The evolution of XPS depth profiling

Significance of the topic

X-ray photoelectron spectroscopy (XPS) depth profiling is a cornerstone technique for characterizing the chemical composition and electronic states of surfaces and thin films with nanometre-scale depth sensitivity. Its ability to deliver elemental quantification and chemical-state information is essential across materials science, polymers, semiconductors, coatings and energy materials. As novel materials and complex multilayer structures emerge, reliable depth-resolved chemical analysis becomes critical for quality control, failure analysis and research into interfaces and buried layers. Evolving sample removal methods directly influence the accuracy and applicability of XPS depth profiles, particularly for damage-sensitive or compositionally heterogeneous materials.

Objectives and study overview

This white paper reviews the historical and contemporary approaches used to remove material during XPS depth profiling, outlines their strengths and limitations, and highlights the role of recent technologies in expanding the range of analysable materials. The main aim is to explain how monatomic ion beams, gas cluster ion beams (GCIBs), and femtosecond (fs) laser ablation compare in terms of induced damage, preferential sputtering, depth resolution and suitability for different material classes, using representative examples (low-emissivity glass stacks, polymers, LiPON solid electrolytes and TiO2) to illustrate practical outcomes.

Methodology and used instrumentation

The paper describes three principal sample-removal approaches coupled to XPS:

• Monatomic ion beams (commonly Ar+), produced by ion guns that ionize low-pressure gas and accelerate ions to energies typically from ~100–200 eV up to 4–5 keV. Rastering generates an etched area larger than the analysis spot to avoid edge artifacts.
• Gas cluster ion beams (GCIBs), formed by supersonic expansion and ionization of gas to yield clusters comprising tens to thousands of atoms. Cluster mass selection and tuning of energy-per-atom allow gentle etching with limited penetration and reduced chemical damage.
• Femtosecond laser ablation, which uses ultrashort laser pulses to induce nonlinear ionization, rapid electron-lattice coupling and localized ablation with minimal thermal diffusion, enabling high-precision removal of material without significant chemical reduction or preferential loss of volatile species.

The instruments referenced include the Thermo Scientific MAGCIS Dual-Mode Ion Source (capable of both monatomic and gas-cluster modes) and the Thermo Scientific Hypulse Surface Analysis System, which integrates multiple removal modalities and fs-laser capability for flexible profiling strategies.

Main results and discussion

Key comparative findings presented in the paper are:

• Monatomic ion beams are versatile and historically standard for XPS depth profiling, providing controllable removal rates across a wide energy range. However, they can induce significant chemical damage (bond breaking, reduction) and preferential sputtering that distort surface chemistry and depth distributions—critical for polymers, alkali-containing materials and metal oxides.
• GCIBs substantially reduce chemical damage compared with monatomic ions by distributing the impact energy across many atoms in each cluster, limiting penetration and preserving fragile functional groups. Examples show GCIBs better preserve stoichiometry in LiPON solid electrolytes and reduce artifactual Li migration observed with monatomic sputtering.
• Fs-laser ablation offers an alternative where even GCIBs produce artifacts. For materials prone to preferential sputtering or reduction (e.g., TiO2 losing oxygen under ion bombardment), ultrafast laser removal can ablate material cleanly while leaving the underlying chemistry intact and enabling faster access to deeper interfaces.

The paper uses illustrative spectra and depth profiles: a low-emissivity glass stack profiled successfully with monatomic Ar+, polymer (polyimide) spectra demonstrating loss of functional groups after monatomic sputtering versus better preservation with clusters, LiPON profiles showing spurious Li accumulation with monatomic ions corrected by GCIBs, and TiO2 reduction under ion beams reversed by fs-laser ablation. These examples underline that the choice of removal method directly impacts the fidelity of chemical-depth information.

Benefits and practical applications of each method

• Monatomic ion beams: High sputter rates and broad applicability; suitable for robust inorganic multilayers and where some ion-induced modification is tolerable. Widely implemented and simple to operate.
• Gas cluster ion beams: Preferred for polymers, organic-inorganic hybrids and alkali-containing materials where preserving chemical state and functional groups is essential. Improved depth fidelity for delicate films and interfaces.
• Femtosecond laser ablation: Enables profiling of highly sensitive oxides and materials exhibiting preferential sputtering or reduction under ion impact. Provides rapid access to buried interfaces and minimizes chemical alteration of the remaining surface.

Practical profiling strategies benefit from combining techniques (e.g., using GCIB for shallow, chemically sensitive layers and fs-laser for deeper, damage-prone interfaces) and from instrumentation that supports modality switching without exposing samples to ambient conditions.

Future trends and possibilities for application

Anticipated developments and opportunities include:

• Integration of multimodal removal systems into a single UHV platform to enable workflow flexibility and in situ modality selection based on material response.
• Refined cluster-source engineering to control cluster size distribution and energy-per-atom with higher precision, improving depth resolution and reducing residual artifacts.
• Advanced laser protocols (pulse shaping, wavelength tuning, automated spot control) and hybrid laser/ion workflows for improved interface access and minimal collateral modification.
• Machine-learning-assisted depth-profile interpretation to deconvolve sputter-induced effects and reconstruct true compositional gradients by combining experimental metadata with models of preferential sputtering and damage.

These trends will expand XPS applicability to increasingly complex materials used in energy storage, flexible electronics, thin-film coatings and biomaterials.

Conclusion

XPS depth profiling accuracy depends critically on the method used to remove material. Monatomic ions remain useful but can misrepresent chemistry for sensitive materials due to damage and preferential sputtering. Gas cluster ion beams offer a gentler alternative that preserves chemical functionality in many organics and mixed systems. Femtosecond laser ablation further extends capability to systems where even cluster sputtering induces artifacts, enabling reliable interrogation of metal oxides and other challenging materials. Combining these approaches within a single analytical platform provides a practical path to robust, high-fidelity depth profiling across a broad materials landscape.

Reference

1. Baker MA, et al. Femtosecond laser ablation (fs-LA) XPS – A novel XPS depth profiling technique for thin films, coatings and multi-layered structures. Applied Surface Science. 654 (2024). doi:10.1016/j.apsusc.2024.159405