Editing Point spread function (section)

=== Lithography ===
[[File:Airy_spot_overlap.png|thumb|left|300px|'''Overlapped PSF peaks.''' When the peaks are as close as ~ 1 wavelength/NA, they are effectively merged. The FWHM is ~ 0.6 wavelength/NA at this point.]]
The PSF is also a fundamental limit to the conventional focused imaging of a hole,<ref name=nat_res>[http://www.lithoguru.com/scientist/litho_tutor/TUTOR23%20(Fall%2098).pdf The Natural Resolution]</ref> with the minimum printed size being in the range of 0.6-0.7 wavelength/NA, with NA being the [[numerical aperture]] of the imaging system.<ref>[https://www.weizmann.ac.il/mcb/ZviKam/ALM/L2_Resolution.pdf Principles and Practice of Light Microscopy]</ref><ref>[http://ww.lithoguru.com/scientist/litho_papers/2000_103_Corner%20Rounding%20and%20Line-end%20Shortening%20in%20OL.pdf Corner Rounding and Line-end Shortening]</ref> For example, in the case of an [[extreme ultraviolet lithography|EUV]] system with wavelength of 13.5 nm and NA=0.33, the minimum individual hole size that can be imaged is in the range of 25-29 nm. A [[phase-shift mask]] has 180-degree phase edges which allow finer resolution.<ref name=nat_res/>