Automatic electron beam alignment and astigmatism correction in scanning electron microscope

A method for automatically aligning electron beam axis to an objective lens axis in a scanning electron microscope, the method comprising the steps of:

• imaging a specimen at a first point within a focal range of an objective lens to form a first image sample defining a straight edge oriented in a first direction;
• generating a first indication signal of the first image at the first point within the focal range of the objective lens, the first indication signal indicative of a first position of the straight edge within a field of view of the microscope;
• imaging the specimen at a second point within the focal range of the objective lens;
• generating a second indication signal at the second point within the focal range of the objective lens, the second indication signal being indicative of a second position of the straight edge within a field of view of the microscope;
• comparing the first and second indication signals for determining translation of the straight edge;
• adjusting alignment of the electron beam axis to the objective lens axis in the first direction;
• repeating the steps of imaging at a first point, generating a first indication signal, imaging at a second point, generating a second indication signal, comparing and adjusting for subsequent image samples of the specimen defining a straight edge, wherein for each subsequent image sample the straight edge is in a direction orthogonal to the direction of the previous sample, said repeated step of adjusting for each one subsequent sample comprising adjusting alignment of the electron beam axis to the objective axis in the direction of the straight edge of said one subsequent sample, and wherein said repeating occurs until the first position differs from the second position by less than a predetermined threshold.