Simulation study on image contrast and spatial resolution in helium ion microscope

We performed Monte Carlo simulation of helium (He) ion induced secondary electron (SE) emission in order to compare the secondary electron image characteristics between He and gallium (Ga) scanning ion microscopes (SIM) and scanning electron microscope (SEM). For 10-50 keV He ion bombardment SE yield increases gradually with increasing the atomic number, Z2, of the target, as well as for the electron bombardment. However, for 30 keV Ga ion bombardment, SE yield shows an opposite Z2 dependence. The calculated SE yield is much larger than that for both electron and Ga ion bombardment. The incident angle dependence of the SE yield approximately obeys the inverse cosine law even at high angles of 85 degrees and more. On the other hand, for electron bombardment, the incident angle dependences are much weaker for low energy and high Z2. These indicate that the image contrast on He-SIM is clearer than those of SEM. Among the electron excitations by incident He ions, recoiled target atoms and excited electrons, the first one having narrow excitation volume dominates the SE yield, so that the spatial image resolution in SIM using zero-diameter He beams with the energies of 10-50 keV is prospected to be smaller or better (<0.1 nm) than for 30 keV Ga ion and 1 keV electron beams.     

Response function and optimum configuration of semiconductor backscattered-electron detectors for scanning electron microscopes

A new highly efficient design for semiconductor detectors of intermediate-energy electrons (1?C50 keV) for application in scanning electron microscopes is proposed. Calculations of the response function of advanced detectors and control experiments show that the efficiency of the developed devices increases on average twofold, which is a significant positive factor in the operation of modern electron microscopes in the mode of low currents and at low primary electron energies.     

Optimizing routing in packet-switched networks with non-Poisson offered traffic

Routing models for packet-switched networks have traditionally been based on the assumption that the offered traffic is Poisson. This is unrealistic since for many applications, real life data streams tend to be extremely bursty, contradicting the Poisson assumption. This paper introduces a centralized bifurcated routing optimization model for networks with significantly non-Poisson offered traffic. The central purpose of the work is to assess the impact of non-Poisson traffic on routing algorithm developed for Poisson flows. A systematic testing plan for networks with a simple topology and variable levels of capacity, demand and packet length was designed. The results confirmed that Poisson delay formulas together with classical Poisson routing are not appropriate when significant demand is highly variable. The results also show that non-Poisson delay formulas are crucial to the assessment of average delay in these cases.     

Monte Carlo simulation of topographic contrast in scanning ion microscope

Topographic contrast of secondary-electron (SE) images in a scanning ion microscope (SIM) using a focused gallium (Ga) ion beam is investigated by Monte Carlo simulation. The SE yield of heavy materials, in particular, due to the impact of 30 keV Ga ions increases much faster than for the impact of electrons at < or =10 keV as a function of the angle of incidence of the primary beam. This indicates the topographic contrast for heavy materials is clearer in a SIM image than in a scanning electron microscope (SEM) image; for light materials both contrasts are similar to each other. Semicircular rods with different radii and steps with large heights and a small wall angle, made of Si and Au, are modeled for comparison with SE images in SEM. Line profiles of the SE intensity and pseudo-images constructed from the profiles reveal some differences of the topographic contrast between SIM and SEM. We discuss not only the incident-angle effect on the contrast, but also the effects of re-entrances of primary particles and SEs to the neighboring surface, the effect of a sharp edge on the sample surface, and the effects of pattern size and beam size.     

测量热阴极次级发射特性的扫描电子探针法

本文介绍了一种适用于量测热阴极次级发射特性的扫描电子探针法,原电子探束束斑直径为50100m,束流小于10-7A,原电子能量不大于3kV。该装置能定点测量热阴极表面上感兴趣点的次级发射系数与原电子能量间的关系曲线(-Ep);也能以扫描方式在短时间内测出整个热阴极表面的次级发射分布,从而求得次级发射的定量分布曲线(f-)。该装置在电视制式的扫描方式工作时,能观察热阴极表面的次级发射图象。 对纯镍样品的测量表明:该装置具有比单枪定点法更多的优点和更高的精度。由于该装置简便灵活,便于与热阴极研究的特殊需要相结合,因此,可望在热阴极的研究中发挥作用。     

Comparative study of depth and lateral distributions of electron excitation between scanning ion and scanning electron microscopes

