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]