| Charging effect is originated from the trapping of charges in the sample,and can be frequently encountered in the characterizations for insulating materials using electron-beam-based experimental techniques.It may impact the experimental result greatly,such that the attempt to obtain the material's property would become difficult or even fail.Due to the extensive attentions caused by charging effect,it is necessary to study it systematically.However,charging effect is concerned with complex physical processes,such as electron transport and charge trapping,it would not be adequate to reveal its characteristics merely from the experimental perspective.This thesis is intended to study the charging effect of insulating materials induced by electron beam irradiation from the theoretical perspective by using the Monte Carlo method,with the hope that the study performed herein could make up for the deficiency of experimental studies and deepen people's knowledge on charging effect.The study in this thesis includes mainly the following aspects:1.A Monte Carlo modeling was proposed to study the charging effect for the targets with complicated geometries or structures.The available Monte Carlo modelings by other authors focus mostly on the charging effect for the simple targets,including the semi-infinite bulks or those ones whose geometry or structure are not very complicated,and cannot be directly applied to the complicated targets.The modeling proposed in this thesis enlarges the application scope of Monte Carlo method to the study of charging effect.In this modeling,the finite triangle mesh(FET)method was used to construct any complicated geometry by covering the target's surface with a non-fixed number of planar triangles whose shapes can be varying.In tracing the electron transport,the Mott cross section was used to describe electron elastic scattering,and a dielectric function approach was used to describe electron inelastic scattering,with the use of a Lorentz-oscillator-type dielectric function;besides,the interaction of low-energy electrons with longitudinal optical(LO)phonons was taken into account.Tracing the electron transport would be terminated once their energy is decreased below the low cutoff energy.Those energy-exhausted electrons,in conjunction with the generated holes,would drift under the electric field until they are trapped,which was also taken into account.In addition,obtaining the spatial potential distribution is more difficult for complicated targets.and therefore,a self-consistent method was particularly developed for the accurate and convenient calculation of that.This modeling has been successfully applied to study the charging effect for several complicated targets.Meanwhile,it has alsobeen applied to investigate the charging effect involved in nano-manipulation and the simulation shows that the charging effect is the physical basis for manipulating the Pd nanoparticle in the liquid water environment.(Chapter 3)2.The application of the peak-shift method to measure the surface potential was studied.In charging effect,surface potential is a special physical quantity of a vital importance,but is difficult to measure.The peak-shift method has become a very suitable method in surface potential measurement,in which the surface potential is derived from the shift magnitude of the emitted secondary electron peak.This study is to elucidate the influence of charging effect on the emitted electron energy spectrum and the applicability of the peak-shift method.The modeling used here was modified from that used in Chapter 3.The simulation used the electric-field-dependent charge drift velocity and trapping cross section,and considered the secondary electron trapping near the sample surface and the charge detrapping.In positive charging,the secondary electron peak is shifted towards the lower energy side,corresponding to positive surface potential;in negative charging,the secondary electron peak is shifted towards the higher energy side,corresponding to negative surface potential.The peak-shift method is more suitable for measuring negative surface potential,rather than positive surface potential,as its applicability in measuring positive surface potential is limited to the stage that the secondary electron peak has not passed through the zero-energy point.(Chapter 4)3.The trapped charge distribution in the sample induced by electron beam irradiation was studied.Considering that the trapped charge distribution is essentially responsible for the appearance of charging effect,the study of the trapped charge distribution is expected to be of great help for better understanding charging efffect.Meanwhile,it is required to know the trapped charge distribution in using available or developing new experimental methods to reduce or even eliminate charging effect.The modeling used here was identical to that used in Chapter 4.It has been found that the positive charges are mostly trapped in the inner region,while the negative charges are mostly trapped in the outer region.More importantly,along the incidence axis of the primary electron beam,six alternating positive and negative charge layers are formed beneath the sample surface from up to down.These charge layers are parallel to the sample surface,and have approximately the uniform thickness of about 0.1 ?m.The toppest layer is positive and is formed due mainly to secondary electron emission(SEE),the deepest charge layer is negative and is formed due mainly to primary electron trapping,and the middle charge layers are formed due mainly to the charge drift.However,the total number and the distribution range of the charge layers remain unchanged with increasing primary energy,which is because the primary electrons of different energies would be decelerated to the similar effective landing energy on the sample surface in negative charging.(Chapter 5)4.The time-dependent SEE characteristics were studied.It is intended to obtain the duration that SEE will last after the primary electron is penetrated into the sample and elucidate the difference on emission energy,emission depth,emission angle,surface emission position and generation position among the secondary electrons emitted at different time instants.The Mott cross section was used to describe electron elastic scattering,and a dielectric function approach was used to describe electron inelastic scattering,in which the energy loss function was calculated by the full Penn algorithm(FPA).It has been found that the SEE duration is only several fs.Due to this rather limited time scale,the assumption used in simulating charging effect that SEE is finished instantaneously after the primary electron is penetrated into the sample would not influence the simulation result on charging effect severely.Meanwhile,compared to the secondary electrons emitted at a later time instant,the secondary electrons emitted at an earlier time instant have higher emission energies,shallower emission depths,more localized surface emission positions and generation positions;but,the emission angle does not show any time dependence,and the cosine law is satisfied throughout the entire SEE process.In addition to elucidating the related problem in simulating charging effect,the simulation results shown herein would be helpful for modifying the available and developing new time-resolved scanning electron microscope(TR-SEM).(Chapter 6)... |
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