Characterization of Silicon Fusion Bonds Using a Four-Point Bend Specimen
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CHARACTERIZATION OF SILICON FUSION BONDS USING A FOUR-POINT BEND SPECIMEN Kevin T. Turner1, Arturo A. Ayon2, Dongwon Choi3, Bruno Miller4 and S. Mark Spearing4 Massachusetts Institute of Technology, Cambridge, MA. ABSTRACT The increased number of MEMS devices that are fabricated by bonding two or more bulk micromachined silicon wafers has highlighted the need to produce reliable silicon fusion bonds. The current study focuses on employing a four-point bend delamination specimen to measure silicon fusion bond strength as a function of processing conditions. The specimen, which is composed of two bonded layers and an initial notch, permits the measurement of a mixed-mode critical strain energy rate, GC, at the interface. This specimen geometry is advantageous because it does not require measurement of crack length to calculate the strain energy release rate and is insensitive to damage near the specimen edges. The fact that the interface is loaded under mixed-mode conditions presents difficulties in achieving stable crack propagation in well bonded specimens. Attempts were made to eliminate this problem by reducing the effective bond toughness by etching shallow grooves in the wafer surfaces to reduce the bonded area. Testing revealed that while this approach reduced the effective toughness of the interface, it did not prevent crack deflection in well bonded samples. Despite the limitations of the specimen, data was obtained for silicon fusion bonds fabricated under various annealing and contacting conditions. Test results indicate an increase in bond toughness with annealing temperature and time. The data also suggests that the contacting pressure and duration have little effect on bond quality. The specimen, while limited to characterizing bonds with lower toughness, proved straightforward to fabricate and test. INTRODUCTION The development of MEMS devices consisting of bonded bulk micromachined silicon wafers has been steadily increasing in recent years. The success of these multi-wafer devices relies on advances in etching technology and reliable bonding between wafers [1, 2]. Silicon remains the choice material for bulk micromachining operations, thus a significant amount of work has been devoted to developing robust bonds between etched silicon wafers. Numerous techniques have been used to bond silicon wafers reliably, including field assisted silicon-toglass bonding (anodic bonding), gold-silicon eutectic bonding, and silicon-silicon direct bonding (fusion bonding). Silicon-silicon direct bonding is advantageous because it eliminates thermal mismatch problems and has the potential to create bonds with strengths comparable to that of the bulk silicon layers. There has been a considerable amount of work with regard to characterizing bond quality as a function of process conditions. Gösele and Tong [3] offer a comprehensive review of this work, which demonstrates the profound effect processing route can have on bond integrity and highlights the need for accurate measurement of bond quality. The current study
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