(SEM VI) THEORY EXAMINATION 2022-23 INTRODUCTION TO MEMS

INTRODUCTION TO MEMS (KOE-063)

Important Questions & Answers – Section-wise

SECTION A

(Attempt all questions – 2 × 10 = 20 marks)

a. Types of MEMS fabrication

MEMS fabrication techniques are broadly classified into bulk micromachining, surface micromachining, and LIGA processes. These methods are used to create micro-scale mechanical structures on silicon substrates.

b. Piezoresistive sensor

A piezoresistive sensor works on the principle that electrical resistance of certain materials changes when mechanical stress is applied. It is widely used in pressure and strain sensors.

c. Hooke’s law and modulus of elasticity

Hooke’s law states that stress is directly proportional to strain within elastic limit. Modulus of elasticity is the ratio of stress to strain and represents material stiffness.

d. Strain from bending stress

Strain due to bending stress is calculated using the relation

ε=σE\varepsilon = \frac{\sigma}{E}ε=Eσ​

where σ is bending stress and E is Young’s modulus.

e. Load carrying capacity of a cantilever

The maximum load a cantilever can carry depends on material properties, length, width, thickness, and boundary conditions. Excess load causes permanent deformation or fracture.

f. Effect of damping in structural dynamics

Damping reduces vibration amplitude and energy in MEMS structures. It improves stability but reduces sensitivity in resonators.

g. Couette flow vs Poiseuille flow

Couette flow occurs due to relative motion between surfaces, while Poiseuille flow occurs due to pressure gradient in a stationary channel.

h. Electrostatic force with example

Electrostatic force is the attraction between charged bodies. In MEMS, it is used in parallel plate actuators where voltage causes plate movement.

i. MEMS resonator: principle and types

MEMS resonators operate by mechanical vibration at resonant frequency. Types include cantilever resonators, tuning fork resonators, and bulk acoustic resonators.

j. Thermocouple

A thermocouple measures temperature based on voltage generated at the junction of two dissimilar metals when heated.

SECTION B

(Attempt any THREE – 10 × 3 = 30 marks)

Q2(a) Signal transduction methods for micro pressure sensors

Pressure sensors use piezoresistive, capacitive, and piezoelectric transduction. Piezoresistive sensors are simple but temperature-sensitive. Capacitive sensors offer high sensitivity but are noise-prone. Piezoelectric sensors are suitable for dynamic pressure but not static pressure.

Q2(b) Operating principle and fabrication of pressure sensor

MEMS pressure sensors use diaphragm deformation under pressure. Fabrication involves substrate preparation, diaphragm formation, doping, metallization, and packaging.

Q2(c) PVD technique in MEMS fabrication

Physical Vapor Deposition deposits thin films by vaporizing material in vacuum. Step coverage refers to uniformity of coating, while shadowing occurs when deposition is blocked by structures.

Q2(d) Design of MEMS pressure sensor

A MEMS pressure sensor consists of a silicon diaphragm, sensing element, and signal conditioning circuit. It is used in automotive and biomedical applications.

Q2(e) Airflow in micro tube (Numerical)

Airflow ratio is calculated using Poiseuille’s law. Substituting given values yields the airflow rate for micro-scale tube.
(In exam, write formula → substitute values → final answer)

SECTION C

Q3(a) Micro-actuation techniques

Electrostatic actuation uses electric field for motion, while thermal actuation uses expansion due to heating. Both are widely used in MEMS devices.

Q3(b) Substrate materials in MEMS

Common substrates include silicon, glass, polymers, and metals. Silicon is most widely used due to mechanical strength and compatibility with IC fabrication.

Q4(a) Incompressible flow in micro conduits

In micro channels, incompressible flow is governed by laminar behavior and viscous forces dominate over inertial forces.

Q4(b) Bulk vs surface micromachining

Bulk micromachining removes material from substrate, while surface micromachining builds structures layer by layer. Sacrificial layers are removed to release structures.

Q5(a) Cantilever working and fabrication

Cantilever bends under applied force. Fabrication involves deposition, lithography, etching, and release steps.

Q5(b) MEMS accelerometer design

A MEMS accelerometer uses a proof mass suspended by beams. Acceleration causes displacement which is sensed electrically.

Q6(a) Electrostatic force on plate electrodes (Numerical)

Electrostatic force is calculated using

F=12εAV2d2F = \frac{1}{2} \varepsilon A \frac{V^2}{d^2}F=21​εAd2V2​

Misalignment reduces effective area. Final force is obtained after substitution.

Q6(b) Electrostatic vs electromagnetic actuation

Electrostatic actuation is preferred in MEMS due to scalability and low power. Electromagnetic forces dominate at macro scale but are inefficient at micro scale.

Q7(a) Micro pressure sensors with different transduction methods

Piezoresistive sensors offer simplicity, capacitive sensors provide high sensitivity, and piezoelectric sensors suit dynamic measurements.

Q7(b) Gas flow in nano tube (Numerical)

Flow rate is calculated using modified Poiseuille equation considering nano-scale effects.
(Show steps clearly in exam)