Successful CO2 snow cleaning, as for any cleaning process, requires attention to process parameters and cleaning methods. The major factors to consider include recontamination risks, moisture condensation, CO2 purity, static charge, cleaning methods, potential damage, and more. By addressing these and other issues, CO2 snow cleaning becomes routine and relatively simple. This makes it possible to scale up a lab method for production.
Recontamination risks of any cleaning process must be investigated before implementation. There are four major sources for recontamination - particulate redeposition, carbon dioxide impurities, the cleaning equipment and fixtures, and the cleaning process.
Particle Redeposition - Once a particle is removed from the surface, the set up must insure that it is swept away and not redeposited on the cleaned surface. Generally, the high CO2 gas high flows insure this given adequate space. If cleaning in a hood, insure adequate space and flows. Make sure you always clean "into dirtier" areas and never spray towards areas already cleaned. For table top systems, just insure the stream can flow away from the cleaning area without striking other objects.
Cleaning Equipment - Recontamination from the cleaning system itself, the fixtures and materials used to hold the sample, nozzles, and environmental chamber, can occur unless attention is paid to material selection, and surface finishes. Material selection is important. The material choice and surface finish must be specified. Cleaning fixtures and other items within the cleaning region is suggested.
Cleaning Method - The cleaning process, after removing particles and organic residues, must not allow for these particles to land back on the part that was cleaned. To insure this in critical cleaning operations, several steps should be taken, including:
If using a hood, the make sure the HEPA filters and flows are operating.
Clean all sample holders and other items with CO2 snow .
If using additional purge gases, make sure they are dry and flow away from cleaned areas.
The cleaning direction will be from a clean area to a dirty area.
No uncleaned parts will ever be allowed to pass over cleaned areas of the sample
Your experience and thinking always help.
CO2 Purity - This was investigated separately in two studies, Whitlock in 1989 and Sherman in 1994. Initial tests in a cleanroom on new wafers determined that residual heavy hydrocarbon within the cylinder can recontaminate. The best results at the time (1989) showed about an increase of about 1 submicron particle per square centimeter of a heavy hydrocarbon-based compound. High temperature baking in air reduced this particle population also suggesting a hydrocarbon-rich contaminant. The sources are believed twofold, hydrocarbon impurities in carbon dioxide or from the cleanroom ambient air. Improved purification methods and operating methods since then will yield better results. Since then Bowers has shown better results.
Sherman in 1994 studied the effects of different CO2 grades; sources included welding grade, food grade, instrument (Coleman) grade, and supercritical fluid chromatography grade (SFC). The cylinders were all gas fed except for one listed as "liquid". XPS was used on the same region for new Si wafers, and then analyzed after CO2 snow cleaning.
Results showed that the SFC grades led to cleaning of a clean wafer while other grades led to increases in surface carbon content. Liquid CO2 source led to a higher increase than the same CO2 gas source. It should be noted that users report excellent results even with non SFC grades. Contact us for details.
A proper cleaning setup is vital and safety is paramount. This includes a cylinder properly secured or external source with proper safety shut off connections. All fittings should be checked to make sure they are tight and no leaks are in the lines. Your sample should be secured, and heated as necessary to minimize moisture condensation on the surface. As discussed in our direction (Email Us for a copy), the actual setup and operation is simple and straightforward. Proper cleaning requires attentions to the issues discussed above and in the following. Cleaning should be systematic, from a region already cleaned to regions waiting cleaning. Never have the spray aimed at a dirty surface so that the contamination removed can land on a cleaned area. Never use a "here and there" approach to cleaning. Complete, systematic cleaning methods are needed.
The cold CO2 snow stream lowers the surface temperature and room moisture can condense. Generally, moisture condensation does not interfere with simple cleaning in some applications, unless the moisture "freezes" or stays on the surface too long. Cleaning setups usually have a method to minimize moisture condensation - the easiest method is use a hot plate or heat lamp as part of the setup. Usual set point for the hot plate set about 40 – 50 C, or a hot air gun, but note higher temperature may be necessary. The samples can be held on the hot plate by vacuum chucks or other special rigs. Generally, for samples with good thermal conductivity, moisture condensation does not occur. For samples with poor thermal conductivity, such as thick glass, an overhead hot air gun or lamp can be used. Other good choices for moisture control are dry boxes, enclosed hoods or environmental chambers that are purged or heated - all have been successfully incorporated. In critical cleaning situations, i.e. submicron particle removal from wafers, cleaning should be done in dry chambers.
Critical cleaning applications require filtering the CO2. The cylinders, hoses and all the equipment have particles and the flowing gas or liquid will transport these particles out of the nozzle and on to the sample. Generally, the cleaning process will remove these particles, but there are risks that some will stay or land nearby. For critical cleaning applications, it is imperative to have in-line filters at the point-of-use; this means placing the filter right before the nozzle. We offer three different filters as options on our units - see equipment page and price list.
There is a potential for static charge buildup on surfaces during cleaning. This is caused by the ionization of a flowing CO2 gas. Obviously, this static charge buildup is not a problem for metal samples. From our experience, if the sample is grounded, static charge is not a problem. Charging is usually worse for glass samples or for electrically isolated parts on complex structures. For these cases, commercially available positive ionization sources can be obtained for charge compensation. We recommend static control devices be used in critical cleaning applications.
The feed pressure is important to maintain. If the feed pressure decreases, stream velocity decreases and organic removal efficiency decreases or stops. With liquid fed sources, pressure reductions will lead to a higher snow percentage having a lower velocity; with gas fed, reduced snow output can result. If reduced pressures are needed to minimize damage to surface feature, you can increase the working distance or contact us for ideas.
Carbon Dioxide Snow Cleaning can damage samples if proper care is not exercised. First and foremost, since CO2 cleaning uses a high velocity stream, the sample MUST be supported, held or fixed. Further, coatings that are poorly adhered can be blown off. As a rule, if a coating is properly deposited and bonded, it should survive cleaning. Kodak, in a 1998 patent, studied CO2 cleaning of a gold coated mirror and found that overall, reflectivity did not change, there was no abrasion, and the film was smoother as determined by AFM RMS values. Cleaning loose fibers may pose a challenge though braids have been done. One customer even cleaned a Si based MEMS devices, another cleaned historic Native American Baskets. Thermally sensitive material also poses challenges. Please note, most all metals, ceramics, optics, polymers, etc., can be cleaned. Delicate art work can be cleaned and see the art page for details.
Certain optical crystals are susceptible to thermal shock, though much less than expected. As for glasses, to our knowledge, only one bulk glass has shown thermal effects - NPK52. Bulk fluoride crystals, such as LiF can be damaged, but users have successfully cleaned MgF2 and LiF thin films. For these thin films, a lower velocity nozzle or a greater working distance will assist Testing is required for synthetic fluoride glasses (FPL53) and the low velocity nozzles may work. Testing is always suggested for every application.