First experimental test of a new monochromated and aberration-corrected 200 kV field-emission scanning transmission electron microscope

The first 200 kV scanning transmission electron microscope (STEM) with an imaging energy filter, a monochromator and a corrector for the spherical aberration (Cs-corrector) of the illumination system has been built and tested. The STEM/TEM concept with Koehler illumination allows to switch easily between STEM mode for analytical and TEM mode for high-resolution or in situ studies. The Cs-corrector allows the use of large illumination angles for retaining a sufficiently high beam current despite the intensity loss in the monochromator. With the monochromator on and a 3 microm slit in the dispersion plane that gives 0.26 eV full-width at half-maximum (FWHM) energy resolution we have obtained so far an electron beam smaller than 0.20 nm in diameter (FWHM as measured by scanning the spot quickly over the CCD) which contains 7 pA current and, according to simulations, should be around 0.12 nm in true size. A high-angle annular dark field (ADF) image with isotropic resolution better than 0.28 nm has been recorded with the monochromator in the above configuration and the Cs-corrector on. The beam current is still somewhat low for electron energy-loss spectroscopy (EELS) but is expected to increase substantially by optimising the condenser set-up and using a somewhat larger condenser aperture.     

Electron optical property of a charged foil,observed with a transmitted electron detector device in an SEM

Experimental evidence is presented for the electron optical behaviour of a charged foil area, using the transmitted electron detection device of the scanning electron microscope JSM 50 A (JEOL). The primary electron beam scanning a thin pioloform foil on the one hand produces a charged foil region which on the other hand acts as an electron lens to the primary and scattered electrons. Scanning electron microscopical investigations of air particulates in the submicron size range can be eased by using a transmitted electron detection device both of the bright and dark field operation mode. The image contrast thus may be improved by orders of magnitude, also allowing on line operation of an image analysis system. Using a special preparation technique, depositing the particles on a thin supporting foil which is also used for LAMMA analysis – Wieser et al. 1980, the x-ray spectra of single particles provided by an energy dispersive x-ray spectrometer may be quantitatively interpreted on the basis of the peak-to-background method (Statham and Pawley 1978, Small et al. 1979). Figure 1 shows a schematic of the transmission detector device of the JSM 50 A when operated in the dark field mode. Geometrical dimensions and apertures also are given in Fig. 1. The dark field diaphragm (DFD) on the optical axis of the microscope blocks all electrons (primary electrons and scattered electrons) within an angle of about 10^?2 rad from contributing to the video signal. As long as magnifications above about 350 × are used the primary electron beam hits the DFD thus yielding a transmission scanning electron micrograph in dark field mode. Below this limit or above the corresponding maximal scanning angle (about 7 × 10^?3 rad) of the primary electron beam the rim of the DFD becomes visible in the displayed image as shown in Fig. 2a. At the same magnification Figure 2b shows the sharpened contours of the DFD as obtained by focussing the primary electron beam to the plane of the DFD by lowering the objective lens excitation. By means of the thin bar attached to the DFD (left hand upper corner of Fig. 2b) the DFD may be centered to the optical axis or exchanged to the bright field aperture. Looking to the circular center of Fig. 2a, we recognize the black grid bars and a few black particles whereas the supporting foil looks bright. No video signal can be obtained, because both the grid bars, and to a lesser extent the particles, are not transparent to the primary electrons of 15 keV energy. On the other hand all electrons scattered by the thin foil to an angle of more than 10^?2 rad are seen by the scintillator and hence accumulate a measurable video signal: This is also favoured by the large solid angle outside the DFD, which is about 30 times the solid angle of the DFD itself.     

On “parallel” illumination in the transmission electron microscope

It has been generally assumed that, in a transmission electron microscope, there exists some combination of lens excitations that will yield almost parallel illumination over a large area of the sample. This assumption is incorrect, when the objective is an immersion lens, due to the beam rotation in the magnetic field. The lack of parallelism is outside the control of the operator and is proportional to the diameter of the illuminated area. It has been measured, in a typical case, as 0.4 mrad/μm, a value that agrees well with calculation. This effect is large and needs to be taken into account when using techniques where parallel illumination is desired.     

