Residual stress relaxation and microstructure in ZnO thin films

Stability under normal environmental conditions over a long period of time is crucial for sustainable thin-film device performance. Pure ZnO films with thicknesses in the 140 - 450 nm range were deposited on amorphous glass microscope slides and (100)-oriented single crystal silicon wafers by radio frequency magnetron sputtering. The depositions were performed at a starting temperature of 200 oC. ZnO films had a columnar microstructure strongly textured along the direction. XRD peak-shift analysis revealed that the films were under residual, compressive, in-plane stress of -5.46 GPa for the glass substrate and -6.69 GPa for the Si substrate. These residual stresses could be completely relaxed by thermal annealing in air. When left under normal environmental condition over an extended period of time the films failed under buckling leading to extensive cracking of the films. The XRD and SEM results indicated different mechanisms of stress relaxation that were favored in the ZnO thin films depending on the energy provided. Although thermal annealing eliminated residual stresses, serious micro-structural damage upon annealing was observed. Thermal annealing also led to preferential growth of some ZnO crystals in the films. This kind of behavior is believed to be indicative of stress-induced directional diffusion of ZnO. It appears that for the extended stability of the films, the stresses have to be eliminated during deposition

RF magnetron-sputtered ZNO thin films: on the evolution of microstructure and residual stresses

Thin ZnO films (200-500 nm) were deposited onto glass, mica, and Si(100) substrates, to study the relations between microstructure and residual stresses. The ranges for the substrate temperature, chamber pressure, and RF power were room temperature-200 °C, 0.009-0.4 mbar, and 100-125 W, respectively. The strain measurements by x-ray diffraction and the biaxial stress model showed that the films were under residual compressive stresses from -2 to -8 GPa. 5-11 percent of those stresses were induced by the thermal expansion coefficient mismatch, while the so-called growth stresses formed the majority. The films were strongly textured along the (002)-direction with additional (lOl)-texture under specific conditions. The texture of the film-substrate interface played a significant role in the average film texture and the residual stress. Variations from the previously developed structure zone models (SZM) and the microstructures in the pressure ranges beyond those SZMs were investigated by scanning electron microscopy. Several mechanisms of microstructure evolution and consequent stress development were proposed. Thermal annealing and aging removed the growth stresses and improved the texture. Annealing caused pore clustering and crack formation. Thus, annealing should be treated with caution when used as a method for stress relaxation. Buffer layer deposition and in-chamber heat treatment were proposed as alternatives to post-deposition annealing.Annealing led to structures on the ZnO film surface up to 100 nanometers in width and 1 micrometer in height. This was attributed to the stress-induced directional diffusion