Köhler illumination in the TEM: Fundamentals and advantages

Köhler illumination is the most favourable design for the illumination path of an electron microscope with a condenser objective lens. The new illumination system of the EM 910 and EM 912 OMEGA allows both wide area (Köhler) illumination for TEM operation and spot illumination for analytical investigations. Compared to conventional systems and objective lenses with a condenser mini lens, this system offers many advantages. In addition to the homogeneous, highly coherent and parallel illumination of every point in the specimen, it offers advantages for selected area diffraction and spot scan mode. Combined with the electron optical selection of a condenser aperture, this illumination system provides the flexibility necessary to achieve optimum illumination for the specimen.     

Very low voltage electron microscopy.

We conclude that a 150 V scanning microscope with a resolution of 10 A is quite feasible and could have considerable value. It might consist of a field emission source, an electron gun to decelerate the electrons, a condenser lens to produce a parallel beam, a multipole corrector and a short focal length objective lens. Electrons reflected from the specimen surface would pass through a spectrometer whose principal features would be a large collecting power and low (1/200) energy resolution. Finally, we should add that such a microscope presents a considerable challenge and new opportunities for the electron optician in both lens and spectrometer design. We cannot refrain from pointing out that the Scherzer theorem does not necessarily hold for such a lens since the constraints of the theorem do not apply to this case.     

An optical configuration for fastidious STEM detector calibration and the effect of the objective‐lens pre‐field

In the scanning transmission electron microscope, an accurate knowledge of detector collection angles is paramount in order to quantify signals on an absolute scale. Here we present an optical configuration designed for the accurate measurement of collection angles for both image‐detectors and energy‐loss spectrometers. By deflecting a parallel electron beam, carefully calibrated using a diffraction pattern from a known material, we can directly observe the projection‐distortion in the post‐specimen lenses of probe‐corrected instruments, the 3‐fold caustic when an image‐corrector is fitted, and any misalignment of imaging detectors or spectrometer apertures. We also discuss for the first time, the effect that higher‐order aberrations in the objective‐lens pre‐field has on such an angle‐based detector mapping procedure.     

A high-resolution add-on in-lens attachment for scanning electron microscopes

A compact add-on objective lens for the scanning electron microscope (SEM) has been designed and tested. The lens is < 35 mm high and can be fitted on to the specimen stage as an easy-to-use attachment. Initial results show that it typically improves the spatial resolution of the SEM by a factor of three. The add-on unit is based upon a permanent magnet immersion lens design. Apart from the extra attachment to the specimen stage, the SEM with the add-on lens functions in the normal way. The in-lens unit can comfortably accommodate specimen heights up to 10 mm. The new add-on lens unit opens up the possibility of operating existing SEMs in the high-resolution in-lens mode. By using a deflector at the top of the add-on lens unit, it can also operate as a quantitative multichannel voltage contrast spectrometer, capable of recording the energy spectrum of the emitted secondary electrons. Initial experiments confirm that a significant amount of voltage contrast can be obtained.     

A method for producing hollow cone illumination electronically in the conventional transmission microscope.

An electronic device manipulates the primary beam in the conventional transmission microscope to produce a hollow cone of illumination with its apex located at the specimen. The device uses the existing tilt coils of the microscope, and modulates the D.C. signals to both x and y tilt directions simultaneously with various waveforms to produce Lissajous figures in the back-focal plane of the objective lens. Electron diffraction patterns can be recorded which reflect the manner in which the direct beam is tilted during exposure of a micrograph. In the bright-field imaging mode the device provides a microscope transfer function without zeros in all spatial directions and has been used to obtain high resolution images which are also free from the effect of chromatic aberration. A standard second condenser aperture is employed and the width of the cone annulus is readily controlled by defocusing the second condenser lens. The cone azimuthal angle is also controlled electronically; hence the device can also be used in the dark-field imaging mode. This device has been applied to imaging both amorphous and crystalline materials including biomolecular specimens.     

Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, part II: inelastic scattering

The implementation of spherical aberration-corrected pre- and post-specimen lenses in the same instrument has facilitated the creation of sub-Angstrom electron probes and has made aberration-corrected scanning confocal electron microscopy (SCEM) possible. Further to the discussion of elastic SCEM imaging in our previous paper, we show that by performing a 3D raster scan through a crystalline sample using inelastic SCEM imaging it will be possible to determine the location of isolated impurity atoms embedded within a bulk matrix. In particular, the use of electron energy loss spectroscopy based on inner-shell ionization to uniquely identify these atoms is explored. Comparisons with scanning transmission electron microscopy (STEM) are made showing that SCEM will improve both the lateral and depth resolution relative to STEM. In particular, the expected poor resolution of STEM depth sectioning for extended objects is overcome in the SCEM geometry.     

