Resolution test specimen for evaluating the performance of electron deflection systems

Abstract

A resolution test pattern is described which was developed to evaluate the performance of a large field deflection system incorporating dynamic astigmatism correction. The spot diameters of interest ranged from 1 to 10 μm and a convenient, rapid, repeatable measurement method was required. A further requirement was that the test specimen should respond to an astigmatic spot, and give a clear indication of the magnitude and direction of the astigmatism, so that stigmator corrections could be quickly determined. Because of these requirements conventional test specimens such as metallic grids could not be used, and so a special test specimen was designed. This consists of a series of concentric octagonal gold rings, fabricated on a silicon substrate. The width of the rings varies uniformly from 0.9 to 7.0 μm, and the diameter of the complete test specimen is 140 μm. Three principal applications have been investigated. Firstly, the magnitude and direction of deflection astigmatism have been measured by raster scanning the tangential and saggital line foci over the test specimen. Secondly, spot diameters habe been estimated by raster scanning over the test specimen, and noting the radius of the octagon at which contrast reversal occurs: this is dependent on the spot diameter. Thirdly, to make quantitative spot diameter measurements, the test specimen has been scanned in a line and the resulting video signal displayed as an ’’A’’ trace on an oscilloscope: modulation of the waveform is a measure of the diameter of the spot in the direction of scanned line.