the ions penetrate the material, they loose their energy and remove substrate atoms. FIB has proven to be an essential tool for highly localized implantation doping, mixing, micromachining, controlled damage as well as ion-induced deposition. The technological challenge to fabricate nanoholes using electron beam lithography and the
- 18. Micromachining Techniques for Fabrication of Micro and Nano Structures 6 minimal feature size accessible by these techniques is typically limited to tens of nanometers, thus novel procedures must be devised (Zhou, 2006). The patterning of samples using the FIB (focused ion beam) technique is a very popular technique in the field of inspection of integrated circuits and electronic devices manufactured by the semi-conductor industry or research laboratories. This is the case mainly for prototyping devices. The FIB technique allowing us to engrave materials at very low dimensions is a complement of usual lithographic techniques such as optical lithography. The main difference is that FIB allows direct patterning and therefore does not require an intermediate sensitive media or process (resist, metal deposited film, etching process). FIB allows 3D patterning of target materials using a finely focused pencil of ions having speeds of several hundreds of km s−1 at impact. Concerning the nature of the ions most existing metals can be used in FIB technology as pure elements or in the form of alloys, although gallium (Ga+ ions) is preferred in most cases. Many device fabrication techniques based on electron beam lithography followed by reactive-ion etching (RIE), chemical methods, and evaporation using hard Si shadow masks, and including lithography-free fabrication, have been reported. The procedures, however, are complex and yield devices with dimensions of ~5 to 50 nm, which are restricted to simple geometries. RIE creates disordered edges, and the chemical methods produce irregular shapes with distributed flakes, which are not suitable for electronic-device application. Practically, FIB patterning can be achieved either by local surface defect generation, by ion implantation or by local sputtering. These adjustments are obtained very easily by varying the locally deposited ion fluence with reference to the sensitivity of the target and to the selected FIB processing method (Gierak, 2009). The FIB milling involves two processes: 1) Sputtering, ions with high energy displace and remove atoms of substrate material, and the ions lose their energy as they go into the substrate; 2) Re-deposition, the displaced substrate atoms, that have gained energy from ions through energy transfer, go through similar process as ions, sputtering other atoms, taking their vacancy, or flying out. A focused gallium ion beam having an energy typically around 30 keV is scanned over the sample surface to create a pattern through topographical modification, deposition or sputtering. A first consequence is that, mainly because of the high ion doses required (~1018 ions cm−2) and of the limited beam particle intensity available in the probe, FIB etching- based processes remain relatively slow. We may recall that for most materials, the material removal rate for 30 keV gallium ions is around 1–10 atoms per incident ion, corresponding to a machining rate of around 0.1–1 μm3 per nC of incident ions (Gierak, 2009). The second consequence is that for most applications the spatial extension of the phenomena induced by focused ion beam irradiation constitutes a major drawback. In addition, there have been few reports of the fabrication of nano-structured materials, nano devices, and hierarchical nano-sized patterns with a 100 nm distance using a focused ion beam (FIB). Fabrication of graphene nanoribbons and graphene-based ultracapacitors were also reported recently. The above-discussed methods were followed by the two- dimensional (2D) fabrication methods and required extensive efforts to achieve precise control. Hence, a novel three-dimensional (3D) nanoscale approach to the fabrication of a stack of graphene layers via FIB etching is proposed, through which a thin graphite flake can be etched in the c-axis direction (stack height with a few tens of nanometers). Also the main purpose of describing graphite and other BSCCO based superconducting nanoscale devices is that these layered structured materials have shown an excellent device structures
- 19. Focused Ion Beam Based Three-Dimensional Nano-Machining 7 during fabrication and their electrical transport characteristics were interesting which will be useful to future works. 2.2.1 Nanoscale stack fabrication by focused-ion-beam Using an FIB, perfect stacks can be fabricated more easily along the c-axis in thin films and single-crystal whiskers. FIB 3D etching has been recognized as a well-known method for fabricating high-precision ultra-small devices, in which etching is a direct milling process that does not involve the use of any masking and process chemicals and that demonstrates a submicrometer resolution. Thus, these our proposal is focused on the fabrication of a nanoscale stack from the layered structured materials like thin graphite flake and BSCCO, via FIB 3D etching. The detailed schematic of fabrication process is shown in Fig. 3. The 3D etching technique is followed by tilting the substrate stage up to 90° automatically for etching thin graphite flake. We have freedom to tilt the substrate stage up to 60° and rotate up to 360°. To achieve our goal, we used sample stage that itself inclined by 60° with respect to the direction of the ion beam (fig 3a). The lateral dimensions of the sample were 0.5×0.5 μm2. The in-plane area was defined by tilting the sample stage by 30° anticlockwise with respect to the ion beam and milling along the ab-plane. Fig. 3. FIB 3-D fabrication process (a) Scheme of the inclined plane has an angle of 60° with ion beam (where we mount sample). (b) The initial orientation of sample and sample stage. (c) Sample stage titled by 30° anticlockwise with respect to ion beam and milling along ab- plane. (d) The sample stage rotated by an angle of 180° and also tilted by 60° anticlockwise with respect to ion beam and milled along the c-axis.
- 20. Micromachining Techniques for Fabrication of Micro and Nano Structures 8 The in-plane etching process is shown in Fig. 3(a)–(c). The out of plane or the c-axis plane was fabricated by rotating the sample stage by an angle of 180°, then tilting by 60° anticlockwise with respect to the ion beam, and milling along the c-axis direction. The schematic diagram of the fabrication process for the side-plane is shown in Fig. 3(d). The dimensions of the side-plane was W=0.5 μm, L=0.5 μm, and H=200 nm. The c-axis height length (H) of the stack was set as 200 nm. An FIB image of fabricated stack is shown in Fig. 4 in which the schematic of stack arrangement (graphene layers with interlayer distance 0.34 nm) was also shown in the inset (top right) in Fig. 4. The vertical red arrow indicates the current flow direction through the stack. 2.2.2 Transport characteristics of nanoscale graphite stacks The electrical transport characteristics (including ρ-T and I-V) can be performed for the fabricated stack using closed-cycle refrigerator systems (CKW-21, Sumitomo) at various temperatures from 25 to 300 K with the use of the Keithley 2182A nanovoltmeters and AC & DC current source (6221). The I-V characteristics of the fabricated stack are shown in Fig.4. Fig. 4. FIB image of the nanoscale stack fabricated on a thin graphite flake along the c-axis height of 200 nm (image scale bar is 2 μm). Inset shows the schematic diagram of stack arrangement along the c-axis. (Venugopal et al, 2011). The vertical red arrow indicates the current flow direction through the nanoscale stack. I-V characteristics at various temperatures of the fabricated nanostack are also shown (right). The FIB ion damage effect can be avoided if the device is fabricated at a 3D angle, in which the top layer of ab-plane will act as a masking layer and the ion beam is exactly perpendicular to the milling surface. The expected ion damage effect was simulated using the TRIM software (Ziegler, 1996) and the fabrication parameter of etching process for the 30 keV Ga+ ions was optimized. It was found from the simulation results that the depth of ion implantation is consistent with 10 nm. Majority (>95%) of the Ga+ ions are expected to be implanted within 10 nm of the side walls of stack surface, with a much smaller fraction, eventually stopping at as deep as 10 nm into the surface. Therefore, the proportion of the fabricated stack affected by ion beam damage is not very large, and it does not affect the quality of graphite devices in the c-axis direction.
- 21. Focused Ion Beam Based Three-Dimensional Nano-Machining 9 By varying in-plane area (A) and stack height (H), several stacked-junctions with the dimensions of W = 1 μm, L = 1 μm, and H = 0.1 μm (denoted as J4) and W = 2 μm, L = 1 μm, and H = 0.3 μm (denoted as J5) were fabricated. The electrical transport characteristics were performed for these stacks and compared their results. The current-voltage (I-V) characteristics of the nanostack with in-plane area (A) of 0.25 µm2 (J2) at various temperatures, are presented in Fig. 4. The stack showed a nonlinear concave-like I-V characteristics at all studied temperatures (25, 50, 110, 200, 250 and 300 K). At 300 K, the stack resistance was found as 75 Ω. The stack resistance found increases when the temperature goes down to 25 K. The electrical characteristics of nanostack (J2) were analyzed and compared with bigger junctions J4 (1 × 1 × 0.1 µm3) and J5 (2 × 1 × 0.3 µm3). From the data analysis, it is clear that the stack with larger height and reduced in-plane effective area (A) has shown higher resistance than the stack with larger in-plane area (A). The I-V characteristics of junctions J4 and J5 at different temperatures are shown in Fig. 5 (a) and (b) respectively. A typical c-axis transport characteristics similar to junction J2 was observed. However the nonlinear I-V characteristics were not observed at 300 K, but ohmic like-linear behavior is observed. When the temperature goes down, this behavior is turned into curve-like nonlinear characteristics. Fig. 5. (a) I–V characteristics of a bigger stacked-junction with A of 1 µm2 (J4) at different temperature from 25 K to 300 K. (b) I–V characteristics of another bigger junction with A of 2 × 1 × 0.3 µm3 (J5) at different temperature from 25 K to 300 K. Both the junctions show ohmic like behavior at 300 K; however the same behavior turned into nonlinear characteristics when the temperature goes down (Venugopal et al, 2011). There is a significant overlap of I-V curves for temperatures 110, 75 and 25 K. For graphite stacks with A ≥ 1 µm2, there was no nonlinear I-V characteristics observed at 300 K even at high biasing. With a decrease of the stack size down to 0.25 µm2, the junction shows clear nonlinear concave-like I-V characteristics for both 300 K and 25 K. Since the fabricated stack contains multiple elementary junctions along the c-axis, the nonlinear concave-like tunneling characteristics appeared from the I-V characteristics (Venugopal et al, 2011). 2.2.3 Temperature dependent resistivity of nanoscale graphite stack Fig 6 represents the ρ–T characteristics of stacked-junction (J2). The junction J2 shows a semiconducting behavior for T > 65 K and metallic characteristics for T < 65 K. Above 65 K,
- 22. Micromachining Techniques for Fabrication of Micro and Nano Structures 10 thermal excitation of carriers plays a major role in semiconducting temperature dependence. However below 65 K, the interlayer hopping conduction combined with scattering of carriers by phonons can be responsible for the metallic-like temperature dependence. The ρ– T characteristics along the ab-plane transport are shown as inset in Fig. 6. A well understood metallic behavior was observed. This behavior is well agreed with earlier observations on c- axis characteristics of bulk graphite material (Matsubara, 1990). An electron motion parallel to its plane is not affected by the stacking faults, however, but an electron motion in the c-axis direction is strongly impeded by the faults. The combined effects of impurity-assisted hopping, tunneling current, and the thermal excitation of the carriers on the plane of a stack play important roles in this temperature-dependent conduction mechanism in layered structured materials such as graphite. Fig. 6. The resistivity–temperature (ρ-T) characteristics of nanostack which shows a clear c-axis characteristics of graphite. A well agreed curve fitting to experimental data is also shown. A clear metallic behavior is observed for ab-plane transport of bare graphite flake which is shown as inset. (Venugopal, 2011) 2.3 FIB nano fabrication on superconducting devices Considering Bi-family as a layered structure material, there are three compounds in the Bi- family high-temperature superconductors, differing in the type of planar CuO2 layers; single-layered Bi2Sr2CuO6+δ (Bi-2201) single crystal, double-layered Bi2Sr2CaCu2O8+δ (Bi- 2212) single crystal, and triple-layered Bi2Sr2Ca2Cu3O10+δ (Bi-2223) single crystal (Saini, 2010). This Bi-family material is a one of the famous emerging material for electron tunneling devices, such as intrinsic Josephson junctions (IJJ) in layered high-Tc superconductors. The spacing of consecutive copper-oxide double planes in the most anisotropic cuprate superconductors is greater than the coherence length in the out-of-plane c-direction. When a current flows along the c-direction in such a material, it therefore flows through a series array of “intrinsic” Josephson junctions (IJJs) (Kleiner, 1992). These junctions and junction arrays are showing promise for a wide variety of applications, including as voltage standards and sub-mm-wave oscillators (Wang, 2001). For sub-micron intrinsic junctions, there is an additional range of potential applications exploiting the Coulomb blockade effect, when the Ec is charging energy Ec ≥ EJ, KBT, where EJ is the Josephson energy & kBT is thermal energy. These applications include electric-field sensors
- 23. Focused Ion Beam Based Three-Dimensional Nano-Machining 11 and quantum current standards (Bylander, 2005). In long arrays of junctions, Ec is enhanced by electron-electron interactions (Likharev, 1989, 1995) by a factor [C/C0]1/2, where C is the junction capacitance and C0 is the stray capacitance to ground. The large ratio C/C0 ∼ 106 for intrinsic junctions makes them particularly suited to the applications involving Coulomb blockade effects. The features of the single Cooper-pair tunneling effect from the layered structure of Bi-family as well as for YBCO will also be discussed in detail. Superconductivity is a phenomenon when the resistance of the material becomes zero and it expels all the magnetic field below a certain temperature usually at very low temperature. The phenomenon of superconductivity was discovered in 1911 by the Dutch physicist H. Kamerlingh Onnes. The quantum application of superconductivity was introduced in 1962. B. D. Josephson discovered a tunnel junction consists of two strips of superconductors separated by an insulator where the insulator is so thin that electrons can tunnel through it known as Josephson junction. The schematic of different types of Josephson junctions are shown below in Fig.7. S stands for superconductor, S’ for a superconductor above Tc, N for normal metal, Se for semiconductor, and I for an insulator. Fig. 7. The schematics of different types of superconducting devices. The term high-temperature superconductor was first introduced in 1986 to designate the new family of cuprate-perovskite ceramic materials discovered by Johannes George Bednorz and Karl Alexander Müller [J. G. Bednorz, K. A. Mueller (1986) "Possible high TC superconductivity in the Ba-La-Cu-O system", Zeitschrift für Physik B 64 (2) 189–193 doi:10.1007/BF01303701] for which they won the Nobel Prize in Physics in the following year. Their discovery of the first high-temperature superconductor, LaBaCuO, with a transition temperature of 30 K, generated great excitement. In 1988, BSCCO (Bi2Sr2Can- 1CunO2n+4+x, with n=2 being the most commonly studied compound, though n=1 and n=3 have also received significant attention) as a new class of superconductor was discovered by Maeda and coworkers [H. Maeda, Y. Tanaka, M. Fukutumi, and T. Asano (1988) "A New High-Tc Oxide Superconductor without a Rare Earth Element" Jpn. J. Appl. Phys. 27 (2) L209– L210. doi:10.1143/JJAP.27.L209.] at the National Research Institute for Metals in Japan, though at the time they were unable to determine its precise composition and structure. The discovery of these high temperature superconductors gave a path for the application of the superconductivity at higher temperature.
