Angle Resolved XPS

Importance of the Topic

Angle resolved X-ray photoelectron spectroscopy (ARXPS) offers a non-destructive approach to probe chemical composition within the top 10 nm of a surface without sputtering. It can reveal sub-surface chemical states and quantify ultrathin film thickness or layered structures. Applications span semiconductor wafer analysis, corrosion studies, thin film growth monitoring and quality control in industrial and research laboratories.

Objectives and Overview

This application note reviews the mathematical foundations and practical implementation of ARXPS using Thermo Fisher Scientific instrumentation. It covers the Beer-Lambert law applied to photoelectron attenuation, single and multiple overlayer thickness models, factors complicating analysis such as elastic scattering and angular asymmetry, and methods for constructing concentration depth profiles.

Applied Methodology and Instrumentation

ARXPS exploits the exponential attenuation of photoelectrons through matter. Key parameters include:

• Inelastic mean free path (IMFP) and attenuation length (AL) governing electron escape probability
• Beer-Lambert formalism to relate photoelectron intensity ratios to film thickness
• Relative depth plots for qualitative ordering of surface species
• Single and multi-overlayer models to derive quantitative thickness via linearized plots against 1/cosθ
• Maximum entropy and genetic algorithm techniques to reconstruct continuous concentration profiles from angular data

Used Instrumentation

Data and examples derive from Thermo Scientific ARXPS systems with angle-resolving lenses and two-dimensional detectors (Theta Probe, Theta 300) as well as conventional spectrometers (ESCALAB 250). Parallel-acquisition ARXPS collects a 60° angular range simultaneously without sample tilt, enabling analysis of full semiconductor wafers and small features under constant charge compensation.

Main Results and Discussion

Calibration on SiO2 films on Si demonstrates strong linear correlation (gradient ~1.08) between ARXPS and ellipsometry thickness measurements, with offset attributable to airborne contamination layers. Determination of the intensity ratio R∞ from atomic number densities and experimental data yields accurate single-layer thicknesses. Multi-overlayer models successfully quantify stacked films such as ALD-grown Al2O3 on SiO2. Elastic scattering and angular asymmetry corrections are required for data above 60° emission angles, and asymmetry factors are adjusted using literature parameters and Monte Carlo–based algorithms.

Benefits and Practical Applications

ARXPS enables:

• Non-destructive depth profiling of ultrathin films and buried interfaces
• Chemical state identification of elements within sub-surface layers
• Accurate thickness measurements independent of surface contamination
• High surface sensitivity by varying collection angle
• Routine analysis of full semiconductor wafers and micro-features

Future Trends and Possibilities

Continued advances are expected in parallel-acquisition optics, software algorithms for scattering corrections and depth profile inversion, and integration with complementary techniques (e.g. spectroscopic ellipsometry, ion scattering). Emerging applications include in situ monitoring of thin-film growth, time-resolved reaction studies, complex multilayer and organic material analysis, and machine-learning driven spectral interpretation.

Conclusion

ARXPS provides a powerful toolbox for nondestructive, quantitative analysis of ultrathin films and depth profiles with chemical specificity. By combining robust physical models, advanced instrumentation and optimized data processing, ARXPS supports critical applications in materials science, semiconductor manufacturing and surface engineering.

References

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4. Jablonski A. Surface and Interface Analysis 1989, 14, 659.
5. Opila R.L.; Eng J. Jr. Progress in Surface Science 2002, 69, 125.