X-Ray Photoelectron Spectroscopy Instrumentation Features

The most crucial component of a modern XPS instrument is its software, which handles all aspects of operation, data interpretation, and reporting. All Thermo Scientific XPS systems use Thermo Scientific Avantage Software for instrument control, data processing, and reporting. Whether working in a dedicated research lab or in a multi-user environment, analysts of all abilities can rely on the flexibility, feature set, and intuitive operation of Avantage Software to obtain the maximum information from their samples.

• Instrument control for all Thermo Scientific surface analysis systems:
  • Thermo Scientific K-Alpha XPS System
  • Thermo Scientific ESCALAB QXi XPS Microprobe
  • Thermo Scientific Nexsa G2 Surface Analysis System
• Fully flexible experiment design, including multi-technique integration
• Import and export of analysis positions for SEM correlation
• Automated data acquisition including data processing functions and reporting
• Thermo Scientific Knowledge View, references, and intelligent algorithms for data interpretation
• Advanced data reduction tools
• Easy export to reporting software

Unravelling the complexity of surfaces and interfaces can be a daunting task, especially for newcomers to XPS and surface analysis. Within Avantage Software, Knowledge View acts as a comprehensive guide to all aspects of the analytical process, including:

• Sample mounting
• Experiment design
• Data processing
• Material-specific workflows
• Element-specific guides

Knowledge View is built upon decades of surface analysis experience and is always available within Avantage Software to help you do your best work.

Ultra-high vacuum (UHV) is required in the analysis chamber to ensure that the emitted photoelectrons can travel from the surface to the detector without losing energy through a collision with a gas molecule and to maintain sample surface cleanliness during analysis. The UHV condition is usually achieved by using a combination of pumps to remove gas molecules from the vacuum chamber. A load-lock or pre-pumping chamber is usually attached to the system so that samples can be introduced without exposing the analysis chamber to air. If the analysis chamber needs to be vented for service, it is typically heated to a temperature above 100°C while pumping down to remove adsorbed water, which would limit the achievable vacuum.

Modern lab instruments generally use a monochromated X-ray source that gives a narrow spread of X-ray photon energies, which results in better resolution of the peaks in the spectrum. The X-rays are usually Al K-alpha radiation generated by electron bombardment of an aluminum target. The X-ray beam is monochromated using a quartz crystal, which has a lattice spacing suitable for Bragg diffraction of the desired photon energy. The crystal is generally shaped to refocus the X-ray beam to a point on the sample.

The electron analyzer is a multi-component part, comprising the lens system, hemispherical analyzer, and detector. The lens collects the emitted photoelectrons, steering and focusing them into the entrance of the analyzer. The hemispherical analyzer acts as a band-pass filter, allowing photoelectrons with a particular kinetic energy (the pass energy) to reach the detector. Electrons with an energy above the pass energy will strike the outer hemisphere, while those with a lower kinetic energy will hit the inner hemisphere. The detector counts the signal. Data is collected in a serial fashion by changing the kinetic energy that can reach the detector along a range to create a spectrum.

The ion source is used to gently remove material from the surface of the sample, either for cleaning of adventitious contamination or for depth profiling. In the past, most XPS systems were equipped with a monatomic ion source, which bombards the surface with single ions. These types of sources are standard equipment on an XPS system. However, monatomic ion beams can damage polymers and some other materials. A source that can use gas cluster ion beams (GCIB), such as the optional Thermo Scientific MAGCIS Ion Source, can avoid this problem. A GCIB source uses large clusters of gas atoms (typically around 2,000 atoms), which include just one charged particle, to bombard the surface instead of a single ion. This minimizes the damage to the surface chemistry, allowing depth profiles of materials that otherwise could not be analyzed.

The essential piece of equipment for insulating material analysis is a low-energy electron flood gun. This allows analysis of insulating samples by replacing the emitted photoelectrons and preventing a positive charge from building up at the surface, which would distort the spectrum. Without the flood gun, it would be impossible to get good, high-resolution spectra of oxides, polymers, or other non-conducting materials.

With the increased size of sample holders permitting several specimens to be introduced into an instrument at one time, automated sample loading and positioning has become a key feature of XPS systems. This capability permits both automated and remote operation, increasing the analytical capacity of the tool. In addition to the sample handling capabilities, it is also essential to be able to see the samples optically in the system to identify the features of interest. Typically, instruments use stored views of the whole sample holder for navigation from sample to sample, and a live, in-chamber view for fine positioning.