Advanced Magnetron Sputtering for Thin-Film Deposition
Abstract: Sputtering magnetrons are widely used to deposit thin films. The technology is considered mature. However, with the advent of high-power impulse magnetron sputtering (HiPIMS), new process parameters allow us to tune film microstructure and related properties such as film stress and texture. HiPIMS was designed to deliver not only film-forming atoms but also film-forming ions, which result in film densification and the possibility of obtaining desired properties at generally lower substrate temperature. This is particularly interesting for deposition on temperature-sensitive substrates including flexible polymer sheets (webs). Pulsed high-power operation leads not only to an increased degree of ionization of sputtered atoms but also to dissociation and ionization of the process gas, which is most relevant for reactive sputtering (e.g., in the presence of oxygen and/or nitrogen).
Besides film-focused research, the introduction of HiPIMS also spurred much research on the magnetron discharge itself. It was found that magnetrons show surprisingly rich physics based on plasma instabilities. Without these instabilities, magnetrons would generally not work. The energy needed to make plasma (i.e., to ionize atoms of the process gas and sputtered from the target) is delivered by "hot" electrons. The key to a functioning magnetron is thus the electron heating mechanism. Recent theoretical, spectroscopic, and probe data prove that most of the electrons' energy comes from the presheath and is provided by localized electric fields concentrated at the edge of "ionization zones" or "spokes." This is closely related to self-organization and turbulence as visualized in interesting high-speed images of magnetron plasmas.