EECS 598: Literature Review Ali Besharatian March 16, 2008 First Paper A HARPSS Polysilicon Vibrating Ring Gyroscope Farrokh Ayazi and Khalil Najafi University of Michigan 2001 What is a Gyroscope? A gyroscope is a device for measuring or maintaining orientation,
It senses angular velocity, Based on principles of conservation of angular momentum. A macroscale gyro is essentially a spinning wheel or disk whose axle is free to take any orientation. A gyroscope in operation with freedom in all three axes. The rotor will maintain its spin axis direction regardless of the orientation of the outer frame. Foucault Pendulum, a device demonstrating the effect of the Earth's rotation, by rotating 360o in its plane, every 24 hours: This is because plane of the pendulum's swing, like a gyroscope, tends to keep a fixed direction in space, while the Earth rotates under it. MEMS Ring Gyros
No Moving parts! (It doesnt rotate!) Based on the same principle: conversion of angular momentum. Applications: Traction control, Ride stabilization, Roll-over detection, Digital camera stabilization Automotive applications (bias stability of 0.5 deg/s) Guidance of missiles (improved performance) Coriolis Force (the same as Foucault Pendulum) is generated in case of rotation. The 1st MEMS Ring Gyro (Electroplated Nickel on Silicon) Fully Silicon Ring Gyro MEMS Ring Gyros: Principle of Operation
Forced resonance by drive electrodes In case of angular momentum, Coriolis force transfers energy from primary mode to secondary flexural mode This causes secondary resonance in another axis (usually 45 o apart) Can be sensed by change in capacitance Advantages are: Symmetry Vertical Capacitors => Very large Sensitivity is amplified by Q Low temperature sensitivity (both modes experience the same expansion) Ease of control and compensation: electronic tuning The drawback is small effective mass Anchor Drive mode Sense mode
Eight Springs: Symmetric with 2 identical elliptical resonance modes Capacitive read-out Concept: Change in capacitance based on change in gap, overlap or both. Here: Change in gap for vertical electrodes. Parasitic capacitance is the drawback. To reduce the electronic noise floor: Should be maximized Reduce the gap Increase the height and radius Increase Q Reduce wo, but keep it beyond env. interferences and below Brownian noise floor
Minimize the input referred noise of the interface circuit Increase the drive amplitude (q d) But only below nonlinear effects Increase the polarization voltage Should be minimized Estimated/Simulated Parameters Resonant Frequency Material Quality Factor: M = Effective Mass D = Damping Coefficient k = Spring Constant Fabrication Process (HARPSS)
High Aspect Ratio combined Poly and Single Crystal Silicon A combination of BULK and SURFACE micromachining Steps: Deep Boron Doping (P++) Deep Etching (20:1 80um) Trench Refill Oxide Deposition Poly-Silicon Deposition Metallization EDP Etch Sacrificial Oxide Etch (Release) Advantages Single wafer Simpler than Nickel gyro (i.e. electroplated) No bonding Stress Cancelation by touching Poly-Silicon films
Fully Silicon Low TCE mismatch, No bonding Poly-Si springs: High Q (Cos4q mismatch is caused by crystal asymmetry of SC Si.) Orientation independent Better material properties than Ni (higher Q) Tall structures (100s of um): Large capacitances for measurement By changing the oxide thickness, the gap can be controlled easily from sub- to 10s of um. Large capacitances for measurement Challenges Void in poly-silicon trench refill process
can be a source of energy loss (lower Q) Excessive undercut of the Si substrate may cause the be soft and dissipates more energy. voids Compensation (Tuning) By applying a CMOS level DC voltage the degenerate frequency can be canceled: 0.9V at 22.5o axis 0 at 45o axis (this would be 15.5V for the Ni gyro) Test Results
Vacuum (1mTorr) Q = 6000 (lower due to voids) Modification in etch/refill process increases Q to 10000-20000 range. (up to 85000) Open Loop (low vac off chip circuit): Q = 250 (poor vacuum 10 times reduce) Measured Capacitance: 500fF Parasitic Capacitance: 2pF (output affected by 4 times) Drive Amp: 150nm 200uV/deg/sec Resolution <1deg/s (BW: 1Hz) Limitation: Ckt Noise Dynamic range: 250deg/sec (BW: 5Hz) Future Work: Parasitic Capacitance Elimination 0.01 deg/s/(Hz)0.5 for next generations Second Paper Batch-Processed Vacuum-Sealed Capacitive
Pressure Sensors Abhijeet Chavan and Kensal D. Wise University of Michigan 2001 General Device Info. Capacitive sensing (~2pF change) Advantages: High Pressure Sensitivity Low Temperature Sensitivity Low Power Diameter: 920-1100um 500-800Torr Res: 25mTorr
(1ft!) Vac. sealed ref. cavity: Lower trapped gas effects Wider BW (low damping) No Stiction Applications: Automotive, Environmental Medical Industrial Proc. Control Distributed Weather Forecasting Networks Different curves for different operating points Tensile Stress (~25MPa) ~3um
~10um Barometric Pressure Sensors Two fabricated devices: Single lead (metal on glass) Multiple leads (better parasitic cancelation) Barometric (absolute) pressure sensors. Both hermetically sealed with PolySi / Glass bonding Poly is used for lead transfer Fabrication Process
DWP Process Anodically bonded to a glass wafer. Std. CMOS - Wafer Level! Fully integrated ckt possible! Single Lead Detail: 8 masks: Recess Etching (KOH) P++ Boron Doping ONO Deposition Poly-Si Deposition and lightly doping (lower temp) Optional CMP Metal Connections (lift-off) Metal On the glass Anodic Bonding EDP Release Optional parylene coating Since the bonding is done in vacuum, membrane is deflected upon release.
Multiple Leads Single Lead Multi Lead Device 2 levels of poly Leads: between second poly and glass Ti/Pt on glass getters out diffusing oxygen. Leak rate < 1.1e-8 atm.cm3/s Lead Transfer: Glass electrode / poly1/ poly2 / poly1/external-metal Poly ring is isolated => tests needed to verify. Switched Capacitor
Sensing CS and CF can exchange their roles: output will be inversely proportional to CS, resulting in linear measurement! Test Results TCO: thermal coefficient of Offset TCS: Thermal Coefficient of Sensitivity Co = 12pF Single Lead Single Lead Multi Lead 27 39 TCO (ppm/oC)
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