Creep-controlled diffusional hillock formation in blanket aluminum thin films as a mechanism of stress relaxation

Hillock formation, a stress-induced diffusional relaxation process, was stu died in sputter-deposited Al films. The grain sizes in these films were sma ll compared to those in other sputter-deposited Al films, and impurities (O , Ti, W) were incorporated during the preparation of the films. Stress and hardness measurements both indicate that the Al films were strengthened by the small grain size and incorporated impurities. We observed a new type of hillock in these Al thin films after annealing for 2 h at 450 degrees C in a forming gas ambient. The hillocks were composed of large Al grains creat ed between the substrate and the original Al film with its columnar grain s tructure, apparently by diffusion from the surrounding area. By modifying t he boundary conditions of Chaudhari's killock formation model [P. Chaudhari , J. Appl. Phy. 45, 4339 (1974)], we have created a new model that can desc ribe the experimentally observed hillocks. Our model seems to explain the e xperimentally observed abnormal hillock formation and may be applied to oth er types of hillock formation using different creep laws.