Control of calcium phosphate thin film stoichiometry using multi-target sputter deposition

RF magnetron sputtering of calcium phosphate (CaP) thin films has been shown to offer significant advantages over other deposition techniques due to the fact that it can offer significant control over the processing condition. Specifically, this technique allows the properties of the deposited coatings to be tailored by varying either the target composition or the in-situ processing parameters. This novel work presents the results from a detailed study of the coatings created by co-sputtering from three targets with different Ca/P ratios. Target combinations were chosen from hydroxyapatite (HA), α tri-calcium phosphate (TCP) and di-calcium phosphate (DCP), with average target stoichiometries of between 1.0 and 1.67 before sputtering. The target combinations chosen provide scope in relation to control of the CaP surface properties than those previously available. The samples have been analysed both in the as-deposited state and after thermal annealing to 500 °C using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). As-deposited coatings were all amorphous in nature, as confirmed by FTIR and XRD analyses. The Ca/P ratios of the as-deposited coatings were generally below those of the average starting target stoichiometry. This was not unexpected for the processing conditions employed here. However, after thermal processing, most of the coatings produced were shown to be crystalline, with an observed increase in the coating Ca/P ratios. However, the coating with an average starting Ca/P ratio of 1.22 did not see any enhancement in its crystallinity. The results from this study show how the varying target compositions control both the annealed coatingʹs stoichiometry and crystallinity, with all coatings also displaying varying levels of preferred orientation. Therefore, the results described here provide a route to control the properties of a CaP surface to meet the requirements of the user, with specific emphasis on critical parameters such as phase, crystallinity and stoichiometry.