Device Photolithography: Thin Photosensitive Materials

In this paper we discuss some recent developments in thin recording media in light of their suitability for exposure in the new step-andrepeat camera sj^stem. The requirements of integrated-circuit pattern generation have led to the development of a wide-field objective lens for this camera having high numerical aperture (N. A.) corrected for diffraction limited performance using monochromatic light. Specifically the lens has a 7.1-mm field diameter, //1.5 at 10:1 conjugate ratio, and is corrected for A = 436 nm. This lens has a depth of focus shallower than the thickness of the photosensitive emulsion on the thinnest high-resolution photographic plates. Kodak High Resolution Plates (KHRP) consist of a 6-/xm-thick Lippman-type emulsion of small ( 0.1 /xm) silver halide grains in gelatin on a flat glass substrate. For any projection lens of /-number less than f/1.7 the depth of focus is less than 6 //.m. This is illustrated in Fig. 1 which is an idealized ray diagram, drawn to scale, showing in cross section a 6-/xm-thick photographic emulsion of refractive index 1.56 into which a linear array of diffraction-limited spots on l-//,m centers have been projected through an f/1.5 lens using 436-nm light. Even a perfect lens images a point source as a diffraction patch, the Airy Disc, having a radius r to the first dark ring given by: 2170