Case Studies in MEMS Case study Pressure sensor

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Case Studies in MEMS Case study Pressure sensor Accelerometer Technology Transduction Bulk micromach. +

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

Analog Devices: Capacitive Accelerometer - Microsystems have a smaller mass and are more sensitive

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

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 www. analog. com E-book/Digital magazine Integrating ADXL 311 with

ADXL accelerometers/inertial sensors: new applications www. analog. com E-book/Digital magazine Integrating ADXL 311 with Toshiba’s Portégé M 200/205 series tablet PCs Hard-drive protection technology IBM Think. Pad® (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) ADXL 202 E (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 (Dx) can be used to measure acceleration Hooke’s law for a

Principal Concept Displacement (Dx) can be used to measure acceleration Hooke’s law for a spring: F = k. Dx = ma acceleration x Proof mass • Sensing of acceleration by sensing a change in position • Sensitivity dictated by mass (m) and nature of spring (k: material dependent) For dynamic loads (Simple Harmonic Motion): a = w 2 x

Position control system Set point Position error + Disturbance In - + Out Controller

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

Modeling a MEMS accelerometer 1 mg - 220 picograms F: Applied force Fn: Johnson/Brownian

Modeling a MEMS accelerometer 1 mg - 220 picograms F: Applied force Fn: Johnson/Brownian motion noise force wo: resonant frequency a: acceleration temperature bandwidth @ 24. 7 k. Hz, noise = 0. 005 g/ Hz Good signal to noise ratio Greater sensitivity (x) by increasing wo, e. g 50 g accelerometer: (wo ) 24. 7 k. Hz, xmax: 20 nm 1 k. Hz, xmax: 1. 2 mm • Design the accelerometer to have a resonance frequency (wo) > expected maximum frequency component of acceleration signal

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

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

Electrical capacitance change can be used to measure displacement Two schemes used for position

Electrical capacitance change can be used to measure displacement Two schemes used for position sensing: Parallel plate Inter-digitated electrodes Dx g Co = e. A C 1 = e. A g g - Dx DC = C 1 - Co Change in Current (DI) = DQ can be measured t by an ammeter DQ = D C V

The parallel plate capacitor I + V - A force of attraction z Area

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

A MEMS cantilever Mechanical displacement using an electrical voltage Voltage source V Si substrate

A MEMS cantilever Mechanical displacement using an electrical voltage Voltage source V Si substrate Spring + + - - +Q -Q Applied voltage (Electrostatics) causes a Mechanical force which moves the cantilever Fmech = k Dx; Felectrostatic = Q 2 2 e. A Displacement (Dx) = Q 2 Q= CV 2 e. A k 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) e.

The parallel plate capacitor Charge stored (Q) = C (capacitance) · V (voltage) e. A z Electrical work (d. W) = ∫ V d. Q = Q 2 2 C Electrostatic force (Fel) = d. W= Q 2 dz 2 e. A = Q 2 z 2 e. A Mechanical force (Fmec) = k z At equilibrium, electrostatic force (Fel) = mechanical force (Fmec) Dispacement (z) = Q 2 2 e. Ak Charge controlled 2 e. AV = 2 g 2 Voltage controlled

Electrostatic virtual work + V - C Increased stored energy due to capacitance change

Electrostatic virtual work + V - C Increased stored energy due to capacitance change (DU) = 1 V 2 DC 2 Work done, due to mechanical force (Wmech) = F Dx Wmech + Wsource = DU Work done by voltage source (Wsource) = V·DQ = V 2·DC Electrostatic force (Fele) = - 1 V 2 ∂C ∂x 2

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

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

ADXL Accelerometers - Construction

ADXL Accelerometers - Construction

Differential Capacitive Sensing Slide courtesy: M. C. Wu

Differential Capacitive Sensing Slide courtesy: M. C. Wu

Differential Capacitive sensing

Differential Capacitive sensing

Electrical capacitance change as a function of displacement x g C = e. A

Electrical capacitance change as a function of displacement x g C = e. A g-x Electrostatic force (Fele) = - 1 V 2 ∂C ∂x 2 ∂C = ∂x eo. A (g – x)2 Restoring force (Fmec)= - k x Equating, Fele = Fmec we get, (g-x)2 x = e AV 2 2 k At a critical voltage, Vpull-in when x = g/3 the capacitor plates touch each other

Bi-stable operating regime of electrostatic actuators

Bi-stable operating regime of electrostatic actuators

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

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

ADXL Accelerometers - Construction

ADXL Accelerometers - Construction

Process flow: i. MEMS technology -24 mask levels (11: mechanical structure and interconnect 13:

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

(2) (3) (4) Deposition and patterning of CVD oxide and nitride, opening of contact

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

Functional block diagram www. analog. com

Functional block diagram www. analog. com

Electrical detection of signal

Electrical detection of signal

ADXL Accelerometers www. analog. com 100 million acceleration sensors shipped through September, 2002

ADXL Accelerometers www. analog. com 100 million acceleration sensors shipped through September, 2002

ADXL Accelerometers

ADXL Accelerometers

ADXL accelerometers/inertial sensors: new applications www. analog. com E-book/Digital magazine Integrating ADXL 311 with

ADXL accelerometers/inertial sensors: new applications www. analog. com E-book/Digital magazine Integrating ADXL 311 with Toshiba’s Portégé M 200/205 series tablet PCs Hard-drive protection technology IBM Think. Pad® (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) ADXL 202 E (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

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

Electrostatic model of comb drive actuator Movable electrode t Fixed electrode Ct = 2

Electrostatic model of comb drive actuator Movable electrode t Fixed electrode Ct = 2 Ct gt gs Cs = 2 Cs w ehw gt - x e h (t + x) X Nteeth gs x Scale: 5 mm w: width, h: height t: initial overlap displacement Higher N, lower gt and gs higher Force

Comb-Drive Actuators: Push-Pull/linear operation VR (Vbias + v) VL (Vbias – v) (Felec)L VL

Comb-Drive Actuators: Push-Pull/linear operation VR (Vbias + v) VL (Vbias – v) (Felec)L VL 2 (Felec)R VR 2 (Felec)total (Felec)R – (Felec)L (VR 2 – VL 2) 4 Vbias· v

Displacement vs. Applied voltage -Expanded linear range - bias voltage to control gain 100

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

Comb-Drive Actuators

Comb-Drive Actuators

Comb-Drive Actuators: Fabrication

Comb-Drive Actuators: Fabrication

Instabilities in comb-drive actuators Lateral instability - increases at larger voltages - proportional to

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

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 VB (breakdown

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

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

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

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