Encapsulation of vacuum sensors in a wafer level package using a gold-silicon eutectic

A vacuum package based on gold-silicon eutectic wafer bonding has been developed and evaluated using high sensitivity poly-Si Pirani vacuum sensors. Encapsulation of the devices was achieved by bonding a silicon cap wafer to a device wafer using a Au-Si eutectic solder at or above 390°C in a vacuum bonder. The Au-Si eutectic solder encircled the devices, providing an airtight seal. Strong bonds could be achieved using ≥2.5 μm of gold on the cap wafer, to bond to either a poly-Si film of 0.5 μm or less, or a gold thin film patterned to the same dimensions as the cap wafer gold. Pirani gauges were encapsulated and tested over several months. Devices packaged without getters produced initial pressures from 2 to 12 torr with initial leak/outgassing rates of -0.073 to 80 torr/year. Devices packaged with Nanogetters™ provided by ISSYS produced pressures as low as 5 mtorr with leak/outgassing rates of <10 mtorr/year.