The thin film solar thermal absorber consists of two layers deposited onto a material with highly infrared reflectivity such as copper or aluminium. The base layer of the coating is metallic with high absorption while the surface oxide layer serves as an antireflection film. A reactive magnetron sputtering of Ni80Cr20 target in argon/oxygen atmosphere was developed for industrial high-rate deposition. Optimum properties and composition of the coating were studied previously. In this study, a model of the sputtering process is reported.
A key feature of the reactive sputter deposition process is the existence of gradients across the deposition chamber, with respect to the deposition rate and gas pressure. This enables the synthesis of graded coating in a single step process using only one magnetron, but posses difficulty for modelling. Existing models assume uniform distribution of the reactive gas in the sputtering chamber and uniform deposition rate. An extended model taking into account distribution of the sputtered material and partial pressure gradients has therefore been developed in this study. The flux of reactive gas is described by the diffusion equation solved by the finite element method, and Berg’s model has been used for the reactive sputtering.
The simulations closely reproduce observed behaviour. For flow of reactive gas below the critical value needed to operate in compound mode, a pressure gradient develops across the substrate. This results in observed composition distribution deposited at different positions. When the critical flow is reached, sputtering target is poisoned and hence the deposition rate is substantially reduced. This leads to reduced sorption of reactive gas, abrupt increase in its partial pressure, negligible pressure gradients and homogenous composition of deposited film.
The presented model may also explain the frequently observed deviations between simulated and measured partial pressure of reactive gas when operating in the metal mode. The observed partial pressure is always higher than predicted. According to presented calculations, this may be a result of pressure gradient in the presence of metal surfaces with very high gettering capacity.
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