The analysis approach for high collection efficiency of secondary electron using Wien filter and in-lens collection system based on differential algebra method

In this work, an analysis approach based on the differential algebra method is proposed to achieve high collection efficiency of secondary electron (SE) using Wien filter in retarding field SEM. In the approach, the influence of Wien filter on electron optical properties of primary electron (PE) is also comprehensively considered. Due to the unmatched fringe fields in Wien filter, the optical axis of primary electron will be deviated from the central axis, but it can be deflected back to the central axis again by the developed Wien condition. And the electron optical properties are calculated and analyzed referred to this curved optical axis. Then, based on the calculation results of SE at detector plane, the expressions about orientation angle and excitations of Wien filter for high SE collection efficiency are derived. By further considering the influence of Wien filter on electron optical properties of PE, the optimized orientation angle and excitations can be obtained. Taking a retarding field SEM with Wien filter as an example, high SE collection efficiency is achieved by optimizing the appropriate orientation angle and excitations of Wien filter, with the analysis of the proposed approach. Finally, experiments are carried out with varying excitations of the Wien filter, and the experimental results are well explained by the calculated current density distributions of SE and the electron optical properties of PE, which validates the proposed analysis approach.