Cleaning prior analysis
When is it useful?
The combination of high sensitivities for elements and molecules with extreme surface specificity (information depth of approx. 1 nm) makes ToF-SIMS an ideal screening method for the resolution of surface-specific issues. However, in order to be able to successfully use the method even in the case of unwanted surface contamination caused by handling after sampling, packaging or transport, it is necessary to clean the surface before the actual analysis. Such cleaning should meet the following criteria:
- targeted removal of contamination without damaging deeper structures
- flexible cleaning depth
- rapid processing
In-situ Ar-cluster cleaning of contaminated surfaces
In the past, chemical removal (flushing) or mechanical (scratching, cutting) cleaning approaches were usually used to eliminate contamination in the area of surface analysis. However, these types of preparation entail the danger of new contaminations, they are time-consuming and can only be used to a limited extent in structured systems (for example semiconductor structures). In contrast, in-situ cleaning by bombardment with Ar cluster ions allows particularly efficient, gentle and rapid removal of surface contaminants. By choosing suitable bombardment conditions, it is possible to selectively remove molecular contamination layers with thicknesses from a few nm to many μm, without damaging deeper-lying molecular structures.
The following example of a component based on polypropylene (PP) illustrates this mode of action of the in-situ Ar cluster beam. After prolonged storage, the surface of the black injection-molded component was coated with a visually discernible, dark coating, which in investigations using confocal interferometry represented a crystalline material with mean heights of about 10 μm (see round figures).
Additional ToF-SIMS analyses should be performed to identify the chemical composition of this overlayer. A direct analysis of the surface in an area with optically recognizable residues (red mark in the micro image and Images upper line) provided evidence of a homogeneous polysiloxane (PDMS) layer but no evidence of crystalline structures. However, after only a few seconds of in-situ Ar cluster cleaning of the sample surface it was possible to expose deeper layers and to image glycerol monostearate (GMS) containing domains (images bottom line). A comparison of the imaged GMS structures with the microscopic images clearly shows that the lateral distribution of the GMS coincided with the optically discernible discoloration.