Understanding intrinsic stress evolution in polycrystalline thin films that grow by the Volmer-Weber method

In thin films, intrinsic stresses created by the growth processes are often difficult to assess and control. We investigate the causes for intrinsic stress generation by observing their evolution in two types of Volmer-Weber (islanded) films: diamond grown by chemical vapor deposition (CVD) and aluminum nitride (AlN) grown by molecular beam epitaxy (MBE). In islanded films, a major source of tensile intrinsic stress is the formation of grain boundaries during the coalescence of individually nucleated clusters. Earlier models treat tensile stress evolution as an instantaneous process of single-step coalescence. However, the CVD grown diamond films show a continuously increasing tensile stress with film thickness. Hence an improved model is developed, that treats intrinsic stress evolution as a process of continuous coalescence. Coalescence grain boundary formation is accompanied by a reduction in island free surface energy. This is offset by the generation of a strain energy in the coalescing islands. Island stress contours and average stress values are calculated using a finite element (FE) model that is coupled to an island growth scheme. Inferences drawn from the model help in the development of interrupted growth experiments that are used to grow good quality diamond films with a lower intrinsic stress. In situ curvature measurements made during MBE growth of AlN show a compressive stress evolution after a tensile peak. An intermediate annealing stage, introduced after the initial grain coalescence produces no change in the residual stress. In these films, stress evolution is governed by competing tensile and compressive sources. Short range attachment of adsorbate atoms at the grain boundary is believed to produce the film compressive stress.