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 degrees C for 10 min and development with the 2.38 wt% TMAH( aq) at room temperature. A fine-positive image with 3-mu m line-and-space patterns was obtained on a 3-mu m thick film exposed to UV light at 365 nm in the contact-printed mode. After thermal curing at 350 degrees C for 1 hr, the resulting PSPI features excellent mechanical strength and elongation.