Surface Science Applications

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.  Next, they cleaned a self-adsorbed monolayer without damage or removal.  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 cleaning 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 overlayers8.  The above work was mostly all on polymeric systems, but these ideas have been extended to peptides and proteins.

 

Methods - 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 good 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.  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.  

 

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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Vacuum Technologies Applications

Numerous applications of CO2 snow cleaning vacuum technologies were developed including cleaning stainless steel vacuum components, electron, ion and x-ray optics, residual gas analyzers (RGA), UHV and HV parts, and many others.  Hydrocarbon removal from electropolished stainless steel parts is as effective as high purity reagent grade solvents such as acetone and methanol. For small complex structures CO2 snow cleaning is better than air blowing or ultrasonics, as discussed below.  Snow cleaning can remove the machining oils, particle residues, and other debris from the stainless steel surfaces and even weld rods, tips, and surfaces before welding.

 

Cleaning vacuum assemblies also works well to remove contamination left from assembly or repair. An early example was given by Layden comparing CO2 snow cleaning to solvent cleaning on a RGA. Initial pump down times were slow after solvent cleaning, which involved total disassembly and ultrasonically cleaning. Repeated solvent cleaning led to an improved pump down time but still not as fast as needed. For CO2 cleaning, just the filament, grids, and aperture were removed. The remaining assembly was cleaned as one piece. Analysis of outgassing elements after solvent cleaning showed hydrocarbons and alkaline based contamination; and after CO2 snow cleaning, these peaks were reduced or eliminated. This demonstrates the ability to clean complex vacuum equipment with CO2 snow.  The same concepts here can be applied to other electron and ion sources, and many other vacuum parts.  Another set of comparable results have been shown by Vondrasek in several papers related to an ECR for a breeder reactor.  Work done by many high energy physics labs mirror these results. 

 

One user in 1993 has remarked that CO2 Snow Cleaning a SIMS source after rebuilding led to reduced high voltage arcing when energized. This was attributed to reduced particle populations that can be sources of high voltage discharge. A similar result has been mentioned for high voltage equipment at William and Mary (unpublished) for vacuum equipment. These later results were taken and expanded for RF cavities at Jefferson Lab2.  In fact, this has been used when cleaning similar synchrotron RF cavities in Europe4.  This application was documented by Dangwal3,5 and included data on removing field emitter spikes.  Other high energy physics sites have incorporated these cleaning procedures6,7.   Similar work has been done on other instruments and parts related to high energy physics and other areas 8,9.  CO2 snow has also been used to clean large vacuum chambers using both the standard and LASU nozzles. 

 

Jantzen et al.10 studied CO2 snow cleaning of cast, small, and complex machined parts.  They compared particle removal efficiencies of blowing air, ultrasonics, and CO2 snow cleaning on these small parts.  Overall, CO2 snow cleaning outperformed the other methods for particle removal regardless of material or geometry.  This implies that CO2 snow cleaning can assist in cleaning HV and UHV parts. 

 

A customer involved in vacuum system fabrication relayed information that CO2 cleaning as a prep step before welding led to less staining near the weld zones and less weld defects during fabrication.  There are patents on cleaning welding equipment.

 

The cited references can be obtained from co2clean@co2clean.com

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Surface Analysis Applications

CO2 snow cleaning applications within the surface analysis community were developed by early users and customers. The early papers by Sherman used XPS to quantify CO2snow 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, in order 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 https://www.co2clean.com/metals .  In these and other cases, ultrasonic solvent cleaning may take too long, or may not be able to adequately 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.

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 {{above HERE}}.

Cleaning of AFM samples can be found on another page https://www.co2clean.com/afm and XPS examples can be found at first 4 links on the Applications https://www.co2clean.com/applications

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