Investigation of reflectometry for in situ process monitoring and characterization of co-evaporated and stacked Cu-Zn-Sn-S based thin films

During preparation of compound semiconductor thin films undesired secondary phases and inhomogeneities may occur. Therefore, process control and monitoring are important aspects towards thin film optimization. In this study, white light reflectometry (WLR) was investigated as an in situ non-destructive optical technique to monitor thin films during thermal vacuum evaporation (PVD) of kesterite-type Cu-Zn-Sn-S thin films. The impact of composition on optical properties was studied ex situ by Raman spectroscopy and reflectometry to identify possibilities for in situ detection of secondary phases. A WLR setup was designed and integrated with the PVD system. Four Cu-Zn-Sn-S films were prepared by evaporation and direct reflection was monitored in situ. Reflection spectra were analyzed to identify the influence of process parameters and phases such as CuS and ZnS. Transfer matrix simulations were performed to further explain experimental observations. It is shown that the occurrence of different phases, growth rate/thickness are well related to reflection spectra. Time-dependent reflection spectra reveal specific properties that could serve as characteristic fingerprints for process control and monitoring. Therefore, this study extends the possibilities for reflectometry as a straightforward, low cost method for in situ process control of Cu-Zn-Sn-S based films.