Theta Probe: Small Spot XPS Spectrometer with Parallel ARXPS Capability

Importance of the Topic

The characterization of ultra-thin films and complex multilayer structures is critical in fields ranging from semiconductor fabrication to surface engineering. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) offers depth-sensitive compositional information, while small-spot XPS and mapping modes enable high spatial resolution. The Thermo Scientific Theta Probe integrates these capabilities with a parallel ARXPS method, delivering rapid, non-destructive analysis across large samples and paving the way for advanced materials research and quality control.

Objectives and Study Overview

This application note presents the design, capabilities, and performance of the Theta Probe instrument. Key aims include demonstrating its parallel angle-resolved XPS (PARXPS) acquisition without sample tilting, high-resolution small area XPS (SAXPS), mapping and depth profiling functionalities, and comparing thickness measurements against established methods such as ellipsometry.

Methodology

Parallel ARXPS data are acquired simultaneously over a 60° angular range using a two-dimensional detector, eliminating mechanical tilting. Small area XPS employs a microfocused monochromatic X-ray beam with selectable spot sizes (15–400 µm). Mapping is achieved by rastering the sample stage beneath the fixed X-ray spot, producing elemental and chemical state images with per-pixel spectra. Depth profiling combines ion sputtering with ARXPS-based reconstruction via maximum entropy methods.

Instrumentation Used

• Electrostatic Radian lens with 60° acceptance and 180° hemispherical analyzer fitted with a 2D multichannel detector
• Monochromated Al Kα X-ray source with movable anode and external probe light for alignment
• EX05 ion gun with floating drift tube for low‐energy sputtering (down to 100 eV)
• Motorized five-axis sample stage, accommodating up to 70 mm × 70 mm samples and multi-sample holders including tilt/rotate options
• Avantage software for instrument control, data acquisition, spectral processing, thickness calculation, and multivariate analysis

Key Results and Discussion

PARXPS snapshots of SiO₂/Si samples yielded clear separation of oxide and substrate signals across emission angles. Overlayer thicknesses determined by Theta Probe matched ellipsometry with excellent linearity (slope ≈ 1), accounting for a known ~0.8 nm contamination offset. Layer sequencing of Al₂O₃/HfO₂/SiO₂ stacks was visualized in relative depth plots. SAXPS mapping of copper grids achieved ~8 µm spatial resolution. Multi-pixel mapping of masked tin oxide samples generated quantitative chemical maps and corresponding oxide thickness distributions. Sputter profiling of multilayer hard disk structures demonstrated high sensitivity and depth resolution.

Benefits and Practical Applications of the Method

Theta Probe’s parallel ARXPS method drastically reduces acquisition time by capturing angular and energy data in a single snapshot, enhancing throughput for depth profiling and mapping. Small spot capability allows targeted analysis of micro-features. Large sample accommodation and unattended multi-sample routines streamline QA/QC. Applications include corrosion studies, thin film uniformity, semiconductor bond pad inspection, and advanced coatings analysis.

Future Trends and Applications

Emerging developments may integrate faster detectors, machine‐learning-based spectral deconvolution, and in-situ environmental cells for real-time surface chemistry monitoring. Expanding angular and energy channel counts will improve angular resolution. Coupling with complementary microscopies and spectroscopies could yield multimodal datasets, driving innovations in nanomaterials, energy storage, and catalytic surface research.

Conclusion

The Theta Probe represents a versatile XPS platform, combining parallel ARXPS, small-spot analysis, mapping, and depth profiling in a single instrument. Its unique design accelerates data acquisition, enhances spatial and depth resolution, and supports robust quantitative analysis, meeting the needs of modern surface science and industrial laboratories.

Reference

• Thermo Fisher Scientific Application Note AN31014: Snapshot Spectrum Acquisition in PARXPS
• Thermo Fisher Scientific Application Note AN31038: Principles of X-ray Monochromator Operation
• Thermo Fisher Scientific Application Note AN31033: Small Area XPS Methodology
• Thermo Fisher Scientific Application Note AN31017: EX05 Ion Gun Description
• Thermo Fisher Scientific Application Note AN31005: Overlayer Thickness Calculator Principles