Vacuum wafer-level packaging for MEMS applications

For several kinds of, MEMS (gyrometers, accelerometers, RF MEMS, bolometers...), vacuum allows a significant improvement of performances. Led has developed a high performance sensor operating at a pressure lower than 10(-3) mbar. In a first phase, a ceramic vacuum packaging has been-developed: the device is encapsulated in a cavity containing a getter. However, this technique increases considerably the fabrication costs, because it is made at the chip level. For that reason, Led has also developed wafer-level vacuum packaging process. The process to manufacture encapsulated devices is presented in this paper. The vacuum function is obtained thanks to an additional wafer (glass or silicon wafer), which supports getters. This wafer is bonded by an hermetic bonding. Characterisation of different kinds of bonding, in term of hermeticity, is presented. First chips manufactured with this process have been tested. The vacuum level in the cavities has been measured, and was lower than 10-3 mbar. Moreover, vacuum evolution during 6 months does not show pressure increase. This process can be easily adapted to several MEMS applications. With these experiments, Leti has so proved the possibility of manufacturing low cost vacuum packaged MEMS.