Experimental determination of reference intensity ratio essential for accurate thickness measurement of HfO2 ultrathin films by XPS

When measuring the thickness of ultrathin overlayer films using X-ray photoelectron spectroscopy (XPS), accurate values of the reference intensity ratio (R₀) and the effective attenuation length (L) are essential. By definition, R₀ is the peak intensity ratio for an overlayer and the substrate in “bulk” phases. Two issues need to be addressed in experimental determining R₀ for ultrathin films: (i) How might a contamination layer on the sample used for measuring peak intensities impact R₀? And (ii) do differences in the structure or chemistry of an ultrathin film make it inappropriate to determine R₀ using bulk forms of the overlayer? In this study, we demonstrate the experimental determination of the R₀ for an ultrathin HfO₂ film on a Si(100) substrate with a 2 nm SiO₂ layer. The values of R₀ were determined using (i) the bulk materials of the HfO₂ film and substrate and (ii) the ultrathin HfO₂ films after different cleaning treatments. The results show that the R₀ determined by the ultrathin films is higher than that determined by the bulk materials. Also, keeping the same level of carbonaceous contamination on the sample surface by cleaning as much as possible is essential for an accurate experimental determination of R₀. In addition, the effective attenuation length was obtained using samples with known thicknesses measured by X-ray reflectometry. The thicknesses and uncertainty budget of the ultrathin HfO₂ films were then evaluated.