Image Contrast In Electron Microscopy Research Articles

High contrast at low dose using a single, defocussed transmission electron micrograph

For many soft-matter specimens, transmission electron microscopists face the double-bind of low contrast images, due to weakly-scattering specimens, alongside severe limits on the electron dose that can be used before the specimen is damaged by the electron beam. The combination of these effects causes the resultant micrographs to have very low signal-to-noise. It is well known that varying the defocus aberration can enhance image contrast in electron microscopy. For single-material objects where the variation of absorption and phase contrast are functions of one another, since both are governed by the variation in thickness profile, we show that the thickness profile can be reconstructed at very low dose. The algorithm, first established in X-ray imaging, requires some a priori information but only a single defocussed image of the region of interest, making it more dose efficient than either a conventional transport-of-intensity phase reconstruction (which would require two images and tends to amplify noise), or an absorption-contrast analysis of a single in-focus image recorded at the same electron dose (which does not benefit from the significant signal-to-noise enhancement of the present algorithm). These findings are presented through both simulations and experimental data.

Conductive contact area estimation for carbon nanotube via interconnects using secondary-electron imaging

A major challenge for carbon nanotube (CNT) to become a viable replacement of copper and tungsten in the next-generation on-chip via interconnects is the high contact resistance between CNT and metal electrodes. A first step in meeting this challenge is an accurate characterization of via contact resistance. In this paper, the scanning electron microscope (SEM) image contrast at low landing energy is employed to estimate the conductive CNT area inside vias. The total conductive CNT area inside each via is deduced using SEM image with 0.1 keV landing energy and a specified threshold brightness, yielding via resistance versus CNT area behavior, which correlates well with electrical nanoprobing measurements of via resistance. Monte Carlo simulation of secondary electron generation lends further support for our analysis and suggests that the residue covering the CNT does not affect the conduction across the contact for residue thickness below 1 nm. This imaging and analysis technique can add much value to CNT via interconnect contact characterization.

Electron dose dependence of signal-to-noise ratio, atom contrast and resolution in transmission electron microscope images

In order to achieve the highest resolution in aberration-corrected (AC) high-resolution transmission electron microscopy (HRTEM) images, high electron doses are required which only a few samples can withstand. In this paper we perform dose-dependent AC-HRTEM image calculations, and study the dependence of the signal-to-noise ratio, atom contrast and resolution on electron dose and sampling. We introduce dose-dependent contrast, which can be used to evaluate the visibility of objects under different dose conditions. Based on our calculations, we determine optimum samplings for high and low electron dose imaging conditions.

Phonon Scattering in High-Resolution Electron Microscopy

It is generally accepted that the lattice fringe contrast in experimental high-resolution electron microscope (HREM) images is lower than that in image simulations by a factor of between 2 and 6 [1]. One likely reason for this discrepancy is the contribution phonon, or thermal diffuse, scattering makes to lattice images. Electrons scattered by phonons are visible as diffuse scattering between the Bragg spots in the diffraction pattern and the loss in intensity of the Bragg spots is modeled by the Debye-Waller factor in image simulations. However, the additive effect of this diffuse scattering on images is not taken into account by image simulations. The amount of phonon scattering as a function of specimen thickness has been measured both for Si by convergent beam diffraction [2] and for ZrB12 by electron holography [3] and in both cases found to contribute about 10% of the total intensity at a thickness typically used in high resolution imaging.

Thickness-fringe Contrast Analysis of Defects in GaN

ABSTRACTThe analysis of thickness-fringe contrast in weak-beam transmission electron microscope (TEM) images has been shown to be a reliable method for the complete determination of the character, as well as the magnitude, of a dislocation Burgers vector. By selecting multiple diffraction conditions and, for each condition, determining the number of terminating thickness fringes at the exit of a dislocation from a wedge-shaped sample, the Burgers vector can be unambiguously determined. Defect analysis of GaN pyramids grown on (111)Si by the lateral epitactic overgrowth (LEO) technique reveals a core region which contains a relatively high density of dislocations and a lateral-growth region where the defect density is decreased. The thickness-fringe contrast technique was used in the lateral growth regions of the pyramids to analyze the dislocation Burgers vectors.

On the analysis of diffuse intensity and striation contrast in electron microscopy images of Al–Cu–Co–Si decagonal phases

Electron microscopy and diffraction experiments were carried out to analyze diffraction features and correlate with the striation contrast observed in the images of Al–Cu–Co–Si decagonal phase. There are two important diffraction features identified which include the diffuse streak and the extra satellite spots. The extra satellite spots can indicate some sort of superlattice chemical ordering among the clusters while the diffuse streaks indicate presence of some kind of phason disorder leading to the observation of striation contrast in the imaging modes.

Interpretation of High Resolution Transmission Electron Microscope Images of Short Range Ordered Ni<SUB>4</SUB>Mo

High resolution transmission electron microscope (HRTEM) image contrast of short-range order (SRO) in Ni 4 Mo alloys has been investigated by means of multi-slice simulations. The HRTEM image contrast of SRO in Ni 4 Mo coincides with a projection contrast of an SRO structure which involves D1 a , D0 22 and Pt 2 Mo-type subunit cells in a disordered matrix. Multi-slice simulations revealed that the SRO structure exhibit a projection contrast in the HRTEM image when the fundamental fcc lattice reflections contribute sufficiently to HRTEM imaging. The results indicate that the image of SRO in Ni 4 Mo can be interpreted in terms of projected potential of atomic columns under an appropriate imaging condition.

