Comparison to Other Cleaning Methods
Many cleaning methods exist and each has their own advantages and disadvantages. We will discuss water and solvent-based methods, ultrasonics, wiping, high speed air blowing, vacuum methods, lasers, replica tape, and argon-nitrogen cryogenic aerosol. when choosing a cleaning method, the most important aspect is can the method clean the part to your needs without damaging the surface in the time frame required. Other factors include capital and operating (labor and consumables) costs, time per sample, safety, and cleaning efficiencies. It is imperative to note that no one method is perfect, they all have their limitations. Vendors have to be honest with customers, and if they fail to mention what can be an issue, that cleaning method should be avoided. An excellent summary, though aimed at optics has been authored by Schalck and published by SPIE
Aqueous and solvent-based cleaning methods, with and without ultrasonics, are batch methods, meaning one or a large number of samples can be cleaned simultaneously. This contrasts with CO2 snow cleaning, which is an in-line method, meaning only one sample can be cleaned at a time per nozzle. The obvious advantage of these wet methods is volume production, and the limitations include drying, staining, potential damage, and chemical disposal. A manufacturer cleaning hundreds of 10-cm2 optical glass samples a day will most likely choose ultrasonics due to the volume throughput and will have to insure proper drying to avoid stains and spots.
Manual wiping with solvents is usually quick, and common in the optics industry. The limitations are labor costs and drying issues. Yet, when labor is not an issue, it is acceptable and can remove particles and organics. Potential surface marring from wiping hard particles on soft optical surfaces can occur, and users must be aware of this limitation. Additionally, stains from improper drying can occur. This process can be rapid per part and the user can quickly inspect each sample after cleaning.
Air blow-off with compressed air or nitrogen is quite common and works well for particles larger than several micrometers. Cost per sample is quite low. The main concerns are oil impurities within the air compressors and lack of effective filtration. Air blow-off does not work on hydrocarbons or particles smaller than a few micrometers. Sample geometry can pose problems too. The study on this page clearly shows this point.
Vacuum methods include plasma, glow discharge, sputtering, etc. These methods work phenomenally well on many substrates and they all require a vacuum system. Plasma may be the most effective method for organic removal, but potential substrate oxidation is a concern. Plasma is not effective for inorganics. UV-Ozone for organic removal can be done outside a vacuum system, but lacks any ability to remove inorganic particles.
Argon-nitrogen cryogenic aerosol cleaning is quite similar to CO2 snow cleaning. It was first reported by McDermott back in 1991 and developed over the years. Essentially, it is a similar process in that a cryogenic particle is aimed at the surface and relies upon momentum transfer for cleaning. Many patents have been granted. Compared to CO2 snow cleaning, results should be similar for particle removal..
There are a few replica-based cleaning companies that offer polymers in a solvent- or water-based solution. Cleaning is effective, but the drying times make the process slow in comparison to most all other techniques. Cost per sample can be quite high too. Issues exist, as Kohli stated: “the coating tends to cling to the irregular surface, loses its coherence, and breaks off in pieces when peeled. Residual polymers have been noted by XPS studies and visual examination. Again, as for all cleaning methods, testing is required before application.
Cost per part is an important consideration. Large automated cleaning systems are quite expensive and are not suitable for cleaning the occasional sample unless there are special considerations that justify the cost. Automated CO2 snow cleaning systems costs are high, and their applications have been directed at high-value samples such as photomasks, back-end wafer processing, and unique high-value parts (gyroscopes). The equipment costs are justified in these cases, as they are for in-line ultrasonics or solvents, or any automated cleaning process. Laser particle removal systems fall in this category too because large capital expenses must be taken into consideration.
Other CO2 cleaning methods include liquid and supercritical CO2 cleaning, and dry-ice pellet blasting. Liquid CO2 cleaning uses the excellent solvent properties of the liquid for degreasing and cleaning. Adding surfactants or co-solvents has made liquid CO2 suitable for textile dry cleaning or metal degreasing. Inorganic particle removal is a weakness here. Systems can be expensive. Supercritical CO2 refers to CO2 exceeding the critical temperature (305 K or 32 °C) and pressure (74 bar or 7.4 MPa). Here, the CO2 enters a unique phase that combines the best properties of both liquid and gas.. Supercritical CO2 has phenomenal solvent properties and the penetrating properties of a gas having zero surface energy. Applications abound in chemistry and biochemistry, along with a few semiconductor applications. The main limitations are no particle removal, and high temperature and pressure requirements make the systems more complex and costly. Dry-ice pellet blasting uses compressed air to accelerate macroscopic pellets of dry ice. The dry-ice impact energies are much higher than with snow and can remove gross contamination in many industrial applications where CO2 snow lacks the impact energy. The need for compressed air makes these systems more expensive.
Overall, all cleaning methods have their advantages and disadvantages and the user should expect the suppliers to be honest about the limitations.