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As the feature sizes of semiconductor devices continue to shrink, there is an increasing interest in thin film imaging approaches such as silicon-based bilayer resists. We have developed such a resist based on a copolymer of 4-hydroxystyrene with a silicon-containing monomer, which functions simultaneously as the acid-sensitive component and a source of O2 etch resistance. In an attempt to understand the reactions that occur in the photoresist film, the acidolysis reactions of the 2-[tris(trimethylsilyl)silyl]ethyl moiety have been studied in solution. Acid-catalyzed cleavage of the model 2-trimethylsilylethyl acetate in solution proceeds via a nucleophilic attack on the silicon atom of the protonated acetate. Protonation of 2-[tris(trimethylsilyl)silyl]ethyl acetate is postulated to lead to a bridged siliconium cation, which reacts with nucleophiles along three pathways and yields products in which a nucleophile is attached to a silicon atom. This mechanism is consistent with the silylation of phenolic hydroxyl groups in the photoresist film consisting of a copolymer of 4-hydroxystyrene with 2-[tris(trimethylsilyl)silyl]ethyl methacrylate, observed during photolithographic processing.

Porous, low dielectric constant polyimide films have been made by a “nanofoam” approach. The pore sizes generated in the polymer films are in the tens of nanometers range, hence the term “nanofoams”. The nanoporous foams are generated by preparing triblock copolymers with the majority phase comprising polyimide and the minor phase consisting of a thermally labile block. Films of the copolymers are cast and then heated to effect solvent removal and annealing, resulting in microphase separation of the two dissimilar blocks. The labile blocks are selectively removed via thermal treatments, leaving pores the size and shape of the original copolymer morphology. The polyimide derived from 2,2-bis(4-aminophenyl)hexafluoropropane (6FDAm) and 9,9-bis(trifluoromethyl)xanthenetetracarboxylic dianhydride (6FXDA) was used as the matrix material for the generation of nanofoams, and specially functionalized poly(propylene oxide) oligomers were used as the thermally labile constituent. The synthesis and characterization of the copolymers were performed and the process for obtaining nanofoams was optimized. The foams were characterized by a variety of techniques including thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic mechanical thermal analysis (DMTA), density, small-angle X-ray scattering (SAXS), refractive index, and dielectric constant measurements. Thin-film, high-modulus nanoporous films with good mechanical properties and dielectric constants ~2.3 have been synthesized by the copolymer/nanofoam approach.

A means of generating foams of high-temperature polymers, polyimides, has been developed for use in dielectric layers in microelectronics. In these systems, the pore sizes generated are in the tens of nanometers range, thus the term “nanofoams”. The foams are generated by preparing phase-separated block copolymers with the majority phase comprised of polyimide and the minor phase consisting of a thermally labile block. Films are cast, solvent is removed, and the copolymers are cured, causing phase separation of the two blocks. The labile blocks are subsequently removed via thermal treatments leaving pores having a size and shape commensurate with the size and shape of the original copolymer morphology. The polyimide derived from pyromellitic dianhydride (PMDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF) was used as the matrix materials for the generation of nanofoams, and poly(propylene oxide) oligomers were used as the thermally labile constituent. The synthesis and characterization of the copolymers were performed, and the process for obtaining nanofoams was optimized. The foams were characterized by a variety of techniques including TEM, SAXS, WAXD, DMTA, density, and refractive index measurements. Thin-film, high-modulus foams with good mechanical properties can be synthesized using the copolymer/nanofoam approach.