MEMS Overview. What is MEMS?

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1 MEMS Overview SPEAKER Andrew Mason, Asst. Professor in Electrical and Computer Engineering TOPIC Overview of Micro-Electro-Mechanical Systems (MEMS) OUTLINE Overview of MEMS & Microsystems Navid Yazdi Micromachining & MEMS process technology Navid Yazdi Micro-electro-mechanical devices & microsensors Inertial sensors Navid Yazdi Pressure sensors Navid Yazdi Bio-sensors Andrew Mason Shock sensors Andrew Mason Integrated Microsystems Andrew Mason MEMS Overview, Prof. A. Mason Page 1 What is MEMS? MEMS = Micro-Electo Electo-Mechanical Systems creation of 3-dimensional 3 structures using integrated circuits fabrication technologies and special micromachining processes typically done on silicon or glass (SiO 2 ) wafers MEMS Devices and Structures transducers microsensors and microactuators mechanically functional microstructures microfluidics: : valves, pumps, flow channels microengines: : gears, turbines, combustion engines Integrated Microsystems integrated circuitry and transducers combined to perform a task autonomously or with the aid of a host computer MEMS components provide interface to non-electrical world sensors provide inputs from non-electronic events actuators provide outputs to non-electronic events MEMS Overview, Prof. A. Mason Page 2 1

2 Why Use MEMS? Motivation and Benefits Small Size Light Weight Enhanced Performance & Reliability high resolution devices array of devices Low Cost (from batch fabrication) Applications Automotive System Health Care Automated Manufacturing Instrumentation Environmental Monitoring & Control Consumer Products Aerospace Applications MEMS-based Microsystems highly integrated systems sensing actuation computation control communication MEMS Overview, Prof. A. Mason Page 3 Example MEMS-Based Microsystem Micro Cluster Environmental Monitoring Microinstrument (developed at U-Mich U in the 1990s, A. Mason, K. Wise, et. al.) Pressure Sensor Humidity Sensor Interface Chips Accelerometer On/Off Switch 60mm Power Management Microcontroller RF Transmitter I/O Connector 35mm Integrated Features - Control - Microcontroller - Power Management - Communication - RF Transceiver - Sensing - Pressure - Humidity - Temperature - Vibration - Packaging MEMS Overview, Prof. A. Mason Page 4 2

3 MEMS Fabrication Technologies Applying Micromachining to create 3-D structures using 2-D processing 2-D IC fabrication 3-D structures technology micromachining Micromachining Processes bulk and surface micromachining isotropic etching anisotropic etching dissolved wafer process deep reactive ion etching anodic and fusion bonding micromolding MEMS Overview, Prof. A. Mason Page 5 Surface vs. Bulk Micromachining Bulk Micromachining: Backside etch Si Substrate Bulk Micromachining: Front-side Etch pit typically polysilicon Surface Micromachined Structure Si Substrate MEMS Overview, Prof. A. Mason Page 6 3

4 Isotropic Etching of Isotropic etchant etches in all directions forms rounded pits in surface of wafer Most common solution HNA: Mixture of HF, HNO3, Acetic acid (CH3COOH) With agitation: Good reactant mass transport Without agitation MEMS Overview, Prof. A. Mason Page 7 Anisotropic Etching of Anisotropic etchant directional-dependant etch; based on crystal planes forms flat-surface pits in surface of wafer Common anisotropic etchants EDP, KOH, TMAH (111) Surface orientation (100) Surface orientation Anisotropic wet etching using EDP, KOH: (100) surface - Etch stop on (111) plane MEMS Overview, Prof. A. Mason Page 8 4

5 Anisotropic Etching: Convex vs. Concave Corners masking layer not attacked by Si etchant Top View Concave corner Convex corner Cantilever Beam (111) (100) Masking layer Side View Buried etch stop layer (SiO2 in SOI wafers) MEMS Overview, Prof. A. Mason Page 9 Anisotropic Etching of : Example bulk micromachined silicon proof mass MEMS Overview, Prof. A. Mason Page 10 5

