Free radical graft polymerization lithography and modeling base quenchers in photoresists
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The process of cationic graft polymerization lithography was developed at The University of Texas at Austin. This process is a top surface imaging scheme that involves using photogenerated acid to initiate the graft polymerization of a gas phase monomer onto a surface. This technique relies on the cationic initiation of polymerization and, hence, is limited to a certain set of monomers and initiators. There are very few photoacid generators strong enough to initiate cationic polymerization. Also, the amount of photoacid generator that can be added to a polymer film is limited by its solubility in the film. Furthermore, only vinyl ethers and epoxides have been found to be active towards this type of polymerization. Because of these limitations, an alternate, free radical based scheme for graft polymerization lithography was desired. Free radical graft polymerization lithography would allow a multitude of initiators to be used, including polymer bound initiators. It would also allow access to a new set of monomers. This document describes the development of a free radical graft polymerization lithography scheme. Custom synthesis of both the initiator and monomer, design and automation of the grafting chamber, studies of grafting kinetics, and imaging experiments are described. This dissertation also describes fundamental research into the function of base in photoresists. As critical dimensions in microlithography become ever smaller and the importance of line-edge roughness becomes more pronounced, it is becoming increasingly important to gain a fundamental understanding of how the chemical composition of modern photoresists influences resist performance. Modern resists contain four basic components: polymer, photoacid generator, dissolution inhibitor, and base quencher. Of these four components, the one that is least understood is the base quencher. This dissertation examines the influence of base additives on line-edge roughness, contrast, photospeed, and isofocal critical dimension (CD). Furthermore, the function of acid/base neutralization products in photoresist films is examined.