The crossover of a thermionic electron gun acts as the virtual source in conventional electron beam lithography systems employing critical illumination as well as those employing Koehler illumination The concept of critical Koehler illumination combines critical illumination and Koehler illumination to image the cathode surface as the real source of the electrons into the probe shaping aperture and finally onto the target plane (window ray trace). Further on, the crossover as the virtual exit pupil of the gun lens is imaged into the entrance pupil of the final lens (pupil ray trace). Thus, in critical Koehler illumination, the linked beam concept consequently correlates conjugate planes of a probe forming system to those of the electron gun. The implementation of critical Koehler illumination in shaped beam lithography systems requires a special imaging mode of the electron gun. A typical triode gun can provide imaging of the emitter surface. Critical Koehler illumination has a high current efficiency, and the cathode supplies only the number of electrons needed in the target plane, so that maximum cathode lifetime and minimum electron-electron interaction in the beam result. Beam stability and spot image quality are substantially improved.