- 24. Micromachining Techniques for Fabrication of Micro and Nano Structures 12 2.3.1 FIB nanomachining of Intrinsic Josephson Junctions (IJJs) on BSCCO and Y123/Pr123 multilayered thin films Many fabrication methods based on high-resolution patterning have been applied to develop high-Tc superconducting devices. Very small structures are needed in the fabrication of tunneling devices, such as intrinsic Josephson junctions (IJJ) in layered high Tc superconductors Bi2Sr2CaCu2O8+δ (Bi-2212). Perfect stacks are more easily obtained in c-axis high-quality thin films than in a-axis films or single-crystal whiskers. However, the IJJ fabrication process using c-axis thin films and single crystals requires intricate processes and limits the junction size in mesa structures. As per previous reports, the fabrication of IJJs by the focused ion beam (FIB) etching method using single-crystal whiskers as a base material requires some complicated processes, including turning over of the sample. As an alternative approach, in this chapter, a three- dimensional IJJ fabrication method is presented using c-axis thin films. The fabrication steps using c-axis single crystal are also simplified by the in situ process. Here, the 3D FIB etching methods using YBCO thin films and Bi-2212 single-crystal whiskers were described as examples with a successive decrease of their in-plane area, S, down to a submicron scale. Also, there was a possibility to identify the features of the single Cooper-pair tunneling effect from the layered structure of Bi-2212 with very narrow interval between layers. FIB image of a submicron stack fabricated on Bi-2212 single crystal whiskers with in-plane area of 0.4 µm × 0.4 µm and schematic of the IJJs configuration are shown in Fig. 8, in which FIB fabrication procedures followed as described in section 2.1.2. Fig. 8. FIB image of a submicron stack fabricated on Bi-2212 single crystal whisker. The red color circular part shown in stack contains many IJJs. FIB image of submicron stack (scale bar of 1 µm) and schematic of the Josephson junctions configuration in the submicron stack fabricated on a-axis oriented YBa2Cu3O7/PrBa2Cu3O7 multi layered thin films are shown in Fig.9. The arrow indicates the direction of current to observe the effect of Josephson junctions. The axial direction of thin film is shown in the expended view.
- 25. Focused Ion Beam Based Three-Dimensional Nano-Machining 13 Fig. 9. FIB image of a submicron stack fabricated on a-axis oriented Y123/Pr123 multi layered thin films. 2.3.2 Electrical transport characteristics of Josephson junctions fabricated on multi layered thin films of Y123/Pr123 Fig. 10 represents R-T characteristics of the Josephson junctions fabricated on multi layered thin films of Y123/Pr123 which shows Tc about 83 K. Fig. 10. R-T characteristics of the device show Tc about 83 K. I-V characteristics of the same device were studied without microwave irradiation at different temperature of 10, 20, and 30 K, shown in Fig. 11. As temperature decreases, the critical current density of superconducting device is increases gradually. The above discussed nanomachining/milling techniques followed by focused ion beam 3-D technique shall be applicable to other layered-structured materials rather than graphite flake, BSCCO, YBCO and multilayered thin films, etc,. This may have great potential in future nanodevice development and applications.
- 26. Micromachining Techniques for Fabrication of Micro and Nano Structures 14 Fig. 11. I-V characteristics of the device without microwave irradiation at different temperature of 10, 20, and 30 K. The critical current density Jc about 2.2 X 105 A/cm2 is measured at 20 K. 2.4 Future advances In the future, micromachining is destined to improve upon its shortcomings, as the various micromachining processes become accurate, reliable, versatile and cost-effective. In India, BARC has established premier micromachining and nano-finishing facilities along with state-of-the art metrology systems. On the other hand, IIT Bombay has also taken a lead in establishing tool-based micromachining facilities. Even at South Korea, the technology towards nanomachining becomes popular nowadays and the active research is now under progress through which an interesting studies may be explored in near future. FIB technology is still relatively young compared with other semiconductor fabrication processes. One of the major challenges for all of the microfabrication and nanofabrication technologies is to downscale the feature size while maintaining a high throughput. To increase the throughput and the ability to be used in production, the milling rate of the existing FIB milling systems has to be improved. A variable-diameter beam system should be developed to provide multi-resolution milling to cope with different accuracy or tolerance requirements. It is ideal that the beam diameter can be continuously changed in situ. (Tseng, 2004). This type of system has been available for many macro-scale fabrication processes. With this system, a larger beam can be used for roughing ‘cut’ (milling) to increase the milling rate in regions where only lower resolution is needed. The advantages to use a heavy-duty two-lens system with improved automation should be examined with the goal to develop a system for limited production usage first. Once the high-performance FIB system is used in production, it can be a vital candidate to become the mainstream tool for the future microtechnology and nanotechnology industry. With an increasing awareness about the advantages of manufacturing micro-components indigenously instead of importing at high costs, the researchers and industrialists are in need of the knowledge of micromachining technology.
- 27. Focused Ion Beam Based Three-Dimensional Nano-Machining 15 3. Conclusion In conclusion, the focused ion beam based nanomachining have been discussed in detail for the layered structured materials, BSCCO superconducting devices, YBCO based thin film devices, and a-axis oriented Y123/Pr123 multi layered thin film devices. The development of focused ion beam technology based nanomachining is one amongst many examples on how research results may have found unexpected applications in totally different application areas. This is particularly true for the FIB technology development itself that has benefited from all the previously made advances in field emission physics, charged particle optics theory or modelling and in fundamental instrumentation or applied metrology. All these advances were very quickly and efficiently integrated into FIB instruments, so that in less than one decade FIB instruments have moved out from some specialist laboratories to enter almost every modern laboratory, research institute or processing environment. This is also true for the semiconductor industry that has been almost immediately applying FIB systems for device inspection failure analysis and reverse engineering with roaring success. The FIB processing methods which we discussed in this chapter, appear now to be well suited and very promising for several diverse nanotechnology applications, and may be of major interest for future applications to spin-electronics, nano-electronics, nano-optics or nanomagnetism. 4. Acknowledgment This research was supported by National Research Foundation of Korea Grant under contract numbers 2009-0087091 and 2011-0015829 through the Human Resource Training Project for Regional Innovation. A part of this research was also supported by the 2012 Jeju Sea Grant College Program funded by Ministry of Land, Transport and Maritime Affairs, Republic of Korea 5. References Bylander, J. (2005). Current Measurement by Real-time Counting of Single Electrons.. Nature., Vol.434, pp. 361-364. Daniel, J. H. (1997). Focused Ion Beams in Microsystem Fabrication. Microelectron. Eng., Vol.35, No.1-4, pp. 431-434. Gierak, J. (2009). Focused Ion Beam Technology and Ultimate Applications. Semicond. Sci. Technol., Vol.24, pp. 043001-043022. Kim, S. J. (2001). Fabrication and Characteristics of Submicron Tunnelling Junctions on High Tc Superconducting c-axis Thin Films and Single Crystals. J. Appl. Phys., Vol.89, No.11, pp. 7675-7677. Kim, S. J. (1999). Submicron Stacked-junction Fabrication from Bi2Sr2CaCu2O8+δ Whiskers by Focused-Ion-Beam Etching . Appl. Phys. Lett., Vol.74, No.8, pp. 1156-1158. Kim, S. J. (2008). Development of Focused Ion Beam Machining Systems for Fabricating Three-dimensional Structures. Jpn. J. Appl. Phys., Vol.47, No.6, pp. 5120-5122. Kim, S. J. (1999). 3D intrinsic Josephson junctions using c-axis thin films and single crystals. Supercond. Sci. Technol., Vol.12, pp. 729-731. Kleiner, R. (1992). Intrinsic Josephson Effects in BiSrCaCuO Single-Crystals. Phys. Rev. Lett., Vol.68, pp. 2394-2396.
- 28. Micromachining Techniques for Fabrication of Micro and Nano Structures 16 Latyshev, Y. I. (1997). Intrinsic Josephson Effects on Stacks Fabricated from High Quality BSCCO 2212 Single Crystal Whiskers. Physica C., Vol.293, pp. 174-180. Langford, R. M. (2001). Preparation of Site Specific Transmission Electron Microscopy Plan- view Specimens using a Focused Ion Beam System. J. Vac. Sci. Technol. B., Vol.19, No.755. doi:10.1116/1.1371317. Likharev, K. K. (1989). Single-electron Tunnel Junction Array: An Electrostatic Anolog of the Josephson Transmission Line. IEEE Trans. Mag., Vol.25, pp. 1436. DOI: 10.1109/20.92566. Likharev, K. K. (1995). Electron-electron Interaction in Linear Arays of Small Tunnel Junctions. Appl. Phys. Lett., Vol.67, pp. 3037-3039. Matsubara, K. (1990). Electrical Resistance in c-direction of Graphite. Phys. Rev. B., Vol.41, pp. 969-974. Melnagilis, J. (1998). A Review of Ion Projection Lithography. J. Vac. Sci. Technol. B., Vol.16, No.3, pp. 927-957. Odagawa, A. (1998). Characteristics of Intrinsic Josephson Junctions in Thin Stack on Bi-2223 Thin Films. Jpn. J. Appl. Phys., Vol.37, No.1, pp. 486-491. Saini, S. (2010). Characterization of Submicron Sized Josephson Junction Fabricated in a Bi2Sr2Ca2Cu3O10+δ (Bi-2223) Single Crystal Whisker. J. Supercond. Nov. Magn. Vol.23, pp. 811-813. Seliger, R. L. (1979). High‐resolution, Ion‐beam Processes for Microstructure Fabrication. J. Vac. Sci. Technol. B., Vol.16, No.6, pp.1610-1612. Tseng, A. A. (2004). Recent Developments in Micromilling using Focused Ion Beam Technology. J. Micromech. Microengg., Vol.14, pp. R15-R34. Venugopal, G. (2011a). Fabrication of Nanoscale Three-dimensional Graphite Stacked- junctions by Focused-ion-beam and Observation of Anomalous Transport Characteristics. Carbon, Vol.49, No.8, pp. 2766-2772. Venugopal, G. (2011b). Temperature Dependence of Transport Anisotrophy of Planar-type Graphite Nano-structures Fabricated by Focused Ion Beam. J. Nanosci. Nanotechnol. Vol.11, No.1, pp. 296-300. Venugopal, G. (2011c). Fabrication and Characteristics of Submicron Stacked-Junctions on Thin Graphite Flakes. J. Nanosci. Nanotechnol. Vol.11, No.2, pp. 1405-1408. Wang, H. B. (2001). Terahertz Responses of Intrinsic Josephson Junctions in High Tc Superconductors. Phys. Rev. Lett., Vol.87, pp. 107002-107005. Ziegler, J. F; Biersack, J. P. & Littmark, U. (1996). The Stopping and Range of Ions in Solids, Pergamon, New York. Zhou, J.; Yang, G. (2006). Proceedings of the 7th ICFDM 2006 International Conference on Frontiers of Design and Manufacturing, pp. 453-458, Guangzhou, China, June 19-22, 2006
- 29. 2 Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining Fei Xu, Jun-long Kou, Yan-qing Lu and Wei Hu College of Engineering and Applied Sciences and National Laboratory of Solid State Mi-crostructures, Nanjing University, Nanjing, P. R. China 1. Introduction Optical fibers have been the basis of the modern information technology since Kao and Hockham proposed glass waveguides as a practical medium for communication in 1965. A lot of different optical fiber active/passive devices including couplers, interferometers, gratings, resonators and amplifiers have been widely employed for applications on telecommunications and sensing networks (Agrawal, 2002). For a number of applications, it is important to reduce the device’s size. Small size is often attractive for particular sensing applications because of some benefits such as fast response to detecting small objection with little perturbation on the object being measured. There are two steps to obtain fiber devices as small as possible. First, it is to taper or etch the fiber and reduce its diameter. A subwavelength-scale microfiber is the basic element of miniature fiber devices and sub- systems (Tong et al., 2003; Brambilla et al., 2004, 2005, 2010). The second is to engineer the microfiber to realize miniature version of conventional fiber devices. There are various fabrication methods to engineer the microfiber, such as CO2 laser, femtosecond (fs) laser, HF acid etching, arc splicing and focused ion beam (FIB). Most of these techniques have the difficulties in carving the microfiber freely because of the resolution. The latest progress in FIB technique has opened a new widow for ultra-small size fiber devices. So far, FIB is the most flexible and powerful tool for patterning, cross-sectioning or functionalizing a subwavelength circular microfiber due to its small and controllable spot size and high beam current density. FIB systems have been produced commercially for approximately thirty years, primarily for large semiconductor manufacturers. FIB systems operate in a similar fashion to a scanning electron microscope (SEM) except, rather than a beam of electrons and as the name implies, FIB systems use a finely focused beam of ions that can be operated at low beam currents for imaging or high beam currents for site specific sputtering or milling (http://en.wikipedia.org/wiki/Focused_ion_beam). The fine and controllable ion spot size and high beam current density are perfect for micro- and nano-fabrications with high spatial resolution (~ 10 nm). As a result, FIB has recently become a popular candidate for fabricating high-quality micro-devices or high-precision microstructures. Originally, FIB processing was used for mask repair (Liang et al., 2000), integrated circuit chip repair/modification (Liu et al., 2006), cross-sectional imaging of critical parts of
- 30. Micromachining Techniques for Fabrication of Micro and Nano Structures 18 semiconductor devices and sample preparation for transmission electron microscopy (Daniel et al., 1998; Hopman et al., 2008; Jeon et al., 2010). Besides these applications, FIB milling can also be used to assist carbon nanotube growth and manipulation (Hofmann et al., 2005; Deng et al., 2006), pattern magnetic data storage media (Terris et al., 2007) and structure hard-to-etch materials like SiC or LiNbO3. In the field of optoelectronics, there have been extensively studies toward utilizing the FIB as a machining tool to fabricate planar micro-optical components with low surface roughness for integrated optical circuits, for example, the end facet mirrors, ring resonators, gratings and photonic crystals (Hopman et al., 2008). Obviously, FIB processing can and in fact has been widely applied to fabricate microfiber based devices to reduce the size of fiber devices as much as possible. In this chapter, we will review several kinds of ultra-small engineered tapered fiber tip (TFT) devices including interferometers and gratings by FIB micromachining and their characteristics and sensing applications. 2. Fabrication and measurement Standard optical TFT is an optical microfiber with only one output or input end and a taper transition. The taper transition is connected to untapered fiber at the extremities which can easily be connected to other fiber optic components. The taper is etched or pulled from a standard single mode fiber when heated by a CO2 laser, electrical microheater or a small flame. Since the TFT is for analyte detecting rather than launching the light, it should be short enough in order to be rigid. However, too short and sharp shape results in high losses due to the poor ‘adiabaticity’ of the taper profile which couples light to lossy unbound modes (Love et al., 1991). During the last decade, much work has been carried out to study and optimize TFT profiles for telecom devices. Technology development allows manufacturing tapers with diameters well below 100 nm and it is possible to tailor the taper shape to an ideal profile (Brambilla & Xu, 2007). The quickest and simplest way to manufacture short TFT relies on using a commercially available pipette puller. This method is often used to manufacture fibre tips for optical tweezers and scanning near-field optical microscopy (SNOM) tips. In this chapter, we make TFTs using a commercial pipette puller (model P-2000, Sutter Instrument). The P-2000 is a microprocessor-controlled CO2 laser- based micropipette puller. The bare fibre is held on two puller stages. The P-2000 can also be used to pull tubes and optical fibres to extremely small diameters. The pipette puller has five parameters which can be adjusted to achieve the wanted profile. The fabrication process is simple, convenient and extremely fast, which usually takes less than 0.3 second. The obtained TFT is then checked under a high-magnification optical microscope. Figure 1 shows a microscope image of a typical TFT with a sharp profile. Fig. 1. Microscope image of a typical TFT, five photographs separated by four dashed vertical lines are used to show the whole profile of the TFT. The black arrow indicates the milling location (Kou et al., 2010b). Reprinted with permission. Copyright 2010 Optical Society of America 125 μm
