Evaluation of lens heating effect in high transmission NTD processes at the 20nm technology node

The NTD (Negative Tone Developer) process has been embraced as a viable alternative to traditionally, more conventional, positive tone develop processes. Advanced technology nodes have necessitated the adopting of NTD processes to achieve such tight design specifications in critical dimensions. Dark field contact layers are prime candidates for NTD processing due to its high imaging contrast. However, reticles used in NTD processes are highly transparent. The transmission rate of those masks can be over 85%. Consequently, lens heating effects result in a non-trivial impact that can limit NTD usability in a high volume mass production environment. At the same time, Source Mask Optimized (SMO) freeform pupils have become popular. This can also result in untoward lens heating effects which are localized in the lens. This can result in a unique drift behavior with each Zernike throughout the exposing of wafers. In this paper, we present our experience and lessons learned from lens heating with NTD processes. The results of this study indicate that lens heating makes impact on drift behavior of each Zernike during exposure while source pupil shape make an impact on the amplitude of Zernike drift. Existing lens models should be finely tuned to establish the correct compensation for drift. Computational modeling for lens heating can be considered as one of these opportunities. Pattern shapes, such as dense and iso pattern, can have different drift behavior during lens heating.