Method and apparatus for automatically correcting astigmatism of scanning electron microscope or the like

A method of automatically correcting the astigmatism of a scanning electron microscope or other similar apparatus equipped with a focusing lens system for focusing an electron beam onto a specimen and equipped with an xy solenoid stigmator for correcting the astigmatism of the focusing lens system, comprising the steps of:

• (a) scanning the specimen with the electron beam in a first direction;
• (b) supplying correcting currents I.sub.x and I.sub.y to the x-axis and y-axis coils, respectively, of the xy stigmator;
• (c) converting the signal emanating from the specimen into a signal D indicating the diameter of the electron beam in the direction of scan and storing the resulting data in a memory;
• (d) forming a first I.sub.x -I.sub.y -D map regarding the first direction of scan by repeating the steps (b) and (c) while systematically varying the values of the correcting currents I.sub.x and I.sub.y fed to the x-axis and y-axis coils, respectively;
• (e) forming a second I.sub.x -I.sub.y -D map regarding the second direction of scan perpendicular to the first direction of scan by repeating the steps (b) and (c) while systematically varying the values of the correcting currents I.sub.x and I.sub.y after changing the direction of scan to the second direction;
• (f) finding such combinations of correcting currents I.sub.xa, I.sub.ya and I.sub.xb, I.sub.yb flowing into the x-axis and y-axis coils which minimize the diameter of the beam, based upon the two I.sub.x -I.sub.y -D maps, for both direction of scan;
• and
• (g) setting the correcting currents I.sub.x and I.sub.y fed to the x-axis and y-axis coils, respectively, substantially to (I.sub.xa +I.sub.xb)/2 and I.sub.ya +I.sub.yb)/2, respectively.

An apparatus using an electron beam for scanning a specimen, said apparatus comprising:

• (a) a focusing lens system for focusing the electron beam onto the specimen;
• (b) an xy solenoid stigmator for correcting the astigmatism of the focusing lens system;
• (c) a scanning means for repeatedly scanning the specimen with the beam;
• (d) a converter means for converting the signal emanating from the specimen into a signal D indicating the diameter of the beam moving in a direction of scan;
• (e) a means for systematically varying the values of the correcting currents I.sub.x, I.sub.y fed to the x-axis and y-axis coils, respectively, of the stigmator for every scan and for obtaining I.sub.x -I.sub.y -D maps from the converted signals D;
• (f) a means for finding such combinations of the correcting currents I.sub.xa, I.sub.ya and I.sub.xb, I.sub.yb fed to the x-axis and y-axis coils which minimize the diameter of the beam, based upon the two I.sub.x -I.sub.y -D maps regarding two perpendicular directions of scan;
• and
• (g) a means for setting the correcting currents fed to the x-axis and y-axis coils to (I.sub.xa +I.sub.xb)/2 and (I.sub.ya l +I.sub.yb)/2, respectively.