Aluminum thin films are frequently used as an interconnect material in integrated circuits. During the fabrication of semiconductor devices the Al interconnects are subjected to a number of thermal cycles. In this process the difference in the thermal expansion coefficients between the various materials induces thermal stresses in the Al film, which - in the case of compressive stresses - may be released by hillock formation. Such protrusions on the film surface can penetrate through dielectric layers leading to electrical shorts, thus producing a potential failure mechanism in electronic devices. Therefore a detailed understanding of hillock formation and its underlying mechanisms is of particular interest. Aluminum films of various film thicknesses were deposited on Si substrates by magnetron sputtering and the surface morphology was characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). To investigate the thermo mechanical behavior of the films different heat treatments thermal cycling and isothermal annealing with simultaneous stress measurements have been accomplished using a wafer curvature system. During heating compressive stresses occur and result in hillocking. This new grown hillock volume has been investigated as a function of temperature and film thickness by means of SEM and Focused Ion Beam (FIB) microscopy using various analysis programs. Finally the stress relief by hillock formation was estimated and the activation energy for hillocking determined.