Effect of bias voltage on microstructure and optical properties of Al2O3 thin films prepared by twin targets reactive high power impulse magnetron sputtering

Abstract

Al2O3 thin films were deposited on silicon (100) and tin-doped indium oxide (ITO) substrates by twin targets reactive high power impulse magnetron sputtering (TTR-HiPIMS). Effects of the substrate bias voltage (Vb) on microstructure, surface morphology, chemical composition and optical transmittance of the deposited films were investigated by grazing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), X-ray photoelectrons spectroscopy (XPS) and Ultra violet-visible-near infrared (UV-visible-NIR) spectra, respectively. The AFM scans showed that increasing the bias voltage (Vb) from 0 to −40 V caused the weakly crystalline film to evolve into fully crystalline Al2O3 film at temperatures as low as 230 °C. However, when Vb reached −60 V, the crystallinity decreased. This result was also confirmed by GIXRD patterns, which showed that the best crystallinity of Al2O3 in γ phase was obtained for film deposited at a bias of −40 V. Besides, with the increase of Vb, the deposition rate decreased from 88 to 70 nm/h, and the O/Al ratio of the as-deposited films reduced from 1.53 to 1.44. The binary collision approximation (BCA) Monte-Carlo code SDTrimSP was also used to calculate the sputter yield of Al and Al2O3. It was found that the sputter yield of Al2O3 was about 1/8 of Al, which is supposed to be one of the main reasons for the observed low deposition rate. UV-vis-NIR transmittance spectra of Al2O3 films deposited under different bias voltages were evaluated after deposition on ITO substrates. The results revealed that all the as-deposited films had high optical transmittance (≈80%) in the wavelength range of 200–800 nm. Moreover, a marginal decrease of the transmittance was also observed with increasing Vb.