Characterization of Low Temperature, Wafer-Level Gold-Gold Thermocompression Bonds
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Abstract Low temperature, wafer-level bonding offers several advantages in MEMS packaging, such as device protection during aggressive processing/handling and the possibility of vacuum sealing. Although thermocompression bonding can be achieved with a variety of metals, gold is often preferred because of its acceptance in die bonding [1] and its resistance to oxidation. This study demonstrates that the simultaneous application of moderate pressure (0.5 MPa) and temperature (300'C) produces strong wafer-level bonds. A four-point benddelamination technique was utilized to quantify bond toughness. Test specimens exhibited constant load versus displacement behavior during steady state crack propagation. Two distinct fracture modes were observed: cohesive failure within the Au and adhesive failure at the Ti-Si interface. The strain energy release rate for Au-Au fracture was found to be higher than that associated with Ti-Si fracture, consistent with the greater plastic deformation that occurs in the metal during fracture.
Introduction Packaging poses unique challenges in MEMS device design. The package has to meet the electrical, thermal and mechanical requirements of the devices and provide protection from the often harsh environments in which they operate. Moreover, it has to be manufactured and assembled cost effectively. A packaging technology under investigation is low temperature, wafer-level thermocompression bonding. This approach is attractive since the low temperature minimizes diffusion in previously fabricated components and allows the possibility of combining integrated circuitry and MEMS components. More importantly, wafer-level sealing can be used to protect the device during potentially damaging final processing steps such as die-sawing. Vacuum sealing of devices may also be implemented. Although thermocompression bonding can be performed with a variety of metals, gold is often chosen because of its use in die bonding [1] and its oxidation resistance. There has been little reported work on gold thermocompression bonding at the wafer level, and most of that which has been published has focused on temperatures above the Au-Si eutectic *Department of Materials Science and Engineering, Microsystems Technology Laboratories tDepartment of Electrical Engineering and Computer Science, Microsystems Technology Laboratories IDepartment of Aeronautics and Astronautics
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Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
of 363°C [1, 2, 3]. The principal goal of the present study is to demonstrate that the simultaneous application of pressure and temperatures below the Au-Si eutectic can achieve reliable bonds. There are several methods available with which to quantify bond strength. These include: pressure burst tests [4], tensile/shear tests [5] and knife-edge tests [6]. However, these methods are sensitive to the loading configurations and/or accuracy in the crack length measurements [7]. An alternative method is employed in this study, namely the four-point bending-delamination technique
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