Separator of primary and signal electrons for very low energy SEM

High resolution can be obtained in the scanning electron microscope (SEM) for a very low landing energy of electrons, even for that below 50 eV, when a cathode lens is used with the specimen as a cathode held at a high negative potential but the detection of signal electrons is totally different compared with classical SEM. Primary electrons with an energy of the order of tens of keV are decelerated in the field of the cathode lens to a very low landing energy and signal electrons originating in the specimen are accelerated and collimated by the same field to a narrow beam with an electron energy nearly the same as that of the primary beam. To detect these signal electrons we must deflect them from the optical axis without deteriorating the properties of the primary beam. The design of a novel type of separator of the primary and signal electrons consisting of two stages, each of them formed by the electric and magnetic crossed fields, is presented, together with calculated trajectories for both primary and signal electrons.Presented at the Seminar on Secondary Electrons in Electron Spectroscopy, Microscopy, and Microanalysis, Chlum (The Czech Republic), 21–24 September 1993.For computations of fields and trajectories, the software package of the Delft Particle Optics Foundation developed by Dr. B. Lencová and Dr. G. Wisselink [10] was used. The design was consulted with Dr. M. Lenc.     

Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

Secondary electron yield measurements of carbon covered multilayer optics

Carbon contamination on extreme ultraviolet (EUV) optics has been observed in EUV lithography. In this paper, we performed in situ monitoring of the build-up and removal of carbon contamination on Mo/Si EUV multilayers by measuring the secondary electron yield as a function of primary electron energy. An electron beam with an energy of 2 keV was used to simulate the EUV radiation induced carbon contamination. For a clean EUV multilayer, the maximum secondary electron yield is about 1.5 electrons per primary electron at a primary electron energy of 467 eV. The maximum yield reduced to about 1.05 at a primary electron energy of 322 eV when the surface was covered by a non-uniform carbon layer with a maximum thickness of 7.7 nm. By analyzing the change in the maximum secondary electron yield with the final carbon layer thickness, the limit of detection was estimated to be less than 0.1 nm.     

基于维格纳方程的电子的古斯-汉欣位移

通过求解电子的维格纳方程研究二维电子气中电子的输运性质.我们发现电子在倾斜入射到势垒界面并反射时,出现与光波类似的古斯-汉欣位移.通过维格纳方程可以得到电子的瞬态演化,不仅可以计算古斯-汉欣位移还能研究电子在势垒内部的运动轨迹以及出现稳定古斯-汉欣位移的时间.与稳定相位法得到的古斯-汉欣位移对比发现,考虑古斯-汉欣位移...     

Special techniques for the Auger analysis of microelectronic devices

Microelectronic devices are becoming more complex and device features are getting smaller as the level of integration continues to increase. Although scanning Auger microscopy has been applied extensively to the analysis of microelectronic devices with a great deal of success, the analysis of current and future devices is presenting new challenges. The major limitations are (1) features of interest in microelectronic circuits are often comparable in size to the beam diameter of commercial Auger microprobes, and (2) the electron beam tends to drift about on the specimen surface because of mechanical instability and differential thermal expansion of the apparatus. In this paper, we present two different techniques developed to overcome these limitations. In the specimen modulation technique, the modulating signal is applied to the electrically isolatable regions of a device instead of to the electron energy analyzer. This method of modulation permits the detection of only the Auger electrons that are emitted from the modulated region. Spurious contributions from adjacent areas inadvertently illuminated by the analyzing beam are suppressed. In the position modulation technique, the analyzing beam is scanned repetitively across the feature to be analyzed and the Auger signal is synchronously detected at the scan frequency. The resulting Auger signal magnitude is shown to be unaffected by beam drift. This method of signal detection eliminates the error and uncertainty caused by beam instability during long-term depth profiling, but is applicable only to specimens with certain geometries.     

Low energy electron microscopy of surfaces

In low energy electron microscopy (LEEM) surfaces are imaged with LEED electrons. Either the (00) beam (bright field mode) or one of the other diffracted beams (dark field mode) can be used for producing a true (non scanning) image of the surface. One can also obtain the LEED pattern of the illuminated area (typically 5–10 μm diameter) on the final screen.     

