Most commonly in XPS instruments the electron energy analyser is positioned so that most of the detected electrons have originated from the sample surface with a trajectory in line with the surface normal. This is called normal emission, and the electrons are said to have a take-off angle of 90° to the surface.
Under these conditions XPS is the most bulk sensitive it can be for a given X-ray energy, with an information depth generally less than about 10nm. It also tends to give the maximum signal, and the geometry is generally well-understood.
Surface sensitivity can be increased
Surface sensitivity can be increased in XPS by varying the angle at which electrons are detected from the surface. This technique is known as Angle-Resolved XPS (ARXPS) and usually requires that samples be tilted to a range of angles from 0° to maybe 80°.
Electrons can only travel a certain distance through a material before some sort of interaction occurs, which is governed by the inelastic mean free path (IMFP) of the electron. By increasing the tilt angle, and therefore increasing the emission angle, we are effectively reducing the information depth of the technique. This is because we are forcing the detection of electrons that have had to travel through more of the material laterally, and therefore can only have escaped from a shallower depth.
This is depicted in the above three graph schematic for the very similar technique of Parallel ARXPS (PARXPS) as employed by the Theta Probe XPS instrument at NEXUS. In this instrument there is no need to tilt the sample, as electrons from emission angles of 20-80° are collected simultaneously.
In this schematic the maximum information depth is indicated by the length of the dashed line in the central graph into the sample surface, and is directly determined by the IMFP. As the emission angle is increased, the length of this line remains the same, as it is governed only by the IMFP, however the depth into the sample that is reaches is now reduced.
At 80° the technique is at its most surface sensitive, and at 20° it is at its most bulk sensitive. By comparing spectra collected at a range of angles, we can identify where a certain species is located in terms of depth. This enables us to:
- perform depth profiling of chemical species without destroying the sample
- calculate the thickness of an overlayer such as an oxide
In the spectra provided, we can see how oxide and metal components on a sample change in relative intensity with emission angle. Since the metal peak is strongest at 20° and weakest at 80°, we can say that the metal is in the bulk, and the oxide is at the surface (as expected).
What can it do for me?
It can:
- provide non-destructive depth-profiling of the top 10nm of a surface
- provide thin-film thickness measurements
- clarify contributions in spectra that come from surface versus the bulk
What are the typical applications?
Typical applications include:
- depth-profiling through thin layers and interfaces with a calculated depth scale
- thin oxide or overlayer thickness measurements
- stratification of multiple thin layers on a substrate
