In situ measurement of mechanical stress in polycrystalline zinc-oxide thin films prepared by magnetron sputtering

Abstract

The generation of stress in magnetron-sputtered undoped and aluminum-doped zinc-oxide (ZnO:Al) films has been measured by an in situ technique. Radio-frequency magnetron sputtering from ceramic targets in an argon atmosphere was used for film deposition. The stress was measured by optically detecting the deflection of a thinned silicon cantilever on which the films were deposited. This in situ stress measurement setup was tested by investigating the sputtering pressure dependence of stress in molybdenum films. For zinc-oxide films it was found that by increasing the argon-sputtering pressure during deposition the intrinsic compressive stress decreased by about a factor of 2, both for undoped and doped zinc-oxide films. The undoped films exhibit significantly higher compressive stresses (1–2.5 GPa) compared to the aluminum-doped films (0.7–1.5 GPa). This fact was ascribed to a surfactant-like effect of the dopant aluminum, which improved the crystallinity of the doped films. In contrast to magnetron-sputtered metallic films (like molybdenum) the zinc-oxide films did not show a transition from compressive to tensile stress when the argon-sputtering pressure was increased. It is assumed that this behavior of the zinc-oxide films is due to an additional incorporation of oxygen into the films. By adjusting the oxygen partial pressure during the deposition at a fixed argon pressure, a minimum of the compressive stress in ZnO: Al films was found which coincided with a minimum of the film resistivity. This observation stresses the role of even small changes in deposition parameters on the mechanical and electrical properties of zinc-oxide films. The possibilities of the in situ stress measurement are demonstrated during the deposition of zinc-oxide multilayers.