Advanced Anodic Bonding Processes For MEMS Applications
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Advanced Anodic Bonding Processes For MEMS Applications V. Dragoi, P. Lindner, T. Glinsner, M. Wimplinger* and S. Farrens* EV Group, 1 DI Erich Thallner, 4780 - Schaerding, Austria * EV Group Inc, 3701 E. University Drive, Phoenix, AZ 85034, USA ABSTRACT Anodic bonding is a powerful technique used in MEMS manufacturing. This process is applied mainly for building three-dimensional structures for microfluidic applications or for wafer level packaging. Process conditions will be evaluated in present paper. An experimental solution for bonding three wafers in one single process step (“triple-stack bonding”) will be introduced.
INTRODUCTION Due to the specific characteristics of MEMS devices (3D architectures, frequently containing mobile mechanical parts, controlled atmosphere encapsulation, etc.) device manufacturing and packaging imposes very specific constraints. New processes were developed for accommodating the new requirements in terms of packaging. Being a wafer level process used for wafers stacking, wafer bonding has increasingly become a key technology in various fields related to MEMS devices fabrication and packaging. Different wafer bonding approaches are currently used in the MEMS industry: fusion, adhesive, eutectic, thermo-compression bonding. These are processes mainly used in device fabrication and generation of 3D structures, while anodic bonding is one of the most commonly used wafer level packaging procedures. Among the different wafer bonding techniques currently used in MEMS devices manufacturing, anodic bonding is the most mature process. Wallis et al. [1] reported in 1969 their results in metal/glass sealing by using the combined action of an electric field applied under heating. Anodic bonding is also known as “field assisted bonding” or “electrostatic bonding”. The term “anodic bonding” is used today mainly to identify the bonding of silicon wafers to glass wafers with high content of alkali oxides (the glass materials most used for anodic bonding are Borofloat® from Schott Glass - Germany, and Pyrex® 7740 from Corning Inc., USA). The bond occurs when the two wafers are heated after being brought in contact and an electric field is applied. At a certain temperature (depending on the glass composition) oxides dissociate and the mobile alkali ions are driven by the electric field into the glass, creating an oxygen rich layer at the silicon-glass interface. Oxygen ions are driven by the electric field to the silicon surface and produce oxidation of Si. The resulting bond strength is very high and the process is irreversible. Currently the most used materials combination for anodic bonding is Si-glass [2-4]. Some special applications require the use of different materials in thin films form (evaporated glass layers [5], polysilicon or metal thin films [6] and spin-on glasses [7]) or special process conditions (vacuum [8] or very low temperatures [9]). Some MEMS devices like accelerometers, gyros and microfluidic devices are using two anodic bonding processes in order to obtain a 3 wafer
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