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Atomic Force Microscopy
Several customers have developed and shown that CO2 snow cleaning is effective for AFM samples. The work includes studies by William Morris at GE R&D, scientists at the former Digital Instruments, and Ray Eby. Since then, many other scientists have used CO2 snow cleaning as a sample preparation for AFM samples. Applications include cleaning standards and sample preparation. Bill Morris of GE R&D was the first to use carbon dioxide snow cleaning as an aid in cleaning AFM samples. He found that it removed particles and reduced the "nanoscum" on the surface. Bill said that he got better images and less tip-substrate artifact events. Ray Eby found better imaging and cleaner samples on polymers and Si wafers. Applied Surface Technologies also thanks Dr. Karin Jacobs for the elegant quartz work shown below and the Park Scientific employee at a trade show for the gold images. Lastly, Chernoff and Sherman looked at CO2 snow cleaning of an AFM step height standard and showed that cleaning is effective down to 3 to 5 nanometers and that contaminated standards can be returned to pristine conditions,
AFM Step Height Standard
Chernoff and Sherman in 2010 (Chernoff and Sherman, D. A. Chernoff and R. Sherman, J. Vac. Sci. Technol. Vol B 28, 2010 “Resurrecting dirty atomic force microscopy calibration standards”demonstrated that contaminated AFM step height standards can be cleaned without any material removal. In other words, the step height after cleaning was within the original value. Particle removal was seen down to 3 to 5 nanometers, as discussed in the paper (see Bibliography). The initial image below shows a heavily contaminated surface in both height and phase. Scan was 15 microns and height scale is up to 50 nanometers.
After CO2 snow cleaning, we see that the contamination is gone, even down to 3 to 5 nanometers. Furthermore, the step height remained unchanged.
The best example of image improvement was from the above publication - D. A. Chernoff and R. Sherman, “Resurrecting dirty atomic force microscopy calibration standards”J. Vac. Sci. Technol. Vol B 28, 643, 2010. Here a grid image has many artifacts and after CO2 snow cleaning, it is free of artifacts. This image is typical of how CO2 snow cleaning can improve imaging by removing the "nanoscum" and trace impurities.
These images are courtesy of the research group of Prof. Stephan Herminghaus now at Univ of Ulm. The researchers were studying polymer film dewetting from silica surfaces and the first step in their process was to cut and clean the quartz substrates. They ultrasonically cleaned the quartz samples in spectroscopic acetone, ethanol, and toluene. Initial AFM examination of the quartz films revealed extensive contamination all over the samples. In this image, many particles ranging in size as small as 15 - 20 nanometers are seen in the 2µ by 2µ image. The CO2 snow jet and was able to clean the surfaces to acceptable levels as in the image. Imagesw can be magnified.
Before CO2 Snow Cleaning
After CO2 Snow Cleaning
The importance of clean surfaces in research cannot be neglected as illustrated in the image set below from Dr. Jacob's (a member of the above group) thesis. The top image was typical of some of the ultrasonically cleaned surfaces. Wiping removed some particles; snow cleaning led to almost total cleaning. After spin coating with polystyrene and annealing the films, extensive "dewetting" or polymer film decohesion is seen for the "wiped" surface while the CO2 snow cleaned surface shows no signs of film failure. The results were used to question the standard spinodal dewetting mechanisms for polymer film failure, and instead, a mechanism related to surface defect densities was suggested. Further details are in Langmuir, Vol14, pages 965-969, 1998.
Cleaning nanoprobes always present a problem. They are small and delicate. Several researchers and users have used the snow jet to clean these probes. Dylan J. Morris at the NIST published a paper detailing the results. The reference is Cleaning of diamond nanoindentation probes with oxygen plasma and carbon dioxide snow, REVIEW OF SCIENTIFIC INSTRUMENTS 80, 126102 (2009). He found "CO2 snow cleaning was found to much more effective at particle and light contamination removal than solvent washing” and "the results presented here suggest that CO2 snow cleaning of diamond probe and material under test may be good nanoindentation practice” Later work by Xiaobing Feng et al. using special nozzles from us displayed nanoprobe cleaning. The paper is “Development of CO2 snow cleaning for in-situ cleaning of micro-CMM stylus tips” Measurement Science and Technology 28, (2017) 015007. Below is an example of probe cleaning from another user.