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We have shown that particles can be theoretically rolled off semiconductor wafers using a turbulent flow of supercritical C02 over them. Flow speeds of a few hundred cm/s will be required to remove particles less than a tenth of a micron in radius. The relative merits of using supercritical carbon dioxide over air include the use of lower flow velocities with supercritical C02, it is pointed as well as an advantage in terms of chemistry over air. Supercritical C02 dissolves organic molecules, implying that it can loosen the adhesion between organic contaminants (particles) and the semiconductor wafer.
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Surfactants and Microemulsions in Supercritical Fluids
Kevin Jackson and John L, Fulton
A new class of cleaning solvents is described in which the unique solvent power of a supercritical fluid is united with the broad solvation capabilities of a reverse microemulsion. The application of these systems to cleaning processes is discussed. A reverse microemulsion has a structure similar to that of a conventional water-based surfactant system with the exception that the nanometer-sized structures are inverted—the core of the micelle is an ultra-small droplet of water and the exterior is the oil or supercritical fluid phase. The advantages of supercritical microemulsions over conventional liquid or aqueous based systems are (i) energy savings (no drying is required), (ii) environmental benefits since the fluids may be benign and the contaminants or surfactants can more easily be recovered,
(iii) microemulsions have a high capacity for oils or other lipophilic
materials, (iv) the selectivity for the contaminants may be adjusted by density manipulation, and (v) improved cleaning or extraction efficiency because of the high diffusivities and low viscosities inherent in supercritical fluids.
A potent, new solvent is evolving—a supercritical carbon dioxide microemulsion. A C02-based microemulsion is especially attractive since C02 is very abundant, relatively inexpensive, and environmentally benign at this scale of use. Applications of this system to cleaning processes appear very promising.
The range of industrial applications that supercritical microemulsions aspire to cover is extensive and includes areas such as separations, reactions, and spray coatings (paints, lacquers, enamels and varnishes). Applications to cleaning could include dry cleaning, the separation of dyestuffs, the extraction of contaminants from soils, the regeneration of activated carbon/catalysts, and the removal of strongly polar or ionic species from printed circuit boards, polymers, foams, aerogels, porous ceramics, and laser optics. The combination of colloidal technology with the technology of supercritical fluids greatly enhances potential applications in cleaning processes.
Supercritical fluids are attractive cleaning solvents because of their excellent mass transport characteristics and their unique physical properties that allow extracts to be isolated with relative ease. Carbon dioxide is especially attractive since it is inexpensive and environmentally benign in comparison to liquid solvents that it might replace. It also has a relatively low critical temperature which makes it an ideal solvent for extracting thermally labile substances. There is however a limitation of supercritical fluids having moderate critical temperatures in that they are poor solvents for polar or ionic species that are typically one of the target components in a cleaning operation. One strategy to overcome this basic limitation of supercritical fluids is the incorporation of a microemulsion phase to greatly enhance the solvent power of the solution. The combination of the unique properties of supercritical fluids (e.g., low viscosities, high diffusion