The effect of surface aluminum oxide films on thermally induced hillock formation

Al films were vapor deposited onto oxidized silicon wafers in high vacuum (HV) and ultrahigh vacuum (UHV). The as-deposited HV films showed large numbers of hillocks while the as-deposited UHV films showed no hillocks even after in-situ vacuum annealing. The evidence indicates that compressive intrinsic stresses are responsible for hillock formation during HV film growth. Moreover the present study on UHV deposited films shows that in this case one needs both a surface Al oxide layer and compressive differential thermal expansion stresses in order to form hillocks. In the absence of the oxide layer on the UHV films, the application of compressive differential thermal expansion strains causes stress relief via surface-diffusion-produced grain boundary grooving rather than hillock formation.