6 Dissolved Wafer Process Structure created by diffusion masking layer heavily p-dope silicon (p++) Dissolve bulk of silicon to release the p++ structure Released p++ structure P++ Dopant selective etch (e.g. EDP) K.D. Wise, K. Najafi, Univ of Michigan MEMS Overview, Prof. A. Mason Page 11 Dissolved Wafer Process Example Shock Switch weighted cantilever beam with contacts that close by acceleration (shock) Fabrication Flow create anchor, weight, support beam, and contact on Si create cavity and contact on glass bond wafers and then dissolve the Si wafer LPCVD layer deposition for beams Anodic bonding glass to silicon Dissolve silicon bulk and release structure MEMS Overview, Prof. A. Mason Page 12 6

7 Deep Reactive Ion Etching (DRIE) Reactive Ion Etching = RIE mechanical (ion) etching in plasma for chemical selectivity Deep RIE creates high aspect ratio patterns, narrow and deep Top view A A mask A-A cross section Trench-Refill process can fill the etched trench with another material MEMS Overview, Prof. A. Mason Page 13 Glass-Si Anodic Bonding Bonding a glass wafer to a silicon wafer both wafer can (and generally are) patterned with structures Application creating sealed cavities on a wafer surface can be sealed in vacuum hermetic packaging Lead Transfer need to bring the metal leads out of from sealed cavities Need sealing Metal Interconnect Glass MEMS Overview, Prof. A. Mason Page 14 7

8 - Fusion Bonding Two silicon wafer with/without SiO2 can be bonded Advantages: No thermal mismatch Needs contamination free, smooth, and flat wafers (e.g. surface roughness ~5 A) Process Flow Clean wafers Make the surfaces hydrophilic (e.g. dip in Nitric Acid) Rinse-Dry Place the wafers together apply pressure H2 or N2 anneal at C MEMS Overview, Prof. A. Mason Page 15 Combined Bulk-Surface Process: Molding Etch silicon with high aspect ratio (e.g., DRIE) Refill partially with sacrificial layer (e.g. silicon oxide) Refill completely with structural layer (e.g. polysilicon) Example: U-Mich Precision Inertial Sensor Polysilicon Electrode with Damping Holes Nitride Standoff Polysilicon Electrode Damping Holes Air Gap Air Gap Nitride Standoff Electrode Dimple Proof Mass Air Gap Vertical Stiffener Proof Mass Vertical Stiffener N. Yazdi & K. Najafi, Transducers 97. MEMS Overview, Prof. A. Mason Page 16 8

9 Combined Bulk-Surface Process: Precision Inertial Sensors Dielectric Layer Metal Pads Metal Pad A A Proof mass Support Rim Damping Holes N. Yazdi, A. Salian & K. Najafi, MEMS 99. MEMS Overview, Prof. A. Mason Page 17 LIGA Process LIGA: Lithographie, Galvanoformung, Abformung Form high aspect ratio structures on top of wafer Uses molding and electroplating Synchrotron Radiation (X-Ray) used uses multiple polymethyl metharylate (PMMA) layers Features Aspect ratio: 100:1 Gap: 0.25µm Size: a few millimeters MEMS Overview, Prof. A. Mason Page 18 9

10 LIGA Process: Example Guckel, IEEE Proceedings, Aug MEMS Overview, Prof. A. Mason Page 19 Why Monolithic? Performance: - Reduce parasitics due to interconnecting devices - Reduce noise & crosstalk Size: Monolithic Integration of MEMS and ICs - Reduce pin count - Reduce package volume Cost: - Integration with signal-processing better functionality - Reduce packaging cost - Self test & calibration at wafer level MEMS Overview, Prof. A. Mason Page 20 10

11 IC + MEMS Process Examples UC Berekely Integrated CMOS & surface micromachining technology CMOS first and MEMS second CMOS circuit passivated using silicon nitride Tungsten interconnects for CMOS J. Bustillo, R. Howe, R. Muller, IEEE Proceedings Aug. 98 Sandia Integrated CMOS & surface micromachining technology MEMS first in recessed cavity CMOS second after planarization J. Smith et. al., IEDM 95 MEMS Overview, Prof. A. Mason Page 21 Neural Recording Probes MEMS Examples Monolithic Integration of Wafer-Dissolved Process and IC Technology OUTPUT LEADS P ++ RIM INTERCONNECTING LEADS SIGNAL PROCESSING CIRCUITRY RECORDING/STIMULATING SITES - Neural probes with signal processing SUPPORTING circuitry SILICON SUBSTRATE - P++ shanks - P++ rim and timed etch to protect active circuitry Najafi, Wise, JSSC-21 (6), May 1986 MEMS Overview, Prof. A. Mason Page 22 11

