(SEM VI) THEORY EXAMINATION 2021-22 INTRODUCTION TO MEMS

INTRODUCTION TO MEMS (KOE063)

Section-wise Detailed Answers – B.Tech Semester VI

SECTION A

(Attempt all questions – brief but descriptive answers)

Q1(a) What do you mean by fabrication? Explain

Fabrication refers to the process of manufacturing micro-scale structures and devices used in Micro-Electro-Mechanical Systems. It involves a sequence of steps such as material deposition, patterning, etching, and bonding to create mechanical and electrical components on a substrate, typically silicon. MEMS fabrication borrows techniques from integrated circuit manufacturing and adapts them to create movable microstructures like beams, membranes, and cavities.

Q1(b) Define air damping

Air damping is the resistance offered by air to the motion of a moving microstructure. In MEMS devices, when a component vibrates or moves, the surrounding air opposes this motion, causing energy loss. This damping significantly affects the dynamic behavior of MEMS devices, especially resonators and sensors, by reducing vibration amplitude and quality factor.

Q1(c) What is a sensor? Explain

A sensor is a device that detects physical quantities such as pressure, temperature, displacement, or acceleration and converts them into electrical signals. In MEMS, sensors are miniaturized devices that offer high sensitivity, fast response, and low power consumption. MEMS sensors are widely used in automotive systems, medical devices, consumer electronics, and industrial automation.

Q1(d) Explain moment of inertia

Moment of inertia is a physical quantity that measures the resistance of a body to rotational motion about an axis. In MEMS structures like beams and plates, moment of inertia depends on geometry and material distribution. It plays a critical role in determining stiffness, deflection, and vibration behavior of microstructures.

Q1(e) What is Hooke’s Law? Explain

Hooke’s Law states that within the elastic limit, the deformation of a material is directly proportional to the applied force. In mathematical terms, stress is proportional to strain. In MEMS devices, Hooke’s Law is used to model elastic behavior of beams, springs, and membranes under small deformations.

Q1(f) Define viscosity

Viscosity is a measure of a fluid’s resistance to flow. It represents internal friction between layers of fluid moving at different velocities. In MEMS, viscosity of air or liquid significantly affects damping forces, especially in micro-scale gaps where viscous forces dominate over inertial forces.

Q1(g) How will you define electrostatic force? Explain

Electrostatic force is the attractive or repulsive force between electrically charged bodies. In MEMS actuators, electrostatic force is generated when a voltage is applied between conductive microstructures, creating an electric field that induces motion. This force is widely used for actuation due to low power consumption and ease of integration.

Q1(h) What do you mean by vibration frequency?

Vibration frequency is the number of oscillations a system completes per unit time. In MEMS devices, vibration frequency depends on mass, stiffness, and damping. Resonant frequency is particularly important in MEMS sensors and resonators as it determines sensitivity and performance.

Q1(i) Define thermo-electricity

Thermo-electricity refers to the generation of electrical voltage due to a temperature difference across a material. This phenomenon is known as the Seebeck effect. In MEMS, thermoelectric principles are used in temperature sensors and energy-harvesting devices.

Q1(j) What do you mean by step voltage? Explain

Step voltage refers to a sudden change in voltage applied to a system. In MEMS actuators, applying a step voltage causes a transient response followed by steady-state behavior. Step voltage analysis helps in understanding dynamic response, settling time, and stability of MEMS devices.

SECTION B

(Attempt any three – detailed explanations)

Q2(a) Processes for Micromachining with example

Micromachining is the process of creating microstructures on a substrate. It is broadly classified into bulk micromachining and surface micromachining. Bulk micromachining involves removing material from the substrate to form structures like cavities and diaphragms, often using wet or dry etching. Surface micromachining builds structures layer by layer on the substrate using thin-film deposition and sacrificial layers. For example, pressure sensors commonly use bulk micromachining to form thin diaphragms.

Q2(b) Strain in a bent beam with suitable example

When a beam is bent due to an applied load, strain is induced within the material. The top surface experiences compressive strain while the bottom surface undergoes tensile strain. The magnitude of strain varies linearly across the beam thickness and is maximum at the outer surfaces. In MEMS cantilever sensors, strain is measured using piezoresistive elements to detect applied forces or acceleration.

