High emittance electron gun for projection lithography

The source requirements for a high‐throughput projection electron‐beam stepper are radically different than those for more typical focused beam systems. The SCALPEL®, proof‐of‐concept stepper requires a uniform, 1 mm2 parallel beam at the mask plane with a convergence angle of 0.5 mrad and a beam current on the order of 10 μA. These requirements translate to an axial brightness of ∼1000 A/cm2 sr which is many orders of magnitude less than a typical direct‐write electron‐beam system. Another characteristic of a radiation source is emittance, the product of beam size, and angular extent. Unlike axial brightness, this property is not conserved throughout the optical column but is reduced by the presence of apertures. The required emittance of the SCALPEL system is ∼700 μm mrad at the mask and wafer plane. For comparison, a typical direct‐write system might have an emittance of ∼1 μm mrad at the wafer plane. In principle, the gun emittance requirement can be met with any combination of source size and angular...