Graft polymerization lithography
Brodsky, Colin John
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Today’s state-of-the-art microelectronic devices are manufactured with circuit elements having dimensions that are on the order of 130 nm and the quest is to make them ever smaller. These elements are created by a process known as microlithography. The photoresist systems used in microlithography have traditionally relied on the photochemically induced switch in solubility through the entire depth of a polymer film to yield the three-dimensional relief images required for subsequent pattern transfer. Over the past two decades, in order to print smaller feature sizes, exposure tools have progressively incorporated shorter exposure wavelengths. However, as the wavelength is reduced, the number of materials available for resist design diminishes rapidly due to their inherent opacity. This dissertation outlines the development of a microlithography scheme in which the photochemistry is confined to the air-resist interface. Irradiation generates an acidic surface that catalyzes the polymerization of silicon-containing monomers, resulting in formation of a protective graft layer. This surface modification provides a mechanism for generating lithographic images via anisotropic oxygen reactive ion etching. This process offers a way to circumvent the transparency requirements associated with single layer resists. The success of this strategy is largely dependent upon finding appropriate chemistries for the deposition process. Silicon-containing vinyl ethers and epoxides have been designed to meet the required process specifications and synthesized. Base layer polymer and photoacid generators used in this work were also synthesized. Several analytical techniques were developed for study of the process, including a quartz crystal microbalance deposition system for monitoring dynamic deposition rates, and a volumetric sorption cell for monitoring equilibrium sorption isotherms. A recurring non-linear dependence of the deposition rate on the monomer pressure was observed for all systems studied. Fundamental studies identified the mass transport of the gaseous, siliconcontaining monomers into the base layer as a key limiting step in the deposition process. Proof-of-concept imaging experiments are presented for the graft polymerization lithographic process. Challenges to achieving high resolution, high contrast imaging have been identified.