Measurement Of Optical Performance Of Photolithographic Lenses

Abstract

In order to verify that the optimum performance of step-and-repeat camera lenses has been achieved, the imaging characteristics of the lens must be evaluated during the manufacturing cycle using methods which are independent of the camera or recording medium. At Tropel the image quality of each lens produced is quantified and correlated to design values by measuring 1) the optical transfer function (OTF) and 2) the residual distortion errors. The definition and significance of these parameters as they relate to the photolithographic process are discussed. This paper describes the OTF and distortion measuring equipment developed at Tropel and the process by which it is integrated into our manufacturing and quality control operations. This equipment 1) provides correlation data for tolerance analysis, 2) allows us to monitor final lens correction which might be required to compensate for any adverse build-up of manufacturing tolerances and 3) provides quantitative test results used for customer acceptance. The features and operation of this test equipment are discussed in detail as well as the procedures which we have used to achieve the level of accuracy required by manufacturers of cameras for direct-step-on wafers. Measured test data for several lens types are compared to computed values as well as measurements obtained from photographic test patterns taken after installation in a camera. These test results show OTF performance to be within 5% of design values and OTF measurement accuracy to be better than 2%. The distortion measurement accuracy is within 0.05pm. The effects that partially coherent pupil illumination can have on both OTF and distortion measurements are also described.