Case Studies in MEMS - University of California, San...
Transcript of Case Studies in MEMS - University of California, San...
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 can
proof of mass motion
Electrostatic Surface micromach. Electrostatic torsion of Glass bonded
projection displays + XeF2 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
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, 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 (Dx) 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 = w2x
Hooke’s law for a spring: F = kDx = 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 feedback
Closed loop, no force feedback (most accelerometers on the market)
MEMS device
Object
Modeling a MEMS accelerometer
2
o
n
ω
a
k
FF x
F: Applied force
Fn: Johnson/Brownian motion
noise force
wo: resonant frequency
a: acceleration
• Design the accelerometer to have a resonance frequency (wo) > expected maximum frequency
component of acceleration signal
Greater sensitivity (x) by increasing wo,
e.g 50 g accelerometer: (wo ) 24.7 kHz, xmax: 20 nm
1 kHz, xmax: 1.2 mm
(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
Dx
Co = eA
gC1 = eA
g - DxDC = C1 - Co
Change in Current (DI) DQ
can be measured
by an ammetert
DQ = D 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 substrateV
Spring
+ + + +
- - - -
Fmech = k Dx; Felectrostatic = Q2
+Q
-Q
2eA
Displacement (Dx) = 2eA 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)
eAz
Electrical work (dW) = ∫ V dQ = Q2
2C
= Q2z
2eA
At equilibrium, electrostatic force (Fel) = mechanical force (Fmec)
Electrostatic force (Fel) = dW
dz
= Q2
2eAMechanical force (Fmec) = k z
Dispacement (z) = Q2
2eAk
eAV2
2g2=
Charge controlled Voltage controlled
Electrostatic virtual work
Increased stored energy due to capacitance change (DU) V2 DC
Work done, due to mechanical force (Wmech) = F Dx
Work done by voltage source (Wsource) = V·DQ = V2·DC
1
2
CV
+
-
Wmech + Wsource = DU
Electrostatic force (Fele) = - V2
2
1 ∂C
∂x
Principle of capacitive sensing-Differential sensing (Overcomes common mode noise, with linearization)
Electrical capacitance change as a function of displacement
g
x C = eA
g - x
Electrostatic force (Fele) = - V2
2
1 ∂C
∂x
∂C
∂x= eoA
(g – x)2
Restoring force (Fmec)= - k x
Equating, Fele = Fmec we get,
(g-x)2x = e AV2
2k
At a critical voltage, Vpull-in
when x = g/3 the capacitor plates touch each other
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
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, 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
Ct = 2gt - x
e h w
Cs = 2gs
e h (t + x)X Nteeth
w: width, h: height
t: initial overlap
displacement
Scale: 5 mm
Electrostatic model of comb drive actuator
Fixed electrode
Cs
Ct
wx
t
gt
gs
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
-Expanded linear range
- bias voltage to control gain
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: pressure
d: gap distance
Very few ionizing
collisions
1 mm @
1 atmosphere
Many ionizing collisions
Why electrostatic actuators are better than
magnetic actuators for micro-systems
- larger energy densities can be obtained