Wafer Level Micropackaging of MEMS Devices Using Thin Film Anodic Bonding
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Wafer Level Micropackaging of MEMS Devices Using Thin Film Anodic Bonding Lauren E. S. Rohwer, Andrew D. Oliver, and Melissa V. Collins Sandia National Laboratories Albuquerque, NM 87185, U.S.A. ABSTRACT A wafer level packaging technique that involves anodic bonding of Pyrex wafers to released surface micromachined wafers is demonstrated. Besides providing a hermetic seal, this technique allows full wafer release, provides protection during die separation, and offers the possibility of integration with optoelectronic devices. Anodic bonding was performed under applied voltages up to 1000 V, and temperatures ranging from 280 to 400oC under vacuum (10-4 Torr). The quality of the bonded interfaces was evaluated using shear strength testing and leak testing. The shear strength of Pyrex-to-polysilicon and aluminum bonds was ~10-15 MPa. The functionality of surface micromachined polysilicon devices was tested before and after anodic bonding. 100% of thermal actuators, 94% of torsional ratcheting actuators, and 70% of microengines functioned after bonding. The 70% yield was calculated from a test sample of 25 devices. INTRODUCTION Wafer level packaging can enhance the yield, reliability, and functionality of MicroElectroMechanical systems (MEMS). Wafer bonding enables the fabrication of microsystems that combine dissimilar materials and multiple functions, e.g., electrical and optical, in a more compact, fully integrated package. Wafer level packaging is more robust than traditional die level packaging. Some of the common causes of MEMS device failure such as particle contamination, excessive handling, and moisture-induced stiction [1] can be mitigated by wafer level encapsulation. The packaging technique described herein can be applied to numerous MEMS devices. Anodic or field-assisted bonding is known for producing hermetic, strong, irreversible glass-to-silicon bonds [2]. Other common wafer bonding technologies [3,4] require mirror polished surfaces (silicon direct bonding), high temperature (silicon direct, eutectic), flatness
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