Combining real and reciprocal space information for aberration free coherent electron diffractive imaging

Information from imaging and diffraction planes, or real and reciprocal spaces, of transmission electron microscopes (TEM) can be combined using iterative transformation algorithms to reconstruct the complex wave function, to improve image resolution and to remove residual aberrations in the case of aberration corrected TEM. Here, we describe the experimental and computation techniques needed for combining real and reciprocal space information. We demonstrate these techniques by reconstructing the complex wave function of quantum dots and carbon nanotubes beyond the microscope's resolution limit.     

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.     

Electron diffraction from nanovolumes of amorphous material using coherent convergent illumination

Using a conventional transmission electron microscope that incorporates a field emission gun it is possible to focus an electron beam to form a small probe (<1nm full-width at half-maximum). Such a probe can then be used to perform high spatial resolution diffraction experiments. The high spatial resolution allows technologically interesting amorphous volumes, such as those found in glassy intergranular phases or in semiconductor implantations, to be investigated directly. In order to achieve the probe characteristics necessary to investigate nanovolumes of material the probe must be highly convergent which results in it being highly coherent. In this paper we examine the effect of coherent convergent illumination on electron diffraction data taken from nanovolumes of amorphous material. It is shown that, for amorphous volumes as small as 1.2nm in diameter, the additional interference effects induced in the diffraction data by the use of coherent convergent illumination are largely suppressed by the lack of order in amorphous materials. This allows the use of deconvolution techniques, developed for the correction of broadening of the diffraction pattern in the case of incoherent illumination, and the subsequent application of reduced density function (G(r)) analysis, to also be used for coherent illumination.     

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.     

Coherent low-energy electron diffraction on individual nanometer sized objects

Today's structural biology techniques require averaging over millions of molecules to obtain detailed structural information. Derivation of the molecular structure from a scattering experiment with just one single 3D-molecule imposes major challenges. Coherent and damage-free radiation is needed to ensure sufficient elastic scattering events before destroying the molecule and a means to solve the phase problem is wanted. We have devised such a scheme using coherent low-energy electrons shaped into a collimated beam by an electrostatic microlens. Initial experiments using a carbon nanotube sample demonstrate the feasibility of coherent low-energy electron diffraction on an individual nanometer-sized object.     

Effective phase correction function for high-resolution exit wave reconstruction by a three-dimensional Fourier filtering method

The phase correction function used in the three-dimensional Fourier filtering method (3D-FFM) for compensating lens aberrations was investigated to reconstruct a high-resolution exit wave of a sample. An appropriate function, which hardly suffered from imperfect illumination conditions, was determined by comparing two types of phase correction functions with numerical calculations and experiments using through-focus images of an amorphous thin film and a [110]-oriented Si single crystal taken under tilted illumination or partially coherent illumination. Theoretical calculations indicated that a function in terms of w (an axial Fourier component), available uniquely in the 3D Fourier space, compensated for the phase shift due to the spherical aberration more precisely than did a conventional function in terms of g (the two-dimensional (2D) planar Fourier components). Experimentally, exit waves reconstructed using the w-function showed sample structures at approximately 20% higher resolution than those reconstructed using the g-function. Image contrast simulations proved that the w-function had a significant advantage over the g-function: the former canceled out the effect of illumination divergence, resulting in a high-resolution exit wave. These results demonstrated that exit waves, which are uniquely realized in the 3D-FFM, should be reconstructed using the w-type phase correction function.     

Resolution in light microscopy studied by computer simulation

Microscopic resolution can be characterized by K = x NA/δ, where x is the distance between two objects, or the interval of a grating, just resolved with light of wavelength λ and an objective of aperture NA. Using a computer simulation of imaging the following K values were obtained on the Sparrow resolution criterion for line and grating objects and various imaging methods (figures for the Rayleigh criterion, which assumes a finite contrast-sensitivity of the light detector, are in parentheses). Several results appear to be novel. Due to limitations discussed in the text some data are only approximate. With a grating object K is 1.0 (1.0) for axial coherent illumination, 0.5 (0.5) for coherent illumination at an obliquity NA_obi which just enters the objective aperture, 0.5 (about 0.53) for incoherent illumination, 0.5 (about 0.52) for illumination with a condenser aperture NA_c equal to NA, 0.5 (about 0.515) for transmitted-light confocal scanning, and 0.25 (about 0.38) for fluorescent confocal scanning. If the object consists of two parallel lines K is about 0.68 (0.71) for axial coherent illumination, about 0.44 (0.5) for incoherent illumination, 0.375 (about 0.48) for optimal partially coherent illumination in which NA_c may exceed NA, 0.44 (0.48) for transmitted-light confocal scanning, and 0.32 (0.41) for fluorescent confocal scanning. For inter-object distances of 1, 1.5 and 2 wavelengths, respectively, NA_c values of about 0.69, 0.5 and 0.375 gave optimal contrast and resolution irrespective of NA. The practice of setting NA_c to about two-thirds of the NA of a high-power objective is supported by the fact that a condenser aperture of about 0.69 gives excellent or optimum resolution and contrast for most inter-object distances and objective apertures tested, although with some distances and apertures reducing NA_c improved contrast slightly. The rule (sometimes attributed to Abbe) that resolving power is proportional to the mean of NA and NA_c is correct for oblique coherent illumination in the case of a grating object, provided NA_obl does not exceed NA. In the case of two isolated objects the rule is only approximately correct, but applies even if NA_obl is greater than NA. Coherent light at an obliquity of 0.5λ/x introduces a half-wavelength phase difference between two objects and permits their resolution (with perhaps an incorrect apparent inter-object distance) even with objective apertures approaching zero. In confocal scanning the width of the scanning spots has only a moderate effect on resolution, and two objects can sometimes be resolved with scanning spots wider than the inter-object distance provided the lens apertures are neither too small nor too large.     

