MEMS

Micro-Electro-Mechanical Systems (MEMS) are miniaturized devices that integrate mechanical, electrical, and sometimes optical or chemical components on a single chip using semiconductor fabrication techniques. With dimensions typically in the range of 1 to 1000 micrometers, MEMS devices are found in smartphones, automotive airbag sensors, medical pressure monitors, and inkjet printer heads. Understanding MEMS principles bridges device physics, mechanics, and IC fabrication.

Core Concept: What Makes MEMS Unique

MEMS devices exploit the coupling between electrical signals and mechanical motion or deformation at the microscale. Unlike purely electronic devices, MEMS include movable or deformable mechanical elements such as cantilever beams, diaphragms, suspended masses, and resonators. The critical insight is that at the microscale, surface forces (electrostatic attraction, surface tension, van der Waals) dominate over volume forces (inertia, gravity) because volume scales as L^3 while surface scales as L^2. This is why a small proof mass MEMS accelerometer can detect tiny accelerations.

MEMS fabrication borrows heavily from IC processing: photolithography defines patterns, etching removes material, and deposition adds functional layers. However, MEMS requires additional steps such as deep reactive ion etching (DRIE) for high-aspect-ratio structures and sacrificial layer release to free suspended mechanical elements from the substrate.

Sensing Mechanisms

Capacitive Sensing

In a capacitive MEMS sensor, a movable mechanical element (proof mass or membrane) forms one plate of a parallel plate capacitor. An applied mechanical stimulus (acceleration, pressure, rotation) moves the plate, changing the gap d. Since C = epsilon_0 * A / d, even nanometer-scale gap changes produce measurable capacitance changes. Differential capacitance configurations cancel common-mode errors and improve sensitivity. This principle is used in MEMS accelerometers found in every smartphone for screen orientation and step counting.

Piezoresistive Sensing

The piezoresistive effect refers to the change in electrical resistance of a material under mechanical stress. In silicon, piezoresistance coefficients are very high, making it an excellent material for stress sensing. Resistors are diffused or implanted at high-stress locations on a silicon diaphragm or cantilever. Applied pressure deforms the diaphragm, creating stress at the edges where the piezoresistors are located. The fractional resistance change is:

Delta_R / R = pi_l * sigma_l + pi_t * sigma_t where pi_l and pi_t are longitudinal and transverse piezoresistive coefficients and sigma_l and sigma_t are the corresponding stresses.

Electrostatic Actuation

MEMS actuators most commonly use electrostatic force because it can be generated with very low current. The comb drive actuator consists of interdigitated finger arrays: one array is fixed to the substrate and the other is suspended by springs. Applying voltage V between the comb sets generates a lateral electrostatic force F = n * epsilon * t * V^2 / d where n is the number of finger pairs, t is the structural layer thickness, and d is the gap between fingers. This force moves the suspended comb array laterally and is used in optical switches, gyroscopes, and resonators.

Fabrication: Bulk vs Surface Micromachining

Bulk micromachining etches into the silicon substrate itself to form three-dimensional structures. Anisotropic wet etchants like KOH etch silicon preferentially along crystallographic planes, creating V-grooves, membranes, and through-wafer holes. DRIE (Bosch process) uses alternating etch and passivation cycles to create near-vertical sidewalls with aspect ratios exceeding 20:1, enabling deep cavities and high-aspect-ratio beams.

Surface micromachining builds structures on top of the substrate using deposited thin films. A sacrificial layer (typically SiO2) is deposited first, then the structural layer (polysilicon) is deposited on top. Patterns are defined by lithography and etching. Finally, the sacrificial layer is removed by HF etching, leaving the polysilicon structure suspended above the substrate. This process is used to make free-standing cantilevers, bridges, and resonators.

Given:
MEMS capacitive accelerometer, plate area A = 100 um x 100 um = 1e-8 m^2
Initial gap d0 = 2 um = 2e-6 m, epsilon_0 = 8.85e-12 F/m
Applied acceleration causes gap change delta_d = 0.1 um = 1e-7 m

Why this formula applies:
Capacitance changes with gap: C = epsilon_0 * A / d

Formula:
C0 = epsilon_0 * A / d0 (initial capacitance)
C_new = epsilon_0 * A / (d0 - delta_d) (after deflection)
Delta_C = C_new - C0

Substitution:
C0 = 8.85e-12 * 1e-8 / 2e-6 = 8.85e-20 / 2e-6 = 44.25e-15 F = 44.25 fF
C_new = 8.85e-12 * 1e-8 / (2e-6 - 1e-7) = 8.85e-20 / 1.9e-6 = 46.58 fF

Calculation:
Delta_C = 46.58 - 44.25 = 2.33 fF

Final Answer: Capacitance change = 2.33 fF for 0.1 um displacement, showing high sensitivity of capacitive MEMS sensing

Exam Tip: GATE questions on MEMS often test the scaling law: force scales as L^2 (electrostatic) or L^3 (magnetic), surface tension as L, inertia as L^3. At microscale, electrostatic actuation dominates over magnetic actuation because electrostatic force scales more favorably. Also remember: KOH etches silicon along (111) planes, leaving 54.7 degree sidewalls.

• MEMS couples mechanical motion with electrical signals at the microscale (1 to 1000 um). Key advantage: batch fabrication on silicon wafers reduces cost.
• Capacitive sensing: C = epsilon*A/d. Gap change produces capacitance change. Used in accelerometers, gyroscopes, pressure sensors.
• Piezoresistive sensing: Delta_R/R = pi_l*sigma_l + pi_t*sigma_t. Silicon has large piezoresistance coefficients, ideal for pressure sensors.
• Electrostatic comb drive: F = n*epsilon*t*V^2/d. Dominant actuation method at microscale due to favorable L^2 scaling.
• Bulk micromachining: etches into substrate (KOH, DRIE). Surface micromachining: builds on top then releases sacrificial layer with HF.
• Stiction (unwanted adhesion after release) is a key MEMS fabrication challenge due to surface tension and van der Waals forces dominating at small scales.

Quick Revision

• MEMS: integration of mechanical + electrical elements at 1 to 1000 um scale using IC fabrication techniques.
• Capacitive formula: C = epsilon_0*A/d. Differential sensing cancels common-mode errors.
• Piezoresistance: Delta_R/R = pi_l*sigma_l + pi_t*sigma_t. n-type Si has different pi values than p-type Si.
• Electrostatic force scales as L^2; magnetic force as L^3. Electrostatic dominates below 1 mm scale.
• Bulk micromachining uses KOH (anisotropic, 54.7 deg walls) or DRIE (Bosch, vertical walls). Surface micromachining uses poly-Si over SiO2 sacrificial layer.
• Exam trap: Stiction is NOT friction. It is adhesion caused by surface energy and capillary forces during wet release steps. Supercritical CO2 drying prevents stiction.
• Resonant frequency of a cantilever beam scales as 1/L. Smaller MEMS resonators operate at GHz frequencies (RF MEMS filters).