Monte Carlo Simulation Study On The Generation Of Secondary Electrons In Scanning Electron Microscopy

Secondary electron is the most basic and useful signal in Scanning Electron Mi-croscopy, while the generation process of detected secondary electrons is most com-plex. It is helpful for making a deep understanding of the generation of the secondaryelectrons by comparing the yields and spectrums obtained by the theoretical calcula-tion and experiment. Furthermore, it is also meaningful for generation and applying thesecondary electrons more effectively. In this thesis, the principle and the state-of-the-art development of the Scanning Electron Microscopy would be firstly introduced, thenthe development of the research on electron-solid interaction is summarized, togetherwith some experimental phenomenons related to secondary electrons and correspond-ing mechanism. (1st chapter)The generation of secondary electrons is closely related to the complex transportprocess of the electrons in solid. Theoretically, it is hardly to obtain the yields and spec-trums of secondary electron by using analytical formulas. Therefore, the Monte Carlosimulation is adapted to obtain the results needed. The transport process of the electronscan be simplified to a combination of a series of elastic and/or inelastic collisions be-tween the moving electron and the solid and the free movement of the electron betweentwo sequential collisions. These two types of collisions can be described by corre-sponding theory. Numerous of researches have been done on the elastic collision, andMott elastic cross section is considered as the most accurate description. On the otherhand, the dielectric function model proposed by Penn, which is based on the extrapola-tion of optical data, is reasonable for the description of inelastic collision. Because ofthe restriction of computation ability in the past, an approximation named single-poleapproximation is adopted to simplify the calculation based on Penn model. Accordingto our recent research, the original model named full Penn algorithm is more accuratethan the single-pole approximation for description of the inelastic collision, especiallyfor near-free-electron metal. In this thesis, the full-Penn algorithm would be introducedin detail, and the inelastic mean free path and the stopping power obtained by this al-gorithm would be also given. Based on the above theory, the detailed process of theMonte Carlo simulation would be introduced together with corresponding treatment onparalleled calculation. (2nd chapeter)The yields, spectrums and the space distribution of the secondary electron are firstly calculated. The secondary yield is an important parameter related to the ma-terial. It has been widely researched experimentally for various materials and incidentenergies. The simulated results and the experiment results are in good agreement, whilethe spectrums of the secondary electrons are also in good consistency, which indicatesthe present model is reasonable. Furthermore, the space distribution of the secondaryelectrons is also studied. The space resolution of the information carried by secondaryelectron can be obtained from the space distribution, which is unable to be measuredexperimentally. (3rd chapeter)As the efficiency of the theoretical simulation has been proved, the following phe-nomenons related to secondary electrons are studied:1. Topographic contrast. The topographic contrast is a basic phenomenon in Scan-ning Electron Microscopy. A basic subject corresponding to this contrast is the mea-surement of the resolution of the Scanning Electron Microscopy. There are severalmethods to do the measurement; however, an objective method is still not decided. Forassessing these methods, the experiment images under various condition are needed,which is difficult to realize. An alternative way is to use the simulation images. Thecomplex surface structure generated by a series of formulas is introduced to the MonteCarlo simulation and the images need are obtained through simulation the scanningprocess as in experiment. The simulated image can also give the idea limit resolutionwithout the infection of the experimental parameter. Nevertheless, the difference be-tween the sample size measured from the image and the actual size can be obtainedthrough comparison of the simulated image and the sample topograph used in simula-tion. (4th charpter)2. Dopant contrast. The contrast would be induced by the different secondaryyields in different regions where the dopant concentrations are different, and is pro-portion to the dopant concentration. This phenomenon is valuable for assessing thesemiconductor device. The change of the work function induced by the surface state isconsidered as a main mechanism. The simulated results are in good agreement to theexperimental results. (5th chapter)3. Spin polarization. The unpolarized electron beam would generates secondaryelectrons of great polarization (much more than the bulk magnetization of the mate-rial) in the material like Fe and Ni. a spin-dependent asymmetry of mean free path isusually considered as the main reason. The simulation results indicated that the Stonerexcitation, the spin-?ip inelastic collision, affects the results significantly. (6th chapter)For understanding the relation between plasmon excitation and secondary electron generation intuitionisticly, the coincident spectrum of two emitting electrons generatedby one incident electron is measured through coincident experiment. For the typicalnear-free-electron metal Al, the corresponding coincident spectrum is simulated ac-cording to the actual setting of experiment. The results indicates that the secondaryelectron excitation is closely related to the plasmon excitation. (7th chapter)...