An investigation of some depth of focus issues in high numerical aperture projection lithography system by experiment and simulation

We have examined the imaging of 0.3 μm equal line/space patterns, in 1 μm of Shipley SN248 resist (referred to as XP8843, in this paper) using projection lithography with deep-UV exposure (248 nm). Experimentally, we have observed some interesting development effects for these small features with high aspect ratios, which highlight the role of developer agitation when using this resist. Our simulations, using a vector model to simulate the exposure, show that because of the high contrast of this resist, there is a greater depth of focus for printing 0.3 μm line space patterns when using a numerical aperture (NA) of 0.42 than 0.6.

Interestingly, we see that the process window, when using a NA of 0.6 is only slightly greater when using a thin (0.3 μm) resist than when using a thick (1 μm) resist. This is due to the low absorption of the resist at 248 nm. Unfortunately the low absorption of the resist makes it extremely sensitive to thickness changes when printing on highly reflective substrates, e.g., bare silicon.