具有可变真空结构的物镜优化设计及性能分析

Optimized Design and Performance Analysis of Objective Lenses with Variable Vacuum Structures

环境扫描电子显微镜(ESEM)能够在低真空环境模式下对含水、含油以及生物等样品进行高分辨率观测。由于电子束通道与样品室间存在着极大的真空压差,采用真空差分结构会导致出现物镜的工作距离增加、偏转范围缩小、电子束和气体碰撞概率增加等问题,最终影响成像分辨率和效率。针对这些问题,本文从电子光学理论出发,综合考虑了ESEM中物镜结构和真空差分结构,将两者结合在一起进行优化设计,提出了一种具有可变真空结构的物镜设计方法,并搭建了实验平台,开展了物镜磁场测试、真空压差测试和分辨率测试。测试结果表明,在目前实验条件和133 Pa的低真空环境模式下,工作距离为15 mm时,20μm×20μm的扫描场对应的成像分辨率优于50 nm。

Objective Environmental scanning electron microscopes(ESEMs)are widely employed for high-resolution observation of water containing,oil containing,and biological samples in low vacuum environments.However,at present,the development of ESEMs in China is almost blank,and most of them need to rely on imports.Therefore,the research on ESEMs can help improve China s independent development capability in this field,and provide a theoretical and experimental basis for the development of ESEMs in the future.Compared with conventional electron microscopy,the sample chamber of ESEMs should be in a low vacuum or ambient state.The vacuum value is generally on the order of 100 Pa,while that of the electron beam channel and the electron gun needs to be less than 1×10^(-3)Pa and 1×10^(-7)Pa respectively.The pressure difference between the electron beam channel and the sample chamber is much larger than that between the electron gun and the electron beam channel.The conventional method is to add a throttle tube between the electron beam channel and the sample chamber.Meanwhile,since the large pressure difference remains much greater than that between the electron gun and the electron beam channel,the conventional method is to add a throttle tube between the electron beam channel and the sample chamber,but the large pressure difference will result in a long throttle tube with a small aperture.This will bring practical problems in imaging,such as the longer throttle tube leading to an increase in the working distance of the objective lens.As a result,it increases the spherical aberration,reduces the imaging resolution,and causes a smaller deflection range to a certain extent.Additionally,the long throttle tube will lead to the presence of residual gas inside the tube,the electron beam will drift in a section of the gas space where there is low gas pressure,and the probability of collision between the electron beam and the gas is high,which will have a greater effect on the resolution at low accelerating voltages.Therefore,the comprehensive design of ESEMs,which plays a key role in the system resolution of the objective lens and vacuum differential structure,is the study focus and difficulty.Methods Starting from the theory of electron optics,we consider the structure of the objective lens and the vacuum differential structure in the ESEM comprehensively.Firstly,two throttle tubes are designed between the sample chamber and the electron beam channel(near the lower pole shoe of the objective lens),and a transition zone is added inside the objective lens to form a three-level vacuum differential structure of the sample chamber,the transition zone and the electron beam channel.The vacuum in the transition zone should be two orders of magnitude higher than that in the sample chamber,and that in the electron beam channel should be two to three orders of magnitude higher than that in the transition zone.Considering the processing cost and difficulty of the elongated throttle tube,we adopt the combination of multiple diaphragms,which can more conveniently change the vacuum level by adjusting the aperture and number of diaphragm sheets in the diaphragm groove to achieve the required differential pressure difference.Then,the optimized design of a high-resolution ESEM objective lens and deflector is carried out based on a double-throttle vacuum resistance structure.Finally,an experimental platform is set up,and the objective magnetic field test,vacuum differential pressure test,and resolution test are carried out for validation.Results and Discussions Considering the objective structure and vacuum differential structure in the ESEM,a double throttle tube vacuum resistance structure as shown in Fig.2 is designed to form a three-stage differential test structure(Fig.10).This design can reduce the length of the throttle tube as a whole,which lowers the requirements for the aperture and length of the throttle tube to a certain extent,and thus reduces the influence on the working distance and the deflection field.Meanwhile,it can also reduce the gas residual situation in the narrow throttle tube,and reduce the influence of the electron beam drift in the narrow gas space in the throttle tube.The results of the vacuum differential pressure test show that the vacuum in the transition vacuum zone is two orders of magnitude higher than that in the sample chamber,and the vacuum in the electron beam channel is two to three orders of magnitude higher than that in the transition vacuum zone,which can meet the design requirements.The resolution test results show that in the current experimental conditions and the low vacuum environment mode of 133 Pa,the imaging resolution corresponding to the 20μm×20μm scanning field is better than 50 nm,and that corresponding to the 80μm×80μm scanning field is better than 100 nm when the working distance is 15 mm(Fig.13).Conclusions Starting from the electron optics theory,we consider the objective lens structure and vacuum differential structure in the ESEM comprehensively,and the two are combined for the optimal design to provide a design method for the objective lens with variable vacuum structures.Systematic analyses,calculations,and simulations are carried out.Based on the theoretical analysis and simulation results,a magnetic field test platform and an ESEM experimental test platform are built for experiments,and the results show that in the current experimental conditions and low vacuum environment mode,the imaging resolution of 20μm×20μm scanning field corresponds to a resolution of better than 50 nm when the working distance is 15 mm.The overall closed-loop design and test of the objective lens with variable vacuum structures provide a theoretical and experimental basis for the development of ESEM.