2.3.2 Condenser Lens

As shown in Figure 2.4 the condenser lens is set up in a way that the position of the light source coincides with its focal plane. Such a configuration is called Köhler illumination [11, pp. 524-526] and has the important property that an ideal point source contributes a plane wave disturbance on the mask side of the condenser lens. The entire photomask is thus uniformly illuminated. Any deviation from the plane wave shape causes phase errors in image composition and strongly degenerates imaging quality.