Hermetic vacuum sealing of MEMS devices containing organic components

Traditionally, the use of organics within a vacuum-sealed hermetic electronics package has been avoided. Organics, including adhesives, may outgas and degrade over time, resulting in a rapid reduction in vacuum quality within a sealed device package. However, MEMS device fabrication is now blurring the lines between strictly electronic devices, which contain very few organic components, and electro-mechanical devices, whose secondary assembly steps require the integration of stable organics. The Chip Scale Atomic Clock (CSAC) device developed and implemented with funding from DARPA by a team from the Symmetricom Technology Realization Center, Sandia National Laboratory, and Charles Stark Draper Laboratory, is a prime example of a device that integrates organics and chip scale die into an assembly that requires operation within a good vacuum environment over the lifetime of the device. Through the use of analytical chemistry techniques such as TGA, DSC, and IVA, we measured outgassing of assembly materials to be sealed in the package. We have been able to determine the magnitude of initial outgassing and to measure the stable vacuum pressure of complete sealed devices to within a few milliTorr. Comparison of these results with predictions based on IVA data of specific gas species and getter capacity has allowed us to optimize processing conditions (such as cure schedule for adhesives and vacuum bake-out profile before sealing) for minimal outgassing. This information has allowed us to design and build MEMS devices which require an internal vacuum level of < 20 milliTorr and seal them in hermetic packages without substantial degradation.