Prospects for using existing resists for evaluating 157-nm imaging systems

Lithography at 157 nn represents the next evolutionary step in the Great Optical Continuum and is currently under investigation as a possible successor to 193-nm lithography. If successful, the photoresists used for this technology must be initially capable of 100-nm resolution and be extendable to less than 70 nn. Unfortunately, as with the transition to shorter wavelengths in the past, the photoresist materials developed for longer wavelengths appear to be too absorbent for practical use as a traditional high resolution single layer resist imageable with 157 nm radiation. Until new photoresist materials are developed that are sufficiently transparent to be used as single layer resists, the existing need for a resist to be used to evaluate 157 nn imaging systems, such as the prototype steppers now under development, will have to be met by employing existing resists. We have surveyed the commercial resist market with the dual purpose of identifying the general categories of commercial resists that have potential for use as tool evaluation resist and to baseline these resists for comparison against future 157 nn resist candidates. Little difference was observed in the 157-nm absorbance between different classes of resists with most resists having an absorbance between 6 and 8 per micron. Due to the high absorbance at 157 nn of polyhydroxystyrene, polyacrylate, and polycyclic copolymer based resists, the coated resist thickness will need to be under 100 mm. All four commercial resists evaluated for imaging at 157 nn showed that they are capable of acting as a tool testing resist to identify issues attributed to focus, illumination, and vibration. Finally, an improved tool testing resist can be developed within the existing resist material base, that is capable of 100 nn imaging with a binary mask and 70 nm imaging with a phase shift mask. Minor formulation modification can greatly improve resist performance including improved resolution and reduced line edge roughness.