(SEM VI) THEORY EXAMINATION 2023-24 INTRODUCTION TO MEMS

INTRODUCTION TO MEMS – KOE063

Section-wise Important Questions & Ready Answers

SECTION A

(Attempt all questions in brief – 2 marks each)

(a) Materials Commonly Used in MEMS Fabrication

Commonly used materials in MEMS fabrication include silicon, polysilicon, silicon dioxide, silicon nitride, metals such as aluminum and gold, and piezoelectric materials like PZT. Silicon is preferred due to its excellent mechanical and electrical properties.

(b) Importance of Sensors and Transducers in MEMS Devices

Sensors and transducers are the core elements of MEMS devices. Sensors detect physical parameters such as pressure, temperature, or acceleration, while transducers convert these signals into electrical outputs for processing and control.

(c) Displacement of Beam Structures under Load

When a beam structure in MEMS is subjected to an external load, it bends and undergoes displacement. This displacement depends on material properties, beam geometry, boundary conditions, and the magnitude of applied force.

(d) Effect of Viscosity on Air Damping

Viscosity determines the resistance offered by air to the motion of MEMS structures. Higher viscosity increases damping force, reducing vibration amplitude and slowing system response.

(e) Effect of Air Damping on MEMS Dynamics

Air damping reduces oscillations and energy in MEMS devices. While it improves stability, excessive damping lowers sensitivity and bandwidth of MEMS sensors and actuators.

(f) Thermal Effects in MEMS

Thermal effects include thermal expansion, thermal stress, Joule heating, and temperature-dependent material property variations. These effects influence performance, reliability, and sensitivity of MEMS devices.

(g) Step Voltage Driving in Electrostatic Actuation

In step voltage driving, a sudden voltage is applied to an electrostatic actuator, producing an immediate electrostatic force that causes displacement of the MEMS structure.

(h) Fringe Effects in Electrostatic Actuation

Fringe effects occur when electric field lines extend beyond the edges of electrodes. These effects increase capacitance and electrostatic force beyond ideal parallel-plate assumptions.

(i) Capacitive Sensing in MEMS

Capacitive sensing measures changes in capacitance due to displacement of MEMS structures. It is widely used in accelerometers, pressure sensors, and microphones.

(j) Applications of MEMS in RF

MEMS are used in RF switches, tunable capacitors, filters, and resonators due to their low power consumption, high linearity, and excellent frequency performance.

SECTION B

(Attempt any three – 10 marks each)

2(a) Importance of Material and Substrate Selection in MEMS Fabrication

Material and substrate selection is critical because it determines mechanical strength, electrical performance, thermal stability, and compatibility with fabrication processes. Silicon substrates offer high precision and batch fabrication capability, while metals and polymers are chosen based on flexibility, conductivity, and cost requirements.

2(b) Hooke’s Law and Its Application to Beam Structures in MEMS

Hooke’s law states that stress is directly proportional to strain within elastic limits. In MEMS beam analysis, this law is used to calculate deflection, stiffness, and resonant frequency. It helps relate applied force to deformation in micro-scale beams.

2(c) Drag Effect of a Fluid and Its Relevance to MEMS Devices

The drag effect arises when MEMS structures move through a fluid, producing resistive force. This drag affects response time, damping, and energy dissipation. It is significant in resonators, accelerometers, and micro-mirrors.

2(d) Fringe Effects in Electrostatic Actuation and Their Impact

Fringe effects increase electrostatic force and capacitance beyond theoretical values. While they improve actuation efficiency, they also introduce non-linearity, affecting accuracy, reliability, and pull-in voltage of MEMS devices.

2(e) Limitations of Micromechanical Resonators in RF Applications

Limitations include sensitivity to temperature variations, air damping losses, fabrication tolerances, aging effects, and limited power handling capability. These factors affect frequency stability and quality factor.

SECTION C

3(a) Characteristics of MEMS Devices for Sensing and Actuation

MEMS devices are small, lightweight, low-power, highly sensitive, and compatible with IC fabrication. These characteristics make them suitable for sensors and actuators in automotive, biomedical, aerospace, and consumer electronics applications.

3(b) Principles of Piezoelectricity and Its Significance in MEMS

Piezoelectricity is the ability of certain materials to generate electric charge under mechanical stress. In MEMS, it is used for actuation, sensing, energy harvesting, and vibration control.

4(a) Moment of Inertia in Beam Structures

The moment of inertia represents resistance to bending. It depends on beam geometry and cross-section. Higher moment of inertia results in lower deflection and higher stiffness in MEMS beams.

4(b) Stress and Strain in Beam and Diaphragm Structures

Stress is the internal force per unit area, while strain is the deformation per unit length. In MEMS beams and diaphragms, these parameters determine sensitivity, durability, and failure limits.

5(a) Drag Effect of Fluid and Role of Viscosity

Viscosity governs drag force acting on MEMS structures. Higher viscosity increases resistance to motion, leading to increased damping and reduced dynamic response.

5(b) Reynolds Equation for Squeeze-Film Air Damping

Reynolds equation describes pressure distribution in thin air films between MEMS structures. It is used to calculate squeeze-film damping, which affects stability and frequency response.

6(a) Electrostatic Forces and Their Application in MEMS

Electrostatic forces arise due to electric fields between charged electrodes. They are widely used in MEMS actuators because of low power consumption and easy integration.

6(b) Negative Spring Effect in Electrostatic Actuation

Negative spring effect occurs when electrostatic force reduces effective stiffness of a MEMS structure. It lowers resonant frequency and may cause instability or pull-in.

7(a) Thermocouples and Their Application in MEMS

Thermocouples generate voltage due to temperature difference between junctions. MEMS thermocouples are used for temperature sensing and thermal analysis at micro-scale.

7(b) Modeling of One-Port and Two-Port Micromechanical Resonators

One-port resonators use a single electrode for excitation and sensing, while two-port resonators separate input and output. Modeling helps analyze frequency response, quality factor, and energy loss.