Vacuum measurement in wafer level encapsulations by interference microscopy

Vacuum encapsulation is required for various microsensors to improve their performances. In the case of wafer level vacuum encapsulation, dedicated techniques must be developed to measure the background pressure in the bonded cavities. In this paper we investigated ex situ measurement techniques based on the measurement by interference microscopy of the deformation and vibrations of the caps as function of an external pressure. Because these techniques detect the equality between external and internal pressure, they do not require any modelling nor prior calibration. Application to wafer level encapsulations fabricated by gold-silicon eutectic wafer bonding demonstrates that a detection limit in the 10(-2)-10(-1) mbar range can be reached for millimetre size caps.