Scanning reflection image from a solid specimen in the scanning electron microscope with a condenser-objective lens

To achieve the highest resolution in the scanning electron microscope (SEM) or in the scanning transmission electron microscope (STEM), the sample must be mounted in the high-field region of a condenser-objective lens. A secondary electron (SE) image can then be obtained using a collector before the lens. It is also possible to obtain a scanning reflection image by tilting the specimen so that the second half of the condenser-objective lens field deflects the forward-scattered electrons onto the transmission detector beyond the specimen. Experiments were made with an unmodified commercial SEM fitted with a condenser-objective in the upper stage and with a transmission detector, and it was found that the scanning reflection image from a solid sample can provide additional useful information when used in conjunction with the SE image.     

High-speed operation of scanning electron microscope lenses

The speed at which the focal properties of an iron-shrouded magnetic electron lens can be changed is fundamentally limited by Eddy currents in the magnetic material. The effects of such Eddy currents on the lens properties can be partially compensated by modifying the lens excitation to speed up the lens response. This work considers the application of excitation pre-emphasis techniques to scanning electron microscope lenses for small and large dynamic changes in excitation. Where transient material saturation effects come into play, attempts to further improve the lens response beyond that at the onset of saturation are futile. Under such circumstances, the use of a closed-loop feedback arrangement is proposed that can provide a degree of response improvement.     

Studies on condenser-objective lenses

Scaling down the dimensions of a conventional magnetic objective lens to reduce aberrations is studied in detail. The values of other aberration coefficients for optimum lens size corresponding to desired minimum aberration are estimated. The possibility of using iron-free condenser–objective lenses at high voltage is examined.     

Features of optical surfaces of multifocal diffractive-refractive eye lenses

This paper considers shape features of the surface structures of multifocal intraocular lenses (IOLs), which, unlike bifocal IOLs, generate additional foci or extends the depth of focus, which not only corrects near and far vision but also provides good vision at intermediate distances. Expansion of the field of clear vision is achieved due to the effects of diffraction, interference, and refraction (change in the radius of curvature of the lens surface). The optical characteristics of the most famous multifocal IOLs (trifocal and quadrafocal lenses and lenses with extended focal area) are given.     

磁-电-四极杆级联质谱中的离子光学设计

离子光学设计是质谱仪器的核心技术,为了提高常规质谱的丰度灵敏度指标,设计了磁-电-四极杆级联质谱,并构建了一台原理实验样机。样机中的离子光学设计是仪器设计的关键内容,尤其是与最后一级质谱相关的离子透镜的设计,与常规双聚焦质谱和四极杆质谱中的透镜均有较大差异。本工作介绍了样机中的总体离子光学设计、四极杆前后相关透镜的设计、离子轨迹的 SIMION仿真计算,最后通过实验调试实现了整体设计,并测试了整套设计的总体指标。结果表明,样机比原仪器的丰度灵敏度指标提高约330倍,样机最后一级的传输率约为9%,与仿真计算结果相符。     

Secondary electron detection in the scanning transmission electron microscope

In a dedicated scanning transmission electron microscope (STEM) secondary electron images with high spatial resolution and good contrast can be obtained. Two types of detector are described. These take into account the secondary electrons which depend on the post-specimen field strength of the objective lens. Due to the thinness of the samples and the collection geometry the images differ from those obtained in a convectional scanning microscope. Examples are given where secondary electron images augment the information obtained by the more commonly used imaging modes.     

Calibration of a commercial electron microscope with a grating replica to an accuracy of better than 1%

The method of calibration described in this paper depends on (1) bringing every specimen to the ‘eucentric point’ of a side-entry goniometer stage to achieve a constant objective magnification, and (2) monitoring changes in the magnification of the projector lenses with a selected area (SA) aperture in the objective image plane. Under a wide range of operating conditions the aperture gives a sharp image whose diameter is proportional to magnification, so that the calibration is independent of changes in HT, lens currents and remanent magnetism. The method has been validated using Jeol 100C and 100S electron microscopes. The spacing of the standard cross-hatched grating replica was determined by comparing a low magnification electron micrograph, in which it could be measured, with a calibrated phase contrast optical micrograph of the same area. Elliptical distortion (which ultimately limits the accuracy of calibration) was assessed by rotating the specimen through 90°.     

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy. An introduction to the proceedings of the Second International Symposium and Workshop on Emission microscopy and Related Techniques

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e., an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.     

