Coping with the impact of lens aberrations in the context of wavefront engineering

Abstract

Variations of lens aberrations of optical projection systems can have undesirable effects on critical dimension (CD) uniformity and depth of focus (DOF) of printed microelectronic circuit patterns. The principal objective of this paper is to investigate how lens aberrations along with variations of partial coherence of the illumination source of an optical stepper affect critical dimensions of dark gate lines when using conventional and phase-shifting masks (PSMs) with and without optical proximity corrections (OPC). The investigations are performed using lithography simulation software tools which help to evaluate different optical projection systems and diverse types of masks. For the purpose of accurate evaluation of the effects of different types of aberrations on printed patterns, 37 Zernike polynomial coefficients representing lens aberrations were normalized using the Strehl test. The impact of aberrations on 0.25 micrometer and 0.18 micrometer dark gate lines is studied by analyzing data obtained from simulations using four different optical projection system set-ups. The results show that lens aberrations do not significantly reduce CD uniformity and DOF or destroy the process window if we use an optimal numerical aperture (NA) and high resist contrast. It was observed that high resist contrast is more important than NA in terms of dealing with the impact of lens aberrations. The effectiveness of masks with OPC for aberrated images was analyzed, and we have been able to show that OPC does not always improve CD uniformity and DOF. This paper describes methods for maintaining tighter control of CD errors in the manufacturing process of integrated circuits using optical lithography.