Laser induced charge transfer from Ni to CO adsorbed on (111) and (110) surfaces

Ni(110) and (111) surfaces covered with up to one monolayer of CO were irradiated with the light of a dye laser in the photon energy range 2.0 to 3.4 eV. Two-photon photoemission was observed when the laser light was focussed. Upon defocussing a signal was measured which did not depend on the potential of the sample and showed a linear intensity dependence. It is caused by electrons transferred from the Ni substrate into adsorbate states. The signal vanishes for photon energies below 2 eV. This shows that the adsorbate state lies at most 2 eV above the fermi level. The lifetime τ of the electrons in the adsorbate states is estimated to be 10^?10 < τ < 10^?7 s. No fluorescence in the photon energy range above 1 eV could be detected.     

Biprismainterferenzen mit langsamen Elektronen

Diffraction patterns of low energy electrons (150 eV) were observed with an electron interferometer. An interferometer of special design was developed for this purpose. The electron source is reduced in size by a retardation lens, which simultaneously decelerates the electrons down to about 150 eV. The electron beam is split up by an electrostatic biprism (filament diameter 10 μ ¦) in two coherent waves, which recombine behind the biprism giving a Fresnel diffraction pattern. Then the electrons are accelerated and the interference fringes are electron optically magnified. In comparison with an interferometer for high energy electrons the intensity on the viewing screen is higher.     

低能电子显微术及其应用

低能电子显微术是新发展起来的一种显微探测技术。它的特点是利用低能（１－３０ｅＶ）电子的弹性背散射使表面实空间实时成像，具有高的横向（１５ｎｍ）和纵向（原子级）分辩率，且易与低能电子衍射及其他电子显微术相结合。近年来它已有效地应用于金属和半导体表面的形貌观测、表面相变、吸附、反应及生长过程的研究。     

The EIGER detector for low‐energy electron microscopy and photoemission electron microscopy

EIGER is a single‐photon‐counting hybrid pixel detector developed at the Paul Scherrer Institut, Switzerland. It is designed for applications at synchrotron light sources with photon energies above 5 keV. Features of EIGER include a small pixel size (75 µm × 75 µm), a high frame rate (up to 23 kHz), a small dead‐time between frames (down to 3 µs) and a dynamic range up to 32‐bit. In this article, the use of EIGER as a detector for electrons in low‐energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) is reported. It is demonstrated that, with only a minimal modification to the sensitive part of the detector, EIGER is able to detect electrons emitted or reflected by the sample and accelerated to 8–20 keV. The imaging capabilities are shown to be superior to the standard microchannel plate detector for these types of applications. This is due to the much higher signal‐to‐noise ratio, better homogeneity and improved dynamic range. In addition, the operation of the EIGER detector is not affected by radiation damage from electrons in the present energy range and guarantees more stable performance over time. To benchmark the detector capabilities, LEEM experiments are performed on selected surfaces and the magnetic and electronic properties of individual iron nanoparticles with sizes ranging from 8 to 22 nm are detected using the PEEM endstation at the Surface/Interface Microscopy (SIM) beamline of the Swiss Light Source.     

Surface imaging with LEEM

In low-energy electron microscopy (LEEM) surfaces are imaged with LEED electrons. The physical and electron-optical principles of the method are described as well as a prototype LEEM microscope. Micrographs of Mo, Si, and Au surfaces illustrate the various types of contrast obtained with LEEM and its application.     

飞秒电子衍射系统中调制传递函数的理论计算

主要介绍了飞秒电子衍射系统的组成及设计指标. 包括光电阴极、电子聚焦系统、电子偏转系统、双微通道板(MCP)电子探测器等，并给出了基本的设计思路、设计结果. 光电阴极是由位于蓝宝石晶体上面的银膜构成，为了获得足够小的电子束斑以及减小电子上靶时的角度，紧贴栅极后放置一个100μm的小孔，对电子束的形状和大小进行限制. 采用磁电子透镜进行聚焦，电子衍射图样由放置在样品后面的双MCP像增强器进行探测. 在设计计算时，用Monte Carlo方法对光电子的初能量、初角度以及初位置分布进行抽样，用有限元法计算磁透镜 关键词： 飞秒电子枪 有限元法 Monte Carlo模拟 调制传递函数     

Developing a forming lens for a low-voltage electron beam system with high resolution

An original design for a combined magneto-electrostatic forming lens for a low-voltage electron- beam system with high resolution is presented. The design allows us to achieve complete efficiency in emitting signal detection and to attain less than one nanometer resolution in the energy region of up to 0.04 eV.     

