Exploring the surface in depth with XPS analysis

Significance of the topic

Surface and near-surface chemical composition and structure determine functional properties in fields ranging from microelectronics and photovoltaics to coatings and polymers. X-ray photoelectron spectroscopy (XPS) depth profiling reveals how composition and chemical state evolve from the outermost layers into the bulk or across buried interfaces. Accurate depth-resolved chemical information is critical for failure analysis, materials optimization, regulatory compliance, and the development of multi-layered or composite materials.

Objectives and overview of the study

This document presents the principles, advantages, and applications of XPS depth profiling using the Thermo Scientific Hypulse Surface Analysis System. It describes conventional ion-beam approaches and their limitations, introduces femtosecond laser ablation (fs-LA) as an alternative ablation method, summarizes the Hypulse system’s analytical capabilities, and reviews representative case studies (metal coatings, perovskite photovoltaics, and polymeric coatings) that demonstrate fs-LA’s practical benefits for damage-free, high-throughput depth profiling.

Methodology

Depth profiling concept:

• Alternate material removal and XPS measurement to build a composition-versus-depth profile.
• Data from successive analysis steps are combined into compositional depth plots that identify interfaces and gradients.

Ablation methods compared:

• Monatomic ion sputtering: focused or rastered argon ions (energies typically 100 eV–5 keV). Good removal control but can cause ballistic damage, preferential sputtering (loss of lighter elements like oxygen), and chemical modification—especially problematic for organics and some oxides.
• Gas-cluster ion beams: clusters of tens to thousands of atoms reduce per-atom energy, lowering damage and depth mixing; better for organics but still can be slow for deep profiles.
• Femtosecond laser ablation (fs-LA): ultrafast pulses remove material via non-thermal mechanisms (Coulomb explosion, rapid solid–vapor transition) that minimize chemical modification of the residual surface. fs-LA enables much faster ablation per step, reducing total experiment time and allowing practical analysis to greater depths without the ion-beam–induced artifacts.

Instrumentation used

The Hypulse Surface Analysis System components and relevant capabilities described include:

• Base platform: Thermo Scientific Nexsa G2 surface analysis system offering XPS, with integrated REELS and ISS as standard; optional UPS and ARXPS.
• Femtosecond laser ablation module: 1,030 nm fs-laser source fully integrated for Class 1 operation in a standard lab; software control via Avantage.
• Ion source: Thermo Scientific MAGCIS Dual-Mode Ion Source capable of monatomic and gas-cluster ion sputtering, with helium capability for ISS and selectable cluster sizes.
• Detectors and acquisition: XPS with high sensitivity and spectral resolution; Snapshot Acquisition Mode for rapid spectra and Scanned Acquisition Mode for high-resolution scans; SnapMap and stage mapping for XPS imaging.
• Software and workflow: Avantage software for instrument control, spectral fitting, Knowledge View guidance, and offline processing.
• Accessories: automated sample handling, optional inert transfer to glovebox, sample heating, and safety-integrated laser enclosure.

Main results and discussion

Key technical findings and performance highlights:

• fs-LA reduces ablation time per level substantially compared with ion-beam etching, shortening overall profiling time and enabling deeper analyses (tens of micrometers) within practical timeframes.
• Because fs-LA removes material through ultrafast, largely non-thermal processes, it minimizes chemical damage and preferential sputtering. This preserves stoichiometry and chemical states in metals, oxides, polymers, and complex layered systems.
• Combined with XPS, fs-LA produced high-fidelity depth profiles for diverse materials: a nitrided steel with Ti-doped MoS2 PVD coating (preserved oxide and nitride chemistry across ~11 µm), lead-halide perovskite solar cells (accurate elemental layering without ion-beam–induced loss), and multi-layer polymer paint systems (clear separation of primer and topcoat chemistries and detection of minor contaminants).
• The MAGCIS ion source enhances versatility by permitting both monatomic and gas-cluster sputtering for materials where ion methods remain preferred, while fs-LA provides a complementary, damage-free option for sensitive materials.

Benefits and practical applications of the method

Practical advantages offered by fs-LA XPS on the Hypulse platform include:

• Damage-free depth profiling for polymers, organics, and delicate oxides—preserving chemical states that ion sputtering would alter.
• Faster ablation and therefore higher throughput for multi-level profiles, enabling routine measurement to greater depths and faster turnaround for R&D and QA/QC workflows.
• Improved representativeness of measured composition by avoiding preferential sputtering and mass-dependent removal artifacts inherent to ion beams.
• Integrated multi-technique capability (XPS, REELS, ISS, UPS optional) on a single platform supports complementary analyses without sample transfer.
• Software-guided workflows (Avantage) streamline acquisition, fitting, and interpretation for users across skill levels.

Future trends and potential applications

Anticipated developments and opportunities include:

• Broader adoption of fs-LA XPS for industrial failure analysis, coatings development, battery and photovoltaic research, and polymer/composite characterization.
• Integration of fs-LA depth profiling with automated, high-throughput workflows and advanced data analytics (multivariate analysis, machine learning) to accelerate materials optimization.
• Standardization efforts for calibration of depth scales and matrix effects specific to fs-LA to support inter-laboratory reproducibility and quantitative depth profiling.
• Further hardware and software integration to enable in situ or correlative multimodal experiments (e.g., combining fs-LA XPS with ion scattering, REELS, or downstream microscopy) and to expand operando studies under controlled environments.
• Continued optimization of laser parameters and spot control for improved depth resolution and minimal crater-edge effects, along with predictive modelling to relate ablation conditions to material removal rates.

Conclusion

Femtosecond laser ablation coupled to XPS, implemented in the Hypulse Surface Analysis System, addresses many limitations of ion-beam depth profiling by providing rapid, minimally damaging material removal. The integrated Hypulse platform combines flexible ion-sputtering options, fs-LA, multi-technique analysis, and advanced software to deliver accurate, high-throughput chemical depth profiles for a wide range of materials. This capability expands practical depth-profiling reach, improves data fidelity for sensitive materials, and supports accelerated materials development and diagnostics across multiple industries.

References

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. 2024;654. doi:10.1016/j.apsusc.2024.159405.
2. Chandler CW, et al. Femtosecond Laser Ablation (fs-LA) XPS Depth Profiling of Lead Halide Perovskite Thin Film Solar Cells. Surface and Interface Analysis. 2025;57(3). doi:10.1002/sia.7374.
3. Hinder SJ, et al. Surface and interface analysis of complex polymeric paint formulations. Surface and Interface Analysis. 2006;38(4). doi:10.1002/sia.2325.