Transcript Spintronics Integrating magnetic materials with semiconductors

Case Studies in MEMS
Case study
Pressure sensor
Accelerometer
Technology
Bulk micromach.
+ bipolar circuitry
Transduction
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 + XeF2 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 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 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 (Dx) can be used to measure acceleration
Hooke’s law for a spring: F = kDx = 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 = w2x
Position control system
Set point
Position error
+
Disturbance
In
-
+
In
Out
Controller
External
Object
Actual position
Out
Force
+
+
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 motion
noise force
wo: resonant frequency
a: acceleration
FFn
a
x

2
k
ωo
temperature
bandwidth
Fn  4kBT (BW)
@ 24.7 kHz, noise = 0.005 g/Hz
Good signal to noise ratio
Greater sensitivity (x) by increasing wo,
e.g 50 g accelerometer: (wo ) 24.7 kHz, xmax: 20 nm
1 kHz, 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 noise
Electrical capacitance change can be used to measure displacement
Two schemes used for position sensing:
Parallel plate
Inter-digitated electrodes
Dx
g
Co = eA
C1 = eA
g
g - Dx
DC = C1 - 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 (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
Spring
+ + + +
- - - -
+Q
-Q
Applied voltage (Electrostatics) causes a Mechanical force which moves the cantilever
Fmech = k Dx; Felectrostatic = Q2
2eA
Displacement (Dx) =
Q2
Q= CV
2eA 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)
eA
z
Electrical work (dW) = ∫ V dQ = Q2
2C
Electrostatic force (Fel) = dW = Q2
dz 2eA
= Q2z
2eA
Mechanical force (Fmec) = k z
At equilibrium, electrostatic force (Fel) = mechanical force (Fmec)
Dispacement (z) =
Q2
2eAk
Charge controlled
2
eAV
=
2g2
Voltage controlled
Electrostatic virtual work
+
V
-
C
Increased stored energy due to capacitance change (DU)  1 V2 DC
2
Work done, due to mechanical force (Wmech) = F Dx
Wmech + Wsource = DU
Work done by voltage source (Wsource) = V·DQ = V2·DC
Electrostatic force (Fele) = - 1 V2 ∂C
∂x
2
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
Electrical capacitance change as a function of displacement
x
g
C = eA
g-x
Electrostatic force (Fele) = - 1 V2 ∂C
∂x
2
∂C =
eo A
∂x
(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
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)
(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 holes and metallization
Schematic of final released structure
Functional block diagram
www.analog.com
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
t
Fixed electrode
Ct = 2
Ct
gt
gt - x
gs
Cs = 2
e h (t + x)
X Nteeth
gs
Cs
w
ehw
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
VL
(Vbias – v)
VR
(Vbias + v)
(Felec)L
(Felec)R VR2
VL2
(Felec)total  (Felec)R – (Felec)L  (VR2 – VL2)  4 Vbias· v
Displacement vs. Applied voltage
-Expanded linear range
- bias voltage to control gain
Displacement
gt
Vbias
- gt
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
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 can be obtained
Why electrostatic actuators are better than
magnetic actuators for micro-systems