Application of photon scanning tunneling microscope to measure optical near field for atom manipulation

Photon scanning tunneling microscope has been employed to measure the three-dimensional evanescent optical field of an atom funnel. A 3.8 neV repulsive optical potential has been estimated by a 300 microm long probe with a tip radius of curvature of 21 nm. We have estimated limiting conditions for cold Rb atoms to reflect from the atom funnel. A two-dimensional doughnut-shaped optical near field has also been investigated. An aperture fiber probe is used to profile a focussed TEM(01) beam at the minimum beam waist and measure a dark center of about 10 microm while it is focussed by a converging lens of focal length 8 cm.     

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.     

Polarization dependence in ELNES: influence of probe convergence, collector aperture and electron beam incidence angle

The differential scattering cross section in electron energy loss near edge spectroscopy (ELNES) generally depends on the orientation of the Q wave vector transferred from the incident electron to an atomic core electron. In the case where the excited atom belongs to a threefold, fourfold or sixfold main rotation axis, the dipole cross section depends on the angle of Q with respect to this axis. In this paper, we restrict to this situation called dichroism. Furthermore, if we take into account the relativistic effects due to the high incident electron velocity, this dipole cross section also depends on the angle of Q with respect to the electron beam axis. It is due to these dependences that the shape of measured electron energy loss spectra varies with the electron beam incidence, the collector aperture, the incident beam convergence and the incident electron energy. The existence of a particular beam incidence angle for which the scattering cross section becomes independent of collection and beam convergence semi-angles is clearly underscored. Conversely, it is shown that EELS spectra do not depend on the beam incidence angle for a set of particular values of collection and convergence semi-angles. Particularly, in the case of a parallel incident beam, there is a collection semi-angle (often called magic angle) for which the cross section becomes independent of the beam orientation. This magic angle depends on the incident beam kinetic energy. If the incident electron velocity V is small compared with the light velocity c, this magic angle is about 3.975theta(E) (theta(E) is the scattering angle). It decreases to 0 when V approaches c. These results are illustrated in the case of the K boron edge in the boron nitride.     

Influence of the elliptical illumination on acquisition and correction of coherent aberrations in high-resolution electron holography

In high-resolution off-axis electron holography, the interpretable lateral resolution is extended up to the information limit of the electron microscope by means of a correcting phase plate in Fourier space. A plane illuminating electron wave is generally assumed. However, in order to improve spatial coherence, which is essential for holography, the object under investigation is illuminated with an elliptically shaped electron source. This special illumination imposes a variation of beam directions over the field of view. Therefore, due to the interaction of beam tilt and coherent wave aberration, the effective aberrations vary over the field of view yielding a loss of isoplanicity. Consequently, in the past the aberrations were only corrected successfully for a small part of the field of view. However, a thorough analysis of the holographic imaging process shows that the imaging artifacts introduced by the elliptical illumination can be corrected under reconstruction by means of a phase curvature, which models the illuminating wave front. Applied in real space, this phase curvature is seamlessly incorporated into the correction process for coherent wave aberration resulting in an improvement of interpretable lateral resolution up to the information limit for the whole field of view.     

Effective elimination of laser interference fringing in fluorescence microscopy by spinning azimuthal incidence angle

Laser illumination used in both conventional widefield epi-fluorescence as well as in total internal reflection fluorescence (TIRF) microscopy is subject to nonuniformities in intensity that obscure true image details. These intensity variations are interference fringes arising from coherent light scattering and diffraction at every surface in the laser light's optical path, including the lenses, mirrors, and coverslip. We present an inexpensive technique for effectively eliminating these interference fringes based upon introduction of the excitation laser beam by oblique through-the-objective incidence coupled with rapid azimuthal rotation of the plane of incidence. Although this rotation can be accomplished in several ways, a particularly simple method applicable to a free laser beam is to use an optical wedge, spun on a motor, which diverts the beam into a hollow cone of fixed angle. A system of lenses converts this collimated beam cone into a focused spot that traces a circle at the objective's back focal plane. Consequently, a collimated beam with fixed polar angle and spinning azimuthal angle illuminates the sample. If the wedge is spun rapidly, then the different interference patterns at every particular azimuthal incidence angle average out over a single camera exposure to produce an effectively uniform field of illumination.     

