Surface coatings and thin films are of great commercial importance in many industries and are used to enhance or provide required properties to bulk materials specific to their applications. Thin films can range from tens of Angstroms to several microns in thickness and find application in areas as diverse as optical anti-reflective coatings, architectural glazing and drug eluting thin films in the pharmaceutical industry. X-ray photoelectron spectroscopy (XPS) is ideally suited to characterising the surface chemistry of these thin films and when combined with sputter depth profiling can be used to determine the elemental and chemical composition as a function of depth through the film using either the AXIS Supra or AXIS Nova spectrometers.
X-ray photoelectron spectroscopy
The use of monochromated Al K-alpha (1486eV) excitation source for XPS characterisation 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 and losing energy. For Al K-alpha excitation the generally accepted information depth is 10nm. The technique can be made more surface sensitive by rotating the sample relative to the collection lens, 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 L-alpha excitation (HAXPS)
Kratos has developed the application of higher energy excitation sources, such as the monochromated Ag L-alpha (2984.2 eV) with the advantage that the information depth can be extended to the 15–20 nm range. The 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 Ka source can be measured with the advantage that greater chemical shifts may be observed. Interestingly, in an Ag L-alpha excited spectrum there are photoelectrons from different electron shells such that the high binding (low kinetic) energy electrons will be more surface sensitive than the low binding (high kinetic) energy photoelectrons inelastically scattered from deeper within the material. When using higher energy X-rays it is possible to depth distribution information for elements and their chemical states from the same spectrum, which is often one of the motivations for using much more expensive synchrotron radiation sources.
Into the bulk – Sputter depth profiling
Extending the information depth range to the bulk material beyond that accessible by lab. X-ray sources and electron escape depths requires the use of ion sputter sources to remove material prior to XPS analysis. The development of gas cluster ion sources for sputter depth profiling has caused a step-change in XPS analysis of materials. With conventional monatomic Ar+ depth profiling it is widely understood that the sputtering process, even with relatively low energy (500eV) ions, results in significant chemical damage to the material. The 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. Being able to select n, the number of atoms in the cluster, and the acceleration energy of the cluster ion gives flexibility to select the energy per atom, or partition energy, appropriate for the material that is being depth profiled. 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 with retention of 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 so that partition energies between ca. 1 and 20 eV may be used.
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’ material.