Electron beam source employing a photo-emitter cathode

An electron beam system for exposing at least one target area in a target plane to a beam spot focused in the target plane, said system comprising:

• diode gun emitter means for emitting a beam of electrons in parabolic trajectories, said diode gun emitter means comprising a source of light energy and a photocathode responsive to said light energy for emitting said electrons from an emitter surface of said photo-cathode;
• an apertured anode through which the emitted electrons pass, the tangents to the parabolic trajectories at said anode intersecting in a virtual cathode plane from which the electrons appear to emerge as generally conical beam pencils having a first semi-angle .alpha..sub.1 ;
• wherein said apertured anode includes a spot-shaping aperture through which the electrons pass in passing through said apertured anode;
• focusing means for focusing said electrons passing through said spot-shaping aperture into a plurality of converging beam pencils converging at the target plane at a second semi-angle .alpha..sub.s within a beam spot area in said target plane, said beam spot area having a dimension b defined by said spot-shaping aperture and said focusing means;
• area-defining means for selectively defining an electron-emitting area portion of said emitter surface such that electrons are emitted only from said electron-emitting area, said electron-emitting area having at least one selectable dimension s corresponding to the dimension b of said beam spot area and established equal to or greater than t(b.alpha..sub.1 /p.alpha..sub.s), wherein p is a truncation factor having a value less than one by which said beam pencils are truncated to define .alpha..sub.1, and t is a compensation factor greater than one accounting for imperfect imaging, beam jitter and noise;
• whereby current density and uniformity of the combination of said electrons at said target plane, within said beam spot area, are maximized by reduced electron-to-electron interaction.