Case Studies in MEMS Case study Technology Transduction Packaging Pressure sensor Bulk micromach.Piezoresistive sensing Plastic + bipolar circuitryof diaphragm deflection Accelerometer Surface micromach.Capacitive detection of Metal can proof of mass motion Electrostatic Surface micromach. Electrostatic torsion of Glass bonded projection displays + XeF 2 release suspended tensile beams RF switches Surface micromach. Cantilever actuation Glass bonded DNA amplification Bonded etched glass Pressure driven flow Microcapillaries with PCR across T-controlled zones Lab on a chip Bulk & Surface Electrophoresis & Microfluidics micromachining electrowetting & Polymers

Analog Devices: Capacitive Accelerometer - Microsystems have a smaller mass and are more sensitive to movement - capable of detecting 0.02 nm displacement (10% of an atomic diameter) - Issues: Bandwidth/Speed, Resolution and Accuracy

MEMS Accelerometers Applications & Design goals The detection of acceleration: - useful for crash detection and airbag-deployment - vibration analysis in industrial machinery - providing feedback to stop vibrations ….. Design goals: - Accuracy, Bandwidth and Resolution - Large dynamic range desired ( 1 nanogram – 100 grams) - Minimize drift (time and temperature) Open loop vs. close loop (with feedback) Courtesy: Boser, UCB

ADXL accelerometers/inertial sensors: new applications E-book/Digital magazine Integrating ADXL 311 with Toshiba’s Portégé M200/205 series tablet PCs Hard-drive protection technology IBM ThinkPad ® (The accelerometer detects shocks/free fall conditions, and within a fraction of a second signals the drive’s read/write heads to temporarily park, helping prevent contact with the disk drive until the system is stabilized Digital blood pressure monitors (Omron) ADXL202E (the accelerometer senses the angle and height of the users elbow and starts measurements only after the wrist is set at the right position) Vibration control, optical switching ….

Principal Concept Displacement  x  can be used to measure acceleration Sensing of acceleration by sensing a change in position Sensitivity dictated by mass (m) and nature of spring (k: material dependent) x acceleration Proof mass For dynamic loads (Simple Harmonic Motion): a =   x Hooke’s law for a spring: F = k  x = ma

Position control system Position error Disturbance In Out External Force In Out Actual position Measurement Noise Position Sensor Measured position Set point +-+- In Out Controller Open loop, with force feedback Closed loop, no force feedback (most accelerometers on the market) MEMS device Object

Modeling a MEMS accelerometer F: Applied force F n : Johnson/Brownian motion noise force   : resonant frequency a: acceleration Design the accelerometer to have a resonance frequency    > expected maximum frequency component of acceleration signal Greater sensitivity (x) by increasing  , e.g 50 g accelerometer: (  o ) 24.7 kHz, x max : 20 nm 1 kHz, x max : 1.2  24.7 kHz, noise = g/  Hz 1 mg picograms bandwidth temperature Good signal to noise ratio

Sensitivity - Determined by noise (fluidic damping, circuit noise, shot noise …) Johnson/Thermal agitation noise

Electrical capacitance change can be used to measure displacement Parallel plate Inter-digitated electrodes Two schemes used for position sensing: g xx C o =  A g C 1 =  A g -  x  C = C 1 - C o Change in Current  I  Q can be measured by an ammeter t  Q =  C V

The parallel plate capacitor +-+- V I Area (A) z There are two counter-balancing forces, a electrical force and an mechanical force in a capacitor, an Electro-Mechanical system A force of attraction

A MEMS cantilever Mechanical displacement using an electrical voltage Voltage source Applied voltage (Electrostatics) causes a Mechanical force which moves the cantilever Si substrate V Spring F mech = k  x; F electrostatic = Q 2 +Q -Q 2A2A Displacement (  x) = 2  A k Q2Q2 Q= CV Displacement sensitivity: 0.2 Å (0.1 atomic diameter) - can be used for single molecule sensing (NEMS)

