Degradation of Thermocompression Bonds to Ti-Cu-Au and Ti-Cu Thin Films by Thermal Aging

Ti-Cu-Au and Ti-Cu thin films are among several conductor metallizations which have been investigated as possible lower cost replacements for type B (0.005 / ) Ti-Pd-Au in film integrated circuits. The results of a study of the degradation of thermocompression bonds to Ti-Cu-Au and Ti--Cu and the failure mechanism produced by thermal aging are presented. Ti-Cu-Au was aged before and after bonding with Ni--Au plated Cu leads. For films aged prior to bonding, the effects of Au film thickness and annealing ambient were studied. Ti-Cu films were aged after bonding with both Ni-Au plated Cu and bare Cu leads. The failed bond areas were analyzed by scanning electron microscope (SEM), Auger analysis, and X-ray diffraction. Thermocompression bonds to Ti-Cu--Au thin films with 2 m of Au, thermally aged in air before or after bonding, exhibited a large percentage of film failures within 1 h at 350°C or 4 h at 300°C. Samples annealed in air prior to bonding exhibited more bond degradation than samples annealed in H2. The film pulled out with the leads during a 90° peel test was identified as C3Au, and the platform side of the film failure appeared to consist of craters with Cu at the bottom and Au-Cu phases on the ridges. The results suggest that this failure mechanism is due to Kirkendall porosity. For Ti--Cu films with Ni-Au plated leads, failures were produced by aging for 5 h at 250°C after bonding, while 50 h at 250°C after bonding was necessary for failure with Ti-Cu-Au. With Ti--Cu, only Cu was left on the platform side of the failure, and both Cu3Au and CuAu were pulled out with the leads. No degradation in bond strength was noted when bare Cu leads were bonded to Ti--Cu and aged for 100 h at 250°C. The latter result is consistent with the Kirkendall hypothesis, since there is no Cu--Au couple.