science/technology
Materials Made Under Pressure Supercritical C02's talents go beyond environmental correctness try," said James J. Watkins, one of the organizers of a symposium on the topic held at the American Institute of Chemiupercritical carbon dioxide has cal Engineers' (AIChE) annual meeting come to be known as an environ- in Los Angeles last month. mentally friendly solvent, offered In particular, electronic materials manuup as a cure for the woes caused by indus- facturing is "an area in which supercritical trial processes that rely on noxious apolar fluids are not conventionally involved," solvents such as acetone, methanol, or noted Watkins, an assistant chemical entoluene. While ordinary C0 2 is apolar, benign, and plentiful, ifs also a gas. But under high pressures (1,070 psi) and modest temperatures (31 °C), C0 2 reaches its supercritical point and becomes dense like a liquid yet maintains its gaslike ability to flow with almost no viscosity or surface tension. Although supercritical C0 2 currently is the focus of unprecedented interest, scientists and engineers have been exploiting its benefits for years. Ifs nontoxic; it evaporates instantly once the pressure is released, leaving almost no trace behind; Watklns (left) and Blackburn deposited a nickel and ifs extremely inexpensive. film onto an etched wafer (right) by continuous Those features make it ideal chemical fluid deposition. for industrial food processing, such as coffee decaffeination. gineering professor at the University of But supercritical C0 2 has other un- Massachusetts, Amherst But the area usual advantages that perhaps are not has proven to be an attractive target for quite so well known. Scientists are find- chemical engineering research. ing that supercritical C0 2 is an intriguChemical vapor deposition is a teching medium in which to discover and nique frequently used to deposit metals develop new materials or to improve the for microelectronic structures. In this way old materials are made. vacuum process, a vapor of organometalFor instance, altering the pressure lic precursor molecules passes over a surwithin the supercritical region allows sci- face. The precursor then reacts, usually entists unique control of the chemistry via chemical or thermal reduction, leavof processes involving supercritical ing a thin metal coating on the surface C02, such as nanocrystal growth. And and releasing gaseous by-products. But because the substance can infiltrate the precursors must be extremely volar pores or nanosized canals on a surface, tile in order to get a high enough rate of supercritical C0 2 turns out to be a natu- massflowin an industrial apparatus. If ral in fabricating surface coatings or the rate isn't high enough—a frequent occurrence-^the coatings can be nonunipolymeric foams. During the 1980s, supercritical fluid form, especially within very small or intriresearch was centered primarily around cate surface features. separations, "but now, there's more emWatkins has pioneered a technique phasis on aspects of materials chemis- similar to chemical vapor deposition, but Elizabeth K. Wilson C&EN West Coast News Bureau
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DECEMBER 18,2000 C&EN
with a twist, which he calls chemical fluid deposition. Rather than existing as a vapor, the precursor dissolves in a bath of supercritical C02. Because of the C02's low viscosity and surface tension, the solutionflowseffortlessly over the surface. In addition, the concentration of the precursor is several orders of magnitude higher than that in chemical vapor deposition, which leads to a uniform coating of very complex topographies, Watkins noted. In this manner, Watkins and his colleagues have coated surfaces with an array of metals, including platinum, palladium, rhodium, and nickel. At the AIChE meeting, Jason M. Blackburn, a graduate student in Watkins' group, reported on depositing nickel and platinumfilmson silicon substrates that uniformly coat fear tures 100 nm wide by 1 urn deep. As for industrial feasibility, the cost of maintaining a vacuum environment for the vapor may balance the cost of pressurization needed in the supercritical fluid technique. 'With chemical fluid deposition, you're trying to keep pressure in, whereas with chemical vapor deposition, you're trying to keep the atmosphere out," Blackburn said. Integrated circuits, the mainstay of modern electron-
ics, start their lengthy and solventintensive manufacturing process as silicon wafers coated with polymers impregnated with a radiation-sensitive compound. In some processes, selected portions of the coating are exposed to ultraviolet light, and the light-sensitive compound, known as a photoacid generator, produces an acid that reacts with the polymer, rendering it insoluble. The remaining, unexposed portion of the coating can be washed away. The foundation polymer layers are prepared by a method known as spincoating that works just like it sounds: the coating material—polymer, photoacid
University of North Carolina, Chapel Hill, graduate student Devln Flowers (far left) and NC State's Hoggan Investigate spln-coatlng with liquid CO& such as the spun polymer film at left.
generator, and an organic solvent—is dropped onto the wafer, then spun rapidly around. Centrifugal force spreads the material out into a thin, even layer. liquid C0 2 may be able to replace the organic solvent used in spin-coating, according to Erik N. Hoggan, a graduate student at North Carolina State University who works with chemical engineering professor Ruben G. Carbonell, codirector of the Kenan Center for the Utilization of C0 2 in Manufacturing & Technology. Hoggan reported the development of the group's unique process at the AIChE meeting. After the creation of the film, the next steps can be performed with supercritical C0 2 . But because the usual photoacid generators aren't soluble in supercritical C0 2 , the group had to custom synthesize a new compound, 2-perfluorohexyl-6nitrobenzyl tosylate, which is soluble. Fluorine-based compounds are known to dissolve well in supercritical C0 2 , and consequently scientists frequently consider fluorine in their design of compounds used with the solvent Industrial use of liquid C0 2 to spincoat films is still a few years away, Hoggan noted, in part because researchers still need to improve film quality. Right now, films can be produced that have a 5% variation in film height But for commercial applications, variations below 1% are needed. Supercritical C0 2 may offer itself as a tool for controlling the processing and behavior of nanocrystals. These metal particles, usually between 20 and 100 Â in diameter, have electronic and optical properties that are different from bulk materials and have numerous potential applications in areas such as microelectronics or biological tagging. Nanocrystals are typically grown in acetone or other solvents, but graduate student Parag S.
