Charged particle beam axial calibration

A method of operating a charged particle beam tool including a charged particle beam column configured to generate a charged particle beam, comprising:

• a) capturing a first focused image of the calibration target using the charged particle beam;
• b) capturing an under-focused image of a calibration target using the charged particle beam;
• c) capturing an over-focused image of the calibration target using the charged particle beam;
• d) determining a first offset vector in response to the under-focused image and the over-focused image;
• e) in response to a magnitude of the first offset vector being greater than a threshold, adjusting a charge distribution of one or more alignment electrodes of the charged particle beam column in response to the first offset vector and a previous charge distribution of the alignment electrodes, so that the charged particle beam has an adjusted alignment;
• f) in response to the magnitude of the first offset vector being greater than the threshold, capturing a second focused image of the calibration target using the charged particle beam with the adjusted alignment, determining a second offset vector in response to the first focused image and the second focused image, and moving the stage in response to the second offset vector;
• g) in response to the magnitude of the first offset vector being greater than the threshold, repeating steps b) through g);
• h) in response to the magnitude of the offset vector being less than the threshold, processing the substrate using the charged particle beam column with the adjusted alignment.