Control and Quantification of Residual Stresses in Anodically Bonded MEMS Structures R. Inzinga, T. Lin, M. Yadav, H.T. Johnson, G.P. Horn Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign 1206 W. Green St. Urbana, IL 61801 ABSTRACT Residual stresses in anodically bonded silicon devices can result in quality control and process control deficits if the stresses are not controlled. At the same time several geometries may benefit from a controlled introduction of residual stresses. For example, long, thin structures may utilize a residual tensile stress to minimize the likelihood of buckling, while etched cavities with sharp corners can benefit from a residual compressive stress to suppress crack initiation and growth. In the present work, we quantify the residual stress fields present in silicon wafers that are anodically bonded to virgin Pyrex wafers. Anodic bonding is conducted using standard procedures as well as a proposed alternative method that utilizes differential thermal bonding to control the residual stress state. The experimental stress state is compared to theoretical finite element calculations to determine the capability of controlling stresses based on a simple thermal model. I. INTRODUCTION Anodic bonding is used in the MEMS industry for the production of microfluidic devices [1], pressure sensors [2], and accelerometers [3,4] among other applications. It is a very useful tool for many applications that require the formation of a reliable hermetic seal between silicon and glass wafers (or bare silicon and glass coated silicon wafers for silicon-on-insulator applications) at relatively low temperatures. However, one noteworthy issue with anodic bonding is the unwanted curvature of the bonded structure at room temperatures that results from residual stresses in the bonded wafer pair. As anodic bonding requires the joining of two dissimilar materials at elevated temperatures, the very slight mismatch in coefficient of thermal expansion between the glass and silicon wafers results in residual stresses in every process currently used in industry and academia. Several types of borosilicate glass wafers are manufactured, including the Corning Pyrex 7740, to have a coefficient of thermal expansion that relatively closely matches that of silicon up to high temperatures. However, at temperatures above about 320 °C, the coefficient of thermal expansion of silicon becomes larger than Pyrex 7740, which typically results in wafer bow with silicon in tension (“convex” in our geometry) [5-7]. Bond strength depends strongly on bond temperature, so in most applications temperatures range from about 350 to 450 °C. At 400 °C, the difference in CTE between the two wafers is only about 7%, but creates a significant wafer bow on the order of tens to hundreds of microns deflection over a 100 mm wafer [6,7]. For equally thick bonded silicon and Pyrex wafers, the induced strain in the silicon changes from being in a compressive state to a tensile state above 315°C [8]. The characterization and manipulation of residual stresses in anodically bonded structures can be useful in the production of MEMS devices. Compressive stresses in a slender beam application or pressure sensor membrane [9] can result in buckling of the structure. In these cases, the typical bonding profile with silicon in tension is often useful to ensure that buckling does not occur. On the other hand, the presence of any residual stresses in resonant sensors can alter the resonant frequency of a device [10]. In many applications, the overall wafer bow must be minimized for post processing reasons, particularly if lithography is required. Finally, in applications where etched features are present at the bond interface, cracks have been found to grow from areas of high stress concentration. Processing to introduce a compressive stress in the silicon wafers with these structures can Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 269 T. Proulx (ed.), MEMS and Nanotechnology, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 2, DOI 10.1007/978-1-4419-8825-6_39, © The Society for Experimental Mechanics, Inc. 2011
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