Paper AS-WeA3
Effects of Elastic Scattering and Analyzer-Acceptance Angle on the Analysis of Angle-Resolved XPS Data

Wednesday, November 11, 2009, 2:40 pm, Room C2

Angle-resolved XPS is frequently used to obtain composition-depth information. Reliable analysis of the data, however, is currently based on the validity of a number of assumptions that include neglect of elastic scattering and neglect of the analyzer-acceptance angle. We determined XPS intensities of N 1s, O 1s, Hf 4f, Si (oxide) 2p, and Si (substrate) 2p peaks at selected emission angles for SiO_1.6N_0.4 and HfO_1.9N_0.1 films on Si with thicknesses of 5 Å, 15 Å, 25 Å, 35 Å, and 45 Å. The intensities were calculated using the NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) [1,2]. Simulations were performed for Al Kα X-rays, sample tilting, and differential elastic-scattering cross sections from the relativistic partial-wave expansion method (Mott cross sections). We also performed simulations with the straight-line approximation (SLA) in which elastic scattering is neglected. We will report comparisons of peak intensities for four models: (a) Mott cross sections and an analyzer semi-angle of 12^o, (b) Mott cross sections and a semi-angle of 0.5^o, (c) the SLA and a semi-angle of 12^o, and (d) the SLA and a semi-angle of 0.5^o. The intensity ratios changed appreciably as elastic scattering is switched on and off, but changing the analyzer semi-angle had a smaller effect. These changes are comparable to those found in N 1s and O 1s angular distributions for different N composition profiles in SiON, thus casting doubt on the validity of N composition profiles obtained from conventional analyses of ARXPS data. We will present plots of ratios of intensities for each line obtained from Models (d) and (a) as a function of emission angle for each film thickness. The largest change occurred for the substrate Si 2p line where the ratio increased appreciably with emission angle, reached a maximum in the vicinity of 50^o to 60^o, and then decreased rapidly. These changes were more pronounced in the HfON films than the SiON films, indicating the stronger elastic-scattering effects in HfON than in SiON. We will report similar comparisons with Cu Kα X-rays. Although elastic-scattering effects are less pronounced than at lower energies, they are not negligible. A planned enhancement of SESSA to include polarized X-rays will make it useful for XPS applications with synchrotron radiation. [1] http://www.nist.gov/srd/nist100.htm.