Gold-Silicon Eutectic Wafer Bonding Technology for Vacuum Packaging

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                                   Gold-Silicon Eutectic Wafer Bonding Technology for Vacuum Packaging
                                                                                                                        Jay Mitchell, Gholamhassan Roientan Lahiji, and Khalil Najafi
                                                                                                                            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.

                        Advantages of Au-Au-Si Eutectic Bonding Technology
                                         Si Eutectic                                                                                                                                                                                                                                                                                                                        Localized Heating
                                     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
             the joining of non-uniform surfaces.                                                                                                                                                                                                                                                                                                                                                                                                        3x4
                                                                                                                                                                                                                                                                                                                            Heater                                                                                                                      array


                                                                                                                           Pressure [Torr]

                                                                                                                                                                                                                    Pressure [Torr]
                                                                     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
                                                                                                                                                                       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
                                                                                                                                                                                                                                                                                                                                                     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
                                                                                                                                                                 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
                                                                                                                                                                                                                                                                                                                         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)

                                            Pad                                                                                                                                                                                                                                                         Cap substrate
                                                                                  measurement in the 200 to 0.5 Torr                                                                                                                                                                                                                                                                                                   600µm                          200

                                                                                                                                                                                                                                                                                                                                                         100 μm
                                                                                                                                                                                                                                                                                                                                                                      Device wafer bond ring (Au) sensors

                                                                                  and 5 Torr to 1 mTorr ranges.                  RESULTS SUMMARY/BENCHMARKING                                                                                                                                           Heater substrate           415ºC
                                                                                     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
                  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
                eutectic, a zoomed in view of a package and cross-section view of a
                                                                                                                         This research is supported by the engineering research centers program of the National
                                                                                                                         Science Foundation under award number ERC-9986866.

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