Alkaline‐developable positive‐type photosensitive polyimide with high mechanical strength and high resolution based on chain extendable poly(amic acid), thermally degradable cross‐linker and photoacid generator

A positive‐type photosensitive polyimide ( PSPI ) based on a chain extendable poly(amic acid) ( PAA ), a thermally degradable cross‐linker 1,3,5‐tris[(2‐vinyloxy)ethoxy]benzene ( TVEB ), and a photoacid generator (PAG) (5‐propylsulfonyloxyimino‐5H‐thiophene‐2‐ylidene)‐(2‐methylphenyl)acetonitrile ( PTMA ) has been developed. The chain extendable PAA was prepared from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride ( BPDA ) and 4,4′‐oxydianiline ( ODA ) and end‐capped with di‐ tert ‐butyl dicarbonate ( DIBOC ) in N ‐methyl‐2‐pyrrolidone (NMP), which has a controlled molecular weight for developing in a 2.38 wt% tetramethyl ammonium hydroxide aqueous solution ( TMAH aq ) and undergoes a chain extending reaction during curing stage. The photosensitive resist solution was formulated with the polymerization solution (30 wt% in NMP), TVEB (15 wt% for the polymer), and PAG (4.5 wt% for the polymer). The PSPI showed a sensitivity of 47 mJ cm −2 and a contrast of 5.8 when exposed to 365‐nm light, followed by postexposure baking at 90 °C for 10 min and development with the 2.38 wt% TMAH aq at room temperature. A fine‐positive image with 3‐μm line‐and‐space patterns was obtained on a 3‐μm thick film exposed to UV light at 365 nm in the contact‐printed mode. After thermal curing at 350 °C for 1 hr, the resulting PSPI features excellent mechanical strength and elongation.