In order to study the contrast difference between scanning ion microscopes (SIM) and scanning electron microscopes (SEM), the depth and lateral distributions of secondary electrons escaped from surfaces of 17 metals with atomic numbers, Z2, of 4-79 were calculated for bombardment with 30 keV Ga ions and for 10 keV electrons. For both projectiles, the excitation depth generally decreased with increasing Z2, while showing the same periodic change as the secondary-electron yield. However, an opposite trend in Z2 dependence between the Ga ion and electron bombardments was calculated with the lateral distribution of secondary electrons escaped from the surface. Except for low Z2 metals, the lateral distribution, which is much narrower for 30 keV Ga ions than for 10 keV electrons, indicates that the spatial resolution of the secondary-electron images is better for SIM than for SEM, if zero-sized probe beams are assumed. Furthermore, the present calculation reveals important effects of electron excitation by recoiled material atoms and reflected electrons on the lateral distribution, as well as the secondary-electron yield, for the Ga ion and electron bombardments, respectively.     

Origins of material contrast in scanning ion microscope images

A Monte Carlo simulation of ion-induced kinetic electron emission (KE) was carried out to study the material contrast in scanning ion microscope (SIM) images, i.e. secondary electron (SE) yields decreasing with atomic number Z2 of the target, which is opposite to that for scanning electron microscope (SEM) images. The simulations show that SE yields decrease with increasing Z2 for the targets of Al (Z2 = 13), Cu (Z2 = 29) and Au (Z2 = 79) bombarded by 10 approximately 40 keV gallium (Ga) ions. Details of the SE yield according to the collision partners (i.e. Ga ion, recoiled target-atom and excited electron) clarify the origins of material (or Z2) contrast in the Ga-SIM images. Cause and effect on the material contrast are as follows: the heavier (or slower) collision partner transfers less energy to the excited electrons and leads to a poorer multiplication of other excited electrons in the cascade process. The simulation also predicts that the Ga-SIM images are more sensitive to the outermost target surface than the SEM images. material contrast, atomic number contrast, secondary electrons, secondary electron yield, scanning ion microscope, scanning electron microscope     

Approximation of loss calculation for hierarchical networks with multiservice overflows

This paper studies loss calculation in hierarchical networks with multiservice overflows which have different call arrival rates, mean holding times, bandwidth requirements and share a common link. The loss calculation involves two challenging problems: 1) the computation of the two moment characterizations of multiservice overflow traffic over the shared link, 2) the calculation of the loss probabilities for multiservice non-Poisson overflow traffic in hierarchical systems. An efficient approximation method, known as multiservice overflow approximation (MOA), is proposed to enable multiservice networks designs with hierarchical architecture. Two contributions are included in the MOA method. First, an approximation based on blocking probabilities matching is proposed to compute the variances of multiservice overflows over the shared link. Second, a modified Fredericks & Hayward's approximation is used to calculate the loss probabilities of multiservice non-Poisson over flow traffic. The performance of the MOA method is evaluated in a two-tier hierarchical cellular network and compared with an existing approximation method based on multi-dimensional Markov-modulated Poisson process (MMPP). Verified by simulations, the MOA method achieves better accuracy in the general heterogeneous cases at lower computational cost than the MMPP method.     

Electron microscopy characterization of airborne micro- and nanoparticulate matter

The aim of this work was to offer a state-of-the-art critical survey for characterizing airborne nano- and microparticles by means of electron microscopy (EM) techniques and to highlight advantages and limits of different possible operation modes. Procedures of collection and sample preparation are revisited and improved to analyse airborne particles deposited on filtering membranes by using various sampling methods. Three kinds of electron microscopes are used to this end: scanning electron microscope (SEM), field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). Following and extending previous studies, we optimized procedures by varying both the sample collection/preparation and the operational parameters of the microscopes. In particular, we diversified the sampling methods applied, using ad hoc filters as well as common filters for standard gravimetric measures. This approach enabled us to achieve a simple and clean procedure allowing direct SEM or TEM observation of the collected particulate matter.     

Topics in recent studies with high-voltage electron microscopes

In this article, topics in recent studies with high-voltage electron microscopes (HVEMs) are reviewed. High-voltage electron microscopy possesses a number of advantages that cannot be afforded by conventional electron microscopy, thus providing a unique microscopy technique in both materials science and biological science. One of these advantages is the capability of continuously observing phenomena using a variety of electron microscopy techniques simultaneously with the introduction of the displacement of atoms from lattice points. This has enabled in-depth studies on such fundamental subjects as the crystalline-to-amorphous-to-crystalline transition, the motion properties of point defects and the one-dimensional diffusion of dislocation loops. Electron tomography studies using HVEMs take advantage of the large observable thickness of a specimen. In addition, by combining different advantages, a number of advanced applications in materials science have been carried out, including analyses of the atomic structure of a reduction-induced reconstructed surface and the atomic mechanism behind the self-catalytic vapor-liquid-solid growth of an oxide nanowire. As long as excellent and invaluable studies that cannot be carried out without HVEMs appear in succession, it is necessary to make the utmost efforts to improve these microscopes.     