超导透镜及其聚焦行为

 理论上分析了利用高温超导体作为透镜聚焦强流相对论电子束的机理。实验研究中，研制出的YBa₂Cu₃O₇-d 高温超导体作为透镜的传输和聚焦性能被得到验证，结果表明，超导透镜具有明显的聚焦效果，并可有效控制束流强度分布，使被聚焦电子在束横截面上的分布更为均匀。     

Characterization of semiconductor detectors of (1–30)-keV monoenergetic and backscattered electrons

Semiconductor detectors of backscattered electrons are basic elements of all modern scanning electron microscopes. Their quality is determined by the properties of planar p-n junctions and the parameters of the protective layer on the detector surface. The main characteristics of semiconductor detectors are considered, their response functions are calculated, and the threshold signal cutoff energies are found both for a monoenergetic electron beam and for detection of the total energy spectrum of backscattered electrons. The experimental results are in good agreement with the computational model data.     

考虑到束-波相互作用的速调管离子噪声二维模拟

采用粒子模拟的方法并考虑电子束与电磁波的相互作用，首次直接得到了速调管输出信号的离子噪声图像，阐述了束电子、二次电子、离子、电磁场之间的相互作用的动力学过程. 指出离子噪声所表现出来的相位波动是由电子束速度的波动引起的，电子束速度的变化来源于管内离子数量的变化，离子的数量的变化又与电子束状态变化相互影响，这是离子噪声产生的根本原因. 二次电子对离子噪声产生过程的影响甚微，但是其行为却反映了离子噪声的形成机理. 离子噪声引发的输出信号幅度波动取决于电子束速度和半径的改变，与离子行为密切相关. 关键词： 离子噪声 速调管 粒子模拟 电子束     

低能电子在水介质中的输运过程

考虑到低能电子(能量下限为1eV)在水中输运时的电离,激发,俘获以及超激发引起的自电离等非弹性散射机制,并且考虑到OH⁺,H⁺等自由基的产生和分布,运用Monte Carlo方法模拟了电子在水中输运的径迹结构,揭示了电子在低能情况下输运时单条轨道的空间分布结构特点(云团,团点和短径迹等空间分布实体)和包含在大量轨道中的径迹结构的统计性质,并通过对比截止能量取30和1eV两种情况,分析了30eV以下的低能电子的作用.     

Effect of backscattered electrons on the analysis area in scanning Auger microscopy

A simple analytical model has been used to determine the effects of backscattered electrons on the analysis area in scanning Auger microscopy. For normally incident electrons, the radius r_a of the analysis area is calculated corresponding to detection of 80, 90, and 95% of the total Auger-electron signal as a function of two sample parameters, the backscattering factor R and the Gaussian parameter σ_b describing the radial distribution of the backscattered electrons. For a reasonable range of these parameters, r_a depends linearly on σ_b and to a lesser extent on R. Values of r_a can also be appreciably larger, by more than a factor of 100, than the widths of the incident beam in modern instruments, and need to be considered in quantitative analyses of particles and inclusions. Monte-Carlo calculations are needed for more realistic evaluations of the analysis area and to determine this area for non-normal incidence of the electron beam.     

Microscopy with slow electrons

Low energy microscopy is treated as the low energy limit of electron microscopy as a whole in all its basic branches, i.e. the emission, transmission and scanning microscopies. The instrumental and methodological aspects are briefly discussed. These include the interaction of electrons with a solid, the contrast formation mechanisms, the instrumentation problems and actual progress achieved in all three microscopies, from the point of view of lowering the energy of electrons, impacting or leaving the specimen, down to the low energy range below 5 keV and the very low energy range below 50 eV.Invited lecture presented at the Seminar on Secondary Electrons in Electron Spectroscopy, Microscopy, and Microanalysis, Chlum (The Czech Republic), 21–24 September 1993.