Q2(c) Drag force damping and effect of air damping on micro-dynamics

Drag force damping occurs when a moving microstructure experiences resistance due to viscous forces in the surrounding fluid. At micro-scales, air behaves as a viscous medium, and damping forces dominate system behavior. Air damping reduces vibration amplitude, lowers resonant frequency, and decreases quality factor, thereby affecting sensitivity and response time of MEMS devices.

Q2(d) Electrostatic driving of mechanical actuators

Electrostatic driving involves applying voltage between electrodes to generate attractive forces that cause mechanical motion. In MEMS actuators, this method is widely used due to low power requirements and compatibility with microfabrication. Applications include micro-mirrors, switches, and micro-pumps.

Q2(e) MEMS resonator design considerations

Design of MEMS resonators involves careful selection of material, geometry, and operating environment. Factors such as resonant frequency, quality factor, damping mechanisms, and temperature stability must be considered. High quality factor ensures low energy loss and high sensitivity, making resonators suitable for filters and oscillators.

SECTION C

(Attempt any one from each question set – detailed explanations)

Q3(a) Materials and substrates for MEMS

Common materials used in MEMS include silicon, polysilicon, silicon dioxide, silicon nitride, metals, and polymers. Silicon is preferred due to excellent mechanical properties and compatibility with IC fabrication. Substrates provide mechanical support and electrical isolation, influencing device performance and reliability.

Q3(b) Piezo resistance effect and piezoelectric sensors

The piezoresistive effect refers to the change in electrical resistance of a material when subjected to mechanical strain. This effect is widely used in MEMS pressure and force sensors. Piezoelectricity, on the other hand, is the generation of electric charge due to mechanical stress. Piezoelectric sensors offer high sensitivity and fast response, making them suitable for dynamic measurements.

Q4(a) Strain and stress in MEMS with example

Stress is the internal force per unit area developed in a material due to external load, while strain is the resulting deformation. In MEMS beams and membranes, stress-strain analysis is essential for predicting deflection, reliability, and failure. For example, excessive stress in a micro-beam can lead to fracture or fatigue.

Q4(b) Cantilever beam and bending under weight

A cantilever beam is fixed at one end and free at the other. When a load is applied at the free end, bending occurs with maximum deflection at the tip. Cantilever beams are widely used in MEMS sensors such as atomic force microscopes and chemical sensors due to their high sensitivity.

Q5(a) Squeeze-film air damping and Reynolds equation

Squeeze-film air damping occurs when air is trapped between two closely spaced surfaces and is squeezed during motion. This creates pressure that resists movement. Reynolds equation describes the pressure distribution in the air film and is used to model damping behavior in MEMS devices.

Q5(b) Stokes-flow model

The Stokes-flow model describes fluid flow at very low Reynolds numbers, where viscous forces dominate over inertial forces. This model is applicable in MEMS due to small dimensions and low velocities. It helps in analyzing drag forces and damping effects.

Q6(a) Normal force, tangential force, and fringe effect

Normal force acts perpendicular to the surface, while tangential force acts parallel to it. In MEMS actuators, both forces influence motion and stability. Fringe effect refers to distortion of electric field lines at electrode edges, affecting electrostatic force calculations and actuator performance.

Q6(b) Negative spring effect and vibration frequency

Negative spring effect occurs in electrostatically actuated MEMS when electrostatic force reduces effective stiffness, leading to instability. Vibration frequency depends on effective stiffness and mass, and changes in stiffness directly affect resonant behavior.

Q7(a) Temperature coefficient of resistance and thermal sensors

Temperature coefficient of resistance indicates how resistance changes with temperature. MEMS thermal sensors use this principle to measure temperature accurately. Examples include resistance temperature detectors and thermistors.

Q7(b) Two-Port Microresonator Modeling

Two-port microresonator modeling represents MEMS resonators using electrical equivalent circuits. This approach simplifies analysis of resonance, coupling, and energy transfer, enabling integration with electronic systems.