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Improvement of MEMs Packaging technologies through Residual Gas Analysis solutions
Keywords: mems, rga, residual gas analysis
Optimized MEMS packaging technologies are today a key element to ensure cost competitive MEMS systems. Presently, MEMS are dominated by packaging costs, (it is commonly cited that packaging accounts for 30 up to 70% of the cost of a MEMS device), and many efforts are spent to get close to comparable IC operation. Wafer-scale encapsulation are more and more used to create a device scale hermetic enclosure around the active MEMS elements before the MEMS wafer leaves the MEMS foundry. Within this frame, it is of paramount importance to have available advanced characterization tools allowing to assess long term reliability of MEMS sensors or actuators such as accelerometers, gyroscopes, resonators, RF switches, microbolometers... The present work describes the development of special Residual Gas Analysis (RGA) benches to measure the vacuum level and the nature of the gaseous species present inside MEMS cavities after sealing, taking into account the specific constraints linked to: -the extremely low cavity volumes to be investigated (~1mm3 or less, usually in the 10-2-10-3 mbar vacuum range after sealing by Wafer Level Packaging process), -the accurate in-situ opening of packaged MEMS devices in the ultra high vacuum chamber, -the different natures of gaseous species sources. This paper introduces results recently achieved with a specific RGA test bench designed, developed and validated at the CEA-LETI to meet all these requirements. This Ultra High Vacuum RGA equipment operates with a residual background pressure level in the 10-10 mbar range and uses a quadrupolar mass spectrometer for gas analysis. The experimental results which are presented clearly show the strong interest of the RGA technique to identify the nature and proportions of gaseous species trapped in cavities or outgassed from cap, sealing or substrate materials. Under favorable conditions and using under vacuum special mechanical tools, it is demonstrated that identification of their sources can be achieved by sequentially breaking under UHV spare parts of the device such as the bonding layer, the cap and the MEMS substrate. This tool today allows assessing to which extent a given packaging technology is capable of maintaining the required environmental conditions all along the device lifetime. Extension to new thin film Wafer Level Packaging technologies will be also discussed.
Andre Rouzaud,
Grenoble, Cedex 9

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