The grain growth and thermal stability of Cu films deposited on Ta and TaN underlayers are investigated as a function of the Cu film thickness. Cu films deposited on Ta underlayer are recrystallized at temperatures lower than those on TaN underlayer. It is found that residual internal stresses in Cu films on Ta underlayer are much larger than those on TAN layers, and that, regardless of underlayer, both the internal stress and the hardness increase with decreasing the Cu film thickness. On annealing, the grain size of Cu films increases with increasing the film thickness, whereas the hardness monotonically decreases with increase of the annealing temperature. We note that the hardness of Cu films increases linearly with the inverse of film thickness. This result most likely confirms that the yield stress of Cu films can also increase with the inverse of film thickness. The thermal defects, voids, are formed mostly in or along grain boundaries after annealing above 200 degrees C. It is noted that the thinner the Cu film thickness is, the lower the starting temperature to grow the voids becomes. The formation of voids can be explained by a stress induced diffusional creep.