- 31. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 19 The TFT is then coated with a thin metal layer such as aluminium (Al) or gold (Au, for exciting surface waves discussed in Section 4.2). The coating thickness is around 30 ~ 150 nm and the metal is deposited on only one side of the taper. The metal Al is used as a conductive layer to prevent gallium ion accumulation in the FIB micromachining process. Then, the Al-coated TFT is placed stably in the FIB machining chamber (Strata FIB 201, FEI Company, Ga ions) using conductive copper tape. We generally use a 30.0 kV gallium ion beam with current 60 ~ 300 pA. This enables us to make structures with high accuracy and sharp end-faces. We mill the structures from the taper end with small diameter to that with bigger diameter, because the milled part becomes non-conductive when the metal is removed by the beam. The total micro-machining process takes about 15 ~ 30 minutes depending on the size of the machined structures. Finally, the TFT is immersed in hydrochloric acid for about 15 ~ 30 minutes to totally remove the Al layer before cleaned with deionized water. In our experiment, the cavity or grating is made from a two-step process. Because there are some remains adhering onto the surfaces of the cavity after the first milling step, a second step under the same or smaller beam current is used to improve the surface smoothness. Fig. 2. Experimental setup of an FPMI. In this chapter, we mainly consider the reflected signals. The reflective spectral response of these TFT based devices are measured with a broadband source (1525 ~ 1610 nm) and an Ando AQ6317B optical spectrum analyzer (OSA) through a circulator, as shown in Fig. 2. The TFTs before milling display an ignorable reflection of less than - 100 dB over the whole broadband spectrum. 3. FIB machined micro-cavity TFT interferometers Optical fiber interferometers have been extensively used in various sensing applications due to its advantages of versatility, linear response and relatively simple structure. In the past decades, a lot of efforts have been made to develop intrinsic and extrinsic interferometers, especially the micro-cavity Fabry-Perot interferometers (MCFPIs). MCFPIs with tens-of- micrometer-length cavity are attractive because of the small size, large free spectrum range (FSR) and high sensitivity. The cavity can be assembled by splicing two single mode fibers (SMFs) to a hollow-core fiber (Sirkis et al., 1993), inserting a silica SMF and a multi-mode fiber into a glass capillary (Bhatia et al., 1996), or splicing a SMF and an index-guiding photonic crystal fiber together (Villatoro et al., 2009). Although many progresses have been made, people are still pursuing new micro cavity fabrication techniques to improve the cavity length precision, structure accuracy and the process repeatability. Femtosecond laser Broadband source Optical spectrum analyzer TFT Circulator SMF
- 32. Micromachining Techniques for Fabrication of Micro and Nano Structures 20 technology thus was proposed recently showing great success in micromachining fiber devices. MCFPIs can be quickly fabricated by milling a small-open hole in a SMF for liquid and gas sensing (Rao et al., 2007). However, even the fs-laser machined MCFPIs still show low fringe visibility of several dBs in liquids due to the rugged surfaces inside the cavity; what’s more, it is difficult to focus the laser spot to a sub-wavelength scale due to the diffraction limit. Thus the micromachining accuracy is limited and the size of the micro- cavity is large (tens of micrometers). The latest progress in FIB technique opens a new widow of opportunity for ultra-small size cavity (Kou et al., 2010a, 2010b). Microcavities with nanometer-scale accuracy in a subwavelength microfiber could be fabricated by FIB, which is relatively difficult for fs laser approach. There are several typical geometries which can be realized by FIB machined-TFT as shown in Fig. 3. Among them, an open-notch in one side is the most preferred and easiest to be fabricated. Fig. 3. Illustration of several typical geometries which can be realized by FIB machining, (a) a side open notch, (b) a hole in the middle and (c) a hole in the tip end and parallel to the fiber axis. For geometry (a), due to the low reflectivity of the air-glass interfaces, multiple reflections have negligible contributions to the optical interference. However, a TFT consists of a SMF and a MMF in nature, without splicing. It may hold both the original single core mode and the multi-modes in the cladding at different positions. As shown in Fig. 4, we only consider two reflections I1 and I2 at the two end-faces, respectively. The fundamental LP01 mode can be coupled to high-order LP0m mode in the taper transition or be excited to high-order LP0m mode at the end-faces. I1 or I2 possibly includes LP01 or LP0m mode (Kou et al., 2010b). We also break the cavity and measure the reflection I1 at end-face 1; flat reflective spectrum without obvious interference fringes is observed. Accordingly, a reasonable assumption is to consider only one dominated mode in I1 (LP01 mode) and I2 (LP01 or LP0m mode excited when inputting I2 into end-face 1) (Kou et al., 2010b). We call this kind of device as a hybrid FP modal interferometer (FPMI). The interference spectrum can be modelled using the following two-beam optical interference equation (Kou et al., 2010b): 1 2 1 2 0 2 cos( ) I I I I I (1) The phase difference between two modes in I1 and I2 is (Kou et al., 2010b) (a) (b) (c)
- 33. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 21 Fig. 4. Illustration of the FPMI. I1 and I2 are the reflections at end-face 1 and end-face 2 respectively; Lc is the length of the cavity. When I2 enters end-face 1, the fundamental mode is possible to be excited to a higher-order mode (Kou et al., 2010b). Reprinted with permission. Copyright 2010 Optical Society of America 2 1 1 2 1 2 1 2 ( ( ) ( )) ( (2 / )( ) 2 ) c c n r n r dz q q n L r (2) and FSR is (Kou et al., 2010b) FSR = 2πλ/δ (3) where q = 0 (for LP01 in I2) and 1 for (LP0m in I2); ∆1 (δ1) and ∆2 (δ2) are the optical path length difference (the phase difference) owing to the micro-cavity and the modal difference in the taper transition, respectively; n1(r) and n2(r) are the effective index of LP01 and LP0m modes, respectively, functions of local radius r(z) of the TFT at position z, which can be calculated by three-layer model of finite cladding step-profile fiber with the TFT profile r(z) which can be obtained from the microscope figure of the TFT (Kou et al., 2010b). 3.1 FIB machined FPMI for temperature sensing The FPMI can be applied as a high-temperature sensor. Its extremely small size and especially unique structure offer great potentials for fast-response high temperature sensing particularly in small and harsh area with high temperature gradient, such as micro-flame and high temperature gas-phase/liquid-phase flow in microfluidics channel. Figure 5 shows an SEM picture of a FPMI with a micro-notch cavity from the side view and cross section after cleaving the TFT at the cavity. The end-face is very sharp and smooth. The cavity is 4.4 μm long and 5 μm high, located at the position with the local radius r = 4.6 μm.
- 34. Micromachining Techniques for Fabrication of Micro and Nano Structures 22 Fig. 5. SEM image (a) of the micro-notch cavity from the side view: three arrows show the edges of the cavity at the fiber tip, (b) of the cross section with the fiber tip cleaved at the position indicated in (a) by a dash line (Kou et al., 2010b). Reprinted with permission. Copyright 2010 Optical Society of America The reflective spectral response of this FPMI device is measured with the setup as shown in Fig. 2. The TFT without a cavity displays an ignorable reflection of less than - 100 dB over the whole broadband spectrum. Hence, the detected signal is the light reflected only at the two end-faces of the micro-cavity, and the reflection at the tip end is negligible. The interference spectra of the FPMI device at room temperatures (19 °C) are shown in Fig. 6. The spectra indicates a free spectral range (FSR) of ~ 11 nm and a fringe visibility of ~ 11 dB around 1550 nm, which is larger than some other MCFPI sensors (Choi et al., 2008), and enough for sensing application. δ1 is ~ 12π and δ2 ~ 295π for LP03 mode, and FSR ~ 10 nm, in good agreement with what we obtain in the experiment. In our calculation, λ = 1530 nm, Lc = 4.4 μm and nc = 1 (Kou et al., 2010b). 1540 1560 1580 1600 -35 -30 -25 Wavelength (nm) Reflection (dB) 19 C 305 C 520 C Fig. 6. Interference spectra of the FPMI device in air at different temperatures (Kou et al., 2010b) Reprinted with permission. Copyright 2010 Optical Society of America We characterize the thermal response of the FTMI device by heating it up in a micro-furnace (FIBHEAT200, Micropyretics Heaters International Inc.) and temperature ranging from (a) (b)
- 35. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 23 room temperature (19 °C) to 520 °C is measured by a thermocouple (TES-1310, Type K, TES Electrical Electronic Corp.). The spectrum and temperature were recorded when both of them are stable for several minutes (Kou et al., 2010b). The temperature sensitivity ST is defined as the interference wavelength shift divided by the corresponding temperature change. ST depends on temperature through the thermal expansion and/or thermo-optics effect (Choi et al., 2008; Kou et al., 2010b): 1 2 2 2 1 2 1 2 2 2 ( ) (2 ) ( ) ( ) [ ] T T c T T d d d d S L dT dT dT dT d n n n n dz dT n r (4) where σT (1.1×10-5 /°C) is the thermo-optics coefficient and αT (5.5×10-7 /°C) is the thermal expansion coefficient. There are two contributions from temperature change: the temperature-induced length variation in the cavity, and the temperature-induced index variation and taper volume variation in taper transition. The first one is less than 1 pm/°C and ignorable, it agrees with the fact that those previous micro-cavity FP interferences in SMF by femtosecond laser machining are temperature-insensitive; the second one is about 10 ~ 20 pm/°C and dominates in temperature sensing (Kou et al., 2010b). Figure 7 displays the measured interferometer wavelength shifts (Δλ) and error on temperature (T). As the temperature increases, the interferometer wavelength shifts to longer wavelength. A third-order polynomial was used to fit the wavelength shifts across the entire calibration range. The average sensitivity of the device is ~ 17 pm/°C, which is very close to the theoretical result. Higher sensitivity can be obtained by optimizing the profile of the SMF-TT or using special fiber taper with higher thermo-optics coefficient (Kou et al., 2010b). Fig. 7. Dependence of the measured wavelength shift on temperature. The asterisk represents the measured results while the solid line is the fitting result. The inset shows the dependence of error on temperature (Kou et al., 2010b). Reprinted with permission. Copyright 2010 Optical Society of America 0 200 400 600 0 2 4 6 8 10 Temperature (C) Wavelength shift (nm) experimental data poly fitting =-7.42810 -8 T 3 +5.63610 -5 T 2 +6.52110 -3 T+9.83210 -2 200 400 600 -10 0 10 20 Thermocouple (C) Error (%)
- 36. Micromachining Techniques for Fabrication of Micro and Nano Structures 24 3.2 FIB machined FPMI for refractive index sensing The FPMI also can be employed as a refractive index sensor. Figure 8 shows the SEM picture of another FPMI with a micro-notch cavity from the side view. The cavity is 3.50 μm long and 2.94 μm high, located at the position with the local radius r = 2.4 μm. Fig. 8. SEM image of the micro-notch cavity from the side view (Kou et al., 2010a). Reprinted with permission. Copyright 2010 Optical Society of America 1530 1535 1540 1545 1550 1555 -50 -40 -30 -20 Wavelength (nm) Reflection (dB) air acetone isopropanol Fig. 9. Interference spectra of the MPRI device in air (solid line), acetone (dashed line) and isopropanol (dotted line), at room temperature (25 °C) (Kou et al., 2010a). Reprinted with permission. Copyright 2010 Optical Society of America The reflective Interference spectrum is measured with the same setup as shown in Fig. 2. Figure 9 shows the interference spectra of the MPRI in air, acetone and isopropanol at room temperature (25 °C). The interference spectrum indicates a fringe visibility maximum of ~ 20 dB, which is much higher than those of typical MCFPIs in liquids. The performance of resonant or interferometer refractive index sensors can be evaluated by using the sensitivity SR, which is defined as the magnitude in shift of the resonant wavelength divided by the change in refractive index of the analyte. The sensitivity was
- 37. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 25 measured by inserting the sensor in mixtures of isopropanol and acetone. These solutions were chosen with the objective of simulating aqueous solutions, having a refractive index in the region around 1.33 at a wavelength of 1550 nm. The ratio was increased by adding small calibrated quantities of isopropyl to the solution at a position far from the sensor. The refractive indexes of pure isopropyl and acetone at 1550 nm are 1.3739 and 1.3577 respectively (Wei et al., 2008). Figure 10 displays the shifted spectral wavelength as a function of the liquid mixture refractive index. The asterisks represent the measurement results while the solid line is the best-fitting. As the refractive index increases, the spectrum shows a red-shift. The sensitivity of the device is 110 nm/RIU (refractive index unit) according to Fig. 11. Higher sensitivity can be obtained by optimizing the profile of the microfiber taper probe. Due to its small size, fiber-probe structure, all fiber connection, linear response, low-cost, easy fabrication and high sensitivity, MPRI devices are promising in various chemical and biological applications. It even may offer fantastic potentials sensing inside sub-wavelength liquid droplets, bubbles or biocells because of its unique probe structure and possible smaller size (Kou et al., 2010a). 1.355 1.36 1.365 1.37 1.375 1532 1533 Refractive index Wavelength (nm) experiment data linear fitting (b) Fig. 10. The shifted spectral wavelength as a function of the liquid mixture refractive index. The asterisks represent the measured results while the solid line is the fitting results (Kou et al., 2010a). Reprinted with permission. Copyright 2010 Optical Society of America 4. FIB machined TFT micro-grating Since their discovery in 1978 (Hill et al., 1978), optical fiber gratings have found a variety of applications in telecom and sensing because of their relatively low cost, inherent self- referencing and multiplexing/demultiplexing capabilities. Over the last two decades, fiber gratings including fiber Bragg gratings (FBGs) and long-period gratings (LPGs) have been manufactured mainly by modifying the core refractive index using interferometric or point- by-point techniques; most of interferometric techniques use a phase mask and an ultraviolet (UV) laser (Hill et al., 1993) (typically excimer or frequency doubled Ar+ ion) or femtosecond
