Method and System for Focus Adjustment of a Multi-Beam Scanning Electron Microscopy System

A multi-beam scanning electron microscopy apparatus comprising:

• a multi-beam scanning electron microscopy sub-system comprising:
• a multi-beam electron beam source including an electron gun configured to generate an illumination beam and a multi-lens array assembly configured to split the illumination beam into the plurality of electron beams, wherein the multi-lens array assembly is configured for adjusting focus of one or more lenses of the lens array assembly;
• a sample stage configured to secure a sample;
• an electron-optical assembly a set of electron-optical elements configured to direct at least a portion of the plurality of electron beams onto a portion of the sample;
• and
• a detector assembly configured to simultaneously acquire a plurality of images of the surface of the sample, each image associated with an electron beam of the plurality of electron beams;
• a controller including one or more processors configured to execute a set of program instructions stored in memory for causing the one or more processors to:
• direct the multi-beam scanning electron microscopy sub-system to acquire a first image in an under-focused condition and an additional image in an over-focused condition;
• receive the first image acquired in the under-focused condition and the additional image acquired in the over-focused condition from the detector assembly;
• identify focus drift in a current image by comparing the first image and the additional image to the current image acquired at the current focus;
• and
• direct the multi-lens array to adjust a focus of one or more electron beams to compensate for the identified focus drift.

A multi-beam scanning electron microscopy apparatus comprising:

• a multi-beam scanning electron microscopy sub-system comprising:
• a plurality of multi-beam electron sources configured to form a plurality of electron beams;
• a sample stage configured to secure a sample;
• an electron-optical assembly including a set of electron-optical elements configured to direct at least a portion of the plurality of electron beams onto a portion of the sample;
• and
• a detector assembly configured to simultaneously acquire a plurality of images of the surface of the sample, each image associated with an electron beam of the plurality of electron beams;
• a controller including one or more processor configured to execute a set of program instructions stored in memory for causing the one or more processors to:
• receive the plurality of images from the detector assembly;
• identify at least one of a best focus image or a best astigmatism image of the plurality of images by analyzing one or more image quality parameters of at least some of the images of the plurality of the images;
• and
• direct one or more electron-optical elements to adjust at least one of focus or astigmatism of one or more electron beams based on at least one of focus or astigmatism of an electron beam corresponding with at least one of the identified best focus image or the identified best astigmatism image.

A multi-beam scanning electron apparatus comprising:

• a multi-beam microscopy sub-system to perform a line scan of a plurality of electron beams comprising:
• a multi-beam electron source configured to form the plurality of electron beams;
• a sample stage configured to secure a sample;
• an electron-optical assembly including a set of electron-optical elements configured to direct at least a portion of the plurality of electron beams onto a portion of the sample;
• and
• a detector assembly configured to simultaneously acquire a plurality of images of the surface of the sample, each image associated with an electron beam of the plurality of electron beams;
• a controller including one or more processor configured to execute a set of program instructions stored in memory for causing the one or more processors to:
• receive a plurality of line scans from the detector assembly;
• identify at least one of a best focus line scan or a best astigmatism line scan of the plurality of line scans by analyzing one or more line scan parameters of at least some of the line scans of the plurality of line scans;
• and
• direct the multi-lens source to adjust at least one of focus or astigmatism of one or more electron beams based on at least one of focus or astigmatism of an electron beam corresponding with at least one of the identified best focus line scan or the identified best astigmatism line scan.