State of the art of intraocular lens manufacturing

Intraocular lens implantation surgery is the only approach for cataract treatment at present. Apart from removing the cloudy lens, correction of refractive errors becomes the second main function of intraocular lenses. This paper systematically summarizes the intraocular lenses in terms of its material, design, manufacturing and evaluation. The next generation of intraocular lenses with customized freeform surfaces is highlighted from the lens design viewpoint. The status of processing and measurement methods is presented for both current intraocular lenses and future freeform lenses. Finally, the research perspectives are outlined.     

Measurement of spherical aberration,beam tilt and image rotation in tem with a focussed probe

When a crystalline specimen in a transmission microscope is illuminated by a small focussed probe parallel to a zone axis, electrons diffracted at large angles by planes in higher order Laue zones (HOLZ) are imaged as a circle surrounding the direct beam. The diameter of this HOLZ image circle is directly related to the spherical aberration coefficient C_s of the objective lens. For the Philips EM430 at voltages between 50 and 300 kV, the 〈111〉 zone axis in silicon was used to measure an averaged C_s of 2.12 ± 0.04 mm. A displacement of the HOLZ image circle relative to the central probe image is attributed to a small tilt of the beam axis induced by lens interactions when switching between image and diffraction modes. This tilt could affect the quality of lattice images, where we require that both the beam and zone axes should be parallel with the optic axis. Even in an aberration-free lens, multiple diffracted images are observed when the probe is focussed and imaged in a plane below the specimen. This effect is proposed as a convenient method for measuring the direction and sign of g in two-beam defect analysis.     

大视场投影光刻物镜的研究

大视场投影光刻物镜是光学系统中的一种特殊的形式,其设计加工要求高。针对研制的8英寸视场的投影光刻物镜,从光学设计要求出发,分析了影响光学系统成像质量的各种主要误差因素,通过ZEMAX模拟,确定了加工容差,并对其加工和装校过程做了阐述,对研制完成的物镜检验了其曝光分辨力和畸变特性,达到了系统设计的指标要求。     

Coherent nano-area electron diffraction

We describe the new coherent nano-area electron diffraction (NED) and its application for structure determination of individual nanostructures. The study is motivated by the challenge and the general lack of analytical techniques for characterizing nanometer-sized, heterogeneous phases. We show that by focusing electrons on the focal plane of the pre-objective lens using a 3rd condenser lens and a small condense aperture, it is possible to achieve a nanometer-sized highly parallel illumination or probe. The high angular resolution of diffraction pattern from the parallel illumination allows over-sampling and consequently the solution of phase problem based on the recently developed ab initio phase retrieval technique. From this, a high-contrast and high-resolution image can be reconstructed at resolution beyond the performance limit of the image-forming objective lens. The significance of NED for nanostructure characterization will be exemplified by single-wall carbon nanotubes and small metallic clusters. Imaging from diffraction patterns, or diffractive imaging, will be demonstrated using double-wall carbon nanotubes.     

人工晶状体的发展趋势及展望

随着人口老龄化,年龄相关性白内障患者日益增多,通过植入人工晶状体可以使白内障患者的视力基本得到恢复。由于白内障患者对术后的视觉质量要求越来越高,个性化设计的人工晶状体已逐渐在临床上推广使用,而多焦点人工晶状体的出现,则成功解决了单焦点人工晶状体术后难以解决的近视力问题。对人工晶状体的发展历史做了简要的阐述,详细地说明了单焦点和多焦点人工晶状体的设计思想及特点,介绍了多焦点人工晶状体的发展现状,最后对人工晶状体未来的发展进行了展望。     

Position averaged convergent beam electron diffraction: Theory and applications

A finely focused angstrom-sized coherent electron probe produces a convergent beam electron diffraction pattern composed of overlapping orders of diffracted disks that sensitively depends on the probe position within the unit cell. By incoherently averaging these convergent beam electron diffraction patterns over many probe positions, a pattern develops that ceases to depend on lens aberrations and effective source size, but remains highly sensitive to specimen thickness, tilt, and polarity. Through a combination of experiment and simulation for a wide variety of materials, we demonstrate that these position averaged convergent beam electron diffraction patterns can be used to determine sample thicknesses (to better than 10%), specimen tilts (to better than 1 mrad) and sample polarity for the same electron optical conditions and sample thicknesses as used in atomic resolution scanning transmission electron microscopy imaging. These measurements can be carried out by visual comparison without the need to apply pattern-matching algorithms. The influence of thermal diffuse scattering on patterns is investigated by comparing the frozen phonon and absorptive model calculations. We demonstrate that the absorptive model is appropriate for measuring thickness and other specimen parameters even for relatively thick samples (>50 nm).