Best Focus alignment through pitch strategies for Hyper NA EUV lithography

To print ever smaller features at high contrast, projection lithography technology has been developed to allow use of shorter wavelength light and to increase numerical aperture (NA) from 0.33 to 0.55. After enabling EUV wavelengths, to keep up with the scaling trends the industry would now again like to increase the NA. Since the depth of focus (DoF) is inversely proportional to the square of NA, in hyper NA EUV lithography (EUVL), we anticipate that the total available DoF in the lithography process would be further limited. Therefore, within the constrained DoF budget, it is necessary for lithographers to minimize the unwanted best focus (BF) variations generated from different pitches on a photomask. In this paper we identify how the mask 3D ( M3D) effect induced BF variation through pitch behaves according to changes in the pattern orientation and mask tonality for hyper NA EUVL. We study how various focus shift mitigation strategies can be combined to align best focus and enhance the image contrast for hyper NA EUVL.