BOE063 – INTRODUCTION TO MEMS
Section-Wise Solved Answers
SECTION A (2 × 7 = 14)
(a) Define MEMS and list two applications.
MEMS are micro-scale systems integrating mechanical structures, sensors/actuators, and electronics on a silicon substrate using microfabrication. Applications include smartphone accelerometers and automotive pressure sensors.
(b) Piezo-resistive effect.
It is the change in electrical resistance of a material under mechanical stress. MEMS sensors exploit this to convert stress/strain into an electrical signal.
(c) Hooke’s Law for beams.
Within elastic limits, stress is proportional to strain; for MEMS beams, deflection is proportional to applied load while remaining elastic.
(d) Moment of inertia of a beam.
A geometric property indicating resistance to bending; higher moment of inertia reduces deflection for a given load.
(e) Viscous flow in air damping.
A regime where viscous forces dominate inertial forces, causing energy dissipation around moving microstructures and damping motion.
(f) Squeeze-film vs slide-film damping.
Squeeze-film occurs when air is compressed between approaching surfaces; slide-film occurs when surfaces slide parallel. Squeeze-film is usually stronger.
(g) Fringe effects in electrostatic actuation.
Edge electric fields increase effective capacitance and force, influencing actuator sensitivity and pull-in behavior.
SECTION B (Attempt any three) (7 × 3 = 21)
(a) PVD for MEMS fabrication & advantages.
Physical Vapour Deposition deposits thin films (via evaporation/sputtering) in vacuum. It offers high purity, thickness control, low contamination, and CMOS compatibility.
(b) Cantilever deflection under own weight.
For a cantilever of length LLL with uniform load www:
δmax=wL48EI\delta_{max}=\frac{wL^4}{8EI}δmax=8EIwL4
Used to estimate MEMS beam deflection.
(c) Reynolds’ equation for squeeze-film damping & significance.
It governs pressure distribution in thin air films, enabling prediction of damping, resonant frequency, and Q-factor.
(d) Parallel-plate actuator with capacitive sensing.
Applied voltage creates electrostatic force reducing the gap; capacitance change measures displacement—common in MEMS sensors.
(e) MEMS resonator design & one-port model.
Design considers geometry, material, damping, frequency, and Q. One-port modeling maps mechanics to an equivalent electrical network for analysis.
SECTION C (Attempt any one) (7)
(a) Micromachining processes, materials & substrates.
Bulk micromachining etches into silicon; surface micromachining builds structures layer-by-layer using sacrificial layers. Materials include Si, polysilicon, SiO₂, metals.
(b) Piezo-resistive sensor & applications.
Stress-induced resistance change is sensed in beams/diaphragms; used in pressure sensors and accelerometers.
