Autofocus method for a scanning electron microscope

A scanning electron microscope, comprising:

• a source of primary electrons;
• converging and objective lenses configured to focus the primary electrons into a beam and onto a specimen;
• X and Y deflectors configured to scan the primary electrons across a surface of the specimen;
• a secondary electron detector;
• a computer configured to control the X and Y deflectors and a focus of the objective lens, and to generate images in memory from signals received from the secondary electron detector while using the X and Y deflectors to scan the primary electrons across the specimen;
• wherein the computer is configured to control the X and Y deflectors and focus of the objective lens to perform steps comprising:
• generating a sequence of thinned images of a pattern formed on the surface of the specimen, each thinned image of the sequence of thinned images generated by repeatedly scanning a specimen with an electron beam along nonoverlapping scan lines while shifting a scanning position of the electron beam between scan lines by a predetermined natural number of pixels greater than one in a direction perpendicular to a scanning direction, each thinned image having fewer scan lines distributed evenly over a field of view than a combined image;
• changing a focal position and an irradiation position of the electron beam after generating each thinned image of the sequence of thinned images to interlace the scan lines of the sequence of thinned images;
• calculating a sharpness level of each thinned image of the respective thinned images to provide a plurality of sharpness levels;
• and
• determining an optimum focal setting of the objective lens based on the plurality of sharpness levels.