Optimum condition of convergent beam illumination for observation of local structure by high resolution transmission electron microscopy

We propose the convergent beam illumination as a technique for the local structural analysis by high resolution transmission electron microscopy. The image contrast is lower in the convergent beam illumination than in the parallel beam illumination because of the lower coherency. However the intensity oscillation around an atom image, which appears due to interference effect, is much reduced with the convergent beam illumination, and pseudo-images do not appear at termination of crystal periodicity. The convergent beam illumination, rather than parallel beam illumination, precisely reveals non-periodic local structures, such as interfaces, surfaces and fine particles, which are even embedded in a crystal. From theoretical analysis the optimum condition is derived as divergence of q(s )* = 0.44 and focus of delta(z)* = 1.35 in generalized coordinates. Using the convergent beam illumination the point resolution is improved by 20% compared to conventional parallel beam illumination.     

A new approach for 3D reconstruction from bright field TEM imaging: beam precession assisted electron tomography

The successful combination of electron beam precession and bright field electron tomography for 3D reconstruction is reported. Beam precession is demonstrated to be a powerful technique to reduce the contrast artifacts due to diffraction and curvature in thin foils. Taking advantage of these benefits, Precession assisted electron tomography has been applied to reconstruct the morphology of Sn precipitates embedded in an Al matrix, from a tilt series acquired in a range from +49° to -61° at intervals of 2° and with a precession angle of 0.6° in bright field mode. The combination of electron tomography and beam precession in conventional TEM mode is proposed as an alternative procedure to obtain 3D reconstructions of nano-objects without a scanning system or a high angle annular dark field detector.     

Electron-electron interactions in cathode objective lenses

The impact of electron-electron interactions on the electron-optical performance of imaging cathode objective lenses is evaluated. Three types of cathode objectives are considered: decelerating and accelerating electrostatic, and combined magnetic lenses. The beam blur is calculated for field sizes ranging from 50 microm x 50 microm to 500 microm x 500 microm and total beam currents ranging from 200 nA to 20 microA. The functional dependence of the beam blur upon electron beam current and current density is elucidated in detail.     

Steady rotation of a sphere in a paramagnetic fluid

The steady rotation of a non-conducting sphere in an unbounded electrically non-conducting incompressible paramagnetic fluid was investigated. An external magnetic field acted parallel to the axis of rotation. The non-dimensional partial differential equations governing the magnetic field, magnetization and velocity were solved numerically. The magnetic field is comparatively weak in the radial direction and strong in the transverse direction over regions nearer the sphere. At points equidistant from the sphere the radial component of the magnetic field is maximum at an angle 14° from the axis. At points equidistant from the centre of the sphere the transverse component of the magnetic field is maximum at points near the equator and minimum at points near the axis. The magnetization increases with the applied magnetic field. At points near the equator and the axis the dipoles are oriented almost parallel to the applied magnetic field. In the remaining region they make an angle with the direction of the applied field which decreases with increasing applied magnetic field until at saturation magnetization they are oriented in the direction of the applied field. At points very near to the sphere the velocity is not affected by the magnetic field. Away from the sphere magnetization retards the flow.     

Expanded beam deflection method for simultaneous measurement of displacement and vibrations of multiple microcantilevers

Here we present an extension of optical beam deflection (OBD) method for measuring displacement and vibrations of an array of microcantilevers. Instead of focusing on the cantilever, the optical beam is either focused above or below the cantilever array, or focused only in the axis parallel to the cantilevers length, allowing a wide optical line to span multiple cantilevers in the array. Each cantilever reflects a part of the incident beam, which is then directed onto a photodiode array detector in a manner allowing distinguishing between individual beams. Each part of reflected beam behaves like a single beam of roughly the same divergence angle in the bending sensing axis as the incident beam. Since sensitivity of the OBD method depends on the divergence angle of deflected beam, high sensitivity is preserved in proposed expanded beam deflection (EBD) method. At the detector, each spot's position is measured at the same time, without time multiplexing of light sources. This provides real simultaneous readout of entire array, unavailable in most of competitive methods, and thus increases time resolution of the measurement. Expanded beam can also span another line of cantilevers allowing monitoring of specially designed two-dimensional arrays. In this paper, we present first results of application of EBD method to cantilever sensors. We show how thermal noise resolution can be easily achieved and combined with thermal noise based resonance frequency measurement.     

Vacuum electron acceleration and bunch compression by a flat-top laser beam

The field intensity distribution and phase velocity characteristics of a flat-top laser beam are analyzed and discussed. The dynamics of electron acceleration in this kind of beam are investigated using three-dimensional test particle simulations. Compared with the standard (i.e., TEM(00) mode) Gaussian beam, a flat-top laser beam has a stronger longitudinal electric field and a larger diffraction angle. These characteristics make it easier for electrons to be trapped and accelerated by the beam. With a flat-top shape, the laser beam is also applicable to the acceleration of low energy electron and bunch compression.     

