Initial hillock formation and changes in overall stress in Al-Cu films andpure Al films during heating

The effect of adding a few percent (3 at.%) Cu to Al films on the initial h illock formation and the changes in overall film stress were studied. Al fi lms were evaporated by an electron-beam gun onto Si wafers in an ultrahigh vacuum deposition system and Al-Cu films were coevaporated with a thermally heated source used for the Cu, The as-deposited samples were radiatively h eated at 3 degrees C/s in an air environment. During heating, the initial h illocking and the changes in overall stress were measured simultaneously an d in real time with a specially designed optical instrument. The measuremen t principle of this instrument is based on laser beam deflection, caused by wafer bending due to film stress, and collection of the laser light scatte red off from the hillocks appearing on the film surface. The experimental r esults show that Cu alloying has a strengthening effect on Al films, result ing in delayed and considerably reduced hillock formation. Before heating, the as-deposited Al-Cu samples were investigated by total integrated scatte ring and atomic force microscopy, These investigations showed that Al-Cu fi lms are considerably smoother and have smaller grains than Al films of simi lar thickness (340 nm). It was found that the small grains of Al-Cu films c ontribute to increasing the tensile stress-temperature slope. In addition, Al-Cu films can withstand higher compressive stresses than Al films.