Shah, at the University of Texas, Austin, reported at the AIChE meeting his group's experiments with nanocrystals in supercritical C0 2 . Shah and chemical engineering professors Keith P. Johnston and Brian A. Korgel believe they can control a number of nanocrystal properties with the solvent—for example, altering the pressure or temperature of supercritical C 0 2 changes the solvent's density, which could make it possible to precipitate nanocrystals of a specific size. Nanocrystals are usually coated with molecules that keep the particles from aggregating
into a clump, and the group was able to accomplish such steric stabilization in supercritical C0 2 . The use of supercritical C0 2 in materials research is not reserved exclusively for microelectronics. Because the solvent has such unusual properties, it stands to reason it would affect surface chemistry. Consider the case of additives for copy
C02-soluble photoacid generator "°> ο Q^CH 2 0~|~
-CH3 (CF2)5CF3 ° 2-Perfluorohexyl-6-nitrobenzyl tosylate
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science/technology machine toner, which are generally surface-modified silica powSupercritical CO2 swells chitosan and chitin . . . ders. The surfaces of oxides like silica are typically quite polar, readily adsorbing atmospheric water. Consequently, the additive particles need to be coated with something that will keep them dry. There are a number of standard chemicals used to treat the surface of metal oxides, such as hexamethyldisilazane and octadecyltrichlorosilane. But the solvents used in the process, such as cyclohexane or toluene, present Chitosan Chitin problems even beyond their environmental toxicity. "Most of the . . . facilitating absorption of drugs solvents physisorb to the silica, even after aggressive drying," said chemical engineer James R Combes of Xerox in Mississauga, Ontario. The result is a cake of toner that has to be ground up to return it to its powder state. Acetaminophen Ketoprofen Combes reported that his group can get around many of these processing headaches with super- C0 2 , the group developed alternatives. ther into the body or at a specific site critical C0 2 . Because depressurization in- One of their compounds, ferric triflate, such as a target organ, would help ensure stantly releases the gas, there's little or becomes relatively soluble with the addi- steady concentrations of drugs where no caking, and the process is almost en- tion of a small amount of ethanol. they're needed. A natural polysaccharide tirely waste-free. And C02's relative puriThe group was proceeding with the known as chitin, found in crustacean ty means water can be readily removed development of this process when Erkey shells and insect exoskeletons, and its from silica. and his colleagues discovered in some deacylated derivative, chitosan, transport The success of this approach has im- older literature that plain iodine was a po- oxygen well, making them promising for plications for supercritical C0 2 and sur- tential pyrrole oxidant Even better, they biomedical uses such as drug-impregnatface chemistry in general. The absence found iodine's vapor pressure indicated it ed wound dressings. of normally omnipresent water, along could be soluble in supercritical C0 2 Getting the drugs into the pores of with the solvents pressure tunabil- even without ethanol, Erkey said. And in- a polymer requires swelling and good ity, creates a whole new research land- deed, it was. The solubility was "a lot transport—and supercritical C0 2 may be scape. aK you can get rid of the atmo- higher than ferric triflate—and iodine is one solution. Randy D. Weinstein, a spheric water, it opens unique synthetic dirt cheap," he said. They now have a chemical engineering professor at Vilgreatly simplified, relatively inexpensive lanova University, Villanova, Pa., and his pathways," Combes said. Supercritical C0 2 also can be used in process for making conductive foams in colleagues tested the effect of supercritical C0 2 on chitin, loading the polysacchainvestigations of a type of conductive supercritical C0 2 . foam. The foams are a blend of a polyThey hope the foams might be useful ride with acetaminophen and ketoprofen. pyrrole—a conducting polymer—and an- as "electronic noses"—sensors that They're still not sure whether or not the other polymer such as polyurethane. The could identify chemicals through a drug loading is uniform or what will happen when the C0 2 pressure is released; combination gives the foams elasticity change in conductivity. and processibility. Their syntheses, not Another useful feature of supercritical this is now under investigation. surprisingly, require organic solvents C0 2 is its ability to swell polymers, which The group has a broader goal for its such as methanol. The host polymer is opens their pores for easy chemical ac- experiments, that of using the results to impregnated with an oxidant, which oxi- cess. Of course, many liquid solvents will develop predictive models of how differdatively polymerizes pyrrole to polypyr- do that too, but driving off all residual sol- ent chemical structures affect a drug's role within the host vent traces within a polymer's intricate solubility in supercritical C0 2 and its abilCan Erkey and Robert A. Weiss, chambers can be a chore. With supercrit- ity to get inside a polymer's pores. The rechemical engineering professors at the ical C0 2 , all it takes is a release of pres- searchers would also like to understand University of Connecticut, and their col- sure. And its low viscosity also means it how changes in supercritical C02's temleagues began looking at supercritical can penetrate porous materials more perature and pressure affects a polymer's swelling and ability to accept drugs. C0 2 as a way to circumvent the solvent deeply. problem while working with Rogers These properties lend themselves to a Thaf s the ultimate goal," Weinstein Corp. in Rogers, Conn. Because the com- growing body of research on polymers as said, "to design different polymers that monly used oxidants such as FeCl3 have drug delivery devices. Drug-laden poly- can have different drug loadings just by extremely low solubilities in supercritical mers that release substances slowly, ei- changing C0 2 properties."^ 36
DECEMBER 18,2000 C&EN
