Strip Coated Materials Pre and Post ‘Cleaning’ Surface Analysis

Strip coating materials are typically employed in optic applications for the removal of small particulates and grease from the surfaces of delicate materials. Usually these encompass laser optics, mask gratings, telescope lenses and refractors. Using a small brush, a clear red solution which consists of a blend of polymers is applied to the material and then left to set. After it is set, the polymer coating is peeled away, resulting in a pristine surface which is particulate free.

The main part of this study was the extent to which surface contaminants were removed and if the polymer solution left any trace material on the surface. This could impact upon the optical/electronic properties of the cleaned surface.

Previous XPS studies^1,2 have analyzed the surfaces after application and discovered the surface was depleted in carbon contamination with little subsequent deposition. These XPS studies were carried out using old instruments where the sensitivity performance, and consequently, the detection limit, for low abundance materials is exceptionally smaller than instrumentation used today.

For this study the cleaning solution is applied to three different materials:

• Aluminum metal foil
• Silicon wafer
• Glass microscope slide

The adjustment in surface contamination will be explored and hence, the quantification, pre- and post-cleaning. The strip coating which has been utilized in this study is First Contact^TM Polymer solution which is manufactured by Photonic Cleaning Technologies.

Experimental

Utilizing the state-of-the-art AXIS spectrometer, XPS was achieved. Survey spectra were gathered over a large energy region of 0 to 1350 eV. To mitigate against the loss of photoelectrons and subsequent charge build-up, the coaxial charge neutralizer was employed. The samples analyzed were uncoated (virgin), coated with the solution (coated) and after removing the coating (post coat).

The First Contact^TM layer was applied to all of the surfaces using the technique set out by the manufacturer and then removed using the adhesive tape supplied. To ensure complete drying of the coating, samples were left for a minimum of one hour.

Results

Survey spectra were gathered for both the virgin and post-coated surfaces for the three systems (Al, Si and glass) after being introduced into the analysis XPS analysis chamber. Optical images were also obtained by utilizing the in-situ microscope (see fig.1). The optical images which were gathered for both the pre- and post-cleaned surfaces indicate an exceptional distribution of particulates existing on the virgin surface.

Figure 1. Optical microscope images of Si wafer surface pre (left) and post (right) strip coating cleaning.

Post strip cleaning, the amount of these particles significantly decreases. In Figure 2 the example survey spectra for the Al foil can be observed pre- and post-cleaning. The spectrum for the polymer coating which was applied but not removed is also included. The surface is mainly made up of oxygen and aluminum, formed by the oxidation of the aluminum metal, which is expected for this system.

Carbon is present in its adventitious form, so is a little amount of fluorine and nitrogen, most likely surface contaminants. Magnesium is identified, this is added to enhance material properties through solid solution strengthening and improves the foil’s strain hardening capability.

The survey spectrum shows a complete loss in the Al 2p and 2s signal after adding the coating, suggesting that the film coverage is 100% and that the film thickness is greater than the analysis depth of XPS. As expected, the elements which are present are C, O and N.

Figure 2. Comparison of survey spectra acquired for virgin (blue) coating applied (red) post coating (green) for Al foil.

The film became dry after 20 minutes and could be peeled away using the prescribed technique. The surface was then reanalyzed and the adjustments in surface concentration can be seen in Table 1.

The appearance of Na and Sn is the most significant – comparison post-cleaning spectra is detailed in Figure 3. These elements were discovered on all three strip cleaned surfaces and more importantly, neither element was observed on the original surface of the Si wafer or Al foil.

Table 1. Surface elemental composition of Al foil, Si wafer and glass slide virgin and post coating removal (red).

Figure 3. Comparison survey spectra for the three surfaces post-cleaning.

High-resolution spectrum of the Sn 3d region was gathered (Fig. 4) to further examine the presence of Sn. The peak position of the primary 5/2 transition is at 486.8 eV, typical for Sn⁴⁺ oxidation state perhaps SnO₂. The Na KLL Auger transition line is also present.

Notably, there looked to be little reduction in the overall carbon contamination found on the surface pre- and post-cleaning. This conveys that the strip process cannot remove the adventitious carbon which is adsorbed by exposure to the atmosphere.

Figure 4. High resolution spectra of the Sn 3d region for Si wafer post cleaning.

Conclusion

Strip coating material cleaning is an excellent technique for the removal of small mobile particulate present on delicate surfaces. The surface was left with residual contamination of Sn and Na after application of the strip coating polymer.

References and Further Reading

1. J. M. Bennett, L. Mattsson, M. P. Keane and L. Karlsson, Appl. Opt., 28, 5, 1989.
2. J. M. Bennett and D. Ronnow, Appl. Opt., 39, 16, 2000.

This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.

For more information on this source, please visit Kratos Analytical, Ltd.