Critical connectivity phenomena in multihop radio models

The percolation of a broadcast in a multihop radio network modeled by a spatial Poisson process is studied. The effect of station density and transmission radius on the extent of broadcast percolation is examined. For broadcast percolation in one spatial dimension, analytical expressions for the average extent of percolation are derived. A model for two-dimensional spatial percolation is presented along with related simulation results. The results suggest that in optimizing transmission radius as a function of communication performance measures, the choice of radius may be bounded from below by the need to maintain a desired network connectivity. The connectivity constraint can be relaxed to some extent in certain non-Poisson spatial models     

Effects of Electron Shot Noise and Quantum Field Fluctuations on the Photon Statistics of the Coherent Spontaneous Harmonic Radiation

We present a theoretical analysis of the statistical fluctuations in the coherent spontaneous harmonic radiation (CSHR) generated by a free electron laser (FEL) under the conditions applicable in a previously published experiment [1] where sub-Poisson optical fluctuations were observed. The numerical analyses presented here document the conclusion that the emission of CSHR can serve as a useful test bed for analyses of the nonclassical aspects of electron beam formation and radiation. Two possible explanations for the observed fluctuations in the CSHR are investigated as limiting physical mechanisms: the fluctuations in the number density of the radiating electrons, the so-called "shot noise," and the quantum fluctuations in the radiation field. When the uncertainties in the radiation field are considered as the only noise source as in the "quasi-classical" analysis, the statistical fluctuations in the CSHR would result in Poisson statistics. On the other hand, when the electron shot noise is regarded as the only noise contribution as assumed by the shot-noise model, we find numerically and analytically that the fluctuations, as characterized by the Fano factor, would be much smaller than those due to the quantum uncertainties in the radiation field in the "quasi-classical" analysis. The experimentally observed fluctuations are larger than those due to shot noise, but smaller than those due to quantum fluctuations in the field. Hence, neither the classical shot-noise model nor the quasi-classical analysis can explain the experimental results. Given the results presented in this report, it is clear that attention must be given to the other phenomena-including the quantum fluctuations in the electromagnetic field and the nonclassical electron correlation, which contributed to the observed laser fluctuations.     

Electron holography study of electric field variations

This paper presents a review of the recent electron holography studies on electric field variation that have been carried out using multifunctional specimen holders. In addition to the standard inner potential analysis, studies on electric field variations around field emission tips have been carried out. The electric field variations caused by ballistic emission in the case of a field emitter made of a TaSi(2) nanowire have been analyzed using electron holography. The charges and electric fields in electrophotographic materials such as toner particles and organic photoconductors have been quantitatively evaluated after equipping the specimen holder with a piezodriving probe to shield the specimens from electron irradiation. The conductivity and electric field variations in the case of Ag-based conductive adhesives have been analyzed by applying an electric current through the holder. Finally, the characteristic charging effect induced by electron irradiation in biological specimens has been studied. It has also been pointed out that under certain experimental conditions, the stationary orbits of electron-induced secondary electrons can be located by electric field visualization.     

Complete statistical characterization of signal and noise inoptically amplified fiber channels

A very simple model is found for phase insensitive laser amplification. Signal and noise along the active fiber are modeled as marked Poisson processes (corresponding to flows of photon bunches of random size) and are dealt with as independent processes. A probabilistic approach, founded on the theory of birth-and-death processes, enables us to give a simple characterization of the amplifier in terms of the statistics of the random gain, of the time intensity of the ASE (amplified spontaneous emission) noise bunches, and finally of the statistics of their sizes. The theory is limited to the linear amplification range, while is valid also for nonhomogeneous inversion along the active fiber. The model can be easily applied for the evaluation of the statistics of the global gain and of the accumulated ASE noise in optically amplified links     

低能电子束照射电介质样品的二次电子特性简

 为阐明低能电子束照射下电介质样品的二次电子电流及产额的动态特性,将蒙特卡洛法和有限差分法相结合,建立了较为准确的电子散射、俘获、输运和自洽场等过程的数值计算模型;采用一个改进二次电子检测实验平台,准确测量了二次电子电流.模拟和实验结果表明,相对于电子束脉冲照射模式,电子束连续照射会导致二次电子产额明显降低.在连续照射模式下,随着电子束照射,二次电子电流和产额逐渐减小至一个稳定值.二次电子产额受入射电子束电流的影响较小,但随样品厚度的增大而增大.本文结果为提高扫描电镜成像质量、降低带电效应提供了理论指导,而且提供了依据二次电子特性研究样品参数的新思路.     

