Optimizing a deep-UV projection lens with a 442-nm interferometer

Abstract

Minimization of aberrations of a microlithographic projection lens is one of the more important processes in manufacturing a wafer stepper. It is difficult to optimize a Deep UV (249 nm) projection lens without a DUV interferometer. This paper will present the technique and result of testing and optimizing a DUV projection lens with a Twyman-Green interferometer which employs a HeCd laser (442 nanometers ) for a source. Testing the DUV lens with a visible wavelength interferometer inevitably degrades the sensitivity and accuracy of the interferometric testing process due to a large change in wavelength. The minimum aberration tested and optimized by the HeCd Twyman-Green interferometer will not correspond to the best performance of the DUV projection lens. This is because there is a residue of aberrations caused by testing the lens at a wavelength other than the 249 nm wavelength for which it was designed. To solve this problem, a synthetic wavefront predicted by a lens design program at 442 nm was generated and converted to Zernike polynomial coefficients. The true aberrations of the projection lens are obtained by subtracting the synthetic wavefront from the measured wavefront.