Contact hole imaging characteristics from projection lithography.
Image intensity profiles (IIPs) are calculated for selected cross sections of contact hole arrays for a circular source and objective in the presence of the defocus aberration. The IIPs for selected cross sections of contact holes are compared with IlPs from correspondingly dimensioned gratings. At resolution levels equal to or greater than 0.9 lambda/N.A., the predicted contact hole image quality should be similar to that attainable from correspondingly dimensioned line-space gratings. Although the shapes of the contact holes appear to be predicted well, in some cases the experimental contact hole to grating dimensional comparison indicates significantly less than perfect imaging characteristics. Advanced lens systems with similar resolution ratios but different depth of focus characteristics are also compared for contact hole imaging. Contact hole imaging characteristics are much more sensitive to residual aberrations in the optical lens system and can provide a convenient measure of the relative magnitude of these aberrations.