Emission image reconstruction for randoms-precorrected PET allowing negative sinogram values

Most positron emission tomography (PET) emission scans are corrected for accidental coincidence (AC) events by real-time subtraction of delayed-window coincidences, leaving only the randoms-precorrected data available for image reconstruction. The real-time randoms precorrection compensates in mean for AC events but destroys the Poisson statistics. The exact log-likelihood for randoms-precorrected data is inconvenient, so practical approximations are needed for maximum likelihood or penalized-likelihood image reconstruction. Conventional approximations involve setting negative sinogram values to zero, which can induce positive systematic biases, particularly for scans with low counts per ray. We propose new likelihood approximations that allow negative sinogram values without requiring zero-thresholding. With negative sinogram values, the log-likelihood functions can be nonconcave, complicating maximization; nevertheless, we develop monotonic algorithms for the new models by modifying the separable paraboloidal surrogates and the maximum-likelihood expectation-maximization (ML-EM) methods. These algorithms ascend to local maximizers of the objective function. Analysis and simulation results show that the new shifted Poisson (SP) model is nearly free of systematic bias yet keeps low variance. Despite its simpler implementation, the new SP performs comparably to the saddle-point model which has shown the best performance (as to systematic bias and variance) in randoms-precorrected PET emission reconstruction.     

电子出射波重构的基本原理和应用

随着场发射枪透射电子显微镜的普及，电镜的信息分辨极限和点分辨率成为表征电镜分辨本领的两个重要指标。由于电镜衬度传递函数的振荡特性，用场发射透射电镜拍摄的高分辨像在其点分辨率和信息分辨极限之间的信息很难解释。电子出射波重构就是通过拍摄系列欠焦高分辨像，计算获得电子离开样品表面的出射波函数，将电镜的分辨本领由点分辨率延伸到其信息分辨极限，井消除球差、欠焦、像散等影响，有望在中等电压电镜上获得近埃、乃至亚埃级可直接解释的高分辨像，实现C、N、O等轻元素单原子列探测，为今后定量电子显微学研究奠定基础。     

Development of battery charging system for the FE gun system of HVEM

We have developed a reliable and efficient battery-charging system for the field emission (FE) gun system of high voltage electron microscopes, where the operating condition of the whole FE gun system can be controlled and monitored through a bi-directional optical fiber system, and the control and monitoring circuits located on the high potential of 1 MV are driven by rechargeable Ni-Cd batteries. The power transmission from ground to the high potential is performed through filter condenser circuit. Under the condition that the filter condenser current is limited to 0.2 A(rms) (root-mean-square value), it is possible to transmit the maximum power as high as 65 W, which is enough for the daily operation of the FE gun system. The charging circuit has a function of protecting the batteries from over-charging.     

透射电子显微镜单色器的发展及应用

自1932年透射电子显微镜发明以来,透射电子显微学在基础理论、仪器研制及其在材料科学、生命科学等领域的应用得到了迅速发展,200kV场发射枪透射电子显微镜的点分辨率已达0.19～0.25nm,能量分辨率为0.7～1.0eV。进一步提高透射电子显微镜性能的关键在于降低物镜球差和电子束能量扩散等。球差校正器的发明使透射电镜的点分辨率已突破0.1nm,电子源色差已成为进一步提高电子显微镜信息分辨极限和电子能量损失谱能量分辨率的瓶颈。在场发射枪透射电子显微镜上增加单色器（能量过滤器）可有效降低电子束的能量色散,减小色差对电子显微镜性能的影响。本文介绍了Wien型、Ω型及Mandoline型等几种常见的能量过滤器的工作原理、结构、性能及其应用。     

Spontaneous emission model of surface-emitting DFB semiconductorlasers

A general spontaneous emission model is developed for surface-emitting (SE) distributed feedback (DFB) semiconductor lasers. The frequency distribution of spontaneous emission noise below lasing threshold and the spontaneous emission rate in lasing operation are formulated by using a transfer matrix method combined with the Green's function method. The effective linewidth enhancement factor is obtained from this model in terms of the elements of the transfer matrix. By way of example, the author applies the formulation to a standard SE DFB laser, and a SE λ/4-shifted DFB laser with a distributed Bragg reflector (DBR) mirror. In particular, the author analyzes the below-threshold spectrum, the threshold current density, the differential quantum efficiency, and the spectral linewidth of these lasers