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Vacuum Packaging of MEMS Devices and the Effects of Outgassing and Getter
Keywords: MEMS Devices , Packaging , Outgassing and Getter
Packaging and encapsulation are the major roadblocks to the commercialization of emerging microelectromechanical systems (MEMS). Devices like MEMS gyroscopes, infrared arrays, resonators, and micro-switches for wireless circuits require vacuum/hermeticity at pressures from below 10 mTorr to as high as atmospheric pressure with virtually no oxygen or water vapor exposure. Furthermore, these devices need to be packaged at the wafer-level in order to handle the high volume/low unit price demands of growing commercial applications. We have developed a number of wafer-level solder/eutectic bonding processes for vacuum encapsulating MEMS devices at temperatures from 200XC up to 390XC. These various solder and eutectic processes can be applied to a wide range of device applications depending on the required package pressure and on the maximum temperature compatible with the device materials. In these processes, solder/eutectic bond rings which encircle devices across the wafer are heated above their melting temperatures. These molten alloys conform over insulated feed-throughs, which allows for a vacuum tight seal while providing electrical interconnection from outside of the packages. The packages had volumes of around 510-4 cm3 and the vacuum pressures were monitored using integrated vacuum sensors. In order to achieve vacuum levels below 10 mTorr, titanium getters and commercial NanogettersTM were employed. The final pressures inside the micro-packages were engineered by changing the temperature profile before, during and after bonding. These varied conditions caused increased or decreased outgassing (desorption of atoms from the inside walls which increased the pressure inside of the micro-cavities) and increase or decreased gettering (absorbtion of atoms into the getter which reduces the pressure inside of the micro-cavities). Furthermore, vacuum seals where shown to hold at temperatures ranging from -65 to 150aC, at room temperature for more than 3 years and with yields higher than 90%.
Jay Mitchell, Research Fellow
University of Michigan
Ann Arbor, MI

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