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]