Articles you may be interested inStudy on the activation of styrene-based shape memory polymer by medium-infrared laser light Appl. Phys. Lett. 96, 111905 (2010); 10.1063/1.3353970 Accurate control of remaining resist depth for nanoscale three-dimensional structures in electron-beam grayscale lithography J.A large number of chlorinated styrene-based resists has been prepared, both by the copolymerization of methylstyrene with chloromethylstyrene, and by the direct chlorination of methylstyrene polymers. These resists have been characterized, and lnokuti's equation has been applied to the lithographic data to obtain radiation chemical yields which have been related to polymer structure and composition. Inokuti's theory has also been used to predict the dependence of lithographic performance on polymer parameters. Qualitative and quantitative agreement between these predictions and experimental observations has been demonstrated in several polymer systems, and this theory has been shown to provide useful pointers to resist optimization. A number of comparatively high sensitivity polymers have been found which do not undergo chain scission on irradiation and which accordingly exhibit high contrasts. From this range of polymers several well-suited for use as electron resists have been selected.
Medical-grade poly(methyl methacrylate) (PMMA) is extensively employed in the fabrication of a variety of medical implants, including intraocular lenses (IOLs). However, a postoperative complication that leads to the failure of the implanted intraocular lenses has been recently identified. This process, termed calcification, occurs when calcium-containing deposits accumulate on the surface of the IOL. In this study direct gas fluorination was used to modify the surface of PMMA in an attempt to increase the service lifetime of the material in optical applications. PMMA discs exposed to a 20% fluorine/nitrogen gas mixture for 24 h were compared with untreated PMMA discs serving as control samples. Over time, both surface-fluorinated and untreated PMMA samples immersed in a simulated aqueous humour solution (SAHS) (pH 7.4, 35˚C) were used to carry out in vitro studies. Attenuated total refractive Infrared spectroscopy (ATR-IR) Scanning electron microscopy (SEM), coupled with Energy dispersive X-ray analysis (EDX), showed that calcium-containing surface deposits were less abundant on surface-fluorinated PMMA compared with the control samples, indicating that the fluorinated surface was acting as a barrier to the deposits. Gravimetric analysis data showed that the decreased rate of diffusion compared with that of a control sample was due to the fluorinated surface.
Structurally characterized polymers of o-, m-, and p-methylstyrene, 2,4-and 2,5-dimethylstyrene, and 2, 4, 6-trimethylstyrene prepared by anionic mechanisms and chlorinated selectively at substituent methyl positions, have been assessed as negative-working electron-beam resists. Copo]ymers of vinyl benzyl chloride and o-, m-, and p-methylstyrene prepared by a radical mechanism are similarly assessed. The lithographic data have been correlated in accordance with Inokuti's theory for the radiation-induced simultaneous cross-linking and scission of polymers with generalized Poisson distributions. All the polymers substantially substituted with methyl groups in the ortho position are shown not to undergo radiation-induced chain scission and accordingly display the highest lithographic contrasts. Lithographic reactivities and the radiation chemical yields are further correlated in accordance with the Charlesby-Pinner relationship and are shown to be independent of both molecular weight and polydispersity and therefore characteristic of chain microstructure. Lithographic contrast is shown to arise from variations in gel dose.The application of chlorinated polymethylstyrene derivatives as negative-working electron beam resists has been investigated extensively in recent years. As a class, they offer submicron resolution and display a moderate to high sensitivity to radiation and good dry-etch durability.In order to optimize lithographic performance, a wide range of structures has been accessed through a variety of synthetic approaches:1. Imamura et al.(1, 2) employed Lewis acid catalyzed chloromethylation of polystyrene (CMS) synthesized by anionic methods. This has the advantage of being able to start with a polymer of narrow molecular weight distribution, but the product polymers are structurally ill-defined, and although a range of chlorine contents can be attained, the phenyl rings bear no unchlorinated methyl groups.2. Tarascon and Hartney et al. (3, 4) carried out homogeneous solution-phase free radical-induced partial chlorinations of poly(p-metl~ylstyrene). This allows the ready preparation of narrow distribution polymers through the anionic polymerization of the parent monomer but results in main chain chlorination and di-and tri-as well as monochlorination of the methyl groups. More recently, Jones and Matsubayashi (5) have achieved essentially selective monochlorination of the ring substituent methyl groups of the polymer using sodium hypochlorite and a phase transfer catalyst (PTC). Some broadening of the molecular weight distribution was found to be inevitable and this feature limited the extent of chlorination that could realistically be achieved.3. Ledwith et al. (6) developed copolymers of vinyl benzyl chloride (VBC; a 60:40 mixture of the m-and p-isomers) and p-methylstyrene, which, though offering a degree of substituent positional specificity and completely avoiding the possibility of main chain chlorination over a wide range of chlorine contents, confines the molecular weights of the polymers and the...
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