The optimisation of the lithographic performance of negative-working electron beam resists is developed through consideration of the radiation chemistry of crosslinking of representative materials, typically epoxy-functionalised polymers and polystyrene and its derivatives. Similarly, the lithographic behaviour of positive-working systems based on radiation-induced chain scission reactions is discussed with reference to acrylate and methacrylate polymers and polysulphones. The difficulties encountered in devising desirable working systems based on novolacs are considered and contrasted with the promise offered by recent developments arising from the extension of chemical amplification techniques to the electron beam lithographic domain.
Blends of poly(epichlorhydrin) elastomer [PECH] and polyaniline dodecylbenzenesulfonate [PAni.DBSA], with electrical conductivities up to 6.4×10-7 S cm-1 , have been prepared by solution mixing and casting. The solubility parameters were calculated and the most suitable solvent (tetrahydrofuran) was selected for mutual solubility between PAni.DBSA and the compatible elastomer poly(epichlorhydrin) [PECH]. Fourier-transform infrared (FT-IR) spectra of PECH-PAni.DBSA blends contained features of the spectra of the pure materials, but with significant peak shifts due to changing intermolecular interactions between the polymers. Thermal decomposition steps of the conductive blends were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal stabilities of the blends were dependent on the ratio of PAni.DBSA to PECH. The electrical conductivities of blends with proportions of 1 to 50 wt % PAni.DBSA were measured by 2probe and 4-probe techniques. The electrical conductivity increased with the proportion of PAni.DBSA, showing two percolation threshold stages, which were explained by a change in the conformation of the polymeric chains leading to an increase in the conductivity. The effects of composition on the glass transition in the blends were determined using thermomechanical analysis (TMA).
Within developing countries the most cost-effective polymer used in the fabrication of rigid intraocular lenses (IOLs) is poly(methyl methacrylate) (PMMA), for which gamma irradiation is increasingly being used as the preferred method of sterilisation. Medical grade PMMA samples sterilised using a dose of 29kGy of gamma irradiation, were immersed in a simulated aqueous humour solution (SAHS) (pH 7.4, 35 °C), and periodically removed to assess changes in their thermal and structural properties with respect to control samples. Triple-detection gel permeation chromatography indicated that substantial chain scission had occurred during gamma irradiation, an adverse effect that also resulted in a small increase in the rate of diffusion of the SAHS into the PMMA. The diffusion coefficient of SAHS into the gamma-sterilised PMMA was 2.7 ± 0.6 × 10−12 m2s−1, and that for the control samples was 2.0 ± 0.5 × 10−12 m2 s−1. Thermomechanical analysis revealed a decreasing Tg with time for samples incubated in SAHS, showing that the solution caused plasticisation irrespective of irradiation; however, the γ-sterilised PMMA had a Tg 7 °C lower than the control PMMA, which indicates the effect of the γ-irradiation on the rigidity of the polymer.
Copolymers of methylstyrene and chlorostyrene cross-link when irradiated with 20 keV electrons and hence act as negative-working electron-beam resists. eMethylstyrenelgchlorostyrene and pmethylstyrenelpchlorostyrene copolymers have been prepared by a free-radical mechanism over the entire composition range and the lithographic performance of the materials has been evaluated. Radiation chemical yields for cross-linking and chain scission have also been estimated. None of the materials undergoes significant chain scission upon irradiation. In contrast to the corresponding methylstyrene/chloromethylstyrene copolymer systems, the resist sensitivities maximize at compositions containing ca. 30% chlorostyrene. A cross-linking mechanism involving an excited-state charge-transfer interaction of adjacent methylstyrene and chlorostyrene chain units is proposed.The copolymers of optimal composition display sufficiently high lithographic sensitivities and contrasts to commend their application as electron-beam resists.
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