Abstract: The forbidden pitch "dip" in the critical dimension (CD) through the pitch curve is a well-known optical proximity effect.The CD and CD process window near the "dip" ,usually found near a pitch range of 1.1 to 1.4 wavelength/NA (numerical aperture),is smaller when compared with other pitches.This is caused by inadequate imaging contrast for an unequal line and space grating.Although this effect is relatively well-known,its relationship with typical process condition parameters,such as the effective image blur caused by the photo-acid diffusion during the post exposure bake or the aberration in the imaging lens,has not been systematically studied.In this paper,we will examine the correlation between the image blur and the effect on the CD,including the decrease in the CD value (the depth of the "dip" ) and the CD process window.We find that both the decrease in the CD value and the focus latitude near the forbidden pitch correlate very well with the effective Gaussian image blur.Longer effective diffusion length correlates well with a smaller process window and a deeper CD "dip" .We conclude that the dip depth is very sensitive to the change in image contrast.

Key words: forbidden pitch, effective resist diffusion length, OPC, OAI, deep-UV, 深紫外線