Figure 33 A proposed photolithography based on radical and cationic dual-curing photoresists.[109]
The most commonly-used photoresist of cationic photopolymerization is SU-8, which is a UV photoresist product specially designed for Micro-Electro-Mechanical System applications with high aspect ratio.[119-120] The main component of SU-8 is a multi-function group and multi-branch epoxy resin, which is synthesized by the condensation reaction of bisphenol and glycerol ether, ideal structural formula of SU-8 is shown in Figure 34.[121] The PIs used in SU-8 series photoresist is generally an onium salt, mainly sulfonium salt, which can produce a strong acid under irradiation to polymerize the epoxy groups.[122]
Figure 34 Ideal structural formula of SU-8.

3.3.2. UV nanoimprint lithography photoresist

As an emerging high-resolution patterning method, nanoimprint lithography (NIL) provides a technology that could replicate the nanoscale patterns below 10 nm, and is a promising solution to the restriction of exposure wavelength in photolithography.[123] NIL has overcome tremendous challenges over the past 20 years to become a realistic method for commercial semiconductor production.[124]UV-nanoimprinting lithography (UV-NIL) is regarded as the new next-generation lithography technique due to the advantages of high throughput, good resolution, and low manufacturing cost, room-temperature operation,[123, 125] and has been receiving increasing attention in many fields such as electronic, photonic, LED, magnetic and semiconductor devices.[125-127]
The cross-linker plays a very important role in the component of traditional UV-NIL photoresist, which determines the mechanical and chemical resistance to a certain extent. However, the high cross-linked structure is difficult to strip from the mold of UV-NIL, conversely, the linear structure displays better solubleness in solvent.[128] Yin et al.[129] designed a UV-NIL resist in which the structure contains photoreversible coumarin derivative as degradable cross-linker. The chemical structures of degradable cross-linker (AHAMC), monomer phenoxy ethyleneglycol acrylate (AMP-10G), and the mechanism of photodimerization and photocleavage of the dimer-coumarin-bridged polymer are exhibited in Figure 35. Through photodimerization of coumarin moieties under irradiation at 365 nm UV light, and photocleaved by 254 nm UV light, the crosslinking and uncrosslinking can be implemented to protect the mold.
Based on the photoreversible coumarin derivatives, Wei et al.[130] reported a pH-UV dual-responsive photoresist for UV-NIL that improves mold release, the chemical structures of the photoreversible cross-linker 5,7-diacryloyloxy-4-methylcoumarin (DAMC), acrylic anhydride (ALA), 3,6-dioxa-1,8-dithiooctane (EGDT) and the PI DMPA for the dual-responsive resist are displayed in Figure 36. The mechanism of degradation for dual-responsive resist is displayed in Figure 37, the cross-linked networks can be photocleaved by 254 nm UV light and degraded in alkaline aqueous solution, which contributes to protect UV-NIL mold and reduce damages in process of imprinting patterns.
Figure 35 Mechanism of photodimerization and photocleavage of the dimer-coumarin-bridged polymer.[129]
Figure 36 Chemical structures of each component for the dual-responsive resist.[130]
Figure 37 Mechanism of degradation for dual-responsive resist.[130]
Volume shrinkage is inevitable due to the van der Waals distance before polymerization becomes the covalent distance after polymerization, consequently, volume shrinkage has a negative impact on the patterns transfer in UV-NIL.[131] It is necessary and meaningful to reduce volume shrinkage in UV-NIL field. Based on the disulfide bond of reducing volume shrinkage and endowing materials with the degradability. Sun et al.[125, 132]synthesized two disulfide bond-containing acrylate monomers 2,2-dithiodiethanol diacrylate (DTDA) and disulfanediyl bis (1,4-phenylene) diacrylate (ADSDA) used for UV-NIL. The chemical structures of DTDA and ADSDA are exhibited in Figure 38. For the photoresist containing DTDA, it underwent a repeated “contraction–expansion–contraction” volume-modulatory process during the polymerization because of the dynamic reversible property of homolysis and recombination of disulfide bonds, which was conducive to releasing stress and reducing volumetric shrinkage. Such as the DTDA/MMA/IOBA system, the minimum rate of volume shrinkage can drop to 0.93 %. The mechanism of reducing volume shrinkage is displayed in Figure 38.
Figure 38 Chemical structures of DTDA and ADSDA.[125, 132]