Hexaarylbiimidazole-based polyurethane positive photoresists for UV lithography

Photochromic hexaarylbiimidazoles (HABIs) enable reversible transformation between HABI and triphenylimidazole radicals (TPIRs) upon UV light irradiation, which have been applied in photo-patterning, super-resolution imaging, photoinduced self-healing materials and photomechanical hydrogels. Here, we designed and prepared a novel kind of polyurethane (PU) photoresist based on photo-cleavable HABI cross-linkers (HABI-PU), linear PU backbone and additive tert-butylhydroquinone (TBHQ) radical quencher. Upon UV light irradiation, the C-N bond inside HABI unit is broken, causing the transformation of HABI into two TPIRs which are subsequently quenched by TBHQ. Therefore, the cross-linking points of HABI-PU in UV irradiation area were broken, forming flexible PU polymer chains with declined molecular weight, which can be dissolved in developer. HABI-PU films show excellent photosensitivity as a positive photoresist in photolithography. The reaction mechanism between HABI and TBHQ was studied by UV-vis absorption spectra, electron paramagnetic resonance (EPR), mass spectra and nuclear magnetic resonance spectra. The lithographic patterns were characterized through metallurgical microscope and scanning electron microscope with a resolution at about 5 µm.