Grafted polymer monolayer brush as electron beam resist
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This thesis focuses on the nanofabrication process using novel electron beam resists. First, in Chapter 3 and Chapter 4, the nanofabrication process using grafted polymer monolayer brush as e-beam resist was examined. Then, in Chapter 5, characteristics of the mixture of polystyrene (PS) and polydimethylsiloxane (PDMS) as negative tone e-beam resist with high dry etch resistance were studied. The main advantage of grafted polymer brush as electron beam is that it is applicable on non-flat or irregular surfaces. Although nanofabrication on non-flat surfaces has a lot of applications such as nanofabrication on AFM tips and optical fibers, there is no versatile method for nanofabrication on non-flat surfaces. In this thesis, nanofabrication on non-flat surfaces of an AFM cantilever was demonstrated using grafted polymer brush. Grafted polymer brush includes grafted poly(methyl methacrylate) (PMMA) brush and grafted PS brush; PMMA brush was used as negative tone e-beam resist, and PS monolayer brush was used as both positive and negative tone e-beam resist in this study. When PMMA brush and PS brush are used as negative tone resist, a development method plays an important role. Although solvent development is the most common development method to develop thick PMMA film or PS film as negative tone resist, solvent does not work as a developer for PMMA brush and PS brush. Instead, thermal treatment was used to develop monolayer PMMA brush or PS brush to achieve negative tone behavior. Since cross-linked PMMA and PS has higher thermal stability than uncross-linked ones, only unexposed polymer was vaporized at proper temperature. Thus, the polymer brush works as negative tone resist. Positive tone behavior of PS brush was achieved by changing the development method. When PS is exposed to electron beam, it loses the resistance to wet etch. Therefore, when the PS brush was grafted on an intermediate Al mask layer and developed by diluted HF solution, Al layer underneath the exposed PS brush was etched directly by HF. Since Al layer underneath unexposed PS was protected from wet etch, the resulting patterns on the Al mask layer showed negative tone behavior. In Chapter 5, the mixture of PS and PDMS was studied as negative tone e-beam resist with high dry etch resistance. High dry etch resistance is one of the most important characteristics of e-beam resist because resist with high resistance can be used as dry etch mask, and the patterns on the resist can be transferred to the substrate using direct dry etch. Recently, hydrogen silsesquioxane (HSQ) has been vigorously studied as negative tone resist with high dry etch resistance and high resolution. However, it has several drawbacks: it is very expensive, the shelf life is very short, and the processes should be done very quickly to obtain reproducible results. Here, it was demonstrated that PS that contains PDMS can be used as negative tone e-beam resist with high dry etch resistance. It was confirmed that by adding PDMS into PS, the resistance to the dry etch dramatically increased, but the sensitivity and contrast remained nearly the same. Thus, PS-PDMS can be a low-cost replacement of HSQ resist when negative tone e-beam resist with high dry etch resistance is required.
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Hirotaka Yamada (2017). Grafted polymer monolayer brush as electron beam resist. UWSpace. http://hdl.handle.net/10012/12128