A novel wafer level hermetic packaging for MEMS devices using micro glass cavities fabricated by a hot forming process

Hermetic or vacuum packaging to maintain a controllable cavity pressure and low costs are required by many MEMS devices having moving parts. A novel fabrication technology using micro glass cavities for wafer level hermetic MEMS packaging including accelerometer or gyroscope will be presented. The micro cavities were fabricated by a hot-forming process using Pyrex 7740 since its coefficient of thermal expansion is similar to that of silicon as well as its good sealing properties. Firstly shallow cavities were wet etched fast on the surface of a silicon wafer. Then foaming agents were placed in the silicon cavities. After that the cavities were sealed with a glass wafer by anodic bonding. The bonded wafers were then heated up, and the gas released by foaming agents foamed the glass into spherical bubbles according to the cavity pattern in silicon substrate. Finally, the wafer level spherical glass cavities for packaging MEMS devices were acquired by removing the silicon mold. Then the glass cavities were used to package MEMS. The cap wafer is bonded to the host MEMS wafer by a second anodic bonding process at a relatively low temperature. The MEMS structures were protected by the glass cavities as well as providing a transparent optical window. Mechanical shock test results show that the sealed glass cavities could sustain impact of 30000g acceleration velocity without fracture. LDV test show that the glass cavities could also provide optical window for MEMS devices. The leakage rate of each sealed chamber was tested with He mass spectrometer leak detector, the result show that the leakage rate of the packaged cavity is below 5X 10
^-9
Pa. m / s. The impact test was also did by a bounce table which show that the packaged cavity with the side length of about 3 mm can sustain over 30000g acceleration velocity. The novel package technology has a potential application in optical MEMS and Bio-MEMS.