- 38. Micromachining Techniques for Fabrication of Micro and Nano Structures 26 lasers (near IR or UV). Gratings based on surface etched corrugations have also been demonstrated in etched fibers using photolithographic techniques (Lin & Wang, 2001). However all these gratings fabricated in thick fibers have weak refractive index modulations (Δnmod ~ 10-4 - 10-3) and the related grating lengths are of the order of several millimeters. To reduce the grating length, strong refractive index modulations (Δnmod > 10-2) are necessary. Strong Δnmod can be obtained by alternating layers of different materials, one of which can be air. Although this process in normal optical fibers imposes the removal of large amounts of material (the propagating mode is confined at a depth > 50 µm from the fiber surface), in fiber tapers and tips it requires the removal of small amounts of matter because the propagating mode is confined to the silica/air interface. A few techniques have been proposed for the fabrication of gratings in microfibers, including photorefractive inscription using CO2 lasers (Xuan et al., 2009), femtosecond lasers (Martinez et al., 2005; Xuan et al., 2010) and wrapping a microfiber on a microstructured rod (Xu et al., 2009, 2010). None of them produced strong and short Bragg gratings. In some cases extra polymer coatings are needed, while in others the use of CO2 lasers implies that the grating length is still long (it only can be used to write long period gratings (LPG) or high-order FBG). As a consequence, devices based on gratings tend to have a sizeable length: typically FBGs have lengths in the order of few millimetres. FIB technique provides a powerful way to mill the microfiber with directly and flexibly without a mask and realize compact micro-gratings (tens of micrometers) with colourful structure in the surface. Such small size and unique structure grating offer great potentials for a lot of sensing applications such as high temperature and refractive index sensing with the advantages of fast-response, ability to work in harsh environments and occupying little space. 4.1 FIB machined micro-grating for temperature sensing Figure 11 shows an SEM micrograph of an ultra-short second-order TFT micro-grating (TFT- MG). The grating has 11 shallow corrugations with period Λ = 1.1 μm, providing a total length of ~ 12 μm, two orders of magnitude shorter than FBGs fabricated in conventional optical fiber. Each notch is ~ 1.6 μm deep and ~ 0.6 μm long. The average radius at the position where the notches are located is ~ 2.7 μm. The Bragg wavelength of the grating can be calculated from λg = 2neffΛ/m, where neff is the mode effective refractive index in the equivalent unperturbed geometry, Λ is the period and m is the Bragg order. Unlike conventional circularly-symmetric FBGs, this nanostructured TFT-MG has asymmetric periodic corrugations. The modal field and neff in the nano-structured TFT can be derived analytically from the bare TFT using the method developed by W. Streifer, which considers an equivalent structure, obtained by shifting the boundary between air and silica to compensate for the different geometry. Figure 12 shows the cross-sections of an un-etched fiber, an etched fiber and the equivalent unperturbed geometry with this method, which shifts the boundary between air and silica to compensate for the different geometry (Streifer et al., 1975, 1978), respectively. The effective groove height heff of the equivalent unperturbed geometry satisfies (Kou et al., 2011a): (1 )( sin cos ) sin cos arccos{( ) / } arccos{( ) / } g g g eff eff eff g g eff eff r h r r h r (5)
- 39. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 27 where τ is the grating duty cycle, hg is the groove height and r is the fiber radius, respectively. In our device, r = 2.7 μm, τ = 0.33 and hg = 1.6 μm, we find heff = 1.2 μm by solving Eq. 5 and neff = 1.428 by utilizing a finite element method. Thus, the Bragg wavelength is 1571 nm. It agrees well with the following experimental results. Fig. 11. SEM picture of the nanostructured TFT-MG. The grating has 11 notches and a total length of ~ 12 μm. The notch length and depth are ~ 0.6 µm and ~ 1.6 µm, respectively. The grating period is Λ ~ 1.1 μm (Feng et al., 2011). Fig. 12. The cross-sections of un-etched fiber (a), etched fiber (b) and equivalent unperturbed geometry (c), respectively. hg is the groove height and heff is effective height (Kou et al., 2011a). The reflective spectral response of the TFT-MG in Fig. 10 is measured with the setup shown in Fig. 2. We characterize the thermal response of the TFT-MG by heating it up in a micro- furnace from room temperature (20 °C) to 230 °C. The spectrum and temperature are recorded when both of them are stable for several minutes.
- 40. Micromachining Techniques for Fabrication of Micro and Nano Structures 28 The interference spectra of the TFT-MG at different temperatures (23 °C, 47°C, 104°C, 153°C and 228 °C) are shown in Fig. 13. The Bragg wavelength is ~ 1570 nm, in agreement with our theoretical calculation. The spectra indicate a reflection peak-to-trough ratio around 1570 nm of ~ 10 dB at the Bragg wavelength which is achieved with as few as 11 periods and is similar with or even better than some other long length fiber gratings, enough for sensing applications. The temperature sensitivity ST is defined as (Kou et al., 2011a): 2 g eff eff T T T eff T silica d n n S n r dT m n r (6) where σT (1.4×10-5 /°C) is the thermo-optic coefficient and αT (5.5×10-7 /°C) is the thermal expansion coefficient. According to our calculations, the first part is about 15 ~ 20 pm/°C and dominates in temperature sensing. Thermal expansion effect (the second and third parts of Eq. 6) contributes little to the total sensitivity (< 6%), mainly due to the low thermal expansion coefficient of silica. Moreover, in the first part of Eq. 2, eff silica n n is nearly 1 and does not change much with the microfiber diameter, which means that the most efficient method to increase thermal sensitivity is to use fiber with higher thermo-optics coefficient. 0 50 100 150 200 250 -1 0 1 2 3 4 5 Temperature (C) Wavelength shift (nm) experimental data linear fitting Fig. 13. Dependence of the measured wavelength shift on temperature. The asterisk represents the measured results while the solid line is the linear fitting result (Feng et al., 2011). Figure 13 displays the measured resonant wavelength shifts (Δλ) on temperature (T). As the temperature increases, the interference wavelength shifts to longer wavelength. The average sensitivity of the device is ~ 22 pm/°C, which is very close to the theoretical result, higher than or similar with previous fiber grating sensors. Higher sensitivity can be obtained by use special fiber taper with higher thermo-optics coefficient. First-order micro-grating (m = 1) with smaller period also can be fabricated in TFT by FIB. Figure 14 shows the SEM photography of a first-order TFT-MG from the side view. The grating has shallow corrugations of period Λ = 600 nm with 61 periods. The total length is
- 41. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 29 about 36.6 μm, which is extremely short. Every groove is 200 nm in depth, located at the position with the local radius around r = 3.25 μm. The resonant spectra of the TFT-MG at different temperatures are shown in Fig. 15. The Bragg wavelength is ~ 1550 nm, with excited higher order mode as deduced from our theoretical calculation. The spectra indicate an extinction ratio of ~ 11 dB at the Bragg wavelength which is achieved with a 36.6 um long Bragg grating. The average temperature sensitivity of the device from room temperature to around 500 °C is ~ 20 pm/°C as shown in Fig. 15 (b), which is similar with the second-order TFT-MG. It is reasonable because the main thermal contribution is from the thermo-optic effect (Kou et al., 2011a). Fig. 14. Left: FIB picture of the TFPG with 61 periods (~ 36.6 μm in length and Λ = 600 nm). Right: magnified picture of the grating (Kou et al., 2011a). Reprinted with permission. Copyright 2010 Optical Society of America 1530 1540 1550 1560 1570 0 0.2 0.4 0.6 0.8 1 W avelength (nm) Reflection (a. u.) 21 C 124 C 187 C 100 200 300 400 0 2 4 6 8 Temperature (C) Wavelength shift (nm) Experimetal data Polyfitting Fig. 15. (a) Reflection spectra of the first-order TFT-MG in air at different temperatures. (b) Dependence of the measured wavelength shift on temperature. The asterisk represents the measured results while the solid line is the linear fitting result (Kou et al., 2011a). Reprinted with permission. Copyright 2010 Optical Society of America
- 42. Micromachining Techniques for Fabrication of Micro and Nano Structures 30 4.2 FIB machined metal-dielectric-hybrid micro-grating for refractive index sensing Conventional FBGs have been extensively developed to measure the temperature, pressure or stress. But it is scarcely used to measure the environmental refractive index variation because there is almost no evanescent field penetrating outside of a standard 125 μm diameter FBG. TFT-MG may overcome the drawback with the available evanescence field interacting with the outer environments. The sensitivity of a pure-silica TFT-MG with the diameter of several micrometers is about tens of nm/RIU. By inducing metal-cladding, more cladding modes are possible to be excited and higher sensitivity can be obtained, which is so called grating-assisted surface plasmon-polariton (SPP)-like grating sensor (Nemova & Kashyap, 2006). Figure 16 shows the SEM picture of a metal-dielectric-hybrid TFT-MG (MD-TFT-MG) by FIB milling. The fabrication process is similar with those mentioned ones above. But the fiber tip is coated with a gold layer with thickness of 30 nm on one side by magnetron sputtering and it is kept all the way throughout the experiment. We choose gold due to its relatively low absorption in the infrared and inertness to oxidation when exposed in air. Then a grating is fabricated by FIB milling at the fiber tip with local radius of ~ 3 μm. The grating has shallow corrugations of period Λ = 578 nm with 17 periods. The total length is about 10 μm, which is extremely short with local radius of ~ 3 μm. Fig. 16. SEM picture of the metal-dielectric-hybrid fiber tip grating (~ 10 μm in length and Λ = 578 nm). Right: magnified picture of the grating (Kou et al., 2011b). Optical characterization of the MD-TFT-MG in Fig. 16 is performed using the same setup as shown in Fig. 2. Figure 17 shows the reflection spectra of the MD-TFT-MG in air, acetone, and isopropanol, respectively. The extinction ratio is about ~ 10 dB. There are several valleys and peaks with different characteristics in the spectral range of ~ 100 nm. They shift when the outer environment changes from acetone to isopropanol. However, these valleys and peaks show larger shifts at longer wavelengths, while those at shorter wavelength region shift much less and almost stop at specific wavelengths. This unique response to outer liquid refractive index comes from the fact that the reflected light can be coupled to different modes. In the micrometer-diameter metal-dielectric-hybrid TFT, several modes are probably excited with similar propagation constant because of the metal cladding. Some modes are well confined in the tip and have negligible field overlap with the liquid while some modes
- 43. Miniature Engineered Tapered Fiber Tip Devices by Focused Ion Beam Micromachining 31 are not. The different valleys and peaks correspond to the coupling between these different forward and backward propagating modes, with different response properties for the outer environment. The reflection resonant condition for the grating is: 2 2 [ ] f b g n n (7) where nf and nb are the effective indices of the forward and backward modes, respectively. For simplicity, we assume a theoretical model to explain our experimental results which is simple and not perfectly matched with the experiment but can give the fundamental mechanism of the device. Within the model, the microfiber is 6 μm in diameter with uniform metal cladding (20 nm in thickness). However, the real device is much more complicated, with nonuniform metal cladding and diameter. And if an asymmetrical mode field lies mainly near the grating, leading to a larger modal overlap with the grating, it may result in a higher sensitivity. Figure 3 shows the calculation on the effective index of one cladding mode and one core mode as a function of outer liquid refractive index nl. Due to the existence of the metal layer, the cladding mode has a larger effective index (corresponding to long resonant wavelength) than that of the core mode (corresponding to short resonant wavelength) and has a larger overlap with the taper surface and the outside environment, leading to a higher sensitivity to the surrounding medium which is in coincidence with the spectra of Fig. 2. 1540 1560 1580 1600 -50 -45 -40 Wavelength(nm) Reflection (dB) Acetone Isopropanol a b c d Fig. 17. Measured reflection spectra of the FTG when immersed in acetone and isopropanol (Kou et al., 2011b). The performance of resonant refractive index sensors can be evaluated by using sensitivity S, which is defined as the magnitude in shift of the resonant wavelength divided by the change in refractive index of the analyte. In our experiment, the sensitivity is measured by inserting the sensor in a beaker containing mixtures of isopropanol and acetone, where the isopropanol component has the following ratios: 0, 1/7, 2/7, 3/7, 4/7 5/7, 6/7, and 1 (Kou et al., 2011b). Figure 18 displays measured resonant wavelength shifts of several peaks and valleys and fitting of this FTG on the liquid refractive index (a, b, c, d as marked in Fig. 2, a and c are
- 44. Micromachining Techniques for Fabrication of Micro and Nano Structures 32 peaks, b and d are valleys). As the refractive index increases, the resonant wavelength shifts to longer wavelength. The sensitivities of different modes change severely. It can be as high as 125 nm/RIU (peak a) or as low as 7 nm/RIU (valley d). For peak a (or valley b), both the resonant wavelength and sensitivity are larger than those of peak c (or valley d). According to our theoretical calculation, we believe peak a (or valley b) corresponds to cladding mode while peak c (or valley d) is core mode. The smallest sensitivity can be further decreased to nearly zero by optimizing the tip grating profile and metal coating. Because of many different properties on the outer liquid refractive index, the metal-dielectric-hybrid FTG can be applied as a multi-parameter sensor and the index-insensitive channel can be used to simultaneously measure temperature, pressure, and so on (Kou et al., 2011b). 1.36 1.365 1.37 0 0.5 1 1.5 2 n l Wavelength shift (nm) d c b a Fig. 18. Dependence of wavelength shift on outer liquid refractive index n1. The asterisks represent the experimental results with the solid line of linear fitting (Kou et al., 2010b). 5. Conclusion In this chapter, FIB machined TFT based micro-devices including interferometers and gratings are demonstrated. Being a very flexible, mask-less, direct write process, FIB milling is perfect for carving nanoscale geometries precisely in microfibers. Various miniature fiber devices can be realized and they show great potential in sensing with the unique geometry and size. The sensitivity such as of temperature or refractive index can’t increase too much because it mainly depends on the fiber materials and size. But the ultra-small size is attractive for some special application, in particular for detecting small-size objects. Some novel geometry is possible to be realized in microfiber such as an inline-microring, a slot-microfiber etc. 6. Acknowledgment This work is supported by National 973 program under contract No. 2010CB327803, 2012CB921803 and 2011CBA00200, NSFC program No. 11074117 and 60977039. The authors also acknowledge the support from the Priority Academic Program Development of Jiangsu (PAPD), and the Fundamental Research Funds for the Central Universities. 7. References Agrawal, G. P. (2002). Fiber-optic communication systems. New York, Wiley-Interscience.
