Sputter deposition of crystalline alumina coatings

In the last years a variety of plasma sources have been developed for film deposition by magnetron sputtering. In addition to RF and DC sputter sources, pulsed DC and low-frequency plasma sources are gaining increased attention in sputter technology. This interest is driven by the wish of depositing coatings with superior properties as compared to those deposited by conventional techniques. One prominent example of coatings that are significantly enhanced by the usage of pulsed sputter deposition is alumina. Although crystalline alpha-alumina can be deposited by thermal CVD at temperatures above 1000 degreesC for two decades, no process for the deposition of crystalline alumina at low temperatures is commercially available up to now. In this paper, the results of a detailed study of the plasma parameters during bipolar pulsed magnetron sputtering and their effect on the properties of alumina hard coatings is reported. Langmuir type voltage measurements at the substrate position, optical emission spectroscopy as well as mass spectroscopy were used to monitor the effect of target poisoning on the reactive deposition of alumina. Those principal observations were connected to easily available process parameters like discharge voltage and oxygen partial pressure. Based on these measurements, the deposition of crystalline gamma-alumina with high hardness and good adhesion under technical conditions was achieved. (C) 2002 Elsevier Science B.V. All rights reserved.