Distortion correction and deflection calibration by means of laser interferometry in an electron-beam exposure system

Abstract

In order to assure accuracy in an electron-beam exposure system, it is necessary to correct for deflection distortion relative to the standard axes of the system. Distortion up to the third order is measured and corrected on-line in a vector-scan system, which uses a laser interferometry positioned stage, a fiducial mark, and a digital–analog correction circuit. The programmed precedure involves moving the stage to positions which form 5×5 or 9×9 lattice points in a region about 2 mm square. At each stage position, the beam is scanned to find the fiducial mark at a position in the scan field, by detecting backscattered electrons. Mark searching is done twice at each position, with two, predetermined, linearly independent settings of the correction circuit. A set of linear equations is solved using the least-square method, to find out the distortion coefficient as well as the correction setting to compensate for the distortion. The above routine takes about 2–3 min including the stage motion, mark search, and calculation. It has been proven that distortion can be corrected to within ±0.1 μm ensuring the field stitching accuracy. It has also turned out that this method is useful in investigating the nature of the deflection subsystem.