Tuesday, July 26, 2011

The Fourth State of Matter—Part 2

Plasma, the fourth state of matter, is a powerful tool for critical cleaning, controlling contamination, and achieving the appropriate surface quality. A wide range of applications are in use; even more have been proposed. Examples include removing minute traces of contamination from semiconductor wafers, preparing surfaces for coatings, removing masks and markings, decontaminating and sterilizing medical devices, and skin rejuvenation.
In the previous column we explained that plasma is a mixture of atoms, ions, and electrons that result when sufficient energy (heat or voltage) is applied to a gas. The efficacy of plasma for surface modification and cleaning is due to the energy of the plasma particles and/or the ultraviolet light they emit as well as from chemical interaction of plasma constituents with surface molecules.
There is versatility provided by plasma parameters including the choice of gas and the pressure mode, vacuum or atmospheric. Two out of many areas of applicability include medical device cleaning and surface preparation for bonding and coating.
Because plasma can destroy as well as remove protein, there is potential utility in biomedical applications, including cleaning of reusable medical instruments such as forceps and dental drills. Plasma cleaning could be inserted as an added step between washing and autoclaving for high risk situations. A result of research conducted at The University of Edinburgh1 indicates that plasmas can destroy and remove proteins, including prions that are believed to cause Transmissible Spongiform Encephalopathy (TSE) or “Mad Cow Disease.”
Table 1
Plasma not only can remove trace levels of contaminants but also can “roughen” the surface to improve the adhesion of a subsequent bonding, marking, or coating. Plastics and composite materials that might be damaged by traditional cleaning chemicals are among the materials treated by plasma. Atmospheric plasmas are line-of-sight and can be used for treating localized regions of a surface or can be scanned across a larger region.
Just as there is a large diversity of options for cleaning agents and methods in traditional liquid cleaning, plasma provides many options. Because much of the effect of plasma on surfaces is a combination of physical momentum and chemical reaction, the choice of gas to be used is an important parameter. Table 1 summarizes chemical and physical properties and uses of major gases used in plasma cleaning.
Many of the applications for plasma are still untapped. No cleaning or contamination removal method can fit all applications. Plasma cleaning and surface treatment provides additional tools that can be considered in the quest to use the most efficacious technique to achieve the desired surface properties.
  1. H. Baxter et al., “Application of epifluorescence scanning for monitoring the efficacy of protein removal by RF gas–plasma decontamination,” New Journal of Physics 11 (2009).
  2. K. Sautter and W. Moffat, “Gas Plasma—A Dry Process for Cleaning and Surface Treatment,” in Handbook for Critical Cleaning: Cleaning Agents and Systems, B. Kanegsberg and E. Kanegsberg, editors, CRC Press, 2011.

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