For mirror MEMS structures, curvature due to film stress degrades optical quality. Cr/Au is the usual metal mirror coating, and is usually thought to have increased stress for increased thickness. Here, we show that is not the case, due to the physics of thin film formation that entail a phase transition from island to laminar stages as thickness increases. Maximum stress occurs just before the transition, and minimum stress just after. We show this transition for the first time for Cr/Au films on silicon. Cr thickness is kept at 50 Å, and gold thickness varied. We find that the transition between island and laminar phases occurs at a gold thickness of ~ 300 Å. Thus, minimum stress, and maximum radius of curvature occurs at a gold thickness of 400 Å. We also show that at this gold thickness, reflectivity is 95 % of what it is for very thick gold. Thus, this is an optimum value of gold thickness for MEMS micromirrors.