Assessing surface cleanliness can involve a range of different measurement approaches, depending on the surface and material of interest. It may also involve comparing samples that were processed differently or exposed to different environments. The differences between the test sample and control or reference sample are often best highlighted by this approach. For example a very sensitive measurement may reveal that cleaning chemicals left residues behind because they were not rinsed properly. A less sensitive measurement technique may not have detected the residue.
Assessing cleanliness generally requires some understanding of the possible sources of the contaminants:
- Are elemental or molecular contaminants of interest?
- Is the presence or absence of material to be confirmed?
- Is there a specific level that species of interest must be below?
Investigating very clean surfaces may require techniques that have detection limits in the parts-per-million range, such as Total Reflection X-ray Fluorescence (TXRF), which detects elemental species only, or Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), which looks at both elemental and molecular contaminants. In cases where cleanliness is measured by the number of particles on the surface, Scanning Electron Microscopy (SEM) may be a good option, for both surface imaging and particle counting.
Primary Analytical Techniques
- X-ray Photoelectron Spectroscopy (XPS) and Electron Spectroscopy for Chemical Analysis (ESCA)
Secondary Analytical Techniques
- Auger Electron Spectroscopy (Auger)
- Fourier Transform Infrared Spectroscopy (FTIR)
- Gas Chromatography Mass Spectrometry (GC-MS)