Influence of pellicle on hyper-NA imaging

Abstract

In the case of hyper-NA (NA>1) imaging with the lens magnification keeping 1/4, the angle of light incidence on pellicle becomes bigger. For example, it is up to 19 degrees for NA=1.3 lens. It is already known that the effect of multiple reflections of the light inside the pellicle film becomes obvious, in that the effect contains transmission variation across the light incidence angle on the pellicle. For normal pellicle, transmission of oblique incidence light is lower than the normal incidence light and the difference is about 10% as intensity changes. And pellicle thickness error affects the transmission characteristics. Thus, pellicle thickness error causes change of iso-dense bias (or optical proximity effect; OPE) and dense line CD variation. Specs for CD uniformity in below half pitch (hp) 45nm imaging become tighter, and therefore, pellicle should not be a new root cause of CD error. The solutions for the issue are (1) tighter specs for pellicle thickness or (2) selection of optimal pellicle thickness. The latter is more effective for suppressing CD variation across the exposure field than the former. In our paper, we describe the pellicle effect for through-pitch imaging including below hp45 nm dense L/S using hyper-NA lens. We discuss pellicle thickness optimization for better CD uniformity and the results of simulation for some pellicle conditions.