Microstructural changes in vapour-deposited silver, copper and gold films investigated by internal stress measurements

Structural changes in silver, copper and gold films 100 nm thick evaporated under ultrahigh vacuum conditions were studied by internal stress measurements and by transmission electron microscopy. Tensile stress changes which are not completed until after 5 h for silver and copper and 20 h for gold indicate an increase in the average grain size in these films. The rate of this recrystallization of the film is reduced to about half of its original value when the metal films are coated with a protective layer of MgF₂ immediately after the metal evaporation. When the film recrystallization is not allowed to proceed to completion under vacuum conditions it is again accelerated during exposure of the films to the laboratory atmosphere. From these experiments it can be concluded that the structure of these three metal films as observed using the electron microscope changes from that present at the end of the metal evaporation owing to significant recrystallization. An MgF₂ protective layer can only slow down this recrystallization, not prevent it.