CO2 snow cleaning has effectively assisted many scientists in their research, be it surface analysis methods (XPS, Auger, AFM), substrate cleaning of many different materials, basic surface science studies, and cleaning vacuum assemblies and parts.  The following sections focus on surface science and analysis. Vacuum system applications are here.

 

Surface Science

Most surface science literature mentions of CO2 snow cleaning involve particle removal from substrates, and we will skip over those.  Instead, we focus on literature where CO2 snow cleaning had an impact on the research. 

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Polymers - CO2 snow cleaning has assisted in basic surface science research in many areas beyond substrate and equipment cleaning.  Though substrate cleaning is the major use, and just worthy of a few words or a sentence in most papers, there have been times when CO2 snow yielded cleaner surfaces and led to insights or was a key step in the study, or the subject of the paper that led to better methods. Topics mentioned below include polymer adhesion, wetting, self-adsorbed monolayers (SAM) formation and cleaning, polymer dewetting, demixing, patterning, nanotubes, peptide adhesion, and improved methods.  Many studies involved AFM also.

 

Walheim and Steiner along with others et al. studied SAMs for demixing, patterning, and other aspects of polymers and SAMs on surfaces.  They were the first to publish a study using CO2 snow cleaning to assist in surface preparation for polymers on Si.  They were the first to show that CO2 snow cleaning does not damage or remove a self-adsorbed monolayer.  Jacobs and Herminghaus studied thin polymer films decohesion from Si wafers. They argued that dewetting nucleation from defects and not spinodal dewetting needs attention.  Chow et al. showed that CO2 snow can assist in creating and perfecting self-adsorbed siloxane monolayers.  Their work showed that CO2 snow plays an active role in removing the already hydrolyzed multilayers leaving just one layer chemisorbed on the surface, a SAM layer.  All physically bound overlayers can be removed.  This method has been used by others in similar research fields.  Checco and Ocko cleaned substrates before wetting and dewetting experiments.  The surfaces were hydrophilic and CO2 snow cleaning did not alter this chemistry.  They determined that preexisting contamination could alter the Hamaker constant and CO2 snow cleaning led to more accurate values.  Zhu et al. established a best practice experimental protocol to measure hydrodynamic forces with colloid probe AFM.  They suggested “in the future, … CO2 snow-jet cleaning procedure should be introduced in all force work.”  They showed that contamination can lead to larger slip values.  CO2 snow cleaning has been shown to be more effective than sonification and solvent for removing excess overlayers.  The above work was mostly all on polymeric systems, but these ideas have been extended to peptides and proteins.

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Methods – CO2 snow cleaning has assisted in several surface science methods and procedures. CO2 snow cleaning had an impact on nanotube research.  Structured nanotube arrays and nanotube based FET were made using a photoresist method to protect the nanotubes.  This initial work by Lay, Novack and Snow allowed for selective placing and protecting nanotubes by a photoresist process after nanotube deposition.  Photoresist removal except over the nanotubes of interest led to substrate cleaning of excess nanotubes and debris.  Finally, the protective photoresist was removed yielding the desired nanotube structure.  Other researchers have used CO2 snow to assist.

 

CO2 snow cleaning has also assisted in improving some surface science methods.  D. J, Morris investigated CO2 snow cleaning at nanoindentators and found cleaning both the diamond probe and surface with CO2 snow may be good practice.  Feng et al. used similar ideas to clean micro-CMM stylus tips and found reliable results.

 

Chernoff and Sherman cleaned dirty AFM standards and restored them to usefulness.  As part of this work, they showed no changes in step height due to cleaning and particle removal down to 3 nanometers.  Cleaning of AFM samples can be found on another page: Atomic Force Microscopy and XPS data can be found at various sections within Applications.   CO2 snow cleaning is a viable method for AFM analysis.  In another paper, Hugall et al. showed that CO2 snow cleaning can vastly improve the signal to background ratio on nanostructured Surface Enhanced Raman Scattering while also enhancing the signal.

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We have over 100 papers on the above topics from many authors and institutions.  We can share the referenced papers and other papers by email co2clean@co2clean.com

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Surface Analysis Applications

Early users of CO2 snow cleaning developed and documented applications within the surface analysis community. The early papers by Sherman used XPS to quantify CO2 snow cleaning effectiveness before and after cleaning wafers, metals, ceramics, polymers, and glasses, and even ‘clean’ surfaces. These papers documented the extent of contamination removal along with the lack of any surface chemistry alterations beyond contamination removal.

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Applications within the surface analysis field have included cleaning "clean" and contaminated samples.   Cleaning "clean" samples, to reduce the adventitious hydrocarbon background on surface have been explored and used by many scientists. Hydrocarbon reductions have been observed on many sample classes. Reductions as large as 50% have been noted on certain surfaces. The extent of hydrocarbon reduction by CO2 snow cleaning is comparable to high purity reagent grade solvents. Cleaning standards for AFM, XPS, Auger, and FTIR is common.

CO2 snow cleaning has been discussed as an acceptable sample preparation method for surface analysis as discussed by ASTM E-42 and others.  The removal of inadvertent or intentional contamination without altering the base material is sometimes required by surface analysts. Here, residual contamination from handling, field tests, or just atmospheric exposure can interfere or hide information.  Cleaning with reagent grade solvents can remove most inadvertent contamination, but there are cases where solvent cleaning is inadequate.   We had samples where the extent of hydrocarbon contamination was over 90%. In one example, CO2 snow cleaning removed enough hydrocarbons to allow the analyst to identify silicone based contamination on gold contacts.  In these and other cases, ultrasonic solvent cleaning may take too long, or may not be able to reduce the residues.   In one case, just 10 seconds of CO2 snow cleaning led to acceptable results for automotive and chemical samples.  The cleaning is probably a combination of solvent action and freeze fracture.  CO2 snow cleaning is considered the best method for micron and submicron particle removal for surface analysis samples.   There are limited examples involving Auger and SIMS.  There is even one paper involving RHEED.

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Carbon dioxide snow cleaning is used to clean the equipment.  Applications include cleaning electron guns, ion guns, dual plasmatron sources, X-ray anodes.  One paper showed reduced arcing after reassembly.   The tests have shown that snow cleaning can remove particles and organic residues quickly without much disassembly necessary.  Applications regarding vacuum systems are elsewhere.  Cleaning of AFM samples can be found on another page and XPS examples can be found at middle 3 links on the Applications page.

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We have references related to sample cleaning for surface analysis and surface science.   We can send the references, contact us at co2clean@co2clean.com

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