In situ capacitance studies of thin polymer films during compressed fluid extraction
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I. INTRODUCTION Supercritical fluid extraction (SCFE) describes processes that use fluids above their respective critical points as ordinary chemical solvents. The principle advantages of supercritical fluid extraction, which are normally exploited, arise because supercritical fluids are good solvents only under pressure at liquidlike densities. Consequently, the solvent can extract a solute from a mixture and then be cleanly unloaded by merely throttling to a pressure below the critical temperature or pressure. Carbon dioxide is an extremely useful supercritical fluid solvent because its critical temperature (31.1 °C) is near room temperature and it is nonpoisonous and nontoxic. The accessible critical temperature implies that labile components can be stripped from mixtures if soluble in CO2, while a critical temperature near ambient conditions reduces thermal energy requirements as well as process complexity. Applications that use SCFE can be found in the food, pharmaceutical, and petroleum industries.1'2
single wafer processing. And last, the supercritical fluid extraction should not damage the fragile and thin polymer film, thereby reducing process resolution. Supercritical CO2 has been reported to cause severe swelling of relatively thick, freely suspended poly (styrene) and poly (methy methacrylate) films.4 In addition, Wang etal. and Wang and Kramer have investigated the effect of supercritical CO 2 exposure on polystyrene.5'6 Their results indicated that supercritical CO 2 affects Tg and serves as a crazing agent for polystyrene. Early experiments demonstrated that siloxane monomers and oligomers of interest could be extracted from micrometer thick polymer films. However, the resulting polymer film was often severely pitted after processing if the extraction was done using conditions outside a defined processing window.3 Figure 1 shows scanning electron microscope (SEM) photographs of this extraction-induced film damage. The nature and cause of extraction-induced defect phenomena had to be investigated, understood, and eliminated to use compressed fluid extraction for a microlithographic process.
A. Supercritical fluid extraction of thin films A novel application of supercritical fluid CO 2 was recently proposed in which supercritical and liquid carbon dioxide were used to extract relatively nonvolatile siloxane monomers and oligomers from micrometer thick host organic films.3 This process uses the ability of supercritical fluid CO 2 to penetrate a polymer matrix and selectively remove a nonvolatile siloxane component. However, the demands on the SCFE extraction were quite severe since this application was developed to enhance a microlithographic process. First, the extraction had to be complete and uniform. Unextracted siloxane monomer in the host polymer causes incomplete pattern definition leading to gross defect formation. Second, the overall extraction cycle (compression, extraction, and decompression) had to be minimized to allow 884
J. Mater. Res. 2 (6), Nov/Dec 1987
B. Dielectric measurements Die
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