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from solvent, then selectively imaged and developed (washed off) to create a pattern. To create the pattern, a mask is applied to the polymer layer, after which radiation is employed to either crosslink the accessi- ble areas (leaving the hidden areas uncrosslinked) or degrade the accessible areas (leaving the hidden area intact). The mask is then removed and the soluble material (in either case) is washed away. Photolithog- raphy currently employs significant volumes of either solvent or water to accomplish the developing (wash- ing) step and hence generates a substantial liquid effluent stream. The key to successful developing is to be able to efficiently change the solubility charac- teristics of the exposed portion of the resin. Carbon dioxide is a particularly intriguing solvent for use in microelectronics applications, not only because it is environmentally benign, but also because its vanish- ing low interfacial tension allows it to successful wet and penetrate very small features on a component. Initial work to apply carbon dioxide to the coat- ing and photolithography processes dates to the mid-1990s; researchers at IBM and Phasex Corpo- ration examined the design of resins specifically for use in CO2-based developing [222]—the work by DeSimone’s group on the miscibility of perflu- oropolyacrylates showed the IBM researchers that such as process was feasible. A number of fluorine and silicon-containing polymers were examined, and a photoacid generator employed to develop the pat- terns; the most viable system seemed to be one where a random copolymer of a fluorinated acrylate and t-butyl methacrylate was used. Ober et al. [223] have also designed a photolithography system that could be developed using carbon dioxide. Here, a block copolymer of a fluoroacrylate (CO2 -soluble) and tetrahydropyrano methacrylate was synthesized. The polymer was spun-cast onto a substrate from a con- ventional solvent and a photoacid generator added. The system was masked, patterned (using 193 nm ra- diation) and developed with CO2, demonstrating that 0.2-micron features could be produced. DeSimone has also postulated the design of fluorinated copolymers for use in photolithography [224]; both negative and positive resist systems are described. Interestingly, fluorinated materials are both highly CO2-soluble and are known to be relatively transparent to radiation in the 130–190 nm range [225] (the wavelengths to be employed in next generation systems). DeSimone et al. have described a free-meniscus coating methodology using CO2 to apply polymers to inorganic substrates, potentially eliminating the signif- icant volume of solvent currently used for that purpose [224,226]. DeSimone has demonstrated the concept using fluorinated polyethers, polymers whose high sol- ubility in CO2 is well known. As suggested in a recent articles in Chemical and Engineering News [227] and Technology Review [228], interest in the use of CO2 in microelectronics processing is growing. To date, most of the indus- trial ventures involve partnerships between large, well-known chemical suppliers to the electronics in- dustry (Praxair, Air Products) or microelectronics companies (IBM) and small firms with expertise in the design of high-pressure equipment (Supercritical Systems [229] (Fremont, CA; purchased by Tokyo Electron) and SC Fluids (Nashua, NH)). The efforts to date have focused on the use of mixtures of CO2 and cosolvents, as a means to overcome the feeble solvent power of CO2 without having to resort to the design of CO2 -philic materials. Clearly, technical challenges for the future include the ability to design CO2-philic materials for use in microelectronics processing that are also acceptable (from both technical and envi- ronmental perspectives) to the industry. Indeed, do we possess a firm understanding as to the underlying molecular foundation for high CO2 solubility as well as transparency to radiation of a particular wave- length? Today, the answer is ‘no’. Will these underly- ing mechanisms ultimately conflict with one another? Further, given the rapid throughput in the industry, can high-pressure systems be developed that will al- low use of CO2 at the throughputs required? Finally, the work to date on polymers for use in lithography has created materials where the exposed portion of the polymer is rendered insoluble in carbon diox- ide (through action of a photochemically-generated acid on a protected carboxylic acid). It is somewhat surprising that we have yet to see a system created where the exposed portion of the material is rendered soluble in CO2 instead. It is clear that if CO2 can make significant inroads into the microelectronics processing industry, then po- tentially large volumes of organic solvents and just as importantly water, could be replaced with CO2—once again there are clear technical challenges to be over- come. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 167PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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