12 Example: Capacitive Accelerometer Vertical accelerometer Anchor Substrate (a) Suspension Mass Conductive Electrode Anchor Suspension Substrate (b) Mass Cs Sense Capacitance Anchors One of the sense Bidirectional capacitor elements Sense Fingers Anchors Mass Cs Mass Bidirectional Sense Fingers Lateral accelerometer Suspensions (a) Substrate Suspensions (b) MEMS Overview, Prof. A. Mason Page 23 Example: Z-Axis Torsional Accelerometer Anchor Torque produced by the mass A' wm Support beam Anchor T C T Fixed Sense Fingers Glass Substrate A Inertial Element Support posts on glass Selvakumar, Najafi, JMEMS Anchor Torsional suspension Mass plate Sense fingers MEMS Overview, Prof. A. Mason Page 24 12

13 Capacitive Accelerometer 3-Axis Monolithic Surface Micromachined Accelerometer Analog Devices ADXL50 Sense & Feedback Interdigitated Fingers Proofmass Suspension Courtesy of Lemkin, Boser, Trasnducers 97. Courtesy of Kevin Chau, Analog Devices Inc. MEMS Overview, Prof. A. Mason Page 25 All- Micro-G Accelerometer A Metal Pads Damping Holes Anchor Dielectric Top View Proofmass Polysilicon Top/Bottom Polysilicon Electrode Stiffeners Proofmass Suspension Beams A A-A Cross-Sectional View Stiffened Polysilicon Electrode Proofmass Frame Stiffened Polysilicon Electrode Proofmass MEMS Overview, Prof. A. Mason Page 26 13

14 MEMS Gyroscopes Draper s Tuning Fork Gyroscope Perforated masses (tines) GM & UM Ring Gyroscope Drive Combs Suspension Electrodes Ring structure MEMS Overview, Prof. A. Mason Page 27 Capacitive Pressure Sensors Consists of two components: Fixed electrode Flexible diaphragm forming a moving electrode Sealed vacuum cavity between the two electrodes Diaphragm (Upper electrode) Lower electrode P++ Si Poly-Si Ti/Pt/Au Tab Contact SiO 2 /Si 3 N 4 /SiO 2 External lead for Glass Substrate glass electrode A. Chavan, K.D. Wise, Transducers 97 MEMS Overview, Prof. A. Mason Page 28 14

15 Integrated Microsystems Architecture Flexible Architectures reconfigurable new/different sensors can be added sensor bus Telemetry Power Management Chip Humidity Sensor Pressure Transducer Array Accelerometer Transducer Array Crystal Wired I/O Port Battery SCI EPROM RAM MCU Timer SPI A/D Temp Sensor Analog Interface Bus Interface Analog Interface Bus Interface Capacitive Sensor Interface Chips Digital Interface Bus Interface Sensor Bus MEMS Overview, Prof. A. Mason Page 29 Microsystem Component: Interface Circuit Generic capacitive sensor interface sensor readout sensor bus communication programmable operation useful for range of sensors Sensor1 Sensor2 Sensor5 Sensor6 Element Select MUX Self-Test DAC C f Charge Integrator b2 b1 b0 Self-Test Command Digital Gain Control Variable Gain Amp Programmable Reference Capacitor Digital Control Sensor Bus MUX Element Select MEMS Overview, Prof. A. Mason Page 30 fs Sample & Hold Bus Interface Address Command Data Register Decoder Latch Temperature Sensor 15

16 Microsystem Component: Shock switch System wake-up switch allows events to be captured while system is in sleep mode useful for system-level power management implements several shock thresholds Suspension Beam Anchor Support Altar Air Gap Inertial Mass Cantilevered Mass Symmetric Contacts Metal Pads & Interconnect MEMS Overview, Prof. A. Mason Page 31 16