Investigations on the substrate bias influence on reactive HPPMS plasmas

For the high power pulsed magnetron sputtering (HPPMS), several investigations regarding correlations between plasma and coating properties and the process parameters have been carried out, especially with respect to the HPPMS pulse parameters frequency f and pulse-on-time ton, the power density at the target and the process gas pressure. Another important aspect with respect to industrial coating processes is the substrate bias UB, which is used to accelerate ionized coating forming species to the substrate in order to obtain homogeneous fine columnar coatings. In order to understand this mechanism, the correlation of substrate-oriented plasma properties and the resulting coating properties is important to investigate. For this purpose, the present work focuses on strategies to provide a contribution to quantifiable investigations on the HPPMS plasma properties using substrate bias. These investigations were conducted for a reactive HPPMS (Cr,Al)N process, which was chosen due to the applicability as protective coating for many tool applications. The strategies were developed for the plasma diagnostics optical emission spectroscopy, Langmuir probe and energy resolved mass spectroscopy in an industrial scale coating unit. Using a varying direct current substrate bias with values from UB = 0 V to UB = −250 V, the methods were validated. For example an increasing Debye sheath thickness sD, ion energy distributions and metal-to-ion-flux ratio JM/JG were observed for higher values of UB. These results were correlated with the corresponding coating properties regarding the coating thickness distribution on complex surfaces, the Al/Cr ratio in the coatings, the phase composition, the residual stress and the mechanical properties as well as the coating adhesion to the substrate. Generally, with these correlations it was proven that the methods for the usage of substrate bias during plasma diagnostics are meaningful for an industry-related coating development.