A systematic approach to determining the limits of nanolithography at extreme NA

Abstract

In optical imaging, separating the impact of the projection tool from the characteristics of the detector is key to understanding the limits of any exposure system. In semiconductor photolithography the detector is photoresist, a polymer that undergoes a photochemical reaction on the molecular scale, with typical feature sizes of 32 nm or less. Separating the performance of the optical system and the detector is thus complicated by the lack of availability of any other suitable detector, as well as lack of access to pupil plane of the projector tool. In this research a novel interferometric exposure tool is presented that allows exposure of photoresist with an image of known quality. Exposures can be carried out in the normal process flow of semiconductor lithography, allowing characterization of the detector using a known optical image. The interferometric stepper was designed and fabricated, and experiments were performed to study the resolution of several commonly available photoresists. A technique for characterizing the collected data in the form of Photoresist Modulation Curves was developed and the photoresist performance thresholds are defined. Combination of the exposure and the analysis technique allows for a better understanding of ultimate capability of the photolithography system, and examples are presented.