Evaluation of electron optics with an offset cylindrical lens: Application to a monochromator or energy analyzer

Abstract

Electron beam monochromators (MCs) and energy analyzers (EAs), which are equivalent optical units with energy filtering capabilities, are of importance in electron microscopy and surface analysis instruments. The authors report the evaluation results for electron optics with an offset cylindrical lens (CL) and an additional transfer lens (TL) for application to an MC or EA. The offset CL deflects beams and generates energy dispersions, and the TL enhances their performance through collimating and retarding the beams. Following the previous theoretical studies, the authors have designed and manufactured the optics with a high mechanical accuracy at the micrometer level and integrated it with electronics with low noise and high stability. The authors adopt two methods for evaluating the energy-resolving performance. First, the authors treat the optics as an EA and evaluate the resolution through comparing the measured total energy distributions of Schottky emitters with the results from the theory. The energy resolution dE is estimated to be 88 meV at a beam energy E0 of 3.1 keV, and the index of performance dE/E0 reaches 2.7 × 10−5. Second, the authors treat the optics as an MC and evaluate directly the energy resolution or the energy spread of the monochromatic beams with an additional EA on the downstream side. The energy resolution dE is 73 meV. Both methods show similar energy resolutions, which enhance the credibility of the evaluation results. Charged particle simulation confirms the results and reveals that the resolution is limited by the width of the energy selection slit. As a total measurement system, the accuracy and stability are better than 3 meV. The MC with the optics exhibits high potential and a simple structure and is suitable for practical use. Advantages of the EA are band-pass filtering type, linear optics, compact size, and easy integration into analytical systems. The MC or EA using the optics will open new possibilities in the research fields of materials science, biology, and electronic devices, where advanced EMs or surface analytical instruments play essential roles.