Effect of Process Variables on Glass Frit Wafer Bonding in MEMS Wafer Level Packaging
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Effect of Process Variables on Glass Frit Wafer Bonding in MEMS Wafer Level Packaging S. Sridharan1, J. Henry2, J. Maloney1, B. Gardner2, and K. Mason2 V. Dragoi3, J. Burggraf3, E. Pabo4, and E. Cakmak4 1,2
Ferro Corporation, Electronic Materials System, 7500 E. Pleasant Valley Road, Independence, OH 44136, USA. 2 1395 Aspen Way, Vista, CA 92083, USA. 3 EV Group, DI E. Thallner 1, 4782-St. Florian, Austria. 4 EV Group Inc, 7700 S. River Pkwy, Tempe, AZ 85284, USA. 1
ABSTRACT Among different MEMS wafer level bonding processes, glass frit bonding provides reliable vacuum tight seals in volume production. The quality of the seal is a function of both seal glass material and the processing parameters used in glass frit bonding. Therefore, in this study Taguchi L18 screening Design of Experiment (DOE) was used to study the effect of materials and process variables on the quality of the glass seal in 6” silicon wafers bonded in an EVG520IS bonder. Six bonding process variables at three levels and two types of sealing glass pastes were considered. The seals were characterized by Scanning Acoustic Microscopy (SAM), cross sectional Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDAX). The results were quantified into four responses for DOE analysis. Key results are a) peak temperature has the strongest influence on seal properties, b) hot melt paste has significantly lower defects compared to liquid paste, and c) peak firing temperatures can be as low as 400°C under certain conditions. INTRODUCTION Due to rapid advances in consumer and automotive electronics, and mobile communications the Micro Electro-Mechanical Systems (MEMS) market is growing rapidly [1]. Applications for MEMS cover diverse fields, such as, automotive, aeronautics, consumer, defense, industrial, medical and life sciences, and telecommunications. Various MEMS devices require a vacuum level or controlled atmosphere operation in order to ensure either good performance or acceptable operating life. Different wafer bonding technologies are commercially used for MEMS packaging [2-7]. Among these, glass frit bonding [7] offers many advantages such as superior hermeticity, less stringent requirement on roughness of the surfaces being joined and tailored expansion matching to the materials being joined. In glass frit wafer bonding, sealing glass paste is screen printed into the desired geometry. This layer is dried in an Infrared (IR) oven, and then fired in a furnace to burn the binder and pre-fuse the sealing glass layer. This process is called “glazing”. Glazing is usually done in either a belt furnace or box furnace under atmospheric conditions. Then the pre-glazed wafer is placed in a wafer bonder and bonded to a cap wafer in a temperature-time-pressure cycle under desired vacuum in a process called “frit bonding” to produce a hermetically sealed assembly. The quality of the resultant seal (or bond) is a function of both seal glass material used and the process variables employed in both glazing and frit bonding. Unde
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