Lithography simulation of sub-0.30-um resist features for photomask fabrication using i-line optical pattern generators

摘要

Abstract: The inorganic antireflection coating (AR3-chromium oxide) commonly used on photomask blanks was designed to minimize flare in h-line (405 nm) lithography steppers. The reflection of light (flare) off this coating (air-photomask) increases with shorter exposure wavelengths. High levels of flare occur in 248 nm and 193 nm IC steppers due to reflections off of the photomask surface. The reflections (standing waves) in photomask resists also increase during exposure using lower wavelengths of light. Lithography simulations and photomask manufacturing trials have led to resist processes that can generate sub-0.30 micron resist features on photomasks with I- line optical pattern generators (ALTA 3500). Lower developer concentration, higher exposure doses and the minimization of standing waves by incorporating a post-exposure bake and/or organic antireflection coatings (ARC) maximizes resolution. High resolution photoresists show standing waves on photomasks fabricated with optical pattern generators. Low contrast resist processes show only small standing waves or in certain cases resist 'footing.' The use of organic antireflection coatings can minimize standing waves and allow the use of high contrast resist processes. ARCs reduce the swing ratio, which improves linewidth uniformity. ARCs also improve the adhesion of the resist to the photomask surface. Simulations reveal that the optimal ARC coating thickness is around 46 nm for typical I-line systems and around 50 nm for 257 nm non- chemically amplified photomask resists. Preliminary I-line photomask manufacturing trials have been done with bottom antireflection coatings at the DPI Reticle Technology Center. The first process trials reveal that the standing waves in high resolution resists were reduced but not removed. Very precise control of the ARC thickness must be exercised to completely extinguish standing waves and careful film thickness optimization appears to be necessary to minimize these reflections. !6