Sub‐100‐nanometre resolution in total internal reflection fluorescence microscopy

Combining total internal reflection fluorescence microscopy with structured illumination allows optical wide‐field imaging with sub‐100‐nanometre resolution. We present a novel objective‐launch set‐up for standing wave illumination that takes advantage of a tunable transmission diffraction grating and transparent phase shifters actuated by electro‐active polymers to control the excitation pattern in three dimensions. Image acquisition is completed in less than 1 s. To reconstruct the extended image spectrum, we apply a new apodization function that results in a lateral resolution of 89 nm for green emission wavelength.     

极小刻划间隙下的衍射光强度变化规律分析

讨论了极小刻划间隙下的衍射场光强度分布规律,并以单缝为例分析了当缝宽与波长改变时,相对光强度变化规律,补充和完善了使用普通平行光照明系统时的接近式光刻衍射场理论。     

Optimization of resolution by FDTD analysis in aligner lithography

A mask aligner can transcribe a pattern from a photomask to an exposure substrate by Fresnel diffraction. A diffraction fringe, specific to Fresnel diffraction, appears on a light intensity distribution of the pattern (a diffraction pattern image), and the formation of the pseudo-pattern restricts the resolution performance. The diffraction fringe can be smoothened by expanding the spread of the illumination source, and thus, the pseudo diffraction can be attenuated. However, this also causes a change in light intensity at the pattern edge to be attenuated, and the error of pattern line width to process change becomes large. Since edge diffraction patterns can be calculated by finite difference time domain (FDTD) analysis, the size of light source providing optimum resolution can be predicted by calculating and comparing pattern images corresponding to the size of the light source using this analysis. Therefore, by introducing an illumination optical system that can arbitrarily set the size of a light source to a predicted value, optimum resolution can be obtained without prior trial exposure. This study shows that the resolution of an aligner can be optimized by prior prediction, by introducing a multiple smoothing optical system that has been developed as an illumination system. This system realizes uniform illumination distribution on both a pupil plane and photomask plane and has an adjustable aperture mechanism that, by combining it with FDTD analysis, can arbitrarily set the size of the light source.     

人眼波前像差测量中的照明系统设计

使用Hartmann-Shack传感器测量大像差人眼时,细平行光照明会导致波前像差测量的精度下降。设计并实现了一种自动的可调照明系统,在入射光路中加入了一个位置可以自动调整的准直透镜,调整这个透镜的位置可以准确的改变照明光的光屈度,精密电控平移台可以对准直透镜的位置进行快速准确扫描定位。实验中对模拟人眼像差进行了测量,分别用细平行光照明和自动可调照明系统的实验结果进行了对比。实验结果显示,使用自动可调照明使传感器获得图像的信噪比由2:1提高到10:1,对近视-5D模拟人眼测量的结果也由-4.285D±0.208D提高到-5.041D±0.157D,结果显示,使用自动可调照明系统提高了Hartmann-Shack传感器图像的信噪比,使测量结果更加准确。     

双胶合透镜的三维成像研究

应用标量衍射理论分析了双胶合透镜的三维超分辨成像特性。对于所设计的双胶合透镜,数值模拟表明,焦平面上主瓣的强度是第1旁瓣的57.14倍,是单一个透镜主瓣强度的2.25倍,而主瓣的半径则是单一个透镜的0.49倍;轴线方向上,主瓣与第1旁瓣的强度之比与单一个透镜的基本相同,但是双胶合透镜的焦深则比单一个透镜的小0.25倍。说明双胶合透镜在横向和轴向同时具有较强的超分辨能力。通过与加圆环光瞳或加相位光瞳的方法比较发现,双胶合透镜具有较好的三维成像能力,可以作为共焦3-D成像和近场光学记录的理想光学器件。     

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.     

Electron diffraction analysis of individual single-walled carbon nanotubes

We present a detailed electron diffraction study of individual single-walled carbon nanotubes. A novel sample preparation procedure provides well-separated, long and straight individual single-shell nanotubes. Diffraction experiments are carried out at 60 kV, below the threshold for knock-on damage in carbon nanotubes. We describe experimental parameters that allow single-tube electron diffraction experiments with widely available thermal emission transmission electron microscopes. Further, we review the simulation of diffraction patterns for these objects.     

