Micro Electromechanical Systems - Combining Electrical and Mechanical Components
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Microelectromechanical systems (MEMS) integrate mechanical and electrical components on a single chip, typically ranging from 1 to 100 micrometres in size, allowing for the creation of systems that can sense and control their environment. Key components include microsensors for data gathering, microelectronics for processing information, and microactuators that trigger external devices, leading to applications in medical, marine, military, and automotive fields. The advantages of MEMS include smaller size, cost-effectiveness, system integration, and improved performance in terms of sensitivity and energy efficiency.
Micro Electromechanical Systems - Combining Electrical and Mechanical Components
- 2. INTRODUCTION • MEMS is a technique of combining Electrical and Mechanical components together on a chip, to produce a system of miniature dimensions. • MEMS is the integration of a number of micro components on a single chip which allows the microsystem to both sense and control the environment. • The components are integrated on a single chip using micro fabrication technologies. 2
- 4. INTRODUCTION(CONTD.) • Made up of components between 1 and 100 micrometres in size. • Devices generally range from 20 micrometres to a millimetre in size. • Functional elements of MEMS are miniaturized structures, sensors, actuators and microelectronics. • One main criterion of MEMS is that there are at least some elements having mechanical functionality whether or not they can move. 4
- 5. COMPONENTS Microelectronics: • “Brain” that receives information, processes it and makes decisions. • Data comes from microsensors. Microsensors: • Constantly gather data from the environment. • Pass data to microelectronics for processing. • Can monitor mechanical, thermal, biological, chemical and magnetic readings. 5
- 6. COMPONENTS(CONTD.) Microactuator: • Acts as trigger to activate external device. • Microelectronics will make the microactuator activate the device. Microstructures: • Extremely small structures built onto the surface of a chip. 6
- 7. ADVANTAGES 1. Much smaller area 2. Cheaper than alternatives 3. System integration (All on one chip) 4. Speed: • Lower thermal time constant • Rapid response times (High frequency) 5. Power consumption: • Low actuation energy • Low heating power 7
- 8. ADVANTAGES(CONTD.) 6. Minimize energy and materials 7. Improved reproducibility 8. Improved accuracy and reliability 9. Increased selectivity and sensitivity 8
- 9. APPLICATIONS • Medical Science: 1. Biocavity laser: This device distinguishes cancerous from non- cancerous cells thus aiding the surgeons in operations. 2. Smart Pill: Implanted in the body. Automatic drug delivery (on demand). 3. Sight for the blind: MEMS based array that may be inserted in the retina of a blind person to provide partial sight. • Marine Science: Sensing in marine environments may be done for various reasons- 9
- 10. APPLICATIONS(CONTD.) 1. Oil exploration and related applications 2. Global weather predictions 3. Monitor water quality for any contamination 4. Measure parameters detrimental to the “health” of structures in the sea (like oil rigs and ships) 5. Study of aquatic plants and animals 6. In military operations 10
- 11. APPLICATIONS(CONTD.) • Marine military operations: • An array of MEMS sensors spread on the ocean floor could detect the presence of enemy submarines. • MEMS sensors (pressure sensors, accelerometers etc) are being used in anti-torpedo weapons on submarines and ships. • MEMS sensors in torpedoes are responsible for- 1. Detonating the torpedo at the right time. 2. Hitting the target in a crowded environment. 3. Preventing any premature explosion. 11
- 12. APPLICATIONS(CONTD.) • In automotives: Heavy use of MEMS is found in air bag systems, vehicle security systems, inertial brake lights, rollover detection, automatic door locks etc. • As gyroscope • In microphones 12
- 13. MICRO SENSORS • A sensor is a device used to measure a physical quantity and convert it into an electronic signal of some kind without modifying the environment. • Commonly sensed parameters are:- Pressure, Temperature, Flow rate, Radiation, Chemicals etc. Benefits of using MEMS for sensors:- • Smaller in size • Lower power consumption • More sensitive to input variations • Cheaper due to mass production 13
