Low Temperature Wafer Level Vacuum Packaging Using Au-Si Eutectic Bonding and Localized Heating.

Abstract

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 ºC 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 2.3×2.3 mm wide with a depth of 90 µm. Yields of 84.6% and 94.1% were achieved in packages with bond ring widths of 100 and 150 µm. With the use of getters and a pre-bond outgassing step, pressures from <3.7 to 23.3 mTorr were achieved. Furthermore, pressures were shown to remain stable to within ±5 mTorr for over 3 years of testing, after 100 hours at 150ºC, and after 50 thermal cycles from -50ºC to 150ºC. Using differential localized heating, one of the two wafers to be bonded is heated from the backside, and the other is cooled from the backside, so that heat flows through the bond regions while the device regions stay relatively cool. A bonder test setup was built where integrated temperature sensors on the device wafer were used to measure the temperature at different distances from the bond region during Si to glass and Si to Si bond experiments. This technique was proven successful with temperature of 23% and 41% of the bond ring temperature at 250 and 650 µm from the bond rings for bonds to glass and Si respectively. These temperature rises were within 3% and 9% of those predicted by 3-D FEM thermal modeling. In the Si to glass bond, bond rings were heated to 400ºC allowing for a Au-Si eutectic bond.