Patent ID: 7026614

Claim:
A method for automatically making a focus correction in a scanning charged-particle beam instrument that focuses a beam of charged particles onto a specimen, scans a desired area on the specimen in two dimensions with the beam, detects a signal obtained by the scanning, and displays an image of the specimen according to the detected signal, said method comprising the steps of: varying the strength of a lens, which focuses the beam onto the specimen, in steps such that the state of the focus of the beam on the specimen is varied in steps; scanning the desired area on the specimen with the beam in each step of the focus; obtaining image data from the signal detected and obtained by the scanning with the beam; processing the image data to find a focus evaluation value; finding a focal position from a relation between the strength of the lens and the focus evaluation value; and setting the strength of the lens that focuses the beam to a value corresponding to the focal position, such that when the image data is processed to find the focus evaluation value, image data about one frame of image is processed in each of arbitrary first and second directions that are independent of each other, such that the strength of the lens is found at each of two focal positions in the first and second directions, respectively, according to the focus evaluation value found in each step of the focus, and such that the strength of the lens, which focuses the beam, is set, using the midpoint between the two focal positions as an overall focal position.