Characterization of Silicon Fusion Bonds Using a Four-Point Bend Specimen

Abstract

ABSTRACTThe 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.