This is a plot showing the depth of focus, measured in nanometers, as a function of numerical aperture, for the currently considered leading edge lithography tools. KrF means 248 nm wavelength, ArF means 193 nm wavelength, and EUV means 13.5 nm wavelength. "Wet" means water is used as the imaging medium (n=1.44), while "Dry" means air is used (n=1). EUV requires a vacuum for the imaging medium (n=1). The depth of focus formula used is 0.25*wavelength/(sin(angle/2))^2, where angle corresponds to the numerical aperture.