An investigation into phenomena which influence the optimisation of ion plating systems
Ion plating involves the transportation of vapour through a low pressure glow discharge to form solid coatings on negatively biased substrates. Although the method has gained commercial acceptance, particularly for engineering applications, much of the underlying science of the process is poorly understood. This work is concerned with investigating certain mechanisms occurring within the ion plating environment, and their role in process optimisation.The work has concentrated primarily on electron beam evaporation in thermionically enhanced discharges. Investigations have centred on various aspects of discharges used in ion plating and the influence of gas scattering mechanisms on the vapour species during deposition. This has been achieved by analysing the published literature and performing experiments involving cathode sheath thickness measurements, sputter weight loss determinations, optical emission spectroscopy and coating thickness evaluation.The main findings are:(i) Thermionically enhanced discharges can considerably reduce cathode sheath thickness, providing benefits in terms of bombardment uniformity and energy transportation. However, the influence of plasma bombardment is anisotropic and also falls off exponentially with distance from the thermionic emitter. This can be offset by a comparable reduction in coating flux arrival rates if the thermionic emitter is positioned close to the vapour source.(ii) The incorporation of nitrogen in (reactive) ion plating discharges may reduce the rate of ion generation, particularly in the presence of thermionic emitters. Nitrogen dissociative charge transfer collisions within the cathode sheath may be signif icant but their practical importance is questionable.(iii) There is evidence to suggest that the metal vapour in an argon ion plating discharge transports most of the ion current to the substrate and at least some of the material arrives as atomic clusters.(iv) A model which unifies coating thickness uniformity with source to substrate distance has been developed. This predicts the existence of a virtual source phenomenon.