A uniform, high-yield, reproducible, gold-silicon eutecticbonding technology for wafer-level MEMS vacuum packaginghas been successfully demonstrated. A device wafer containinga polysilicon layer is eutectically bonded to a silicon cap wafercontaining an electroplated gold bond ring. The soft eutecticflows over non-planar surfaces containing insulatedfeedthroughs of polysilicon (1.2μm-thick,). The two wafers arefirst baked in vacuum at 300°C for 60 minutes, brought intointimate contact under a pressure of 1MPa, and subsequentlybonded at ~400°C for 30 minutes in a vacuum of ~0.25mTorr. Abond yield of >95% is achieved across 4” wafers, and excellentreproducibility is achieved from wafer to wafer. The lowpressureinside the vacuum-sealed cavity is monitored using athin (~2.5μm-thick) diaphragm of insulated polysilicon. Afterbonding and diaphragm release, the diaphragm buckles under theone atmosphere of pressure by > 27μm. The buckling in thediaphragms has been maintained for more than 14 weeks.Vacuum sensors are being fabricated to more exactly monitorthe pressure inside the package.
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