A comparative study of hillock formation in aluminum films

Studies were conducted to evaluate the performance of a variety of aluminum alloys with respect to hillock formation. Specifically, the effects of film composition, deposition temperature, and underlayer on hillock formation were investigated. The film compositions included pure Al, Al with 1.5 wt.% Cu (or AlCu), AlCu with 0.2 wt.% W, and AlCu with 0.4 wt.% W. These films were sputter deposited at 300 degrees C and 450 degrees C on either oxide- or Ti-W-coated Si substrates. Following deposition, the films were annealed in air for 20 min at 450 degrees C. For all film compositions, the deposition temperature had the most significant effect on the hillock density. For a given deposition temperature, the addition of Cu to pure Al drastically reduced the hillock density, but the addition of W to AlCu increased the hillock density, The effect of the underlayer on the hillock density was negligible. Cross-sections of the hillocks showed that they were solid from the bottom interface to the top surface, and no voids were found near the hillocks. The grain size did not have a significant effect on the hillock density. However, a correlation between increasing (111) film texture and decreasing hillock density was observed.