Upload
sydney-day
View
245
Download
3
Tags:
Embed Size (px)
Citation preview
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