Aberration-correcting dark-field electron microscopy

A transmission electron microscope comprising:

• an electron beam source to generate an electron beam;
• beam optics to converge the electron beam, the beam optics defining an optic axis of the microscope along which there is substantial cylindrical symmetry of the beam optics;
• an aberration corrector comprising a set of elements approximately concentric about the optic axis, the concentric elements being adapted to have different respective electric potentials applied to them, the aberration corrector being adapted to correct the electron beam for at least a spherical aberration;
• a specimen holder to hold a specimen in the path of the electron beam;
• and
• a detector to detect the electron beam transmitted through the specimen.

A method for diagnosing aberrations in a transmission electron microscope that is adapted to operate in a dark-field mode in which a zero beam of the electron beam is not detected, the method comprising:

• acquiring one or more images, as a function of either illumination tilt or defocus, from the transmission electron microscope;
• extracting a value for the blurring effect of the tilt or defocus from the images;
• and
• calculating a value for defocus or astigmatism based on the value for the blurring effect and the value for either the illumination tilt or the defocus.