The effect of ion current density on the adhesion and structure of coatings deposited by magnetron sputter ion plating

Recent work using an improved design of an unbalanced magnetron in a four-magnetron closed-field arrangement has resulted in exceptionally high ion current densities at the substrates during both ion cleaning and deposition. The ion current at the electrically biased substrate is increased by a factor of close to 100 as compared to balanced magnetrons of the same polarity. The absolute ion current is dependent on a number of factors which are discussed, the most important being the power applied to the magnetrons. The relationship between substrate bias and ion current is presented. It is found that it is possible to achieve very high ion currents at bias voltages as low as - 50 V. The effect of the substrate bias voltage and the substrate ion current on the coating structure have been studied, and it is found that the optimum conditions for the deposition of dense coherent coatings with low internal stresses are a high ion current at a low bias voltage. These conditions are readily achieved using the closed-field unbalanced magnetron technique. It is shown that coating structure is almost independent of deposition rate, which is extremely important for the deposition of coatings with excellent structures at low temperatures. Ion cleaning prior to deposition is carried out with the magnetrons operating at low power. This allows the plasma discharge to strike and be maintained at low arson a pressures, thus avoiding contamination from the chamber walls, etc. The enhancement of the ion current under these conditions is similar to that achieved during deposition, i.e. around 100 x, and so the efficiency of ion cleaning is considerably enhanced, producing a "clean" surface which results in excellent adhesion. Scratch adhesion testing has been used to compare the critical loads for coatings deposited with different cleanings conditions and the excellent adhesion achievable is demonstrated. (C) 1999 Elsevier Science S.A. All rights reserved.