Transcript Spintronics Integrating magnetic materials with semiconductors
Case Studies in MEMS
Case study Pressure sensor Technology Transduction Bulk micromach.
+ bipolar circuitry Piezoresistive sensing of diaphragm deflection Accelerometer Surface micromach.
Capacitive detection of proof of mass motion Packaging Plastic Metal can Electrostatic projection displays Surface micromach. Electrostatic torsion of + XeF 2 release suspended tensile beams RF switches Surface micromach. Cantilever actuation Glass bonded Glass bonded DNA amplification Bonded etched glass Pressure driven flow with PCR across T-controlled zones Microcapillaries Lab on a chip Bulk & Surface micromachining Electrophoresis & electrowetting Microfluidics & 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
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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
(D
x
)
can be used to measure acceleration
Hooke’s law for a spring: F = k D x = 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 + External + Force Object In Out Actual position + Measured position + Measurement Noise In Out 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
x F F n k
F: Applied force
F n : Johnson/Brownian motion noise force
w o
: resonant frequency a: acceleration
a ω o 2 F n
temperature
bandwidth
4 k B T (BW)
@ 24.7 kHz, noise = 0.005 g/
Hz
Good signal to noise ratio
Greater sensitivity (x) by increasing
w o
,
e.g 50 g accelerometer: (
w
o ) 24.7 kHz, x max : 20 nm 1 kHz, x max : 1.2
m
m
•
Design the accelerometer to have a resonance frequency
(w o )
component of acceleration signal > expected maximum frequency
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 sensing:
Parallel plate Inter-digitated electrodes D x g C o = e A g D C = C 1 C 1 = e A g D x - C o
Change in Current
(D I ) D Q
can be measured
t
by an ammeter
D Q = D C V
The parallel plate capacitor
I V + z A force of attraction 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
Si substrate
Voltage source
V Spring + + + + +Q - - - -Q
Applied voltage (Electrostatics) causes a Mechanical force which moves the cantilever
F mech = k
D
x;
F electrostatic = Q 2
2
e
A Displacement (
D
x) = Q 2 2
e
A k
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)
e
A z Electrical work (dW) = ∫ V dQ = Q 2 2C = Q 2 z 2
e
A Electrostatic force (F el ) = dW dz = Q 2
e
2 A Mechanical force (F mec ) = k z
At equilibrium, electrostatic force (F el ) = mechanical force (F mec )
Dispacement (z) = Q 2 2
e
Ak
Charge controlled
=
eA
V 2 2g 2
Voltage controlled
Electrostatic virtual work
+ V C Increased stored energy due to capacitance change
(D
U
)
V 2
D
C 2 Work done, due to mechanical force (W mech ) = F
D
x W mech + W source =
D
U Work done by voltage source (W source ) = V·
D
Q = V 2 ·
D
C Electrostatic force (F ele ) = 1 2 V 2 ∂C ∂x
Principle of capacitive sensing
-
Differential sensing (Overcomes common mode noise, with linearization)
ADXL Accelerometers
- Construction
Differential Capacitive Sensing
Slide courtesy: M.C. Wu
Differential Capacitive sensing
g
Electrical capacitance change as a function of displacement
x C = e A g - x
∂C ∂x =
e o A (g – x) 2
Electrostatic force (F ele ) = 1 2 V 2 ∂C ∂x
Equating, F ele (g-x) 2 x = = F mec
e
AV 2 2k we get,
Restoring force (F mec )= - k x
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
(1)
Process flow: iMEMS technology
-
24 mask levels (11: mechanical structure and interconnect 13: electronics, MOS + Bipolar) Initial electronics layout (2) (necessary to prevent electrostatic stiction) Deposition of poly-Silicon
(structural element) Partially amorphous to insure tensile stress (prevents warping/buckling)
(2) (3) Deposition and patterning of CVD oxide and nitride, opening of contact holes and metallization (4) Schematic of final released structure
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Functional block diagram
Electrical detection of signal
ADXL Accelerometers
www.analog.com
100 million acceleration sensors shipped through September, 2002
ADXL Accelerometers
ADXL accelerometers/inertial sensors: new applications
www.analog.com
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
Electrostatic model of comb drive actuator Movable electrode Fixed electrode w g t t g s C t C s x
e
h w C t = 2 g t - x C s = 2
e
h (t + x) g s X N teeth Scale: 5
m
m
w: width, h: height t: initial overlap
displacement
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 (F V bias elec ) R + v) R
(F elec ) total
(F elec ) R – (F elec ) L
(V R 2 – V L 2 )
4 V bias · v V R 2
Displacement vs. Applied voltage
-
Expanded linear range
-
bias voltage to control gain g t V bias - g t Control voltage (v)
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) P: pressure d: gap distance ln (Pd) + B
Many ionizing collisions Very few ionizing collisions
1
m
m @ 1 atmosphere
Why electrostatic actuators are better than magnetic actuators for micro-systems
- larger energy densities can be obtained