Characterization of the cathode objective lens by Real-Space Microspot Low Energy Electron Diffraction.

Measurement of the geometric aberrations of the cathode objective lens is a key step in setting up a high quality C(c) and C₃ corrected state in state-of-the-art aberration corrected instruments. In addition to Cc and C₃, elimination of sample tilt and astigmatism is necessary for optimum imaging conditions. In transmission electron microscopy (TEM), the use of Zemlin tableaux has become the method of choice for measuring geometric aberrations, while Scanning TEM relies on the use of the ronchigram to assess the properties of the contrast transfer function. These techniques are not applicable to cathode lens instruments such as a low energy electron microscope. Instead, we propose to utilize Microspot Low Energy Electron Diffraction (LEED) with an illumination area on the order of 0.1 μm, where the image locations of the diffracted beams are observed in the Gaussian image plane. This Real-Space MicroLEED (RS-μLEED) pattern is the first derivative of the aberration function with respect to the angle of the diffracted beams. Thus, the geometric distortions observed in the RS-μLEED pattern are a direct measure of the relevant geometric aberrations.