Photosensitive polyimide (PSPI) involves photopatternable polyimide (PI) with photoactive agents and their precursors, such as poly(amic acid) (PAA) or poly(amic ester), and photosensitive compounds [1]. It is a kind of organic material with an imine ring and a photosensitive gene on the polymer chain that has excellent thermal stability, good mechanical properties, and chemical and photosensitive properties. Some of them can be used like photoresists, with both “positive” and “negative” types.
Under UV light irradiation, PSPI resist undergoes chemical reactions involving chain scission, crosslinking, and deprotection that change the solubility of exposed areas and non-exposed areas in the developer, resulting in the pattern of the mask plate transferring to the resist layer and forming a desired pattern on the PI film. The exposure light source usually chooses g-line (436 nm), h-line (405 nm), or i-line (365 nm) UV. Alfa chemistry offers a kind of PSPI that acts as a resist in the process of UV irradiation.
Synthesis
PSPI involves photopatternable PI whose main chain contains an imide structure. The synthesis of PI is generally divided into two steps: polycondensation and cyclization. First, tetracarboxylic dianhydride and diamine are polycondensed to generate the polyimide precursor PAA, and then dehydration and cyclization become polyimide (heat or chemical reaction), as shown in Fig. 1. The photolithographic characterization can be evaluated directly from the polymerization solution (varnish) because of its simple chemistry and environmentally benign process, making this synthesis method suitable for industrial manufacturing [1].
Properties
PI is widely used in many fields such as power electronics, printed circuits, novel displays, and energy conversion due to its excellent thermal stability, mechanical properties, and dielectric properties. It also can be used as a buffer layer, a passivation layer, a planarization layer, or an interlayer dielectric layer in integrated circuits.
Generally, the properties of PSPIs that need attention include thermal stability, mechanical properties, dimensional stability, dielectric properties, adhesive properties, and chemical corrosion resistance, especially photosensitive characteristics and the final patterned film characteristics. The photosensitivity depends on that of the PSPI system and the molecular structure of the polymer while the final patterned film properties are affected by the PI structure and the heat process.
Advantages
PSPI resist has a more simple and convenient photolithography process compared with that of conventional resists. The photolithographic patterning process of the conventional and the PSPI resists are shown in Fig. 2. PSPI still remains in a specific area to form the dielectric insulating layer required for the device after patterning, while conventional photoresist requires removal after transferring patterns to the substrate.
Besides, PSPI resist saves material costs, significantly shortens integrated circuit manufacturing, and improves photolithographic pattern accuracy and yield. Therefore, PSPI resist becomes an ideal material in the electronics and microelectronics fields [2].
