Depth profiling by GDOES: application of hydrogen and d.c. bias voltage corrections to the analysis of thin oxide films

Abstract

Glow discharge optical emission spectrometry (GDOES) has emerged as a major technique for surface and depth profile analysis, owing to the unique combination of fast sputtering rate, high depth resolution, excellent sensitivity and multi-element capability. Compared to direct current (d.c.) glow discharges the more recent radio frequency (r.f.) GD sources offer the advantage that both conducting and non-conducting samples can be analyzed. Recently, an algorithm for r.f.-GDOES has been developed for d.c. bias voltage- and hydrogen-correction [R. Payling et al. J. Anat. At. Spectrom. 16 (2001) 50[1]]. These corrections allowed the combination of samples with different electrical characteristics in a single calibration and the obtainment of depth profiles of hard coatings on steel even in the presence of significant amounts of hydrogen in the coating. In this paper, depth profiles of thin oxide films on the titanium alloy Ti6Al4V measured by r.f.-GDOES are presented. Various oxide films prepared either by anodic or thermal oxidation or by sol–gel deposition were analyzed. In all cases, the emission yields of the different elements were found to depend on the hydrogen content always present as a hydroxide layer on titanium oxides. By applying d.c. bias voltage-, hydrogen- and density-corrections, expected compositions and thicknesses of reference oxide thin films were obtained. The GDOES depth profiles obtained from oxide films prepared by different processing routes demonstrate that the composition, in particular the vanadium content of the oxide layer, can be controlled by means of the oxidation procedure in order to fulfil and guarantee the demands for absolutely biocompatible surface properties of a medical device constructed of the alloy Ti6Al4V.