Optical absorption spectroscopy of facing targets and conventional magnetron sputtering during process of Al-doped ZnO films

Facing targets sputtering (FTS) is a promising process for low-temperature TCO film deposition and for the significant reduction of damage in film structure by suppression of the direct impinging of high-energy particles due to confining plasma between two facing magnetic fields compared with conventional magnetron sputtering (CMS). In this study, FTS was compared with CMS by measuring Zn densities during the deposition process of Al-doped ZnO films with DC and 13.56 MHz RF discharge at various process conditions. Optical absorption spectroscopy (OAS) was adopted for measuring Zn densities, for which a hollow cathode lamp was used as a source light of a 213.9 nm line, which was selected to measure absorption coefficients and to calculate the ground state densities of Zn atoms. When RF power was applied, Zn densities were lower than in DC conditions in both CMS and FTS. Interestingly, in FTS, Zn densities were observed to be lower than in CMS regardless of the RF and DC powers, indicating an increase of plasma density associated with the high ionization rate and collision rate of FTS.