| 摘要 |
Aberration of an electron beam (8) due to X- and Y-direction astigmatism and field curvature of focus is corrected by feeding correction currents through stigmator coils (10X, 10Y) and a focus correction coil (11). These correction currents (Ix, Iy, IF) are determined from measurements of a combination sharpness (P) of the electron beam (8). This combination sharpness (P) is dependent upon both a sharpness (px) measured when the electron beam (8) is deflected in the X-direction, and a sharpness (py) measured when the electron beam is deflected in the Y-direction. For example the combination sharpness (P) is the product of the X and Y direction deflection sharpnesses (P = Px x py). The convergence of measurements of the combination sharpness is very good. It is thus possible using the combination sharpness to calibrate the electron beam (8) for many points in its scanning field. It becomes possible to attain or more nearly attain maximum resolution all over the scanning field. A correction current is obtained from measurements relating to several points around the measuring point, and by using at least squares method approximating the measured values to a quadratic curve. |
