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Wafer Bonding for MEMS Vacuum Packaging
Among the MEMS devices which are built using one or more wafer bonding steps the devices operating under vacuum raised new challenges for bonding-based fabrication: the need for achieving vacuum levels down to 10-4 mbar, long term stable, had to be accommodated.
From the broad range of wafer bonding processes only few are compatible with vacuum applications: fusion bonding, anodic bonding, glass frit bonding and metal-based bonding. The outgassing at the bonded interface or of the bonding layer material is extremely important for vacuum applications as a high level of outgassing has a significant impact on final vacuum level in manufactured device. Additionally it has to be considered that the inner surfaces of a device cavity are expected to outgas so this is another factor imposing the low outgassing amount requirement for bonding process and bonding surfaces.
The vacuum levels required would further restrict the process type for specific applications. Applications requiring high vacuum levels are very sensitive to any outgassing during bonding so in some cases a small amount of getter material may be used inside the device cavity in order to compensate for the inner surfaces’ outgassing.
This work reviews the principles of vacuum encapsulation using wafer bonding. Examples showing the suitability of each process for specific applications types will be presented.
A significant challenge in vacuum MEMS fabrication is the lack of analytical methods needed for process characterization or reliability testing. A short overview of the most used methods and their limitations will be presented. Specific needs to be addressed will be introduced with examples.