Unique optical design features were incorporated into the Model 900 Projection Stepper. The f/4 illuminator uses a pulsed 200 w mercury short arc lamp and a glass light pipe to achieve a uniform intensity of .5 w/cm2 at the reticle. The 1:1 projection lens is a folded, double-pass design which consists of a concave mirror and a cemented achromat-prism assembly. With a numerical aperture of .30, the lens achieves diffraction-limited performance at both the g and h mercury lines. Reticle to wafer alignment is detected through the lens and corrected automatically at each exposure step.