A large field i-line lens has been developed for submicron IC manufacturing. This lens has a field size of 22 mm X 44 mm (total area of 9.68 cm2), which is the largest field size of any current submicron microlithographic lens. A primary consideration in the development of this lens is its overall distortion, which must be small enough to provide total overlay consistent with submicron design rules. This paper presents distortion characteristics of the Ultratech Stepper Model 2244i lens. Models that can be used to predict the lens distortion and result in minimizing absolute distortion have been developed. The technique used to minimize the lens distortion of the 2244i, along with the effectiveness and the application, also is described. We review the utility of this process for matching new lenses to a customer's existing lenses, and we show how this process supports the mix-and-match of this lens's large field size with the smaller fields of reduction lenses.
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.
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