*     Ion sputter depth profiling in AES and XPS

 

The universal method of destructive composition-depth profiling applied in AES and XPS is through the sputtering of thin sections of the sample surface by energetic ion bombardment. Here, the analysis is carried out as a succession of ion sputtering and AES/XPS analysis cycles. The advantages of ion sputter depth profiling are:

 

1.      The information depth is variable (typically between 0.5 and 3 nm)

2.      The analysis is independent of the sputtering yield (for most cases)

3.      The influence of the matrix on the elemental sensitivity factor is small

4.      The analyzed area is small compared to the sputtered area, thus minimizing crater edge effects.

 

The quantification of composition-depth profiles by ion sputtering involves the conversion of the original measured peak area data (in XPS) or peak-to-peak height data (in AES) as a function of sputter time into an atomic concentration profile as a function of depth. This would require an accurate knowledge of the sputtering rate through the material so that a calibration of the depth scale can be performed. Likewise, the intensity of the XPS or AES signal must be calibrated in terms of a local elemental concentration. The goal here is to achieve a measured concentration profile as close as possible to the “true profile” if the sputtering proceeds in an ideal “layer by layer” removal of surface atoms from the sample. In practice, however, due to the various sputtering induced topographical and compositional changes of the instantaneous sample surface as well as changes of the electron mean free path, most analysis are reported as relative atomic composition profiles as a function of the sputter time. It is possible to get an accurate conversion of the sputter time into the depth scale, but this involves extensive measurements of the sputter rate of standard reference materials with accurately known thickness and closely identical composition to the unknown sample. Some of the factors that mitigate the accuracy of a sputter depth profile analysis include:

 

  1. Instrumental factors: recontamination or readsorption of sputtered atoms from the background gas during sputtering; non-uniformity in the sputtered area (static versus rastered ion beam) broadens the depth resolution; variations in the ion beam energy and intensity (flux), and the selection of the photoelectron or Auger electron energy and take-off angle must be considered to minimize the IMFP.
  2. Sample characteristics: surface roughness; crystalline structure and defects; preferential sputtering of one component over the others in a compound or alloy sample; sample surface charge build up due to positive ion implantation into insulators
  3. Radiation induced effects: ion induced mixing of surface atoms between layers; sputter-induced microtopography; decomposition of compounds; radiation induced chemical reaction of surface species; radiation enhanced diffusion and segregation; electron-induced desorption of surface atoms in AES

 

 

 

Suggested reading:

 

*      Dr. Roger Nix’s AES depth profiling chapter in his Introduction to Surface Chemistry Course