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- 47. 3 Fundamentals of Laser Ablation of the Materials Used in Microfluiducs Tai-Chang Chen and Robert Bruce Darling University of Washington, USA 1. Introduction Microfluidics falls into an intermediate range within the spectrum of applications for microfabrication techniques. The width and depth of most microfluidic channels fall in the range of 10-1000 µm, and this feature size is thus small for conventional machine tool microfabrication, but quite large for photolithographically defined etching processes of the type used within the microelectronics industry. In addition, most microfluidic channels occupy only ~10% or less of the surface area of a microfluidic device. Wet chemical or plasma etching processes to produce microfluidic devices therefore take considerable time to complete, based upon the comparatively deep depths that are required for the channels. A comparatively fast wet or dry etching rate of 1 µm/min would still require up to several hours per wafer to achieve these depths. The small surface areas that are etched within this time make conventional batch processing of wafers less attractive economically. In many cases, photolithographically defined microfluidic features with micron scale accuracy are more precise than what is required for these applications. At high volumes, other microfabrication processes become more applicable for the manufacture of microfluidics. Roll-to-roll stamping, lamination, hot embossing, and injection molding of plastic components offer excellent accuracy, repeatability, and cost effectiveness once the non-recoverable engineering (NRE) costs of molds, dies, and master templates have been paid for. However, the cost of these NRE items is comparatively high, and in most circumstances, production volumes of >1 million parts are required to recover this cost. For part volumes from 1 to 1 million, laser microfabrication offers an excellent balance between speed, cost, and accuracy for microfluidics. Laser micromachining is also unmatched in the breadth of different of materials that it can process. A single laser system can micromachine materials all the way from lightweight plastics and elastomers up through hard, durable metals and ceramics. This versatility makes laser micromaching extremely attractive for prototyping and development, as well as for small to medium run manufacturing. The most common criticism of laser micromachining is that it is a serial, rather than batch process, and it is therefore too slow to be economical for high volume manufacturing. While certainly true in some instances, as a generalization, this is not always the case. The processing time per part is the sum of the beam exposure time plus the beam positioning time. For parts which require only minimal volumes of material to be removed, serial
- 48. Micromachining Techniques for Fabrication of Micro and Nano Structures 36 processes such as laser micromachining can indeed be extremely efficient and cost effective. Whereas older laser micromachining systems were often limited by clumsy beam positioning, modern systems incorporate high speed beam positioning and parts handling so that the overall processing time is limited more by the net beam exposure time, which for many applications can be fairly small. A good counter-example to the criticism of serial processing is chip resistor trimming, which is used for almost all 1% tolerance and better metal film chip resistors in the microelectronics industry today and which are produced in extremely high volumes, >10 billion/year. Microfluidics is becoming increasingly used for miniaturized chemical analysis systems, such as the new generations of lab-on-a-chip applications which are rapidly being developed. The fundamental structure used in microfluidics is the flow channel, but integrated microfluidic systems also incorporate vias, T-junctions, sample wells, reaction chambers, mixers, and manifolds, along with some moving mechanical components such as valves, pumps, and injectors, and often some optical and electrical components for integrated control and sensing. Unlike wet and dry etching which must be carefully formulated to achieve the required material selectivity, laser micromachining can be used to process many different materials and structures at a time. For example, a laser can be used to cut a channel to one depth, cut a via to another depth, trim a metal trace, release a check valve structure, and weld two mating elements together all within the same mounting of the part. This illustrates one of the advantages that serial processing has over traditional batch processing of wafers. Another obvious advantage of serial laser processing is that no masking is required, greatly reducing the time and expense for design changes. Different parts can also be individually customized with virtually no extra tooling overhead. Microfluidics and laser micromachining are an excellent marriage of technologies which will prove essential for the rapid development of these applications. This chapter will discuss the fundamentals of laser ablation in the microfabrication of microfluidic materials. After briefly describing the various types of lasers which are used for this purpose, the fundamental mechanisms of laser micromachining will be described, along with some data illustrating the performance of some state-of-the-art laser micromachining systems. 1.1 Lasers for micromachining By far the most common laser used for industrial processing is the carbon dioxide (CO2) gas laser. This popularity comes from its unique combination of high average power, high efficiency, and rugged construction. Unlike the original glass tube style gas lasers, the modern CO2 lasers which are used for materials processing are of a hard sealed waveguide construction that use extruded aluminum RF driven electrodes to excite a CO2/N2/He gas mixture. The lasing transitions are from asymmetric to symmetric stretch modes at 10.6 µm, or from asymmetric stretch to bending modes at 9.4 µm of the CO2 molecule (Verdeyen, 1989). Within each of these vibrational modes there exist numerous rotational modes, and hundreds of lasing transitions can be supported by excitation into the parent asymmetrical stretch mode of the CO2 molecules. This large number of simultaneous lasing modes along with the efficient excitation coupling through the N2 gas is what allows CO2 lasers to achieve power levels up to 1 kW with electrical to optical conversion efficiencies of nearly 10%. CO2 lasers emit in the mid-infrared (MIR), most commonly at 10.6 µm, and they principally interact with their target materials via focused, radiant heating. They are used
- 49. Fundamentals of Laser Ablation of the Materials Used in Microfluiducs 37 extensively for marking, engraving, drilling, cutting, welding, annealing, and heat treating an enormous variety of industrial materials (Berrie & Birkett, 1980; Crane & Brown, 1981; Crane, 1982). For micromachining applications, the long wavelength translates into a fairly large spot diameter of ~50-150 µm with a corresponding kerf width when used for through cutting. The most common solid-state laser used in industry is the neodymium-doped yttrium- aluminum-garnet, or Nd:YAG. The YAG crystal is a host for Nd3+ ions, whose lasing transitions from the excited 4F3/2 band to the energetically lower 4I11/2 band produces emission at 1.064 µm in the near-infrared (NIR) (Koechner, 1988; Kuhn, 1998). Nearly all industrial Nd:YAG lasers are now pumped by semiconductor diode lasers, usually made of GaAlAs quantum wells and tuned to emit at ~810 nm, for optimum matching to the pertinent absorption band of Nd:YAG. Semiconductor diode pumping of Nd:YAG offers much more efficient pumping with minimal energy being lost to heat, since the diode emits only into that part of the spectrum which is needed for the pumping. However, semiconductor diode pump lasers can only be made up to ~100 W, and thus these are used only for Nd:YAG lasers of low to moderate average powers. Most industrial Nd:YAG lasers are also Q-switched, usually by means of a KD*P electrooptic intracavity modulator. When the modulator is in the non-transparent state, the pumping of the Nd:YAG rod allows the population inversion to build up to very high levels. When the modulator is rapidly switched to the transparent state, the energy stored in the inverted population is discharged at once into a single giant pulse of narrow duration and high peak power. Typical Q- switched pulse widths are in the range of ~25 ns, and with firing repetition rates of ~40 kHz, the duty cycle of a Q-switched Nd:YAG laser is ~1:1000. A ~10 W average power Nd:YAG laser can then produce pulses with peak powers of ~10 kW. This high peak power makes Q- switched Nd:YAG lasers ideally suited for nonlinear optical frequency multiplication through the use of an external cavity harmonic generating crystal such as KDP, KTP, LiNbO3, or BBO. Most commonly, the 1064 nm output from the Nd:YAG is frequency doubled to produce a green output at 532 nm. The 1064 nm output can also be frequency tripled to produce 355 nm in the near ultraviolet (UVA band), or frequency quadrupled (using a sequential pair of doublers) to 266 nm in the deep ultraviolet (UVC band). All four of these commonly available Nd:YAG output wavelengths are extremely useful for micromachining purposes (Atanasov et al., 2001; Tunna et al., 2001). Copper vapor lasers have also proven their use in high accuracy micromachining (Knowles, 2000; Lash & Gilgenbach, 1993). Similar to the Nd:YAG, they are Q-switched systems which produce high intensity pulses of typically ~25 ns at rates of 2-50 kHz and average powers of 10-100 W. Unlike the Nd:YAG, they emit directly into the green at 511 nm and 578 nm, and thus do not require a nonlinear crystal for frequency multiplication to reach these more useful wavelengths. Copper vapor lasers also have excellent beam quality and can usually produce a diffraction-limited spot on the substrate with only simple external beam steering optics. The disadvantage of copper vapor lasers is that they tend to have shorter service life and require more maintenance than Nd:YAG lasers. Frequency multiplying crystals have now become a ubiquitous feature of commercial Nd:YAG lasers, and as a result, Nd:YAGs have largely displaced the copper vapor laser for industrial micromachining applications. Excimer lasers have also found wide use in materials processing applications. Excimer lasers operate from a molecular transition of a rare gas-halogen excited state that is usually pumped by an electric discharge. The XeCl excimer laser, which emits at 308 nm, is prototypical of these in which a pulsed electric discharge ionizes the Xe into a Xe+ state and
- 50. Micromachining Techniques for Fabrication of Micro and Nano Structures 38 ionizes the Cl2 into a Cl− state. These two ions can then bind into a Xe+Cl− molecule which will loose energy through a lasing transition as it relaxes back to the XeCl state. The resulting ground state XeCl molecule readily dissociates, and these products are then recycled. Other commonly used excimer lasers are the XeF which emits at 351 nm, the KrF which emits at 249 nm, the ArF which emits at 193 nm, and the diatomic F2 which emits at 157 nm (Kuhn, 1998). Like other laser systems which are well matched to applications in materials processing, excimer lasers produce pulses of ~50 ns with repetition rates of ~100 Hz to ~10 kHz and average powers of up to a few hundred Watts. Excimer lasers are fairly efficient in their electrical to optical conversion efficiency, but their use of highly reactive halogen gases at high pressures requires significantly more servicing and maintenance than other types. One of the most important properties of excimer lasers is their ability to create a rather large spot size which can be homogenized into a high quality flat top beam profile of up to several cm in dimension. Because of this, they have been the pre-eminent source for coherent UV radiation at moderate power levels, they can be used both as a masked or a scanned exposure source, and currently they are used extensively for UV and deep UV lithography as well as several other applications in thin film recrystallization and annealing. At higher beam intensities, they can be used for surface ablation of materials, and due to the short wavelength and short pulse width, they typically produce clean, crisp features in metals, ceramics, glasses, polymers, and composites, making them adaptable for numerous micromachining applications (Gower, 2000). Short laser pulses, on the order of a few tens of nanoseconds, are a desirable feature for laser micromachining applications, and these can be produced with many different laser systems. As will be discussed in more detail later, the short pulse width produces nearly adiabatic heating of the substrate which allows the substrate surface temperatures to quickly reach the point of vaporization with minimal heating effects on the surrounding areas. There has been interest in laser systems which can produce even shorter pulse widths, and the foremost candidate for this has been the Ti:sapphire laser. The Ti:sapphire laser has the unique feature of being tunable over a surprisingly large fluorescence band: from ~670 nm to ~1090 nm. For efficient pumping, it needs to be optically excited in its absorption band, which is centered about 500 nm, and for which argon ion lasers and frequency doubled Nd:YAG lasers provide excellent sources (Kuhn 1998). Most Ti:sapphire lasers are configured into an optical ring resonator arrangement with a set of birefringent filters for tuning. In addition, the ring cavity usually contains a Faraday rotator and wave plates to limit the propagation to only one direction around the ring. This arrangement is well suited for wide tuning and also mode locking, through which very short pulses, on the order of a few tens of femtoseconds can be produced. Ti:sapphire lasers have thus become a key resource for spectroscopy and research on ultrafast phenomena. The Ti:sapphire laser is also capable of average powers of up to several Watts, which makes it a viable tool for micromachining. Although its operation is at longer wavelengths than those normally preferred for micromachining, its capability for tuning and producing ultrashort pulses makes it attractive for research in this area. Since it requires a pump laser of ~10 W which is already in the green, and its more complicated optical system requires more maintenance and user savvy, it is presently not a common choice for industrial micromachining applications, but this may change in the future. There are many other new laser systems under development which offer efficient generation of green light at the power levels and pulse widths required for micromachining. It is worthwhile to realize that the field of laser sources is constantly changing.
