Abstract: The direction, $phi@, and magnitude, A$-1$/, of residual astigmatism in microlithography lenses used for semiconductor circuit fabrication is determined by measuring the focal position, F, of lines orientated at four values of $Theta equals 0 degrees, 45 degrees, 90 degrees, 135 degrees. These parameters are related by F equals A$-0$/ plus A$-1$/ cos2($Theta $PLU $phi@) which is solved for the four measured values of F. If the lens is axially symmetric the angle $phi will be that of the field diameter, but real lenses have fabrication errors that may introduce non-classical astigmatism, so $phi may have values 0 less than or equal to $phi less than or equal to $pi at any point in the field. It is for this reason that conventional resolution reticles with perpendicular resolution targets are inadequate to accurately measure residual astigmatism. Using such a reticle will result in under estimation of the actual astigmatism. Wafers are exposed through focus using a reticle having an array of resolution targets, each having the four orientations. Measuring the focal position of each of the four orientations by examination of their photoresist images with a dark field optical microscope enables determination of astigmatism with a standard deviation 7 nm. Application of this procedure used to evaluate the residual astigmatism in high quality lithography lens is reported. !6
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