Fundamental aspects of HiPIMS under industrial conditions
Sammanfattning: Fundamental aspects of the high power impulse magnetron sputtering (HiPIMS) process and its implication for film growth under industrial conditions have been studied. The emerging HiPIMS technique exhibits a higher plasma density and an enhanced degree of ionisation of sputtered material as compared to conventional direct current magnetron sputtering (DCMS). The increased ionisation permits control of the deposition flux and facilitates an intense ion bombardment of the growing films. The latter allows for growth of well adherent, smooth, and dense thin films. Moreover, the technique offers increased stability of reactive processes, control of film phase constitution as well as tailoring of e.g. optical and mechanical properties.In the present work, it was shown, for eight different metals (Al, Ti, Cr, Cu, Zr, Ag, Ta, and Pt), that films grown using HiPIMS exhibit a 5-15% higher density than films grown using DCMS under otherwise identical conditions. Through simulations of the fundamental ionisation processes in the plasma discharge, a correlation between high ionisation degree and film densification was established. The densification was suggested to be a consequence of increased ion irradiation of the growing films in the HiPIMS case. This knowledge was used to investigate the degree of ionisation in the deposition flux required for film modifications. Using a hybrid process, where DCMS and HiPIMS were combined on a single Cr cathode, independent control of the degree of ionisation from other experimental parameters was achieved. The results showed that the majority of the ion irradiation induced modifications of surface related film properties occurred when ~40% of the total average power was supplied by the HiPIMS generator. Under such conditions, the power normalised deposition rate was found to be ~80% of that of DCMS. This was attributed to a reduction in back-attracted ionised sputtered material, which is considered to be the main reason for the low deposition rate of HiPIMS. Thus, enhanced film properties were attainable largely without sacrificing deposition rate.Compound carbide and boride films were synthesised using both reactive processes and compound sources. Reactive deposition of TiC/a-C:H thin films using C2H2 as reactive gas, i.e. carbon source, was demonstrated. It was found that the high plasma density processes (i.e. HiPIMS) facilitated growth conditions for the film structure formation closer to thermodynamic equilibrium than did processes exhibiting lower plasma densities (i.e. DCMS). This was manifested in a high stoichiometry of the carbide phase, whilst excess a-C was removed by physical sputtering. Moreover, the feasibility of using HiPIMS for thin film growth from a compound source, obtaining the same composition in the films as the sputtering source, was demonstrated through synthesis of ZrB2 films.
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