Lens heating challenges for negative tone develop layers with freeform illumination: A comparative study of experimental vs. simulated results

Abstract

Negative tone development (NTD) processes have been widely explored as a way to enhance the printability of dark field features such as contact holes and trenches. A key consequence of implementing NTD processes and subsequent tone reversal of dark field reticles is the significantly higher transmission of bright field masks and thus higher light intensity in the projection optics. This large increase in mask transmission coupled with the higher throughput requirements of multiple patterning and the use of freeform illumination created by source mask optimization creates a significant amount of lens heating induced aberrations that must be characterized and mitigated. In this paper, we examine the lens heating induced aberrations for high transmission reticles common to NTD using both simulations and experiments on a 193 immersion lithography tool. We observe a substantial amount of aberrations as described by even and odd order Zernike drifts during the course of a wafer exposure lot. These Zernike drifts per lot are demonstrated to have the following lithographic effects: critical dimension shifts, pitch dependent best focus shifts and image placement errors between coarse and fine patterned features. Lastly, mitigation strategies are demonstrated using various controllers and lens manipulators, including FlexWave with full Zernike control up to Z64, to substantially reduce the lens heating effects observed on-wafer. © 2012 SPIE.