LASER, DIRECT-WRITE MICROLITHOGRAPHY OF SOLUBLE POLYTHIOPHENES

Focused laser light of wavelength 442 nm was used to induce photochemical crosslinking in thin films of poly(3-hexylthiophene). Irradiated regions were insoluble and thus patterns of micron resolution could be achieved by dissolution of unexposed regions. The minimum gel dose required to leave an insoluble residue was 7.7 mJ cm-2 and the dose required for 50% gel formation was 61 mJ cm-2. Patterns of the polymer were oxidized with nitrosonium tetrafluoroborate. The resistance of oxidized polymer channels having dimensions 40 mum wide, 90 nm thick and 200 mum long was 150 kOMEGA. The bulk conductivity of deposited and oxidized polymer was approximately 6 OMEGA-1 cm-1. Two dominant photochemical mechanisms are present during photolysis: photosensitization of singlet oxygen and Diels-Alder addition of singlet oxygen to thienyl residues; photo-oxidation of the alkyl side chain. The latter is believed to be responsible for crosslinking and photo-insolubilization.