Case Studies in MEMS Case study Pressure sensor
- Slides: 39
Case Studies in MEMS Case study Pressure sensor Accelerometer Technology Transduction Bulk micromach. + bipolar circuitry 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 + Xe. F 2 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 M 200/205 series tablet PCs Hard-drive protection technology IBM Think. Pad® (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) ADXL 202 E (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 = k. Dx = 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 Actual position Out + + 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 temperature bandwidth @ 24. 7 k. Hz, noise = 0. 005 g/ Hz Good signal to noise ratio Greater sensitivity (x) by increasing wo, e. g 50 g accelerometer: (wo ) 24. 7 k. Hz, xmax: 20 nm 1 k. Hz, 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 = e. A C 1 = e. A g g - Dx DC = C 1 - 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 = Q 2 2 e. A Displacement (Dx) = Q 2 Q= CV 2 e. A 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) e. A z Electrical work (d. W) = ∫ V d. Q = Q 2 2 C Electrostatic force (Fel) = d. W= Q 2 dz 2 e. A = Q 2 z 2 e. A Mechanical force (Fmec) = k z At equilibrium, electrostatic force (Fel) = mechanical force (Fmec) Dispacement (z) = Q 2 2 e. Ak Charge controlled 2 e. AV = 2 g 2 Voltage controlled
Electrostatic virtual work + V - C Increased stored energy due to capacitance change (DU) = 1 V 2 DC 2 Work done, due to mechanical force (Wmech) = F Dx Wmech + Wsource = DU Work done by voltage source (Wsource) = V·DQ = V 2·DC Electrostatic force (Fele) = - 1 V 2 ∂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 = e. A g-x Electrostatic force (Fele) = - 1 V 2 ∂C ∂x 2 ∂C = ∂x eo. A (g – x)2 Restoring force (Fmec)= - k x Equating, Fele = Fmec we get, (g-x)2 x = e AV 2 2 k 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: i. MEMS 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 M 200/205 series tablet PCs Hard-drive protection technology IBM Think. Pad® (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) ADXL 202 E (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 gs Cs = 2 Cs w ehw gt - x e h (t + x) X Nteeth gs 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 VR (Vbias + v) VL (Vbias – v) (Felec)L VL 2 (Felec)R VR 2 (Felec)total (Felec)R – (Felec)L (VR 2 – VL 2) 4 Vbias· v
Displacement vs. Applied voltage -Expanded linear range - bias voltage to control gain 100 V 200 V 300 V 400 V 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
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