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17 October 2008 Effects of heated substrates on bimetallic thermal resist for lithography and grayscale photomask applications
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Bimetallic thin-films of Bi/In act as negative thermal resists when laser exposure pulse (7mJ/sq. cm for 4 nsec) converts the film into a transparent eutectic metallic oxide alloy. Resist transparency varies with exposed laser power, changing from <0.1% (3.0 OD) unexposed to >60% (0.22 OD) exposed. This generates direct-write gray scale photomasks, and adding a feedback system where the transparency is measured and adjusts the writing process to account for local variations in the film, achieves >64 gray level control. These resists are also wavelength invariant, operating from visible to EUV with a resolution >42nm after development using a diluted RCA-2 solution (HCl:H2O2:H20 @ 1:1:48) with a gamma of 2-18. Longer duration exposures with lower instantaneous intensities result in lower gammas, while shorter exposures with higher energies give higher gammas. One limitation on these resists is that the exposure energy must be delivered in a single pulse. This limitation puts pulse energy requirements into the mJ per pulse range: greater than desired for EUV exposure systems. Bimetallic thermal resists remain almost unaffected during a sub-threshold exposure that does not reach the activation energy. It has been shown that the resist and substrate can be heated below the threshold energy, to temperatures of at least 90°C, without creating any exposure of the resist. In this research, Bi/In resists are heated through a range of substrate temperatures, measured for their optical exposure requirements and gammas under these conditions, and used to determine if substrate heating can improve the film's sensitivity.
© (2008) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
James M. Dykes, Polly Tsui, Jenny Leung, and Glenn H. Chapman "Effects of heated substrates on bimetallic thermal resist for lithography and grayscale photomask applications", Proc. SPIE 7122, Photomask Technology 2008, 71220M (17 October 2008);

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