- 51. Fundamentals of Laser Ablation of the Materials Used in Microfluiducs 39 In general, the lasers best suited for micromachining are those that produce short pulses of high intensity at short wavelengths. Pulse widths of less than a microsecond are needed to allow the formed plasma to extinguish in between pulses so that subsequent pulses are not scattered and absorbed. Time for the debris plume to clear takes longer, often up to tens or hundreds of milliseconds, but its optical attenuation is usually less. Concentrating the laser radiation into short pulses of high intensity also has the benefit of more adiabatic heating of the substrate, bringing its temperature up to the vaporization point before too much of the heat can diffuse vertically and laterally away from the intended ablation zone. Shorter wavelengths generally have higher absorption coefficients in most materials, and they are thus absorbed nearer to the surface where the ablation is intended to occur. Shorter wavelengths can also be focused into a proportionally smaller diffraction-limited spot, which improves both the accuracy and precision of the ablation process. Typical working spot diameters for UV lasers in the 350 nm range are ~25 m, although this is larger than the theoretical diffraction limit. 2. Fundamental laser micromachining processes Laser micromachining includes a number of different processes which are differentiated by the feature geometry and the manner in which material is removed from the substrate (Ion, 2005; Schuöcker, 1999). Cutting in this context refers to using the beam to slice all of the way through a thin sheet of substrate material, leaving behind a kerf which extends completely through to the opposite side of the substrate. As is commonly the case in laser cutting of sheet metal, the material removed from the kerf is predominantly ejected out the opposite side. Ablating is usually taken to mean removal of material in a thin layer from one side only, giving only partial penetration into the thickness of the substrate, and the removed material must necessarily be ejected from the same side as which the laser is incident. In both cases, the newly removed material is ejected primarily through the kerf which has just previously been cut and which trails along behind the laser beam as it is moved along the tool path. Whereas cutting and ablating can create geometries of any shape, drilling refers to the creation of a nominally circular hole with minimal lateral translation of the beam, with either through or blind penetration. If the laser beam is held in one fixed position and pulsed, the process often termed percussion drilling, whereas if the beam is swept around in a circular pattern to first roughly remove the bulk material and then completed with a fine finishing pass to accurately define the perimeter, the process is called trepanning. Percussion drilling produces holes whose diameter is roughly the same as the diameter of the laser beam, while trepanning produces holes whose diameter is larger than the beam diameter. Because drilling does not produce a trailing kerf, all removed material must be ejected from the same side as which the laser beam was incident, and drilling is thus necessarily an ablative process, regardless of whether it creates a through or blind hole (Voisey et al., 2003). The removal of material can involve both thermal and chemical processes, depending upon how the laser radiation interacts with the substrate. At longer wavelengths, the photon energy is insufficient to provide anything more than simple heating of the substrate. At sufficiently high intensities, however, the heating can be concentrated enough to first melt the substrate material within a localized zone, and then vaporize it in those areas where the laser intensity and subsequent heating is higher. The substrate material is thus removed via a transition to the gas phase, although the vaporized material is often subsequently ionized
- 52. Micromachining Techniques for Fabrication of Micro and Nano Structures 40 by the laser radiation, leading to a plasma and plume that can have the effect of occluding the incident beam. It is customary to identify three zones around the incident beam: the heat-affected zone or HAZ, the melt zone, and the vaporization zone. Some materials can pass directly from the solid phase into the vapor phase by sublimation, and for these the melt zone is absent. Both melting followed by vaporization or direct sublimation are purely thermal ablation processes. At shorter wavelengths, the photon energy may reach the level of the chemical bond strength of the substrate. Laser radiation may then break these chemical bonds through direct photon absorption, leading to volatilization of the substrate into simpler compounds. For most organic polymers, this photolysis process produces mainly H2O and CO2. This occurs typically for photon energies above 3.5 eV, or for wavelengths shorter than ~350 nm, i.e. into the near UV part of the spectrum. Because the photon energy is lost to chemical bond scission, the heating effects of the beam are greatly reduced, and this regime is sometimes referred to as “cold laser machining,” non-thermal ablation, or photochemical ablation. This greatly reduces the transient thermal stresses that occur as part of thermal ablation, and the result is less bowing, warping, and delamination of the substrate, as well as fewer edge melting effects which degrade feature accuracy (Yung et al., 2002). Since the peak temperature rise is greatly reduced, conductive heat flow away from the irradiation area is also reduced, and better dimensional control of the micromachined structure is obtained. There has been a general trend toward using shorter wavelength lasers for micromachining over the past two decades of development. Currently, UV lasers in the 350 to 250 nm range dominate the industrial market for the above reasons. Thermal ablation and photochemical ablation are two ideal extremes, and laser micromachining can often involve a combination of both for any given material or composite. In addition, there are several secondary processes which can arise due to the steep temperature gradients which are produced. If the laser beam is composed of short, high-intensity pulses, as would be typical for Q-switched systems, then the adiabatic heating of the substrate can cause sufficiently high temperature gradients for which differential thermal expansion and acoustic shock can produce surface cracking or spalling of the substrate (Zhou et al., 2003). Micron-sized flakes of the substrate can be explosively ejected from this process without requiring the additional thermal energy to fully vaporize the material. This is typically more prevalent for brittle materials with low thermal conductivity, e.g. ceramics and some glasses. For materials which readily oxidize, the rapid cycle of laser heating and cooling of the melt zone can cause the formed oxide film to flake off in chips from the compressive stress that was built into the oxide during the process. This is typically more prevalent for reactive metals such as chromium, nickel, iron, and copper. Thermal spalling and oxide chipping both create debris particles which are significantly larger than the redeposition of fully vaporized substrate material. Because both thermal spalling and oxide chipping occur after the melt zone has refrozen, they leave behind a surface finish which is typically more frosted or matte in visual appearance, and microscopically cusped on a smaller scale. Inherent to all laser micromachining is the creation of a plume of ejected material, either fully vaporized or sometimes containing micron-sized debris flakes. This plume requires time to disperse, and if the next laser pulse arrives before this takes place, the laser radiation will usually produce some degree of ionization as it is absorbed by the vapor. This ionization of the vaporized material produces a plasma which, in addition to being fairly energetic and reactive, can absorb the laser radiation further, sometimes occluding the path
- 53. Fundamentals of Laser Ablation of the Materials Used in Microfluiducs 41 for the beam to reach the substrate (Eloy, 1987). This luminous plasma is what is usually responsible for the “sparkles” that mark the travel of the laser beam across the substrate. Achieving beam positioning and pulse timing to avoid the plasma and plume occlusion of the beam is a central part of tuning the recipe for any laser micromachining. This problem is generally severe in continuous wave (CW) laser micromachining, but greatly reduced for pulsed lasers which are Q-switched. While the complete plume of vaporized material usually does not have time to fully disperse in between Q-switched pulses, the more optically opaque and higher density plasma does, and laser ablation can continue onward with usually only minor attenuation. If the beam positioning is not well designed, however, the plasma and plume can become trapped into the confined spaces of the kerf, and greater time will be required for their dispersal. The most common symptom of this effect is a reduced depth of ablation for a given beam transversal rate. 2.1 Ablation process models Laser ablation involves a complex interaction between optical, thermal, and chemical processes, but some simplifications can lead to models which can be useful for characterization, optimization, and troubleshooting of the process. Most such models start with the optics of a Gaussian beam and compute the conductive flow of heat from this source to find the temperature distribution, adding in the thermal effects which are needed to account for melting and vaporization of the substrate (Engin & Kirby, 1996; Kaplan, 1996; Olson & Swope, 1992). An idealized geometry is illustrated in Fig. 1 in which a circularly symmetric Gaussian laser beam is moved across the substrate at a constant speed v in the +x direction. The beam has an average power of P0 = πrB 2I0, where I0 is the peak intensity and rB is the 1/e beam radius. The beam propagates in the +z direction and meets the substrate surface in the x-y plane. The situation is more easily described by using the relative coordinate ξ = x − vt which moves along with the laser beam. The interaction of the laser beam with the substrate first involves absorption of the optical radiation and its conversion into heat for thermal (non-photo-chemical) ablation. Shorter wavelengths are absorbed more strongly at the surface with a higher absorption coefficient , and since this is usually ~104 cm−1 or greater, the heating is effectively concentrated at the surface of the substrate. Volumetric heating effects have been considered by Zhang, et al. (2006). The surface heating density is then 2 2 2 0 2 B y q( ,y) (1 R)I exp [W / m ], r where R is the reflectivity loss from the surface of the substrate. The heat transfer within the substrate is entirely by conduction, so the resulting temperature field is given by a solution to the heat conduction equation (Carslaw & Jaeger, 1959) 2 T D T 0, t where D = κ/ρC is the thermal diffusivity, κ is the thermal conductivity, ρ is the mass density, and C is the specific heat capacity. The surface heating density provides a source boundary condition for the solution of the heat conduction equation. Ashby and Easterling (1984) have shown that a close analytical approximation to the solution of this problem is given by
- 54. Micromachining Techniques for Fabrication of Micro and Nano Structures 42 2 2 0 0 0 1/ 2 2 2 B B y (1 R)P (z z ) T( 0,y,z,t) T exp , 4Dt 4Dt r 2 v t(t r / D) where T0 is the initial temperature of the substrate, and z0 is a parameter chosen to eliminate the surface singularity as t → 0. Fig. 1. Geometry and intensity and temperature profiles for laser ablation.
- 55. Another Random Scribd Document with Unrelated Content
- 56. them, are very unjust to us all. I am always ready to make any sacrifices to do justice to engagements, and would rather sell anything, or everything, than be less than true men to the world." ARCHIBALD CONSTABLE From the painting by Raeburn I have already, perhaps, said enough to account for the general want of success in this publishing adventure; but Mr. James Ballantyne sums up the case so briefly in his deathbed paper, that I may here quote his words. "My brother," he says, "though an active and pushing, was not a cautious bookseller, and the large sums received never formed an addition to stock. In fact, they were all expended by the partners, who, being then young and sanguine men, not unwillingly adopted my brother's hasty results. By May, 1813, in a word, the absolute throwing away of our own most valuable publications, and the rash adoption of some injudicious speculations of Mr. Scott, had introduced such losses and
- 57. embarrassments, that after a very careful consideration, Mr. Scott determined to dissolve the concern." He adds: "This became a matter of less difficulty, because time had in a great measure worn away the differences between Mr. Scott and Mr. Constable, and Mr. Hunter was now out of Constable's concern.[23] A peace, therefore, was speedily made up, and the old habits of intercourse were restored." How reluctantly Scott had made up his mind to open such a negotiation with Constable, as involved a complete exposure of the mismanagement of John Ballantyne's business as a publisher, will appear from a letter dated about the Christmas of 1812, in which he says to James, who had proposed asking Constable to take a share both in Rokeby and in the Annual Register, "You must be aware, that in stating the objections which occur to me to taking in Constable, I think they ought to give way either to absolute necessity or to very strong grounds of advantage. But I am persuaded nothing ultimately good can be expected from any connection with that house, unless for those who have a mind to be hewers of wood and drawers of water. We will talk the matter coolly over, and, in the mean while, perhaps you could see W. Erskine, and learn what impression this odd union is like to make among your friends. Erskine is sound- headed, and quite to be trusted with your whole story. I must own I can hardly think the purchase of the Register is equal to the loss of credit and character which your surrender will be conceived to infer." At the time when he wrote this, Scott no doubt anticipated that Rokeby would have success not less decisive than The Lady of the Lake; but in this expectation—though 10,000 copies in three months would have seemed to any other author a triumphant sale—he had been disappointed. And meanwhile the difficulties of the firm, accumulating from week to week, had reached, by the middle of May, a point which rendered it absolutely necessary for him to conquer all his scruples. Mr. Cadell, then Constable's partner, says in his Memoranda,—"Prior to this time the reputation of John Ballantyne and Co. had been decidedly on the decline. It was notorious in the trade that their
- 58. general speculations had been unsuccessful; they were known to be grievously in want of money. These rumors were realized to the full by an application which Messrs. B. made to Mr. Constable in May, 1813, for pecuniary aid, accompanied by an offer of some of the books they had published since 1809, as a purchase, along with various shares in Mr. Scott's own poems. Their difficulties were admitted, and the negotiation was pressed urgently; so much so, that a pledge was given, that if the terms asked were acceded to, John Ballantyne and Co. would endeavor to wind up their concerns, and cease as soon as possible to be publishers." Mr. Cadell adds: "I need hardly remind you that this was a period of very great general difficulty in the money market. It was the crisis of the war. The public expenditure had reached an enormous height; and even the most prosperous mercantile houses were often pinched to sustain their credit. It may easily, therefore, be supposed that the Messrs. Ballantyne had during many months besieged every banker's door in Edinburgh, and that their agents had done the like in London." The most important of the requests which the laboring house made to Constable was that he should forthwith take entirely to himself the stock, copyright, and future management of the Edinburgh Annual Register. Upon examining the state of this book, however, Constable found that the loss on it had never been less than £1000 per annum, and he therefore declined that matter for the present. He promised, however, to consider seriously the means he might have of ultimately relieving them from the pressure of the Register, and, in the mean time, offered to take 300 sets of the stock on hand. The other purchases he finally made on the 18th of May were considerable portions of Weber's unhappy Beaumont and Fletcher—of an edition of De Foe's novels in twelve volumes—of a collection entitled Tales of the East in three large volumes, 8vo, double-columned—and of another in one volume, called Popular Tales—about 800 copies of The Vision of Don Roderick—and a fourth of the remaining copyright of Rokeby, price £700. The immediate accommodation thus received amounted to £2000; and Scott, who had personally conducted the latter part of the negotiation, writes thus to his junior partner, who
- 59. had gone a week or two earlier to London in quest of some similar assistance there:— TO MR. JOHN BALLANTYNE, CARE OF MESSRS. LONGMAN & CO., LONDON. Printing-Office, May 18, 1813. Dear John,—After many offs and ons, and as many projets and contre-projets as the treaty of Amiens, I have at length concluded a treaty with Constable, in which I am sensible he has gained a great advantage;[24] but what could I do amidst the disorder and pressure of so many demands? The arrival of your long-dated bills decided my giving in, for what could James or I do with them? I trust this sacrifice has cleared our way, but many rubs remain; nor am I, after these hard skirmishes, so able to meet them by my proper credit. Constable, however, will be a zealous ally; and for the first time these many weeks I shall lay my head on a quiet pillow, for now I do think that, by our joint exertions, we shall get well through the storm, save Beaumont from depreciation, get a partner in our heavy concerns, reef our topsails, and move on securely under an easy sail. And if, on the one hand, I have sold my gold too cheap, I have, on the other, turned my lead to gold. Brewster[25] and Singers[26] are the only heavy things to which I have not given a blue eye. Had your news of Cadell's sale[27] reached us here, I could not have harpooned my grampus so deeply as I have done, as nothing but Rokeby would have barbed the hook. Adieu, my dear John. I have the most sincere regard for you, and you may depend on my considering your interest with quite as much attention as my own. If I have ever expressed myself with irritation in speaking of this business, you must impute it to the sudden, extensive, and unexpected embarrassments in which I found myself involved all at once. If to your real goodness of heart and integrity, and to the quickness and acuteness of your
- 60. talents, you added habits of more universal circumspection, and, above all, the courage to tell disagreeable truths to those whom you hold in regard, I pronounce that the world never held such a man of business. These it must be your study to add to your other good qualities. Meantime, as some one says to Swift, I love you with all your failings. Pray make an effort and love me with all mine. Yours truly, W. S. Three days afterwards Scott resumes the subject as follows:— TO MR. JOHN BALLANTYNE, LONDON. Edinburgh, 21st May, 1813. Dear John,—Let it never escape your recollection, that shutting your own eyes, or blinding those of your friends, upon the actual state of business, is the high road to ruin. Meanwhile, we have recovered our legs for a week or two. Constable will, I think, come in to the Register. He is most anxious to maintain the printing-office; he sees most truly that the more we print the less we publish; and for the same reason he will, I think, help us off with our heavy quire-stock. I was aware of the distinction between the state and the calendar as to the latter including the printing-office bills, and I summed and docked them (they are marked with red ink), but there is still a difference of £2000 and upwards on the calendar against the business. I sometimes fear that, between the long dates of your bills, and the tardy settlements of the Edinburgh trade, some difficulties will occur even in June; and July I always regard with deep anxiety. As for loss, if I get out without public exposure, I shall not greatly regard the rest. Radcliffe the physician said, when he lost £2000 on the South Sea scheme, it was only going up 2000 pair of stairs; I say, it is only writing 2000
