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
\Omega
/
Box$^b$
) 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
\mu
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 H
2
. The film pulled out with the leads during a 90° peel test was identified as C
3
Au, 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 Cu
3
Au 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.