Low temperature wafer level vaccum packaging using gold-silicon eutectic bonding and localized heating.

An Au-Si eutectic wafer-level bonding process and a localized heating process, called differential backside heating, were developed for low-temperature vacuum packaging of MEMS devices. Using Au-Si eutectic bonding, devices were encapsulated by bonding a silicon cap wafer to a device wafer. Au-Si eutectic bond rings melt at over 363°C allowing them to conform over topology such as electrical feed-throughs allowing for a vacuum seal. Detailed specifications are given for achieving uniform/strong bonds to poly-Si and Au bond rings in a bond recipe which includes vacuum pumping, an outgassing step, application of the bond pressure (∼2.5 MPa), and heating to 390°C. Micromachined poly-Si Pirani vacuum sensors were developed, characterized and then packaged in the Au-Si eutectic bonding process in order to measuring vacuum pressures. These packages had cavity dimensions of 2.3x2.3 mm wide with a depth of 90 mum. Yields of 84.6% and 94.1% were achieved in packages with bond ring widths of 100 and 150 mum. With the use of getters and a pre-bond outgassing step, pressures from