Cleaning of a contaminated surface with the ToF-SIMS argon cluster gun

The fingerprint or dactylogram is our genetic visa, the probability that two people have the same fingerprint is indeed one chance in 64 billion. The Babylonians used it as early as 5000 BC as a signature on prehistoric pottery. In 1902, the first judicial use of fingerprints was made in a murder case in France. Today, they are still considered to be a key piece of scientific evidence in solving criminal cases. The mark left each time a finger is placed on a surface is an irrefutable means of identification.

The dermis, which makes up 90 to 95% of the skin, contains more than 5 million sweat glands that produce sweat and latent fingerprints. The latter are made up of around 346 organic and inorganic compounds.

In contrast to the forensic environment, in the field of physicochemical characterisation of materials, fingerprints are far from being considered an aid. They are a source of exogenous contamination of samples and distort the results obtained. 

ToF-SIMS is a surface analysis technique that is very sensitive to all types of contamination. To analyse a contaminated surface, cleaning with argon clusters is necessary.

The ToF-SIMS can detect traces of elements down to ppb at the extreme surface (< 1 nm) and up to 20 µm in profile mode on samples of all types compatible with vacuum.

The abrasion speed of the different guns can be modulated and adapted according to the density and chemical nature of the abraded layers; it must be measured for each medium traversed. It is therefore necessary to calibrate the instrument by using deposits of known thickness or by measuring the depth of craters using optical (interferometry) or mechanical (Talystep) methods. Abrasion of the samples will allow to carry out analyses in profile mode but also to clean the surface of samples contaminated by different sources before the analysis.

TESCAN ANALYTICS has more than 20 years of expertise in the use of ToF-SIMS on all types of materials, insulating or conducting... With the latest generation instruments, our team of experts works with all industrial sectors.

 

Figure 1: Before cleaning - MacroRaster image, positive ions, 10 x 10 mm2



Figure 2: MacroRaster image, positive ions, 2.5 x 2.5 mm2




Figure 3: After cleaning - MacroRaster image, positive ions, 2.5 x 2.5 mm2

Figure 1 shows the MacroRaster images (10 x 10 mm2 ) of the positive ions on the contaminated surface of our sample. The fingerprint is clearly visible on all images, thus camouflaging the results of interest of the sample surface, i.e. the characteristic PET ions. The image of the surface was then acquired with a size of 2.5 x 2.5 mm2 , thus revealing a greater level of detail of the fingerprint, Figure 2.

When our team of ToF-SIMS engineers notice such contamination on samples to be analysed, the decision for surface cleaning is taken in consultation with our customers. The GCIB argon cluster gun is then set up so that the abrasion gently cleans the first monolayers of contamination without destroying the chemical signature of interest of the PET surface. The corresponding depth profile is shown next.
In the case of this application, a square of 1 x 1 cm2 was defined in the centre of the print as the area to be cleaned. In Figure 3, the images show the sample after the targeted area has been cleaned, illustrating the disappearance of the contaminants from the print (fatty acid triester, C22H48N, PDMS) and the appearance of the ions characteristic of PET film. The difference in results between the cleaned and uncleaned areas is evident. The trace left by the human hand is no longer visible on the cleaned area.

In this example, the use of the GCIB argon cluster gun to remove surface pollution from an organic sample was illustrated.

These results demonstrate the effectiveness and accuracy of the Argon Cluster Gun in cleaning a contaminated surface.

In other works, the ToF-SIMS profile mode is used to obtain the chemical composition of the different layers making up a stack to a depth of up to 20 µm.

For more analysis applications with ToF-SIMS or our other techniques, click here.

The XPS also has a gun which, like the ToF-SIMS, allows it to clean the first monolayer.