Measurment of astigmatism in microlithography lenses

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) + (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.