- 14. MICRO SENSORS(CONTD.) Types of sensors:- • Mechanical sensors- Strain gauges, accelerometers, pressure sensors. • Optical sensors- Direct sensors (Light -> Electronic signal), Indirect sensors (Light -> Intermediate Energy -> Electronic signal). 1. Pressure sensor • First microsensor developed and used. • Low production cost, high sensitivity and low hysteresis. • Piezoresistive pressure sensor to reduce fuel consumption by a tight control of the ratio between air and fuel. 14
- 15. MICRO SENSORS(CONTD.) • Piezo sensors integrated in the membrane. • Pressure deflects the membrane. • Resistance changes proportional to the deflection and thus to pressure. • Resistance change is measured with Wheatstone’s bridge. 15
- 16. MICRO SENSORS(CONTD.) 2. Chemical sensor • A sensitive layer is in contact with the substance. • Chemical reaction occurs on the sensitive layer. • Due to the reaction, physical, optical or acoustic properties are changed. • Transducer transforms the signal into electrical form. 16
- 17. MICRO ACTUATORS • Actuators use input energy and release output energy in a controlled manner. • Mechanical actuators act upon something and move it with force or torque. • Actuators can be classified on the basis of:- Type of output energy released, the way the output energy is released and the input energy used. • Microactuators produce motions over small distances, of the order of microns to mm. • Produce small forces, of the order of pN to mN. 17
- 18. MICRO ACTUATORS(CONTD.) Characteristics of a mechanical micro actuator:- • Stroke- The maximum displacement possible with an actuator. • Force/Torque- Maximum force/torque generated by an actuator. • Stiffness- The rate at which the generated force/torque decreases with stroke. • Input energy- The energy given to the actuator. • Efficiency- The ratio of the released energy to the input energy. • Linearity- The extent to which the force and stroke are linear. • Hysteresis- The difference in displacement/force for the same values of input energy in up and down strokes. • Response time- The time taken for the actuator to respond from the instant the input signal is given. • Drift- The unintended shift in force even when the input is steady. • Bandwidth- The frequencies at which it can reliably provide rated force/displacement. 18
- 19. MICRO ACTUATORS(CONTD.) Some micro actuators:- • Electrostatic micromotors • Electrostatic comb-drive • Magnetic actuators • Thermal micro actuators • Pneumatic actuators • Piezoelectric actuators • Surface-tension driven fluidic actuators 19
- 20. FABRICATION PROCESSES 1. Deposition: • Deposit thin film of material onto the substrate. • Physical- material placed onto substrate, techniques include sputtering and evaporation. • Chemical- stream of source gas reacts on the substrate to grow product, techniques include chemical vapor deposition and atomic layer deposition. 2. Patterning: • Transfer of a pattern into a material after deposition in order to prepare for etching. Techniques include lithography. 20
- 21. FABRICATION PROCESSES(CONTD.) 3. Etching: • Process of using strong acid to cut into the unprotected parts of a metal surface to create a design in. • Wet etching: dipping substrate into chemical solution that selectively removes material. The process provides good selectivity. • Dry etching: material sputtered or dissolved from substrate with plasma or gas variations. 21
- 22. FABRICATION METHODS Bulk micromachining: • Oldest micromachining technology • Technique involves selective removal of substrate to produce mechanical components • Accomplished by physical or chemical process, chemical being used more for MEMS production • Chemical wet etching is popular because of high etch rate and selectivity • Isotropic wet etching- Etching moves at equal rates in all directions • Anisotropic wet etching- Etch rate depends on crystallographic orientation 22
- 23. FABRICATION METHODS(CONTD.) Surface micromachining: • Process starts with deposition of thin film that acts as a temporary mechanical layer (sacrificial layer) • Device layers are constructed on top • Deposition and patterning of structural layer • Removal of temporary layer to allow movement of structural layer 23