Experimental Beam Studies of Plasma-generated Species Interaction with Polymeric Materials and Biomolecules

Low temperature plasma-based processes are used extensively in many modern technologies. It is thus very important to understand plasma and surface interactions in order to improve plasma processes and design of functional materials. Applying a high vacuum beam system, this dissertation studies the fundamental mechanisms of plasma species-induced modification of materials for two critical applications: manufacturing of semiconductor devices and surface deactivation of infectious biomolecules./nManufacturing of integrated circuits relies on well-controlled film patterning technology. This is currently achieved by photolithography followed with plasma etch. While critical dimension control is acknowledged to be a major challenge for future miniaturization of transistors and other functionalities, degradation and roughening of methacrylate-based 193 nm photoresist (PR) during plasma etch processes result in poor pattern transfer and decreased device performance. The first part of this dissertation addresses the effects of ion bombardment, vacuum ultraviolet (VUV) irradiation, electron exposure, and moderate substrate heating in 193 nm PR surface roughening./n150 eV Ar ion bombardment results in physical sputtering and formation of a carbon-rich layer at the PR surface. The thickness of this surface layer is about 2 nm. This ion-modified layer is expected to bear an intrinsic compressive stress, roughly a few GPa. In contrast, 147 nm VUV irradiation and 1 keV electron exposure both modify the PR film up to the penetration depth, ~100 nm. Enhanced PR surface roughening is only observed when a simultaneous exposure provides a combination of an ion-modified surface layer on top of a scissioned/softened bulk layer, either resulting from VUV irradiation or low fluence electron exposure. 2-methyl-2-adamantyl methacrylate, the leaving group of 193 nm PR, is especially sensitive to VUV exposure. The adamantyl leaving group is shown to be one of the main photolysis products. The loss and detachment of bulky adamantyl groups are highly correlated to the surface roughening of processed PR. These phenomena can be qualitatively explained by a bi-layer wrinkling mechanism. The results demonstrate that PR structure can couple to plasma etch processes and strongly alter the post-etch morphology./nThe second part of this dissertation is motivated by the insufficiency of conventional sterilization methods against bacterial and protein residues. Such residues on the surface of medical instruments increase the risk of healthcare-associated infections for patients. Low temperature plasmas are promising alternative sterilization/deactivation methods. A thorough understanding of plasma and biological target interactions is required to align applications with scientific principles. Lipid A, the immune-stimulating region of lipopolysaccharide, is chosen to be the model molecule. Using a surface-sensitive human whole blood-based assay, the present study shows that VUV photons, oxygen and deuterium radicals can cause deactivation of lipid A film through different mechanisms. Similar to 193 nm PR studies, VUV photons are able to induce bulk modification of lipid A film up to the penetration depth of photons, ~200 nm. VUV photons primarily cleave ester linkages and lead to desorption of aliphatic chains. Loss of phosphate groups and the glucosamine backbone is also observed. In contrast, radicals react at the lipid A film surface, form volatile products, and lead to slow chemical etching. The etch yield of radicals is one order of magnitude lower than that caused by VUV-induced photolysis. In spite of its low etch yield, radical exposure strongly modifies lipid A film surface. This work contributes to the fundamental understanding of plasma interaction with biomolecules. The principle of this study is also relevant to the broader scope of plasma applications on biological targets, including cells and tissues, in the rapidly growing field of plasma medicine.

Advisor: Graves, David B.