Figure 27 Schematic diagram of photolithography.
Negative photoresist can be divided into photoinitiated radical
polymerization (mainly acrylic ester system) and photoinitiated cationic
polymerization (mainly epoxy system). The typical negative photoresist
consists of PISs, acrylate monomer, resin, additive and solvent, it is
mainly used in the field of PCB and display devices.
3.3.1. PCB dry film
photoresist
With the rapid development of numerous industries of electronic
information such as automotive electronics, artificial intelligence,
commercialization of 5G and wearable devices, the Printed Circuit Board
(PCB) industry has witnessed a fast growth in recent years. The
photoresist used in the PCB industry mainly includes liquid photoresist
and dry film photoresist (DFRs). Due to its unique advantages of uniform
thickness, controllable, stability, high reliability, easy operation,
low energy of exposure and high resolution, DFRs has developed rapidly,
and has been widely utilized in PCB manufacturing
process.[105-108] As displayed in Figure 28, the
polyester (PET) film, photoresist layer and polyethylene (PE) film from
bottom to top together constitute a complete
DFRs.[109] The PET film plays the role of bearing
photoresist layer, and PE film is used to avoid the deterioration caused
by the contact between photoresist layer and dusts or oxygen, in
addition, prevents adhesion between multilayer film.
The photoresist layer of DFRs generally composed of resins, monomers,
oligomers, photoinitiators, dyes and other
additives.[109-111] Resin possesses the largest
mass ratio in all components of DFRs, and directly determines the
resolution, adhesiveness, developing time and flexibility, etc. the most
commonly-used PIs for DFRs are hexarylbiimidazole (HABIs)
compounds,[43] among which
2,2-bis(2-chlorophenyl)-4,4,5,5-tetraphenyl-1,2-biimidazole (BCIM) has
been broadly employed due to its merits of good efficiency and
discoloration property. However, it always cooperates with
photosensitizers because the light absorption capacity of BCIM in
near-UV and visible light region is weak.[112] The
structures and photoinitiation mechanism of BCIM/N-phenylglycine (NPG)
systems are shown in Figure 29, firstly of all, BCIM will generate
imidazole radicals at exposure to light source, which reacts further
with NPG to generate carbon-centered radical that can initiate
polymerization reaction rapidly.[112]