Ion controlling electrode assembly for a scanning electron beam computed

Abstract

An electron beam scanning system employing a relatively short housing chamber wherein an electron beam is produced includes an ion controlling electrode assembly. Located in the housing between the electron gun and system beam optics, the assembly includes a generally cone-shaped rotating field ion controlling electrode (or 'RICE') unit comprising cylindrically symmetrical element pairs disposed on opposite sides of the housing Z-axis. Preferably equal and opposite potential sources coupled to elements comprising an element pair create a transverse electric field therebetween. The vector sum of the fields produced by all element pairs is the transverse field created by the RICE unit. The potentials are varied, rotating the overall RICE field to controllably remove most but not all positive ions. The remaining ions improve the electron beam space-charge density, resulting in a sharply focused scanning electron beam. Preferably a disk-like positive ion electrode (or 'PIE') unit coupled to a large positive potential is disposed downstream from the RICE unit to block upstream migration of positive ions. Where discontinuities are present in the housing, a periodic axial field ion controlling electrode (or 'PICE') unit is disposed at the upstream end of the overall assembly. The PICE comprises spaced-apart disks alternately coupled to large and small potentials to create alternating axial fields within a short axial distance, to rapidly sweep away ions. Regions within the overall assembly not otherwise acted upon by fields are covered with one or more conventional ICE units to sweep away positive ions.