The Mechanical Response of Thin Film Substrates Subject to Ultrasonic Joining
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deformation, resulting in dislocation cell structure formation. The Cu substrates showed evidence of a cell structure influence by thermal activation, while the Ni did not. In the present study, we extend the previous work to thin film substrates, which are more commonly found in wire bonding practice. We first examine changes to surface morphology as a result of bonding by etching off the bonded Al wire and using the atomic force microscope (AFM) to examine the surface. The advantage of using a substrate in thin film form is that most sputterdeposited thin films have a readily recognizable columnar surface structure, and deviations from this can be easily identified. The disadvantage of this is that the method is not possible with Al thin film substrates, which would also be etched away. Therefore, for Al films we used scanning and transmission electron microscopy (SEMITEM) to investigate both interface morphology and microstructural changes to the deposited films. EXPERIMENTAL PROCEDURE
Aluminum ultrasonic wire wedge bonding was carried out using 38 and 50 pLrm diameter aluminum wire on a K&S model 4123 wedge bonder. The force, power, and bonding times were set for each experiment, and the details will be provided in the following section. Bonding was carried out at both room temperature and 175TC. Gold ball bonding was done using 33 Pm diameter gold wire on a K&S XQ1488 Turbo bonder at a temperature of 200TC. Substrates for the Al wirebond experiments were fabricated by depositing the desired metal on Si wafers by magnetron sputter deposition. Thin films of pure Cu, a Cu-40%Ni alloy, Ag, and pure Al were deposited. In one experiment, an Al/Ag/Al triple layer structure was fabricated by depositing 800 nm Al, 20 nm Ag, followed by 200 nm of Al. This experiment was to use the Ag as a "marker" layer to follow surface morphology changes, since in Al/Al cross-sections the interface was often difficult to identify. Al wires that were bonded to thin films of Cu, Cu/Ni and Ag were removed to allow AFM examination of the films by immersing the samples in a solution of aqueous sodium hydroxide, followed by rinsing in distilled water and ethanol. This leaves a "footprint" which is used to identify the bond location4 . The film surface was examined using a DI-3000 AFM operating in the tapping mode. Images were collected from areas near the center of the footprint, where bonding is expected have taken place. Approximately 10 AFM images were collected from each sample. The RMS surface roughness was also computed using these images. For Al-wire to Al-thin film specimens, SEM crosssections were made by using a tripod polisher to obtain a section through the center of the bond. Samples were polished through 0.05 pm alumina, but to reveal the true structure it was necessary to ion mill the surface for about 20 minutes. For TEM, additional thinning was conducted by dimpling the sample and then thinning to electron transparency by ion milling. Electron microscopy observations were carried out using a Hitachi H-600 TEM/SEM. RESULTS
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