A negative-type photosensitive poly(ether ether sulfone) (PSPEES) based on a commercially available engineering plastic poly (oxybiphenyl-4,4 0 -diyloxy-1,4-phenylenesulfonyl-1,4-phenylene) (PEES), a cross-linker 4,4 0 -methylenebis[2,6-bis(methoxymethyl)phenol] (MBMP) having good compatibility with PEES, and diphenylidonium 9,10-dimethoxyanthracene-2-sulfonate (DIAS) as a photoacid generator (PAG) has been developed. The resist consisting of PEES (85 wt %), MBMP (10 wt %) and DIAS (5 wt %) showed a high sensitivity (D 0:5 ) of 21 mJ/cm 2 and a contrast ( 0:5 ) of 2.1 when it was exposed to i-line (365 nm wavelength light), post-exposure baked (PEB) at 170 C for 3 min, and developed with N,Ndimethylacetamide (DMAc) at room temperature. A fine negative-image featuring 4 mm line-and-space pattern was obtained on the film. Thus, this photolithographic system has a highly potential to be utilized in the industry for next generation, because most of engineering plastics having aromatic ring are available as matrix polymers, and this process is not necessary high thermal curing treatment after development.KEY WORDS: Photosensitive Polymer / Photoacid Generator / Poly(ether ether sulfone) / Cross-linker / High-performance engineering plastics such as polyimides (PIs), polybenzoxazoles (PBOs), poly(ether ether sulfone)s (PEESs), poly(phenylene ether)s (PPEs), and poly(ether ether keton)s (PEEKs) are continuously receiving considerable attention for their potential application in automobile, electronics, optical devices and related markets. Particularly, photosensitive polyimides (PSPIs) and polybenzoxazoles (PSPBOs) are widely used as stress buffer and insulation layers of multichip package in micro-electronics field, because they possess excellent thermal stability and mechanical properties, and reasonably low dielectric constant.1-3 As PIs and PBOs are generally not soluble in organic solvents, poly(amic acid)s (PAAs) and poly(o-hydroxy amide)s are used as PI and PBO precursors, respectively, and require an extremely high temperature about 350 C to convert into corresponding PIs and PBOs. This high-temperature process is not applicable to electronic applications because the thickness of silicon wafers is less than 100 mm, and thus the less thermal stress is required. Therefore, we have reported a series of low temperature cyclization methods for PSPIs and PSPBOs to produce more useful applications in the micro-electronics industry. [3][4][5][6][7] It is also important to develop a photosensitive and thermally stable polymer (PSTSP) which does not require high thermal curing treatment.In previous papers, we reported a negative-type PSPI based on PAA, a cross-linker 4,4 0 -methylenebis[2,6-bis(hydroxylmethyl)]phenol (MBHP), and (5-propylsulfonyl oxyimino-5H-thiophene-2-ylidene)-2-(methylphenyl)acetonitrile (PTMA) as a PAG.8,9 A negative-type photosensitive polymer based on poly(naphthylene ether), PTMA, and hexa(methoxymethyl)-melamine as a cross-linker was also reported. 10 These findings prompted us to develop a PST...