The parallel plate capacitor Charge stored (Q) = C (capacitance) · V (voltage) AA z Electrical work (dW) = ∫ V dQ = Q 2 2C = Q 2 z 2A2A At equilibrium, electrostatic force (F el ) = mechanical force (F mec ) Electrostatic force (F el ) = dW dz = Q 2 2A2A Mechanical force (F mec ) = k z Dispacement (z) = Q 2 2  Ak  V 2 2g 2 = Charge controlled Voltage controlled

Electrostatic virtual work Increased stored energy due to capacitance change  U  V 2  C Work done, due to mechanical force (W mech ) = F  x Work done by voltage source (W source ) = V·  Q = V 2 ·  C 1 2 C V +-+- W mech + W source =  U Electrostatic force (F ele ) = - V ∂C ∂x

Principle of capacitive sensing -Differential sensing (Overcomes common mode noise, with linearization)

ADXL Accelerometers - Construction

Slide courtesy: M.C. Wu Differential Capacitive Sensing

Differential Capacitive sensing

Electrical capacitance change as a function of displacement g x C =  A g - x Electrostatic force (F ele ) = - V ∂C ∂x ∂C ∂x =  o A (g – x) 2 Restoring force (F mec )= - k x Equating, F ele = F mec we get, (g-x) 2 x =  AV 2 2k At a critical voltage, V pull-in when x = g/3 the capacitor plates touch each other

Bi-stable operating regime of electrostatic actuators

Voltage controlled gap-closing actuator S. Senturia, Microsystem design

ADXL Accelerometers - Construction

Process flow: iMEMS technology -24 mask levels (11: mechanical structure and interconnect 13: electronics, MOS + Bipolar) (necessary to prevent electrostatic stiction) (2) (1) Initial electronics layout Deposition of poly-Silicon (structural element) Partially amorphous to insure tensile stress (prevents warping/buckling)

(3) Deposition and patterning of CVD oxide and nitride, opening of contact holes and metallization (2) (4) Schematic of final released structure

Functional block diagram

Electrical detection of signal

ADXL Accelerometers million acceleration sensors shipped through September, 2002

ADXL Accelerometers

ADXL accelerometers/inertial sensors: new applications E-book/Digital magazine Integrating ADXL 311 with Toshiba’s Portégé M200/205 series tablet PCs Hard-drive protection technology IBM ThinkPad ® (The accelerometer detects shocks/free fall conditions, and within a fraction of a second signals the drive’s read/write heads to temporarily park, helping prevent contact with the disk drive until the system is stabilized Digital blood pressure monitors (Omron) ADXL202E (the accelerometer senses the angle and height of the users elbow and starts measurements only after the wrist is set at the right position) Vibration control, optical switching ….

Comb-Drive Actuators Why? - larger range of motion - less air damping, higher Q factors - linearity of drive (  V) - flexibility in design, e.g. folded beam suspensions

Movable electrode C t = 2 g t - x  h w C s = 2 g s  h (t + x) X N teeth w: width, h: height t: initial overlap displacement Scale: 5  m Electrostatic model of comb drive actuator Fixed electrode CsCs CtCt w x t gtgt gsgs Higher N, lower g t and g s  higher Force

Comb-Drive Actuators: Push-Pull/linear operation V L (V bias – v) (F elec ) L  V L 2 V R (V bias + v) (F elec ) R  V R 2 (F elec ) total  (F elec ) R – (F elec ) L  (V R 2 – V L 2 )  4 V bias · v

Displacement vs. Applied voltage Displacement Control voltage (v) - g t gtgt V bias 400 V 300 V 200 V 100 V -Expanded linear range - bias voltage to control gain

Comb-Drive Actuators

Comb-Drive Actuators: Fabrication

Instabilities in comb-drive actuators Lateral instability - increases at larger voltages - proportional to comb-spacing Courtesy: M. Wu, UCLA

To increase lateral stability, at small gaps - Optimized spring design - Use circular comb-drive actuators

Is there a limit to the gap size? - breakdown Paschen’s law V B ( breakdown voltage ) = A (Pd) ln (Pd) + B P: pressure d: gap distance Very few ionizing collisions 1  1 atmosphere Many ionizing collisions

Why electrostatic actuators are better than magnetic actuators for micro-systems - larger energy densities can be obtained

Why electrostatic actuators are better than magnetic actuators for micro-systems