A model for comparing process latitude in ultraviolet, deep-ultraviolet, and x-ray lithography

Abstract

It is widely recognized that process latitude is an important figure of merit in microlithography. We derive a general expression for process latitude based on a model for lithography that takes into account linewidth control, contrast of the imaging system, diffraction, resist contrast, nonuniformity of illumination, and shot noise. We define a normalized process latitude parameter (NPL) that enables one to compare quantitatively the various optical projection systems and x-ray lithography in the submicron linewidth domain. NPL is plotted as a function of linewidth for two optical projection systems [λ=436 nm, numerical aperture (NA)=0.42; λ=193 nm, NA=0.4] at perfect focus and at 0.5 μm out of focus. These plots are compared with a plot of NPL for x-ray lithography, from 1- to 0.25-μm linewidth, using a mask-to-sample gap of 20 μm and a laser-plasma source centered at λ=1.24 nm. The comparison verifies what others have observed experimentally: that x-ray lithography offers a significant advantage in process latitude.