Concepts for in situ characterization and control of plasma ion assisted deposition processes

In this paper recent results on monitoring and control of plasma ion assisted deposition (PIAD) processes are presented. An industrial PIAD system equipped with diagnostics for deposition rate and optical thickness was augmented by active plasma resonance spectroscopy to access electron density near the substrate. A series of experiments implementing different schemes to control the level of plasma assistance, while depositing a model 5-layer quarterwave stack SiO2/TiO2, were conducted. Data on in situ spectral transmission of the coatings was used to reconstruct optical properties and the thickness of each layer. Conventional control of anode voltage of the assist source Advanced Plasma Source (APS) was found to exhibit remarkable variations of electron density as drifts during deposition and shifts between separate runs. We demonstrate that measuring the electron density and using that signal in the feedback loop control of the APS greatly improves the layer-to-layer repeatability and run-to-run reproducibility of coating performance. Best results were obtained combining electron density control with optical monitoring for endpoint detection.