The stress in the thin films of pure Cu and Cu (Mg) alloy deposited on oxidized silicon substrates has been measured in the range of room temperature to 500 °C by the use of aninsitulaser based curvature measuring tool. Results indicate that small amounts of Mg (∼2–3 at. %) in copper changes the stress‐temperature behavior significantly when compared with that of pure copper. The alloy, after the first annealing cycle which apparently equilibrates the film, shows a predominantly elastic stress‐temperature behavior up to ∼400 °C compared to pure Cu, which shows a departure from linearity at ∼200 °C. Rutherford backscattering and x‐ray diffraction studies were also carried out to elucidate the role of Mg in strengthening the copper film. The results indicate that additions of Mg to Cu increase the mechanical strength of the film and that such behavior is retained in several (subsequent) thermal cycles to 400 °C. ©1995 American Institute of Physics.