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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 FFn a x 2 k ωo temperature bandwidth Fn 4k BT (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