Figure below shows the XPS depth profile of a 22-layer SiO₂ and tantalum oxide (Ta₂O₅) dielectric reflective film on the interior wall of a hollow quartz material. Depth profiles are used to determine the elemental composition as a function of depth. A rastered, differentially pumped argon ion gun at 4keV was used to sputter the surface of the reflective dielectric film. The sputter rate of SiO₂ was used to express the thickness of the layers in the dielectric film.

From the figure, it is observed that there are alternating SiO₂ and non-stoichiometric TaO_x layers in the dielectric film, SiO₂ being the first layer. Also, outer layers are thinner than the inner layers, though the first SiO₂ layer is thicker than the other outer SiO₂ layers. The 8th layer should have been a TaO_x layer, but it was a mixed layer instead. Ta had a local maximum there, which was less than the Si concentration at that depth. It is also observed that the oxygen concentration in the TaO_x layer is less than that in the SiO₂ layer, despite the fact that it should be 25% greater. On the other hand, the carbon concentration in the TaO_x layers is greater than that in the SiO₂ layers. Generally, it was greater in the deeper layers, though it fluctuated from layer to layer. The Ta and C concentration variations were due to a poorly operating valve system in the deposition chamber of our customer’s subcontractor.