Relationship between the microstructure and nanoindentation hardness of thermally evaporated and magnetron-sputtered electrochromic tungsten oxide films

Tungsten oxide (WOx) films were fabricated by (i) reactive thermal evaporat ion (RTE) at room temperature with oxygen ambient pressure P-O2 as a parame ter, and (ii) reactive magnetron sputtering (RMS) with substrate temperatur e T-s as a parameter. The film structure revealed by x-ray photoelectron sp ectroscopy, x-ray diffraction, density measurements, infrared absorption, a nd atomic force microscopy was correlated with the nanoindentation hardness H. The RTE WOx films deposited at high P-O2 were amorphous and porous, whi le H depended appreciably on normalized penetration depth h(D) (indentation depth/film thickness) due to the closing of the pores at the point of inde ntation. Decrease in P-O2 from 10 to 2 x 10(-3) mtorr led to smaller porosi ty, weaker h(D) dependence of H, and higher average H (measured at h(D) app roximate to 0.2 to 0.3, for example). The RMS WOx film deposited at room te mperature was amorphous and denser than all RTE films. The rise in substrat e temperature T-s first densified the film structure (up to 110 degreesC) a nd then induced crystallization with larger grain size for T-s greater than or equal to 300 degreesC. Correspondingly, the h(D) dependence of H became weaker. In particular, H of the RMS sample deposited at 110 degreesC showe d a peak at h(D) slightly above 1 owing to pileup at the contact point of i ndentation. For higher T-s, pileup occurred at shallower h(D) and the avera ge H (measured at h(D) approximate to 0.2 to 0.3, for example) rose, accomp anied by the increase of grain size.