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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.
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Keywords:
- photo-emission electron gun /
- ultrafast scanning electron microscope /
- electron optics /
- finite element analysis methods
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图 1 (a) 超快扫描电子显微镜原理图; (b) 三种激励模式的电子枪原图; (c) 热助光发射电子枪结构图(左)与原理图(右)
Figure 1. (a) Schematic of ultrafast scanning electron microscope; (b) electron gun with three excitation modes; (c) structural diagram (left) and schematic (right) of thermally assisted photo-emitting electron gun.
图 2 偏压对电子光学的影响 (a) 灯丝尖端的电场分布; (b) 电子束轨迹; (c) 电子束的几何光学, 其中z = 0 mm处为韦氏极, z = –0.5 mm处为灯丝尖端, z = 23 mm处为阳极; (d) 交叉点位置、直径、发散角
Figure 2. Effect of bias voltage on the electron optics: (a) Electric field distribution at the filament tip; (b) electron beam trajectories; (c) optics geometry of electron beam, where the wehnelt at z = 0 mm, the filament tip at z = –0.5 mm, and the anode at z = 23 mm; (d) position, diameter, divergence angle as a function of bias voltage.
图 3 (a)截止偏压与韦氏极-阳极间距的关系, 工作电压为30 kV; (b)截止偏压与工作电压的关系, 韦氏极与阳极间距为23 mm
Figure 3. (a) Relationship between cutoff bias voltage and the distance between the wehnelt electrode and the anode at 30 kV; (b) relationship between cutoff bias voltage and working voltage with a distance of 23 mm between the wehnelt electrode and the anode.
图 4 不同灯丝深度的发射状态 (a)—(c) 不同灯丝深度的交叉点位置、交叉点大小、发散角等随偏压的变化; (d) 不同灯丝深度条件下的截止偏压, 当偏压大于截止偏压时处于禁止发射状态
Figure 4. Emission states at different distances between filament and wehnelt: (a)–(c) Variation of positions, diameters and divergence angles of crossover with bias voltage; (d) the cut-off bias voltage at the different distance between filament and wehnelt; when the bias voltage is greater than the cut-off bias voltage, the emission is prohibited.
图 5 反射镜对电子枪内部电场的影响 (a) 不对称放置反射镜的纵向截面电场分布(上图)、电势分布(下图). (b) 阳极平面处的横向截面电场分布 (i)无反射镜; (ii), (iii)阳极与反射镜高度差为H = 0, 1.4 mm
Figure 5. Effect of mirrors on the electric field inside the electron gun: (a) Longitudinal cross-sectional electric field distribution (top) and potential distribution (bottom) for asymmetrically placed mirrors. (b) Transverse cross-sectional electric field distribution at the anode plane: (i) Without mirrors; (ii), (iii) height difference between the anode and mirror H = 0, 1.4 mm.
图 6 偏压对电子漂移时间的影响 (a) 电子漂移示意图; (b) 阴极附近光轴上的电场分布随偏压的变化, 插图为整个加速区间电场分布; (c) 偏压Vbias = 0, 300 V时的电子束时间展宽; (d) 偏压为300 V时, 不同发射分布下的电子束时间展宽
Figure 6. Effect of bias voltages on electron drift time: (a) Schematic diagram of electron drift; (b) electric field distribution along the optical axis near the cathode as a function of bias voltage, with the illustration showing the electric field distribution across the entire acceleration region; (c) temporal broadening of the electron beam at Vbias = 0, 300 V; (d) temporal broadening of the electron beam under different emission distributions at Vbias = 300 V.
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