Effect of thickness on optical and microwave dielectric properties of Hydroxyapatite films deposited by RF magnetron sputtering

This study present findings on the structural, optical and dielectric properties of polycrystalline Hydroxyapatite [HAp, Ca10(PO4)6(OH)2] films, deposited using radio-frequency (RF) magnetron sputtering. The X ray diffraction (XRD) studies revealed that the unit cell volume and crystallite size of the films deposited on quartz substrates enhanced with an increase in film thickness. The Young's modulus (Ehkl) and the Poisson ratio (νhkl) of the thin films along different crystallographic directions have been calculated using the X-ray elastic constants. The Young's modulus of the films exhibited crystallographic direction dependence which suggests that the sputtered films are anisotropic. The dielectric constant εr and the loss tangent tanδ of the sputtered films were in the range 29-85 and 0.0028-0.0014 respectively at a frequency of 1 MHz. These values are by far the best for films deposited under a pure argon environment. The improvement in crystallinity and uniform grain size of the films are used as parameters to understand the variation in dielectric properties of the sputtered films. The best microwave dielectric properties were in the range εr = 75 - 65 and tanδ = 0.014-0.032 when measured at 5 and 10 GHz, employing split post dielectric resonator (SPDR) technique. The obtained results suggest that HAp sputtered films can be promising for optical limiting and applications in tunable microwave devices.