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 ( R 0 ) and the effective attenuation length ( L ) are essential. By definition, R 0 is the peak intensity ratio for an overlayer and the substrate in “bulk” phases. Two issues need to be addressed in experimental determining R 0 for ultrathin films: (i) How might a contamination layer on the sample used for measuring peak intensities impact R 0 ? And (ii) do differences in the structure or chemistry of an ultrathin film make it inappropriate to determine R 0 using bulk forms of the overlayer? In this study, we demonstrate the experimental determination of the R 0 for an ultrathin HfO 2 film on a Si(100) substrate with a 2 nm SiO 2 layer. The values of R 0 were determined using (i) the bulk materials of the HfO 2 film and substrate and (ii) the ultrathin HfO 2 films after different cleaning treatments. The results show that the R 0 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 R 0 . 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 HfO 2 films were then evaluated.