Hermeticity evaluation of MEMS wafer packages by Raman spectroscopy

Abstract

Hermeticity of MEMS wafer packaging has a major impact on the performance and reliability of MEMS devices. The test for hermeticity is usually based on the test method from MIL-STD-883. However, both theory and experiment have shown that this test standard has great limitations for MEMS wafer cavities. Raman spectroscopy has also been used to obtain the quantitative and qualitative information about the molecular composition in the small cavity of MEMS packages and then determine the leak rate of MEMS packages. However, this method is reliant on the package cap being transparent to the probing light and requires a reflective surface. In this paper, the Raman spectroscopy is used to obtain the stress of the surface of the package cap and from the information of stress change to infer the change of the gas pressure in the vacuum encapsulation cavity. The results have shown that the lower the gas pressure in the cavity, the larger the tensile stress; When packaged in the atmosphere, the sample cap has shown a compressive stress (corresponding to the reference), which may result from fabrication process. Based on this method, there is no need to use a transparent package cap, and there is no special requirement for the roughness of the cap surface. Compared with the optical deformation test, this method can directly obtain the surface stress information of the cover without knowing the relevant geometric parameters of the MEMS structure.