Errors Associated With Thin Film Measurements Using XPS at a Single Angle

Significance of the topic

Accurate measurement of thin film thickness is critical in semiconductor fabrication, coatings development and surface engineering. X-ray photoelectron spectroscopy (XPS) is widely used for non-destructive overlayer analysis, but improper data acquisition can introduce significant underestimation errors. Understanding and mitigating these errors ensures reliable process control and material performance assessment.

Objectives and study overview

This application note examines the errors arising when film thickness is computed from a single fixed emission angle XPS measurement. It compares single-angle results with true values obtained via angle-resolved XPS (ARXPS) and quantifies deviations for common oxides. The goal is to demonstrate the importance of selecting appropriate angular ranges or adopting ARXPS to eliminate measurement bias.

Methodology and instrumentation

Measurements were simulated for a spectrometer collecting photoelectrons at 45° ± 20° relative to the surface normal. Overlayers of SiO2, Al2O3 and HfO2 on silicon substrates were modeled. Film thickness calculations used standard single-angle formulas after summing peak areas across the angular window. ARXPS reference data were generated by measuring intensity as a function of emission angle and applying established fitting protocols.

Key results and discussion

• Single-angle measurements consistently underestimated overlayer thickness due to elastic scattering of electrons within the film.
• Underestimation magnitude increased with film thickness and depended on overlayer material. For example:

• SiO2 showed modest errors at a few nanometers, rising steeply beyond ~10 nm.
• Al2O3 and HfO2 exhibited more pronounced biases due to higher scattering cross-sections.

• ARXPS data revealed the optimal angular integration windows: thin films tolerated wider ranges without significant error, whereas thicker films required narrower angular spans to maintain accuracy.

Benefits and practical applications of the method

• Adoption of ARXPS eliminates systematic underestimation and improves thickness accuracy across a wide range of overlayer compositions and thicknesses.
• Flexibility to tailor angular integration for specific film types enhances measurement precision without hardware modification.
• Reliable thickness data support quality control in microelectronics, protective coatings and nanolayer research.

Instrumentation used

• Thermo Scientific Theta Probe XPS
• Thermo Scientific Theta 300 XPS

Future trends and potential applications

Advances in detector design and data processing algorithms will enable faster ARXPS measurements with higher angular resolution. Integration of machine learning may automate error correction for single-angle setups, extending reliable thickness analysis to portable or budget instruments. Emerging layered materials like 2D heterostructures will benefit from tailored ARXPS protocols to probe interlayer interfaces and chemical gradients.

Conclusion

Single-angle XPS thickness calculations are prone to significant underestimation due to elastic scattering effects that vary with overlayer material and thickness. Employing angle-resolved measurements or carefully selected angular integration ranges effectively eliminates these errors, ensuring accurate thin film characterization essential for research and industrial quality control.

Reference

Thermo Fisher Scientific Application Note 31016: Errors Associated With Thin Film Measurements Using XPS at a Single Angle