Measurement of Electron-Optical Parameters for High-Resolution Electron Microscopy Image Interpretation

A method is presented to measure various electron-optical parameters needed for high-resolution electron microscopy image interpretation with high accuracy. The method is based on the measurement of a series of beam-tilt induced image displacements. The displacements are calculated via cross-correlation of the images, and subsequently fitted to a third-order polynomal in the beam tilt. From two series of images (using the x and y beam tilt coils), the spherical aberration constant of the microscope can be measured, as well as the current values of defocus, beam tilt and astigmatism. The spherical aberration constant of three Philips microscopes is measured with a precision better than 1 %, apart from calibration errors.The misalignment in the reference image (i.e. without induced beam tilt) can be measured with an absolute accuracy of 0.05 mrad, while the accuracy in the measured defocus value is 5 nm (at a magnification of 250,000). A computer is used to direct the experiments via remote control of the microscope and perform fast image processing to calculate the cross-correlations