Stress evolution in copper–silver thin films during thermal-cycling

Stress evolution in Cu, Cu–1.4 at% Ag, Cu–3 at% Ag and Cu–6 at% Ag thin films was determined by substrate curvature measurements from room temperature to 400 °C in two cycles, each involving a heating and a cooling stage. Stress hysteresis curves for the Cu–Ag films changed slope significantly within the temperature range 220–250 °C and stress range 100–200 MPa during the heating stage of the first cycle; this occurred at 175 °C and 100 MPa for the Cu film. This was followed by stress relaxation in all the films at about 300 °C. For the as-deposited, annealed and thermally cycled films exposed to low temperatures (−197 to 0 °C), instances of yielding occurred in Cu, Cu–1.4 at% Ag and Cu–3 at% Ag films. However, the as-deposited, annealed and thermally cycled Cu–6 at% Ag film was always elastic and there was no yielding when exposed to low temperatures. Microstructural analyses revealed the presence of Cu and Ag phases in all the Cu–Ag films, irrespective of the thermal history. Despite electron diffraction revealing their existence, Ag particles were not so apparent in the microstructures of the as-deposited and annealed films probably because of their fine sizes. However, after thermal-cycling Ag particles were observed at grain boundaries and inside grains in addition to significant grain growth.

► Stress in Cu and Cu–Ag films determined from −197 to 0 °C and RT to 400 °C. ► Heating to 400 °C, films elastically and plastically deformed. ► Cooling, Cu plastically deformed; Cu–Ag films plastic/elastic deformation. ► At −197 to 0 °C, yielding did not occur in any Cu–6 at% Ag film. ► Cu and Ag existed; grain growth and Ag coarsening occurred.