ABSTRACT
A number of industrial manufacturing processes and imaging systems require lenses capable of very high resolving power (RP) often at the theoretical limits of diffraction-limited lenses. Microimaging lenses are used both to make the mask and for its unit projection. Optical limits, even with positive photoresists of high RP, give a minimum attainable line width of about 0.5 µm. Thereafter, UV, electron-beam and X-ray lithography techniques are required, to utilize shorter wavelengths, giving features of 0.25, 0.1 and 0.1 µm respectively. High resolution implies as short a wavelength as possible, but silver halide maximum resolution materials scatter such light significantly even in their thin emulsions; this can be reduced by the use of anti-scatter agents. Highly symmetrical configurations with negative elements are used to give the necessary low-distortion and flat-field performance, but combinations of retro-focus with double Gauss derivatives as rear groups are also used.
