XPS – Analysis Beyond the Surface

Kratos Analytical Ltd, a wholly owned subsidiary of Shimadzu Corporation, has been manufacturing surface analysis instruments since 1969. Kratos continues to lead the development of new technologies relating to X-ray photoelectron spectrometers and associated accessories for surface and materials characterization.

Overview

X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis), is a surface-sensitive technique capable of providing quantitativechemical-state information from the uppermost 10 nm of a material’s surface. The technique uses incident X-rays to excite photoelectrons, which are ejected from core levels within the elements present in the material. The number of electrons and their binding energy provide quantitative and chemical-state information. Recent developments from Kratos Analytical, such as the laboratory-based high-energy X-ray source and gas-cluster ion source for sputter depth profiling, allow XPS analysis deeper into the bulk of materials. Here we present a brief overview of the exciting potential of these technologies applied to materials surface characterization.

X-ray photoelectron spectroscopy

The use of a monochromated Al Kα (1486 eV) excitation source for the XPS characterization of materials yields quantitative information of all elements, excluding hydrogen and helium. The surface sensitivity of the technique arises from the distance that the photoelectrons can travel through the material without interacting with other atoms. For Al Kα excitation the generally accepted information depth is 10 nm, which can be made more surface sensitive by rotating the sample relative to the collection lens, the so-called angle-resolved XPS. In such experiments the information depth can be decreased to 1–3 nm. Using Kratos’ maximum entropy modelling software it is possible to recreate a concentration depth profile from this data.

Looking deeper – Ag Lα excitation

Kratos has developed higher energy excitation sources, such as the monochromated Ag Lα (2984.2 eV) with the advantage that the information depth can be extended to the 15–20 nm range. Increased information depth arises by virtue of the greater energy of the excited photoelectrons.

The greater excitation energy also means that elemental core levels not accessible using the conventional Al Kα source can be measured. Interestingly, in an Ag Lα-excited spectrum there are photoelectrons from different electron shells where the low kinetic energy electrons will be more surface sensitive than the high kinetic energy photoelectrons inelastically scattered from deeper within the material. When using higher energy X-rays it is possible to obtain the depth-distribution information for elements and their chemical states from the same spectrum, often one of the motivations for using less accessible synchrotron radiation sources.

Into the bulk – sputter depth profiling

Extending the information depth into the bulk material requires the use of ion sputter sources to remove material prior to XPS analysis. The recently developed use of massive Arn+ gas-cluster ions as the projectile allows successful sputtering of “soft” organic materials with the retention of chemistry throughout the depth profile. For organic polymer materials large clusters with low acceleration energies are useful as the material is readily removed and the retention of the chemistry is important. For inorganic materials including metal oxides it is necessary to use smaller clusters with high acceleration energies (20 keV) to achieve a reasonable sputter rate whilst retaining the chemical bonding within the material. Kratos’ gas-cluster ion source (GCIS) allows the selection of cluster sizes up to 5000 and acceleration voltages between 2.5–20 keV, resulting in ca. 1–20 eV per atom for optimized sputtering.

From surface to bulk

XPS is unique in being able to generate quantitative, chemical-state information from a wide range of conducting, semiconducting and insulating materials. The information can be extremely surface sensitive, probing the outermost 1–3 nm of the surface by angle-resolved XPS. Higher energy X-ray excitation sources can be used to generate information from the near surface, up to 20 nm, whilst the destructive sputter depth profiling using Arn+ gas clusters can provide XPS data from several microns into the “bulk” materials.

Kratos Analytical Ltd, a wholly owned subsidiary of Shimadzu Corporation, has been manufacturing surface analysis instruments since 1969. Kratos continues to lead the development of new technologies relating to X-ray photoelectron spectrometers and associated accessories for surface and materials characterization.

 

 

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Web www.kratos.com

 

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