面板检测用显微镜光学系统设计

为了实现对面板上缺陷精细分析和分类,设计了可见光波段、数值孔径为0.42的复消色差平场显微物镜和照明光学系统.通过合理结构优化、光焦度分配和材料的选择,优化出平场复消色差物镜,使其MTF曲线接近衍射极限.采用科勒照明匀光方案,设计照明光学系统,并用Lighttools软件对照明光学系统进行仿真.实验表明,光学系统分辨率...     

Quantitative temperature measurement of an electrically heated carbon nanotube using the null-point method

Previously, we introduced the double scan technique, which enables quantitative temperature profiling with a scanning thermal microscope (SThM) without distortion arising from heat transfer through the air. However, if the tip-sample thermal conductance is disturbed due to the extremely small size of the sample, such as carbon nanotubes, or an abrupt change in the topography, then quantitative measurement becomes difficult even with the double scan technique. Here, we developed the null-point method by which one can quantitatively measure the temperature of a sample without disturbances arising from the tip-sample thermal conductance, based on the principle of the double scan technique. We first checked the effectiveness and accuracy of the null-point method using 5 μm and 400 nm wide aluminum lines. Then, we quantitatively measured the temperature of electrically heated multiwall carbon nanotubes using the null-point method. Since the null-point method has an extremely high spatial resolution of SThM and is free from disturbance due to the tip-sample thermal contact resistance, and distortion due to heat transfer through the air, the method is expected to be widely applicable for the thermal characterization of many nanomaterials and nanodevices.     

SEM investigation of interfacial dislocations in nickel-base superalloys

Chromatography is a widely used separation unit operation for separating nanomaterials such as proteins and enzymes, quantum dots and carbon nanotubes. An understanding of the chromatographic stationary phase on a nanoscale would be extremely helpful in improving the process and developing efficient and new materials. This study is an attempt to characterize the stationary phase in its swollen wet state using environmental scanning electron microscope (ESEM) and atomic force microscopy (AFM). Observation of the wet beads using ESEM is limited to a micron-range resolution. However, AFM can be used in wet mode to characterize the stationary phase in both wet and dry states with nanometric resolution. In the swollen state, microscale cracks were observed on the surface and this may explain the high mass transfer rate and lower back pressures of the stationary phase. The structures on the surface of the stationary phase depict that the micron-sized beads may be composed of nanometric beads.     

An orientation‐independent DIC microscope allows high resolution imaging of epithelial cell migration and wound healing in a cnidarian model

Epithelial cell dynamics can be difficult to study in intact animals or tissues. Here we use the medusa form of the hydrozoan Clytia hemisphaerica, which is covered with a monolayer of epithelial cells, to test the efficacy of an orientation‐independent differential interference contrast microscope for in vivo imaging of wound healing. Orientation‐independent differential interference contrast provides an unprecedented resolution phase image of epithelial cells closing a wound in a live, nontransgenic animal model. In particular, the orientation‐independent differential interference contrast microscope equipped with a 40x/0.75NA objective lens and using the illumination light with wavelength 546 nm demonstrated a resolution of 460 nm. The repair of individual cells, the adhesion of cells to close a gap, and the concomitant contraction of these cells during closure is clearly visualized.     

The design and implementation of a high-fidelity Raman imaging microscope

We describe a Raman imaging microscope that produces high-fidelity, large format Raman images and Raman spectra from samples as small as 1 μm in size. Laser illumination is delivered to the object by means of an infinity corrected microscope objective, either by a galvanometer scanning system or a widefield fibre optic. Wavelength selection of Raman scattered emission is achieved by an acousto-optic tunable filter (AOTF), which maintains image fidelity and provides either continuous or random wavelength selection. The collimated AOTF output is imaged first by a tube lens and then by a projection lens onto a cooled silicon CCD array. Instrument features, including factors that determine the system's spatial and spectral resolution, and design considerations are discussed in detail. Images and spectra of test objects and samples that demonstrate the capability of this imaging spectrometer are presented. The potential of intrinsic chemical imaging is discussed in terms of its use in the analyses of a variety of chemical and biological samples.     

Extended ptychography in the transmission electron microscope: possibilities and limitations

The extended-ptychographical iterative engine (e-PIE) is a recently developed powerful phase retrieval algorithm which can be used to measure the phase transfer function of a specimen and overcome conventional lens resolution limits. The major improvement over PIE is the ability to reconstruct simultaneously both the object and illumination functions, robustness to noise and speed of convergence. The technique has proven to be successful at optical and X-ray wavelengths and we describe here experimental results in transmission electron microscopy supported by corresponding simulations. These simulations show the possibilities - even with strong phase objects - and limitations of ptychography; in particular issues arising from poorly-defined probe positions.