Carving Out Nanostructures Beneath the Surface of Silicon

Modern computer chips can have features built on a nanometer scale. Until now it has been possible to form such small structures only on top of a silicon wafer, but a new technique can now create nanoscale features in a layer below the surface. The approach has promising applications in both photonics and electronics, say its inventors, and could one day enable the fabrication of 3D structures throughout the bulk of the wafer.

The technique relies on the fact that silicon is transparent to certain wavelengths of light. This means the right kind of laser can travel through the surface of the wafer and interact with the silicon below. But designing a laser that can pass through the surface without causing damage and still carry out precise nanoscale fabrication below is not simple.

Researchers from Bilkent University in Ankara, Türkiye, achieved this by using spatial light modulation to create a needlelike laser beam that gave them greater control over where the beam’s energy was deposited. By exploiting physical interactions between the laser light and the silicon, they were able to fabricate lines and planes with different optical properties that could be combined to create nanophotonic elements below the surface.

“Silicon is the bedrock of electronics, photonics, photovoltaics.”

Using lasers to fabricate inside silicon is not new. But Onur Tokel, an assistant professor of physics at Bilkent who led the research, explains that so far it’s been possible to create only microscale-size structures. Extending the approach to the nanoscale could unlock new capabilities, he says, because it allows features that are comparable in size to the wavelengths of incoming light. When this happens, the structures exhibit a host of novel optical behaviors, which, among other things, make it possible to create metamaterials and metasurfaces.

“Silicon is the bedrock of electronics, photonics, photovoltaics,” says Tokel. “If we can introduce additional functionality inside the wafer at the nanoscale that would complement these existing functionalities, this allows an entirely different paradigm. Now you can imagine doing things inside the volume, and maybe even eventually in three dimensions. We believe that would open exciting new directions.”

Previous approaches had been unable to fabricate at the nanoscale because laser light typically scatters once inside the silicon, making it hard to precisely deposit energy. In a paper in Nature Communications, Tokel’s team showed they could get around this by using a special type of laser known as a Bessel beam, which doesn’t diffract. This means the laser fights against the optical scattering effect to remain narrowly focused inside the silicon, making it possible to deposit energy with precision.

When the laser is fired at the wafer it creates tiny holes, known as voids, in the area where the beam is focused. This happened in previous approaches as well, says Tokel, but the smaller voids created by the more tightly focused beam exhibit a “field enhancement” effect that causes the laser’s intensity to increase around them. This modifies the structure of the silicon around the voids, which further reinforces the enhancement effect, creating a self-sustaining feedback loop. The team also discovered that they could modify the direction of the field enhancement by altering the polarization of the laser light.

The end result is the creation of two-dimensional planar or linelike structures in the silicon as small as 100 nanometers. These structures have a different refractive index from the rest of the wafer, but Tokel says it’s not entirely clear what these structures consist of. Based on previous research, he thinks the underlying crystal structure of the silicon has probably been modified. Electron microscopy studies should be able to clarify this in the future, he adds, but ultimately it’s not necessary to understand the exact underlying nature of these structures to create useful nanophotonic elements.

To demonstrate this, the researchers built a nanoscale photonic device known as a Bragg grating, which can be used as an optical filter. This represents the first functional, nanoscale optical element completely buried in silicon, according to the team.

That the researchers were able to achieve nanoscale features is spectacular, says Maxime Chambonneau, a researcher at the University of Jena, in Germany, because the relatively long laser pulses that Tokel’s team uses generally produce large heat-affected zones that result in microscale modifications. (The Bilkent team employed pulses that measure in nanoseconds, while other work in direct laser writing traditionally involves picosecond or femtosecond lasers.) The ability to create features smaller than light waves could open up various possibilities, including boosting the energy-harvesting capabilities of solar cells, says Chambonneau.

Because the fabrication technique doesn’t make any alterations to the surface of the wafer, Tokel says it could one day be used to create multifunctional devices with electronics on the surface and photonic elements buried below. The team is also investigating whether the approach can be used to carve microfluidic channels beneath the surface of a chip. Pumping fluid through these channels could improve heat extraction, says Tokel, which could help cool electronics and allow them to run faster.

The biggest limitation of the approach, Tokel says, is that the researchers don’t have fine control over where voids appear in a given region. At present, a small number are unevenly distributed in the area where the laser beam is focused. If they could more precisely position these voids, Tokel says it would enable them to carry out nanofabrication in three dimensions rather than simply producing lines or planes.

“If you can control these things individually and distribute them like a chain, in the future that would be very exciting,” he adds. “Because then you will have much more control, and that will enable even richer elements or systems.”

This article appears in the September 2024 print issue as “Laser Embeds Nanoscale Device in Silicon.”