Wafer Bonding Technology FOR VACUUM PACKAGING USING GOLDSILICON EUTECTIC
— In this paper successful silicon wafer bonding technology using gold–silicon eutectic is reported. The wafers, device wafer and cap wafer are bonded in vacuum. Device wafer is terminated on the surface with a bond layer that can be single crystal silicon, a layer of polysilicon, gold or any other suitable metal, dielectric or semiconductor layer, and the cap wafer contains an electroplated gold bond ring. Bonding has been carried out in a standard wafer bonder by performing a pre–bake at 300°C and then pressing the wafers together by 1Mpa and subsequent bonding at a temperature of about 400°C for 30 minutes. Bonding yield of more than 95% is achieved on 4” silicon wafers with excellent reproducibility. Excellent coverage of soft eutectic bonding over non–planar surfaces has been investigated using 1.2mm–thick insulated feedthroughs of polysilicon, which shows a good flow of eutectic material over the feedthroughs. Thin film polysilicon diaphragm (~2.5mm–thick) and micro–Pirani gauge have been used to monitor and measure the low–pressure inside the packaged vacuum cavity. It is more than one year that we are monitoring the pressure by the amount of buckling in the diaphragm, and so far no pressure drop is observed. Direct pressure measurement is also underway by the vacuum packaged floating Pirani gauges fabricated inside the device wafer. It also has been shown that polysilicon is a better source material in gold– silicon eutectic formation as it bonds at relatively lower temperature than single crystal silicon.
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