Contrast enhancement of negatively stained macromolecules and biomembranes by single sideband phase contrast interference.

A straightforward procedure is described for the production of contrast enhancement of negatively stained macromolecules and biological membranes by single sideband phase contrast interference (electron optical shadowing). The instrumental adjustment required to produce this type of phase contrast illumination is readily achieved by beam deflection from the strioscopic (dark field) mode. Part of the hollow cone of electrons from the annular condenser aperture that are unscattered by the specimen are permitted to pass through the objective aperture and interfere with the scattered beam. The electron optical shadowing effect is produced because only one side of the unscattered beam is used. Careful adjustment of the beam tilt control, with the ability to tilt in any azimuth, allows optimal illumination conditions to be achieved. The results presented show the increased image contrast obtained using as specimens the purified cylindrical macromolecule from human erythrocyte membranes, purified nuclear envelopes and collagen fibres.     

复合式多点测量速度干涉仪光学系统设计

为满足爆炸冲击作用下物质界面的速度测量需求,设计了一种复合式多点测量的速度干涉仪。采用物方和像方双远心光路,将光纤阵列出射的照明激光定点投射到待测物面上,实现了针状滴注式照明,充分利用了照明激光能量,且保证了待测物面在运动过程中具有恒定的照度。成像系统像面采用末端为大芯径的锥形光纤接收信号光,既保证了物面运动过程中信号光与光纤的有效耦合,又保证了信号的单模输出,以便进入单模光纤马赫-曾德干涉仪进行差频干涉。采用具有微小楔角、沿直径方向镀矩形带状45°反射膜的反射镜,将照明光路与成像光路同轴,并校正了成像系统的大量像散。该干涉测量系统在物面运动10mm的行程中,物面滴注式照明照度保持恒定,像面光斑大小没有超出大芯径的光纤芯径。此光学系统能够满足爆炸冲击界面的大行程速度测量需求。     

Transmission electron microscopy at 20 kV for imaging and spectroscopy

The electron optical performance of a transmission electron microscope (TEM) is characterized for direct spatial imaging and spectroscopy using electrons with energies as low as 20 keV. The highly stable instrument is equipped with an electrostatic monochromator and a C_S-corrector. At 20 kV it shows high image contrast even for single-layer graphene with a lattice transfer of 213 pm (tilted illumination). For 4 nm thick Si, the 200 reflections (271.5 pm) were directly transferred (axial illumination). We show at 20 kV that radiation-sensitive fullerenes (C₆₀) within a carbon nanotube container withstand an about two orders of magnitude higher electron dose than at 80 kV. In spectroscopy mode, the monochromated low-energy electron beam enables the acquisition of EELS spectra up to very high energy losses with exceptionally low background noise. Using Si and Ge, we show that 20 kV TEM allows the determination of dielectric properties and narrow band gaps, which were not accessible by TEM so far. These very first results demonstrate that low kV TEM is an exciting new tool for determination of structural and electronic properties of different types of nano-materials.     

A spectroscopic scanning electron microscope design

This paper describes a proposal to improve the design of scanning electron microscopes (SEMs). The design is based upon using an SEM column similar to the conventional one, magnetic sector plates and a mixed field immersion objective lens. The optical axis of the SEM column lies in the horizontal direction and the primary beam is turned through 90 degrees before it reaches the specimen. This arrangement allows for the efficient collection, detection and spectral analysis of the scattered electrons on a hemispherical surface that is located well away from the rest of the SEM column. The proposed SEM design can also be easily extended to incorporate time multiplexed columns and multi-column arrays.     

长波红外广角地平仪镜头的光学设计

介绍适用于非致冷凝视式焦平面阵列的长波红外(LWIR)广角地平仪镜头的光学设计.其工作波长范围10～16μm,全视场角为135°.采用"负-正-正"型式的反远距像方远心光路镜头结构,仅有三块非球面锗透镜构成.能够很好地解决广角镜头轴外像差校正和像面照度均匀性问题.此镜头结构简单、体积很小、后工作距离大,成像质量接近于衍射极限,在20lp/mm空间频率处的调制传递函数值超过0.6,像高与视场角关系偏离线性的相对误差不超过15%.文中还分析了此镜头的加工和装调公差.