- 61. couplets, and the account is balanced. More of this hereafter. Yours truly, W. Scott. P. S.—James has behaved very well during this whole transaction, and has been most steadily attentive to business. I am convinced that the more he works the better his health will be. One or other of you will need to be constantly in the printing- office henceforward,—it is the sheet-anchor. The allusion in this postscript to James Ballantyne's health reminds me that Scott's letters to himself are full of hints on that subject, even from a very early period of their connection; and these hints are all to the same effect. James was a man of lazy habits, and not a little addicted to the more solid, and perhaps more dangerous, part of the indulgences of the table. One letter (dated Ashestiel, 1810) will be a sufficient specimen:— TO MR. JAMES BALLANTYNE. My dear James,—I am very sorry for the state of your health, and should be still more so, were I not certain that I can prescribe for you as well as any physician in Edinburgh. You have naturally an athletic constitution and a hearty stomach, and these agree very ill with a sedentary life and the habits of indolence which it brings on. Your stomach thus gets weak; and from those complaints of all others arise most certainly flatulence, hypochondria, and all the train of unpleasant feelings connected with indigestion. We all know the horrible sensation of the nightmare arises from the same cause which gives those waking nightmares commonly called the blue devils. You must positively put yourself on a regimen as to eating, not for a month or two, but for a year at least, and take regular exercise—and my life for yours. I know this by myself, for if I were to eat and drink in town as I do here, it
- 62. would soon finish me, and yet I am sensible I live too genially in Edinburgh as it is. Yours very truly, W. Scott. Among Scott's early pets at Abbotsford there was a huge raven, whose powers of speech were remarkable, far beyond any parrot's that he had ever met with; and who died in consequence of an excess of the kind to which James Ballantyne was addicted. Thenceforth, Scott often repeated to his old friend, and occasionally scribbled by way of postscript to his notes on business— "When you are craving, Remember the Raven." Sometimes the formula is varied to— "When you've dined half, Think on poor Ralph!" His preachments of regularity in book-keeping to John, and of abstinence from good cheer to James Ballantyne, were equally vain; but on the other hand it must be allowed that they had some reason for displeasure—(the more felt, because they durst not, like him, express their feelings)[28]—when they found that scarcely had these "hard skirmishes" terminated in the bargain of May 18, before Scott was preparing fresh embarrassments for himself, by commencing a negotiation for a considerable addition to his property at Abbotsford. As early as the 20th of June he writes to Constable as being already aware of this matter, and alleges his anxiety "to close at once with a very capricious person," as the only reason that could have induced him to make up his mind to sell the whole copyright of an as yet unwritten poem, to be entitled The Nameless Glen. This copyright he then offered to dispose of to Constable for £5000; adding, "this is considerably less in proportion than I have already made on the share of Rokeby sold to yourself, and surely that is no unfair admeasurement." A long correspondence ensued, in the course of
- 63. which Scott mentions The Lord of the Isles, as a title which had suggested itself to him in place of The Nameless Glen; but as the negotiation did not succeed, I may pass its details. The new property which Scott was so eager to acquire was that hilly tract stretching from the old Roman road near Turn-again towards the Cauldshiels Loch: a then desolate and naked mountain-mere, which he likens, in a letter of this summer (to Lady Louisa Stuart), to the Lake of the Genie and the Fisherman in the Arabian Tale. To obtain this lake at one extremity of his estate, as a contrast to the Tweed at the other, was a prospect for which hardly any sacrifice would have appeared too much; and he contrived to gratify his wishes in the course of that July, to which he had spoken of himself in May as looking forward "with the deepest anxiety." Nor was he, I must add, more able to control some of his minor tastes. I find him writing to Mr. Terry, on the 20th of June, about "that splendid lot of ancient armor, advertised by Winstanley," a celebrated auctioneer in London, of which he had the strongest fancy to make his spoil, though he was at a loss to know where it should be placed when it reached Abbotsford; and on the 2d of July, this acquisition also having been settled, he says to the same correspondent: "I have written to Mr. Winstanley. My bargain with Constable was otherwise arranged, but Little John is to find the needful article, and I shall take care of Mr. Winstanley's interest, who has behaved too handsomely in this matter to be trusted to the mercy of our little friend the Picaroon, who is, notwithstanding his many excellent qualities, a little on the score of old Gobbo—doth somewhat smack—somewhat grow to.[29] We shall be at Abbotsford on the 12th, and hope soon to see you there. I am fitting up a small room above Peter-House, where an unceremonious bachelor may consent to do penance, though the place is a cock-loft, and the access that which leads many a bold fellow to his last nap—a ladder." [30] And a few weeks later, he says, in the same sort, to his sister-in- law, Mrs. Thomas Scott: "In despite of these hard times, which affect my patrons the booksellers very much, I am buying old books and old armor as usual, and adding to what your old friend Burns[31] calls—
- 64. 'A fouth of auld nick-nackets, Rusty airn caps and jingling jackets, Wad haud the Lothians three in tackets A towmont gude, And parritch-pats and auld saut-backets, Before the flude.'" Notwithstanding all this, it must have been with a most uneasy mind that he left Edinburgh to establish himself at Abbotsford that July. The assistance of Constable had not been granted, indeed it had not been asked, to an extent at all adequate for the difficulties of the case; and I have now to transcribe, with pain and reluctance, some extracts from Scott's letters, during the ensuing autumn, which speak the language of anxious, and, indeed, humiliating distress; and give a most lively notion of the incurable recklessness of his younger partner. TO MR. JOHN BALLANTYNE. Abbotsford, Saturday, 24th July. Dear John,—I sent you the order, and have only to hope it arrived safe and in good time. I waked the boy at three o'clock myself, having slept little, less on account of the money than of the time. Surely you should have written, three or four days before, the probable amount of the deficit, and, as on former occasions, I would have furnished you with means of meeting it. These expresses, besides every other inconvenience, excite surprise in my family and in the neighborhood. I know no justifiable occasion for them but the unexpected return of a bill. I do not consider you as answerable for the success of plans, but I do and must hold you responsible for giving me, in distinct and plain terms, your opinion as to any difficulties which may occur, and that in such time that I may make arrangements to obviate them if possible.
- 65. Of course, if anything has gone wrong you will come out here to-morrow. But if, as I hope and trust, the cash arrived safe, you will write to me, under cover to the Duke of Buccleuch, Drumlanrig Castle, Dumfries-shire. I shall set out for that place on Monday morning early. W. S. TO MR. JAMES BALLANTYNE. Abbotsford, 25th July, 1813. Dear James,—I address the following jobation for John to you, that you may see whether I do not well to be angry, and enforce upon him the necessity of constantly writing his fears as well as his hopes. You should rub him often on this point, for his recollection becomes rusty the instant I leave town and am not in the way to rack him with constant questions. I hope the presses are doing well, and that you are quite stout again. Yours truly, W. S. (Enclosure.) TO MR. JOHN BALLANTYNE. My Good Friend John,—The post brings me no letter from you, which I am much surprised at, as you must suppose me anxious to learn that your express arrived. I think he must have reached you before post-hours, and James or you might have found a minute to say so in a single line. I once more request that you will be a businesslike correspondent, and state your provisions for every week prospectively. I do not expect you to warrant them, which you rather perversely seem to insist is my wish, but I do want to be aware of their nature and extent, that I may provide against the possibility of miscarriage. The calendar, to which you
- 66. refer me, tells me what sums are due, but cannot tell your shifts to pay them, which are naturally altering with circumstances, and of which alterations I request to have due notice. You say you could not suppose Sir W. Forbes would have refused the long dated bills; but that you had such an apprehension is clear, both because in the calendar these bills were rated two months lower, and because, three days before, you wrote me an enigmatical expression of your apprehensions, instead of saying plainly there was a chance of your wanting £350, when I would have sent you an order to be used conditionally. All I desire is unlimited confidence and frequent correspondence, and that you will give me weekly at least the fullest anticipation of your resources, and the probability of their being effectual. I may be disappointed in my own, of which you shall have equally timeous notice. Omit no exertions to procure the use of money, even for a month or six weeks, for time is most precious. The large balance due in January from the trade, and individuals, which I cannot reckon at less than £4000, will put us finally to rights; and it will be a shame to founder within sight of harbor. The greatest risk we run is from such ill-considered despatches as those of Friday. Suppose that I had gone to Drumlanrig—suppose the pony had set up—suppose a thousand things—and we were ruined for want of your telling your apprehensions in due time. Do not plague yourself to vindicate this sort of management; but if you have escaped the consequences (as to which you have left me uncertain), thank God, and act more cautiously another time. It was quite the same to me on what day I sent that draft; indeed it must have been so if I had the money in my cash account, and if I had not, the more time given me to provide it the better. Now, do not affect to suppose that my displeasure arises from your not having done your utmost to realize funds, and that utmost having failed. It is one mode, to be sure, of exculpation, to suppose one's self accused of something they are not charged
- 67. with, and then to make a querulous or indignant defence, and complain of the injustice of the accuser. The head and front of your offending is precisely your not writing explicitly, and I request this may not happen again. It is your fault, and I believe arises either from an ill-judged idea of smoothing matters to me— as if I were not behind the curtain—or a general reluctance to allow that any danger is near, until it is almost unparriable. I shall be very sorry if anything I have said gives you pain; but the matter is too serious for all of us, to be passed over without giving you my explicit sentiments. To-morrow I set out for Drumlanrig, and shall not hear from you till Tuesday or Wednesday. Make yourself master of the post-town—Thornhill, probably, or Sanquhar. As Sir W. F. & Co. have cash to meet my order, nothing, I think, can have gone wrong, unless the boy perished by the way. Therefore, in faith and hope, and—that I may lack none of the Christian virtues—in charity with your dilatory worship, I remain very truly yours, W. S. Scott proceeded, accordingly, to join a gay and festive circle, whom the Duke of Buccleuch had assembled about him on first taking possession of the magnificent Castle of Drumlanrig, in Nithsdale, the principal messuage of the dukedom of Queensberry, which had recently lapsed into his family. But, post equitem sedet atra cura— another of John Ballantyne's unwelcome missives, rendered necessary by a neglect of precisely the same kind as before, reached him in the midst of this scene of rejoicing. On the 31st, he again writes:— TO MR. JOHN BALLANTYNE, BOOKSELLER, EDINBURGH. Drumlanrig, Friday. Dear John,—I enclose the order. Unfortunately, the Drumlanrig post only goes thrice a week; but the Marquis of Queensberry,
- 68. who carries this to Dumfries, has promised that the guard of the mail-coach shall deliver it by five to-morrow. I was less anxious, as your note said you could clear this month. It is a cruel thing that no State you furnish excludes the arising of such unexpected claims as this for the taxes on the printing-office. What unhappy management, to suffer them to run ahead in such a manner!—but it is in vain to complain. Were it not for your strange concealments, I should anticipate no difficulty in winding up these matters. But who can reckon upon a State where claims are kept out of view until they are in the hands of a writer? If you have no time to say that this comes safe to hand, I suppose James may favor me so far. Yours truly, W. S. Let the guard be rewarded. Let me know exactly what you can do and hope to do for next month; for it signifies nothing raising money for you, unless I see it is to be of real service. Observe, I make you responsible for nothing but a fair statement.[32] The guard is known to the Marquis, who has good-naturedly promised to give him this letter with his own hand; so it must reach you in time, though probably past five on Saturday. Another similar application reached Scott the day after the guard delivered his packet. He writes thus, in reply:
- 69. TO MR. JOHN BALLANTYNE. Drumlanrig, Sunday. Dear John,—I trust you got my letter yesterday by five, with the draft enclosed. I return your draft accepted. On Wednesday I think of leaving this place, where, but for these damned affairs, I should have been very happy. W. S. Scott had been for some time under an engagement to meet the Marquis of Abercorn at Carlisle, in the first week of August, for the transaction of some business connected with his brother Thomas's late administration of that nobleman's Scottish affairs; and he had designed to pass from Drumlanrig to Carlisle for this purpose, without going back to Abbotsford. In consequence of these repeated harassments, however, he so far altered his plans as to cut short his stay at Drumlanrig, and turn homewards for two or three days, where James Ballantyne met him with such a statement as in some measure relieved his mind. He then proceeded to fulfil his engagement with Lord Abercorn, whom he encountered travelling in a rather peculiar style between Carlisle and Longtown. The ladies of the family and the household occupied four or five carriages, all drawn by the Marquis's own horses, while the noble Lord himself brought up the rear, mounted on horseback, and decorated with the ribbon of the order of the Garter. On meeting the cavalcade, Scott turned with them, and he was not a little amused when they reached the village of Longtown, which he had ridden through an hour or two before, with the preparations which he found there made for the dinner of the party. The Marquis's major-domo and cook had arrived there at an early hour in the morning, and everything was now arranged for his
- 70. reception in the paltry little public house, as nearly as possible in the style usual in his own lordly mansions. The ducks and geese that had been dabbling three or four hours ago in the village pond were now ready to make their appearance under numberless disguises as entrées; a regular bill-of-fare flanked the noble Marquis's allotted cover; every huckaback towel in the place had been pressed to do service as a napkin; and, that nothing might be wanting to the mimicry of splendor, the landlady's poor remnants of crockery and pewter had been furbished up, and mustered in solemn order on a crazy old beauffet, which was to represent a sideboard worthy of Lucullus. I think it worth while to preserve this anecdote, which Scott delighted in telling, as perhaps the last relic of a style of manners now passed away, and never likely to be revived among us. Having despatched this dinner and his business, Scott again turned southwards, intending to spend a few days with Mr. Morritt at Rokeby; but on reaching Penrith, the landlord there, who was his old acquaintance (Mr. Buchanan), placed a letter in his hands: ecce iterum—it was once more a cry of distress from John Ballantyne. He thus answered it:— TO MR. JOHN BALLANTYNE. Penrith, August 10, 1813. Dear John,—I enclose you an order for £350. I shall remain at Rokeby until Saturday or Sunday, and be at Abbotsford on Wednesday at latest. I hope the printing-office is going on well. I fear, from the state of accompts between the companies, restrictions on the management and expense will be unavoidable, which may trench upon James's comforts. I cannot observe hitherto that the printing-office is paying off, but rather adding to its embarrassments; and it cannot be thought that I have either means or inclination to support a losing concern at the rate of
- 71. £200 a month. If James could find a monied partner, an active man who understood the commercial part of the business, and would superintend the conduct of the cash, it might be the best for all parties; for I really am not adequate to the fatigue of mind which these affairs occasion me, though I must do the best to struggle through them. Believe me yours, etc. W. S. At Brough he encountered a messenger who brought him such a painful account of Mrs. Morritt's health, that he abandoned his intention of proceeding to Rokeby; and, indeed, it was much better that he should be at Abbotsford again as soon as possible, for his correspondence shows a continued succession, during the three or four ensuing weeks, of the same annoyances that had pursued him to Drumlanrig and to Penrith. By his desire, the Ballantynes had, it would seem, before the middle of August, laid a statement of their affairs before Constable. Though the statement was not so clear and full as Scott had wished it to be, Constable, on considering it, at once assured them, that to go on raising money in driblets would never effectually relieve them; that, in short, one or both of the companies must stop, unless Mr. Scott could find means to lay his hand, without farther delay, on at least £4000; and I gather that, by way of inducing Constable himself to come forward with part at least of this supply, John Ballantyne again announced his intention of forthwith abandoning the bookselling business altogether, and making an effort to establish himself—on a plan which Constable had shortly before suggested—as an auctioneer in Edinburgh. The following letters need no comment:— TO MR. JOHN BALLANTYNE. Abbotsford, August 16, 1813.
