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Bonding with Al Metallurgies for 200mm MEMS Devices
Keywords: MEMS Wafer Level Bonding, Aluminum Bonds, Eutectic Seals
Aluminum metallization is used frequently in MEMS device structures and offers the potential to serve as a sealing metallurgy during device packaging steps. The vast majority of MEMS devices are sealed using anodic or glass frit seal rings to create hermetic packages for the accelerometer, pressure sensor or gyroscope. However, metal seals would enable smaller device geometries by providing for high hermeticity levels which enable seal ring shrinkage and device geometry scaling to smaller sizes. There are two broad categories of metal seals utilizing aluminum based metallurgies; diffusion type bonds and eutectic bonds. The main advantage of the eutectic alloys is that the sealing ring will melt and under controlled flow mechanics it will accommodate surface roughness and disparities. In this way the eutectic metal bonds have similar advantages to glass frit processing. Diffusion bonding, on the other hand, requires intimate contact between the two surfaces and places limits on the surface topography that is acceptable. In all types of metal bonding the cleanliness of metal layer is essential for high quality interface formation. Several cleaning methods can be used to remove surface oxides prior to bonding. A popular method used for both bumping technologies and for copper or aluminum bonding is formic acid vapors. The vapor phase limits damage to underlying layers, is very rapid and enhances the bond quality. Because re-oxidation is equally important during the thermal treatment steps in the bond chamber, forming gases utilizing up to 4% hydrogen can be used to suppress and prevent oxide formation. Specifically in the case of automated production on 200mm wafers several advancements are needed to process aluminum metallurgies successfully. For the eutectic category of bonds a very uniform applied pressure and temperature must be applied to the wafers to ensure even alloy formation within the interface and to avoid distortion of the seal rings. For the diffusion based processes it is understood that the aluminum oxide is not soluble in the metal and hence the use of cleaning as well as high applied forces (up to 90KN) are needed to get successful 200mm wafer level aluminum diffusion bonds. Acoustic micrographs 200mm wafers successfully bonded with Al blanket layers show perfectly formed diffusion bonds with no voiding. This paper will focus on the metallurgies used in aluminum processing of MEMS wafers and 200mm production methods for this new bond process technology.
Shari Farrens, Chief Scientist - Wafer Bonder Division
SUSS MicroTec
Waterbury Center, VT

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