Case Studies in MEMS

Case study Technology Transduction Packaging

Pressure sensor Bulk micromach. Piezoresistive sensing Plastic + bipolar circuitry of diaphragm deflection

Accelerometer Surface micromach. Capacitive detection of Metal canproof of mass motion

Electrostatic Surface micromach. Electrostatic torsion of Glass bondedprojection displays + XeF2 release suspended tensile beams

RF switches Surface micromach. Cantilever actuation Glass bonded

DNA amplification Bonded etched glass Pressure driven flow Microcapillarieswith 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

www.analog.com

E-book/Digital magazineIntegrating ADXL 311 with Toshiba’s Portégé M200/205 series tablet PCs

Hard-drive protection technologyIBM 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, helpingprevent 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 ConceptDisplacement xcan 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 = kx = ma

Position control system

Position errorDisturbance

In Out

External

ForceIn Out

Actual position

Measurement Noise

Position Sensor

Measured position

Set point+

-

In Out

Controller

+

++

+

Open loop, with force feedbackClosed loop, no force feedback (most accelerometers on the market)

MEMS device

Object

Modeling a MEMS accelerometer

2o

n

ω

a

k

FF x

F: Applied forceFn: Johnson/Brownian motion noise force: resonant frequencya: 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, xmax: 20 nm

1 kHz, xmax: 1.2 m

(BW) Tk4F B n

@ 24.7 kHz, noise = 0.005 g/Hz

1 mg - 220 picograms

bandwidthtemperature

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

Co = Ag

C1 = Ag - x

C = C1 - Co

Change in Current IQcan 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 forcein 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 substrateV

Spring + + + +

- - - -

Fmech = k x; Felectrostatic = Q2

+Q

-Q

2A

Displacement (x) = 2A k

Q2

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)

Az

Electrical work (dW) = ∫ V dQ = Q2 2C

= Q2z2A

At equilibrium, electrostatic force (Fel) = mechanical force (Fmec)

Electrostatic force (Fel) = dWdz

= Q2

2AMechanical force (Fmec) = k z

Dispacement (z) = Q2

2Ak V2

2g2=

Charge controlled Voltage controlled

Electrostatic virtual work

Increased stored energy due to capacitance change UV2 C

Work done, due to mechanical force (Wmech) = F x

Work done by voltage source (Wsource) = V·Q = V2·C

12

CV

+

-

Wmech + Wsource = U

Electrostatic force (Fele) = - V2

2

1 ∂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 = Ag - x

Electrostatic force (Fele) = - V2

2

1 ∂C∂x

∂C∂x

= oA(g – x)2

Restoring force (Fmec)= - k x

Equating, Fele = Fmec we get,

(g-x)2x = AV2 2k

At a critical voltage, Vpull-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 preventelectrostatic stiction)

(2)

(1) Initial electronics layout

Deposition of poly-Silicon (structural element)

Partially amorphous toinsure 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

www.analog.com

Functional block diagram

Electrical detection of signal

ADXL Accelerometerswww.analog.com

100 million acceleration sensors shipped through September, 2002

ADXL Accelerometers

ADXL accelerometers/inertial sensors: new applications

www.analog.com

E-book/Digital magazineIntegrating ADXL 311 with Toshiba’s Portégé M200/205 series tablet PCs

Hard-drive protection technologyIBM 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, helpingprevent 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 ActuatorsWhy?

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

Movable electrode

Ct = 2 gt - x

h w

Cs = 2 gs

h (t + x) X Nteeth

w: width, h: heightt: initial overlap

displacement

Scale: 5 m

Electrostatic model of comb drive actuator

Fixed electrode

Cs

Ct

wx

t

gtgs

Higher N, lower gt and gs higher Force

Comb-Drive Actuators: Push-Pull/linear operation

VL (Vbias – v)

(Felec)L VL2

VR (Vbias + v)

(Felec)R VR2

(Felec)total (Felec)R – (Felec)L (VR2 – VL

2) 4 Vbias· v

Displacement vs. Applied voltage

Dis

pla

cem

ent

Control voltage (v)- gt

gt

Vbias

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 VB (breakdown voltage) = A (Pd)

ln (Pd) + BP: pressured: gap distance

Very few ionizing collisions

1 m @ 1 atmosphere

Many ionizing collisions

Why electrostatic actuators are better thanmagnetic actuators for micro-systems

- larger energy densities can be obtained

Why electrostatic actuators are better thanmagnetic actuators for micro-systems