用于超快扫描电子显微镜的光发射电子枪及电子光学模拟

Photo-emission electron gun and electron optical simulation for ultrafast scanning electron microscope

超快扫描电子显微镜将泵浦探测技术与显微成像相结合,能够实现高时空分辨率下光诱导表面电荷动力学的可视化研究,对于半导体表面态以及光电器件的高分辨检测具有非常重要的意义.本文基于首台全国产化超快扫描电子显微镜的研制工作,阐述了将热发射电子枪改造成光发射电子枪后的参数化设计,定量分析了偏压,阴极、韦氏极、阳极的空间位置与交叉点位置、大小、发散角的依赖关系.分析结果显示,当韦氏极与阳极位置从8 mm调整到23 mm后,通过将灯丝深度从0.65 mm调整至0.45 mm,配合偏压调节可以实现热发射高分辨成像、低工作电压以及光发射的正常使用.此外,也分析了反射镜分布对电子光路的影响,发现当阳极高出反射镜1.4 mm后,图像畸变几乎消失.还研究了偏置电压对脉冲光电子在时域上的影响,结果表明随着偏压的增大,光发射的时间零点会推后且时间展宽变大.这些工作将为后续超快电子显微镜的发展及光发射电子源的设计奠定基础.

Ultrafast scanning electron microscope(USEM)integrates pump-probe technique with microscopic imaging,enabling the visualizing of photon-induced surface charge dynamics with high spatial and temporal resolution.This capability is crucial for high-resolution detection of semiconductor surface states and optoelectronic devices.This work discusses the parametric design of a thermionic emission electron gun that has been modified into a photoemission electron gun,based on a home-built ultrafast scanning electron microscope.Given that the dose of the photoemitting electron beam is usually much lower than that of thermionic emission,the transition to photoemission requires the removal of the self-bias voltage function of the original electron microscope power supply to ensure the normal operation of the Wehnelt electrode.We quantitatively analyze the dependence of bias voltage,cathode,Wehnelt electrode,and anode on the position,size and divergence angle of crossover,which helps to improve the parameter adjustment of the modified electron gun.The analysis results indicate that if the distance between the Wehnelt electrode and the anode is adjusted from 8 to 23 mm,the distance between the filament and wehnelt can be changes from 0.65 to 0.45 mm to cooperate with the bias adjustment,so that the normal use of high-resolution thermionic emission mode,low voltage mode and photoemission mode can be realized.Subsequently,the effect of the mirror’s position on the electron optical path is analyzed.It is found that when the anode is raised 1.4 mm above the mirror,the influence on the electron optical path can be ignored.Additionally,the zero-of-time and temporal broadening of the photo-electron pulse are further simulated.The results indicate that with the increase of bias voltage,the time zero of photoemission will be delayed and the temporal broadening will become larger.This study lays a foundation for the future development of ultrafast electron microscope and the design of photoemission electron sources.