- 72. Dear John,—I am quite satisfied it is impossible for J. B. and Co. to continue business longer than is absolutely necessary for the sale of stock and extrication of their affairs. The fatal injury which their credit has sustained, as well as your adopting a profession in which I sincerely hope you will be more fortunate, renders the closing of the bookselling business inevitable. With regard to the printing, it is my intention to retire from that also, so soon as I can possibly do so with safety to myself, and with the regard I shall always entertain for James's interest. Whatever loss I may sustain will be preferable to the life I have lately led, when I seem surrounded by a sort of magic circle, which neither permits me to remain at home in peace, nor to stir abroad with pleasure. Your first exertion as an auctioneer may probably be on "that distinguished, select, and inimitable collection of books, made by an amateur of this city retiring from business." I do not feel either health or confidence in my own powers sufficient to authorize me to take a long price for a new poem, until these affairs shall have been in some measure digested. This idea has been long running in my head, but the late fatalities which have attended this business have quite decided my resolution. I will write to James to-morrow, being at present annoyed with a severe headache. Yours truly, W. Scott. Were I to transcribe all the letters to which these troubles gave rise, I should fill a volume before I had reached the end of another twelvemonth. The two next I shall quote are dated on the same day (the 24th August), which may, in consequence of the answer the second of them received, be set down as determining the crisis of 1813. TO MR. JAMES BALLANTYNE.
- 73. Clerkship, £1300 Sheriffdom, 300 Mrs. Scott, 200 Interest, 100 Somers, (say) 200 ——— £2100 Abbotsford, 24th August, 1813. Dear James,—Mr. Constable's advice is, as I have always found it, sound, sensible, and friendly,—and I shall be guided by it. But I have no wealthy friend who would join in security with me to such an extent; and to apply in quarters where I might be refused would insure disclosure. I conclude John has shown Mr. C. the state of the affairs; if not, I would wish him to do so directly. If the proposed accommodation could be granted to the firm on my personally joining in the security, the whole matter would be quite safe, for I have to receive in the course of the winter some large sums from my father's estate.[33] Besides which, I shall certainly be able to go to press in November with a new poem; or, if Mr. Constable's additional security would please the bankers better, I could insure Mr. C. against the possibility of loss, by assigning the copyrights, together with that of the new poem, or even my library, in his relief. In fact, if he looks into the affairs, he will I think see that there is no prospect of any eventual loss to the creditors, though I may be a loser myself. My property here is unincumbered; so is my house in Castle Street; and I have no debts out of my own family, excepting a part of the price of Abbotsford, which I am to retain for four years. So that, literally, I have no claims upon me unless those arising out of this business; and when it is considered that my income is above £2000 a year, even if the printing-office pays nothing, I should hope no one can possibly be a loser by me. I am sure I would strip myself to my shirt rather than it should be the case; and my only reason for wishing to stop the concern was to do open justice to all persons. It must have been a bitter pill to me. I can more confidently expect some aid from Mr. Constable, or from Longman's house, because they can look into the concern and satisfy themselves how little chance there is of their being losers, which others cannot do. Perhaps between
- 74. them they might manage to assist us with the credit necessary, and go on in winding up the concern by occasional acceptances. An odd thing has happened. I have a letter, by order of the Prince Regent, offering me the laureateship in the most flattering terms. Were I my own man, as you call it, I would refuse this offer (with all gratitude); but, as I am situated, £300 or £400 a year is not to be sneezed at upon a point of poetical honor—and it makes me a better man to that extent. I have not yet written, however. I will say little about Constable's handsome behavior, but shall not forget it. It is needless to say I shall wish him to be consulted in every step that is taken. If I should lose all I advanced to this business, I should be less vexed than I am at this moment. I am very busy with Swift at present, but shall certainly come to town if it is thought necessary; but I should first wish Mr. Constable to look into the affairs to the bottom. Since I have personally superintended them, they have been winding up very fast, and we are now almost within sight of harbor. I will also own it was partly ill-humor at John's blunder last week that made me think of throwing things up. Yours truly, W. S. After writing and despatching this letter, an idea occurred to Scott that there was a quarter, not hitherto alluded to in any of these anxious epistles, from which he might consider himself as entitled to ask assistance, not only with little, if any, chance of a refusal, but (owing to particular circumstances) without incurring any very painful sense of obligation. On the 25th he says to John Ballantyne: — After some meditation, last night, it occurred to me I had some title to ask the Duke of Buccleuch's guarantee to a cash account
- 75. for £4000, as Constable proposes. I have written to him accordingly, and have very little doubt that he will be my surety. If this cash account be in view, Mr. Constable will certainly assist us until the necessary writings are made out—I beg your pardon —I dare say I am very stupid; but very often you don't consider that I can't follow details which would be quite obvious to a man of business;—for instance, you tell me daily, "that if the sums I count upon are forthcoming, the results must be as I suppose." But—in a week—the scene is changed, and all I can do, and more, is inadequate to bring about these results. I protest I don't know if at this moment £4000 will clear us out. After all, you are vexed, and so am I; and it is needless to wrangle who has a right to be angry. Commend me to James. Yours truly, W. S. Having explained to the Duke of Buccleuch the position in which he stood—obliged either to procure some guarantee which would enable him to raise £4000, or to sell abruptly all his remaining interest in the copyright of his works; and repeated the statement of his personal property and income, as given in the preceding letter to James Ballantyne—Scott says to his noble friend:— I am not asking nor desiring any loan from your Grace, but merely the honor of your sanction to my credit as a good man for £4000; and the motive of your Grace's interference would be sufficiently obvious to the London Shylocks, as your constant kindness and protection is no secret to the world. Will your Grace consider whether you can do what I propose, in conscience and safety, and favor me with your answer?—I have a very flattering offer from the Prince Regent, of his own free motion, to make me poet laureate; I am very much embarrassed by it. I am, on the one hand, afraid of giving offence where no one would willingly
- 76. offend, and perhaps losing an opportunity of smoothing the way to my youngsters through life; on the other hand, the office is a ridiculous one, somehow or other—they and I should be well quizzed,—yet that I should not mind. My real feeling of reluctance lies deeper—it is, that favored as I have been by the public, I should be considered, with some justice, I fear, as engrossing a petty emolument which might do real service to some poorer brother of the Muses. I shall be most anxious to have your Grace's advice on this subject. There seems something churlish, and perhaps conceited, in repelling a favor so handsomely offered on the part of the Sovereign's representative; and on the other hand, I feel much disposed to shake myself free from it. I should make but a bad courtier, and an ode-maker is described by Pope as a poet out of his way or out of his senses. I will find some excuse for protracting my reply till I can have the advantage of your Grace's opinion; and remain, in the mean time, very truly your obliged and grateful Walter Scott. P. S.—I trust your Grace will not suppose me capable of making such a request as the enclosed, upon any idle or unnecessary speculation; but, as I stand situated, it is a matter of deep interest to me to prevent these copyrights from being disposed of either hastily or at under prices. I could have half the booksellers in London for my sureties, on a hint of a new poem; but bankers do not like people in trade, and my brains are not ready to spin another web. So your Grace must take me under your princely care, as in the days of lang syne; and I think I can say, upon the sincerity of an honest man, there is not the most distant chance of your having any trouble or expense through my means. The Duke's answer was in all respects such as might have been looked for from the generous kindness and manly sense of his character.
- 77. TO WALTER SCOTT, ESQ., ABBOTSFORD. Drumlanrig Castle, August 28, 1813. My dear Sir,—I received yesterday your letter of the 24th. I shall with pleasure comply with your request of guaranteeing the £4000. You must, however, furnish me with the form of a letter to this effect, as I am completely ignorant of transactions of this nature. I am never willing to offer advice, but when my opinion is asked by a friend I am ready to give it. As to the offer of his Royal Highness to appoint you laureate, I shall frankly say that I should be mortified to see you hold a situation which, by the general concurrence of the world, is stamped ridiculous. There is no good reason why this should be so; but so it is. Walter Scott, Poet Laureate, ceases to be the Walter Scott of the Lay, Marmion, etc. Any future poem of yours would not come forward with the same probability of a successful reception. The poet laureate would stick to you and your productions like a piece of court plaster. Your muse has hitherto been independent—don't put her into harness. We know how lightly she trots along when left to her natural paces, but do not try driving. I would write frankly and openly to his Royal Highness, but with respectful gratitude, for he has paid you a compliment. I would not fear to state that you had hitherto written when in poetic mood, but feared to trammel yourself with a fixed periodical exertion; and I cannot but conceive that his Royal Highness, who has much taste, will at once see the many objections which you must have to his proposal, but which you cannot write. Only think of being chaunted and recitatived by a parcel of hoarse and squeaking choristers on a birthday, for the edification of the bishops, pages, maids of honor, and gentlemen-pensioners! Oh horrible! thrice horrible! Yours sincerely, Buccleuch, etc.
- 78. The letter which first announced the Prince Regent's proposal was from his Royal Highness's librarian, Dr. James Stanier Clarke; but before Scott answered it he had received a more formal notification from the late Marquis of Hertford, then Lord Chamberlain. I shall transcribe both these documents. TO WALTER SCOTT, ESQ., EDINBURGH. Pavilion, Brighton, August 18, 1813. My dear Sir,—Though I have never had the honor of being introduced to you, you have frequently been pleased to convey to me very kind and flattering messages,[34] and I trust, therefore, you will allow me, without any further ceremony, to say—That I took an early opportunity this morning of seeing the Prince Regent, who arrived here late yesterday; and I then delivered to his Royal Highness my earnest wish and anxious desire that the vacant situation of poet laureate might be conferred on you. The Prince replied, "that you had already been written to, and that if you wished it, everything would be settled as I could desire." I hope, therefore, I may be allowed to congratulate you on this event. You are the man to whom it ought first to have been offered, and it gave me sincere pleasure to find that those sentiments of high approbation which my Royal Master had so often expressed towards you in private, were now so openly and honorably displayed in public. Have the goodness, dear sir, to receive this intrusive letter with your accustomed courtesy, and believe me, yours very sincerely, J. S. Clarke. Librarian to H. R. H., the Prince Regent.
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