Thickness dependent residual stress in sputtered AlN thin films

Thin aluminum nitride (AlN) films of different thickness are deposited by DC-pulsed magnetron sputtering under identical conditions on sapphire (0001) and silicon (100) substrates. An investigation of the residual stress, morphology and structural properties is carried out. The thickness of the films covers the range from 17 nm to 3.9 μm. A higher compressive residual stress is measured for the thinner films and the presence of a stress gradient is proven. X-ray diffraction (XRD) studies show that all AlN films are achieved with perfect c-axis orientation perpendicular to the film surface and that the films are biaxially strained. XRD rocking curves reveal that AlN films on sapphire are highly oriented for all film thicknesses, whereas AlN film growth on silicon starts highly disoriented and the film quality improves with film thickness. Surface analysis by atomic force microscopy shows a continuous film roughening and decrease of grain boundary density with increasing film thickness.