MEMS packaging reliability assessment: Last development in the field of residual gas analysis tests

The reliability of MEMS sensors or actuators such as accelerometers, gyroscopes, resonators, RF switches, microbolometers... is critically dependent on the vacuum level and on the nature of the gaseous species present inside the MEMS cavity (~1mm³ or less, usually in the 10^-2-10^-3 mbar vacuum range after sealing by Wafer Level Packaging process. Although a lot of methods have been explored [1], the Residual Gas Analysis (RGA) method-however destructive - reveals to be an interesting method to determine the possible sources of gaseous species present in the MEMS cavity, in order to assess to which extent a given packaging technology is capable of maintaining the required environmental conditions all along the device lifetime. This paper introduces results recently achieved with a specific RGA test bench designed, developed and validated at the CEA-LETI to meet these requirements. This Ultra High Vacuum (UHV) RGA equipment operates under a residual background pressure level in the 10^-10 mbar range and uses a quadrupolar mass spectrometer (QMS) for gas analysis. Experimental results confirm that the RGA technique is successfully used to identify the nature and proportions of gases trapped in cavities as small as 1 mm³ under a residual pressure lower than 10^-2 mbar. Under favorable conditions and using UHV-compatible specific mechanical tools, it is possible to identify the sources of the various gases present inside the package by sequentially breaking under UHV spare parts of the device such as the bonding layer, the cap and the MEMS substrate. It also proves to be an interesting tool to study the degassing behavior of materials and their compatibility with the MEMS fabrication process steps.