Monte Carlo Simulation Study On Scanning Electron Microscopy Imaging Including Charging Effect

Scanning electron microscopic (SEM) imaging technique is the most widely used method for observation of microstructure of materials at micrometer and nanometer scales. However, due to the limitation of signal formation mechanism in electron-solid interaction an experimental image cannot reveal fully and accurately the structure and morphology information of the sample. Then accurate length measurement by SEM needs the help from theoretical simulation by comparing the simulated image contrast with experimental image contrast. This way is not only useful for understanding the imaging mechanism, but also for establishing a platform of experimental parameters and material information for characterization and, furthermore, for database construc-tion for dimension metrology by SEM.The 1 st Chapter of this thesis firstly introduces the basic principle of SEM, outlines the electron-solid interaction theories and researches on 3D reconstruction by SEM and charging effect in SEM. It is then summarized the application of Monte Carlo method to SEM image simulation.The accurate description of transport process of electrons in solid forms the basis for scanning electron microscopic imaging simulation. The transport processes of elec-trons in a solid can be described as a series of elastic and inelastic scattering events. The 2nd Chapter describes the treatment of the electron elastic scattering by the use of Mott cross section to event and electron inelastic scattering and the associated secondary electron production by employing the full Penn dielectric functional model.Based on the sophiscated physics model of Monte Carlo simulation, we have ac-complished the following studies:1. By our own developed SEM imaging simulator, we have performed simulation of SEM images of Au nanoparticles in complex shape on a C substrate. The 3rd Chap-ter describes in detail the 3D structure construction method, the finite element mesh modeling, and the space subdivision method for ray-tracing technique, which is used to improve computational efficiency. It is indicated the most accurate method for the sample dimension measurement.2. The 4th Chapter studies the influence of electron beam focusing with finite width and the focusing condition to linewidth measurement. We have made a sim-ple and fast computational model of the electron beam focusing, which can be easily adopted in a Monte Carlo electron trajectory simulation. Simulation results have given the focusing condition for most accurate linewidth measurement. Meanwhile, it is in-dicated that the usual Gaussian profile cannot describe accurately the electron probe shape; therefore, an "effective electron beam shape" (EEBS) should be considered in an analysis instead.3. The 5th Chapter investigates the influence of EEBS to SEM imaging by Monte Carlo simulation. It is found that EEBS strongly depends on local topography of sample and focusing condition and, hence, is very different from the Gaussian probe shape which is invariant with position. For application to image convolution, EEBS is better and more appropriate than Gaussian profile.The Chapters 6 and 7 are related to charging effect, which is the main research aim of this thesis. We have built respective models for charging deposition and dynamical transportation.4. The 6th Chapter studies the charging effect on mask surface in plasma etching process. By a particle simulation method the mechanism for the shape change of mask holes from the round shape to hexagonal shape during a plasma etching process has been explained.5. The charging accumulation in insulating materials during electron beam bom-bardment exerts a significant influence to SEM imaging. In 7th Chapter we have devel-oped a self-consistent Monte Carlo simulation method to study the basic mechanism of charging effect. The method is based on an effective charging deposition and dy-namics model, meanwhile it includes also the cascade secondary electron production. We have calculated for SiO2 the variation of electron yield and surface potential with irradiation time. The deposited charge distribution was studied in various forms. More importantly, we have considered this charging effect in the SEM imaging simulation of linewidth with roughness surface.6. In 8th Chapter we have simulated SEM images of Au nanorods. The study can help to establish a measurement method for aspect ratio of nanorods. We have con-structed structure modeling of Au nanorods with an organic molecular shell. However, the simulated image contrast of Au nanorods is not fully agreement with experimental observation and the reason needs to be clarified further.Chapter 9 summarizes the contents of the studies in the dissertation.