Correction of Nonlinear Deflection Distortion in a Direct Exposure Electron-Beam System

This paper describes a distortion correction technique, used in a high-throughput electron-beam exposure system, which achieves an absolute deflection accuracy of about 30 parts per million (ppm) throughout a 5-mm field. To accomplish this, a cyclic, numerically controlled magnetic deflection with an accuracy of about 1000 ppm and high repeatability is first established. Horizontal and vertical errors in this deflection are measured by sensing the apparent locations of features in a calibration grid that is placed in the exposure field of the beam. The measured error is smoothed by means of a two-dimensional spline-fitting method, and the parameters for the correction surfaces are calculated. Corrections to position and speed, defined by a set of digital tables, are superimposed on the basic deflection, and the process is iterated until acceptable precision is achieved. Parameters relating registration scans to field-writing scans are then calculated for use in the registration of all written fields.