| Scanning electron microscope (SEM) is an important tool to characterize and an-alyze the structure and morphology of nano-scale materials, and secondary electron imaging is the most commonly used imaging mode. The generation of secondary elec-tron signal process is very complex, at the same time, there is no unified standard for the definition and measurement methods of secondary electron imaging resolution. The understanding for the secondary electron imaging in SEM depends on in-depth theoret-ical study for the main physical process, the interaction between electron and solid. Firstly, this paper introduces the basic principle and development of SEM, and the the-oretical methods of electron-solid interaction process. Secondly, we summarize the related to theory and experiment research background of secondary electron imaging, including secondary electron generation mechanism, secondary electron image resolu-tion and sharpness, secondary electron image at atomic resolution, as well as the effect of the incident electron beam shape on the quality of the secondary electron imaging.(1st chapter)The electrons transport process in the solid is the physical basis of secondary elec-tron imaging in the SEM. Because of the complexity of the interaction between elec-tron and solid, and the diversity of the boundary conditions, it is difficult to use analytic method, so we use the Monte Carlo method to simulate the process. Electrons transport process in the solid can be simplified to a series of general elastic scattering and inelas-tic scattering, and the accurate description for the electron scattering process and corre-sponding cross section is the key for the secondary electron imaging simulation. Mott elastic scattering cross section and full-Penn dielectric function method can accurately describe the elastic scattering, inelastic scattering in the solid and secondary electron cascade process. This paper describes in detail the relevant theory and the used electron scattering cross section, and introduces the sampling method and the program architec-ture of the Monte Carlo simulation. To simulate secondary electron imaging of the real sample, we also need to sample the three-dimensional complex structure of the speci-men, introducing two kinds of model:entity structure geometry method combined with ray tracing algorithm, to construct the special geometry structure; finite element method of triangle mesh combined with spatial segmentation algorithm and linear stepper, to approximately construct arbitrarily complex three-dimensional structure. To simulate electron interaction with crystal, we also developed the quantum Monte Carlo model, including Bohm mechanics used to calculate quantum trajectories which describe the electron elastic scattering and diffraction in crystal. Thus complete description of the involved physics and geometry models for Monte Carlo simulation of the secondary electron imaging is given.(2nd chapter)Based on the above simulation model, the research work related with the secondary electron imaging mechanism and image resolution have been performed in this thesis:1. It has been experimentally achieved atomic resolution imaging by using sec-ondary electron signals in a scanning transmission electron microscope (STEM) with aberration correction, challenging the traditional understanding of secondary electron imaging mechanism. We have developed a new theoretical method, the quantum Monte Carlo simulation, to investigate the interaction process of electron beam with crystal. This method combines the Bohmian quantum trajectory method for treating electron elastic scattering and diffraction in a crystal with a conventional Monte Carlo sampling of inelastic scattering events along quantum trajectory paths. The impact parameter dependent inner-shell ionization cross section is introduced for Monte Carlo sampling of high-energy secondary electron excitation in the inner-shell ionization events when the incident electrons travel along the quantum trajectory. We use the quantum Monte Carlo model to simulate the electron scattering and secondary electron generation pro-cess when a convergent electron probe is used. The atomic resolution secondary elec-tron image of Si (110) crystal is successfully calculated and compared with the corre-sponding experimental image, confirming that the high-energy electrons excited in the inner-shell ionization events is the main source of imaging mechanism for the atomic resolution secondary electron image observed in experiment.(3rd chapter)2. Using the secondary electron images by Monte Carlo simulation under the real instrument condition to evaluate several common used image sharpness measure meth-ods. Adding the model of instrument parameters (such as electron beam focusing, astig-matism, drift and vibration) to the Monte Carlo model for SEM imaging, we produced a series of simulated secondary electron images for the real specimen of gold particles on the substrate (Au/C), under different instrument parameters, showing that the real images under various kinds of instrument conditions can be accurately predicted and controlled. The simulation images are used for evaluation of three kinds of secondary electron image sharpness measurement method, Fourier transform method, contrast-gradient method, and differential method. And we studied the response of the image sharpness measurement methods to various instrument conditions. Because the dif-ferent response behaviors of various sharpness measurement methods on experimental parameters, we proposed an average method as a suitable secondary electron image sharpness measurement method.(4th chapter)3. The secondary electron image resolution is closely related to the incident probe shape and size. The actual electron probe size is affected by the electronic source and electron optical system. The traditional Monte Carlo simulation of secondary electron imaging often assumes that the electron beam shape is similar to a two-dimensional Gaussian distribution, but that is different from the actual electron probe shape after focusing in the electromagnetic lens. We have analyzed the electron beam forming pro-cess from the perspective of geometrical optics and wave optics, constructed different electron beam focusing models, and used the Monte Carlo simulated secondary electron image to discuss the effect of the electron probe shape on the quality of the secondary electron imaging.(5th chapter)4. Secondary electron yield is a basic parameter in SEM, closely related with sec-ondary electron imaging. However, because of the complexity of the interaction be-tween electron and solid samples, the secondary electron generation mechanism and the corresponding yield and energy spectrum are not very clear, and the difference of secondary electron yield obtained among various experimental groups is very large. We used Monte Carlo simulation method, in which the MELF-GOS model based on Mermin dielectric function and the full-Penn dielectric function model are applied to describe the electron inelastic scattering respectively, to discuss the effect of the dielec-tric function on secondary electron yield and energy spectrum.(6th chapter)... |
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