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& Q = At − n Gold-Silicon Eutectic Wafer Bonding Technology for Vacuum Packaging Jay Mitchell, Gholamhassan Roientan Lahiji, and Khalil Najafi Microsystems Engineering Research Center for Intergraded Wireless Microsystems University 48109- Electrical Engineering and Computer Science Department, University of Michigan, Ann Arbor, MI 48109-2122 USA Project Description Low-cost, simple, and reproducible hermetic/vacuum packaging technologies are required for many microsystems, including resonant devices and RF MEMS. Of the wafer bonding techniques available, eutectic bonding is one of the most attractive because it is easy to use, it forms a soft eutectic to allow bonding over non-planar surfaces, it can be done at slightly above the eutectic temperature (363°C for Au-Si), and it does not out-gas. Although Au-Si eutectic has long been used for die bonding and packaging, few have reported its successful use in vacuum packaging or even wafer to wafer bonding. There are several reasons for this, including non-uniform eutectic formation, insufficient eutectic material in between wafers causing non-uniform bonding, void formation, oxidation of bond surfaces, and poor surface contact/adhesion. This project aims at developing a uniform, high-yield, reproducible, silicon-gold eutectic wafer-level bonding technology used for vacuum encapsulation of MEMS. Au-Au- Advantages of Au-Au-Si Eutectic Bonding Technology Si Eutectic Localized Heating Au- Why a Au-Si Eutectic? Results Bonder Setup 363° Au- Above 363°C, the Au-Si eutectic liquefies allowing for Long Term Testing (without getters) Long Term Testing (with Getters) 30 0.02 non- the joining of non-uniform surfaces. 3x4 Heater array Heater Non-conformal Pressure [Torr] Pressure [Torr] 0.015 Location of 20 features and/or Presumed Device interconnects 0.01 10 Bond 0.005 Device pads Leads to 0 0 Heat Sink 0 50 100 150 200 250 temperature A 3x4 heater array. 0 75 150 225 300 375 450 525 600 sensors Time [Days] Time [Days] Using resistive backside heating, Heaters arrays sitting on Wafers were aligned in a SUSS bonder and the insulating plate. (a) Silicon is diffuses from both the cap In packages without getters, initial pressures of 1 Packages with getters remains stable even after heat passes through the bond clamps were used to hold their alignment as to 12 Torr and changes of pressure of ~0 to 80 600 days of testing. rings towards the heat sink so they sat atop the copper heat sink. Also, part wafer and the device wafer @ T>363 ºC. Torr/year were measured. that the device stays cool. of the cap wafer was diced away (top left) to allow access to the temperature sensor leads Table 1: A summary of the reliability tests showing how many and wire bonds were made to these leads. devices survived of the ones tested. 50 lb weight Bond Ring Width applied here Test Test Parameters 150µm 300µm 50 lb steel Heater The Au-Si eutectic phase diagram. (b) Upon cooling, a strong diffusional Temperature/humidity weight Device Wafer and chemical bond is obtained. 95ºC 200 h 1/5 4/5 (saline) Cap Wafer Temperature 150 ºC, 100h 1/1 12/12 Glass Substrate Copper Heat Sink Thermal Cycles 50 cycles, -65 to 150ºC - 6/6 Fabrication Poly-Si Feedthroughs Melted solder Low freq. vibration 10 Hz, 2g, 12 hours - 15/15 V Low freq. vibration 30 Hz, 16.5 g, 12 hours - 15/15 Insulator Plate I/O’s for temperature Power to 4) Bond the wafers sensors Heaters 1) Au deposition and patterning Shock 1m (>100g) - 5/12 Design 2 A schematic of the test setup. Shock 3m (>1000g) - 1/12 The total assembly where leads were The total assembly with the 50lb weight on top. Design 1 soldered for connection to the heater. RELIABILITY RESULTS/SUMMARY • Temperature: Elevated temperatures do not appear to affect the pressure. 2) KOH etching of vacuum cavity 5) Dicing off part of the top wafer • Saline Soak: Bond ring width seems to make a difference in life time. • Shock/vibration: Vibration and shock do not effect the pressure unless the cap is a) Device Heat Sink Results 400 Temperature vs. Power Heater Temp 50 um away to allow for access to bond pads. physically broken. Substrate 100 um away 600 um away Temperature [ C] Two different Pirani gauge designs 300 50 um away (model) Bond 110ºC Cap wafer bond ring (Au-Si) 100 um away (model) 3) NanogetterTM patterning were used for pressure 600 um away (model) misalignment Pad Cap substrate measurement in the 200 to 0.5 Torr 600µm 200 100 μm Device wafer bond ring (Au) sensors Device and 5 Torr to 1 mTorr ranges. RESULTS SUMMARY/BENCHMARKING Heater substrate 415ºC 100 Si3N4 passivation • Development of a Au/Si eutectic bonding process. Heater Poly-Si feedthrough • Pressures from 1 to 16 mTorr in a wafer level bonding process with useful reliability test data. Insulator 0 100µm The process flow for device encapsulation. b) 0 30 60 90 120 Table 2: Good results are shown in the literature—but in general, they do not specify the details 50µm Power [W] of their process (*N.I.= no information given). 410ºC Temperature vs. time for the heater, and temperature Leak sensors 50, 100 and 600 µm from the bond ring. (In this Institution Bond Material Bond Temp. Bond Process Pressure Data 100ºC case a heater his used to heat 4 bond rings at a time). A view through the backside of the glass Raytheon Solder (unspecified) N.I.* N.I.* 4 mTorr ~950 Days 135ºC device wafer which was clamped to the Si a) b) c) Si torn cap wafer showing the 100 µm misalignment and temperature sensors at Reflowed Au-Si from cap This Work Au-Si Eutectic 390 Detailed 1 to 16 mTorr 600 Days Au on cap wafer wafer different distances from the bond ring. eutectic Flip Chip Dot Solder (not Device N.I.* N.I.* 7.5 mTorr - Modeling for the Si to glass bond, a) A Com specified) cross section where the arrows indicate heat Cap Cross Section cavity Samsung Glass Frit 450 N.I.* 150 mTorr - flowing to the heat sink. b) The bond ring which reached a temperature of 410ºC while LETI LIR, France Au-Sn Solder N.I.* N.I.* 1 Torr - the center is only at ~100 ºC. With this a) Misaligned Au on the cap and device wafers before bonding and b) geometry, it should take about 750 W to heat after bonding. c) Silicon torn from the cap wafer, adhering to the device 100 bond rings across an entire wafer. Delphi Automotive Solder (unspecified) N.I.* N.I.* 1.5 Torr 42 days wafer via a Au-Si eutectic bond, achieved through localized heating. A wafer with 124 vacuum packaged devices fabricated using a Au-Si Acknowledgments eutectic, a zoomed in view of a package and cross-section view of a package. This research is supported by the engineering research centers program of the National Science Foundation under award number ERC-9986866.
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