The nature of ion milling damage in MoSi 2

Circular defects having strain and displacement contrast in transmission electron microscope images have been frequently reported at dislocations in MoSi . The origin of this contrast has been associated with deformation2 induced effects. However, by comparing samples that had been prepared either by electropolishing or by ion-beam milling during the final stage of thinning, it has been shown that these features are an artefact of ion-beam thinning. A detailed analysis of the contrast from the defects, as well as their thermal stability, suggest that they are platelets of argon introduced during the ion milling process.

Characterization of thin film materials by 400 kV electron microscope images and with an energy filter

Characterization of thin film materials by the contrast of high resolution electron microscope images and by the images of electrons that excite the core electrons of the specimen is discussed. Using current high resolution electron micrographs, which are produced by partially coherent waves, the best focus for obtaining the atomic structure images of three crystals, MgO/MgAl 2O 4/Al 2O 3, has been found using pseudo AFF conditions. Three kinds of electron microscope images of ThO 2 crystal, formed by the zero-loss, plasma-loss and O 4,5 core-loss electrons, are shown. Three similar images of a Si crystal, which precipitated in an Al crystal, are also shown, and the contrast mechanism of core-loss electron images is discussed by deriving the inelastic scattering factors of Si and Al crystals and using multi-slice elastic image contrast calculations. It is noted that the individual spot contrast in Si core-loss images of a small crystal is due to the images of Si atoms in a single atomic column when the thickness is less than 10 nm.

Detection of a Point Defect in a Silicon Single Crystal by a Subtraction Method using High-Resolution Transmission Electron Microscopy

The contrast of high-resolution transmission electron-microscope (HRTEM) images of silicon single crystals in [\bar 110] orientation containing a pair of single 〈111〉 vacancies tetrahedrally bonded (divacancy), triple divacancy clusters in two orientations and an interstitial single Si atom has been calculated. The displacement of atoms around a point defect are deduced by extrapolating the distortion around a stacking-fault tetrahedron (SFT). The small contrast changes and displacements of the images of atoms in the crystal containing a point defect are revealed by finding the difference between the computed image of the point defect and that from the perfect crystal. The visibility of the images and the possibility of the detection of point defects are also discussed.

Atomic resolution electron microscope images formed by SiL 3-ionization electrons

Contrast of energy-filtered electron microscope images with atomic resolution of Si atoms producing SiL 3-ionization electrons in a Si crystal is discussed. The localization width of the inelastic scattering potential is deduced from the inelastic scattering factor for SiL 3-ionization, which is based on a program for a modified hydrogenic L-shell cross section. The image contrast at the bottom surfaces of the Si crystals containing a single inelastic scattering layer at the top, middle and bottom layers showed diffuse, sharp and very sharp image contrast, i.e. top-bottom effect is predominant. Similar images of an atom which scattered electrons inelastically with a very small energy loss in an aluminum crystal were calculated and it is noted that the atomic structure of aluminum is observed.

Interpretation of mullite HREM images using the Potential-exchange-method

AbstractMulti-slice image contrast calculations using the periodic continuation theorem (supercell-model) and a new approximation developed by Hamid Rahman (1989) called the “Potential exchange method” show good agreement in the electron microscope image contrast simulation for mullite. The method can be used to calculate a non-periodic potential and is most accurate when the defects (light atom vacancies) are distributed on (100), (010) or (001) planes and accompanied with cation shifts within the unit cell.Mullite Al2[Al2 + 2xSi2−2x]O10−xis a non-stoichiometric silicate compound wherexis the number of missing oxygen atoms per average unit cell (0.17 ≤x≤ 0.59, Cameron 1977).The average structure of mullite (Fig. 1a,Pbam,a= 7.56 Å,b= 7.68 Å,c= 2.884 Å) was determined by Burnham (1963, 1964), Durovic (1969) and Saalfeld and Guse (1981).The structure analysis shows that the vacancies occur on the tetrahedral linking positions (Oc, Oc* Fig. 1b, c). The loss of the fourth coordinating oxygen atom causes the cation on the tetrahedral sites (Si/Al site) to occupy neighbouring alternative position of the second tetrahedral site denoted by Al*. This means that the contrast variation of the HREM image is mainly affected by the site occupation (cation shift Fig. 1a, b, c) within the unit cell.X-ray (h0l, 0kl,hk1/2,hk1/3andhk2/3) and electron (h0land 0kl) diffraction patterns (x= 0.25 and 0.4) show diffuse streaks and satellite reflections, indicating a modulation (ordering scheme) within the mullite structure which doubles thec-axis and has a composition-dependent, incommensurable periodicity along thea-axis (Guse and Saalfeld, 1976;

On the applicability of bloch’s theorem to the total wave function in the dynamical theory of electron diffraction

The dynamical theory of electron diffraction is widely used in materials science problems such as determining the contrast in electron microscope images of crystal defects, calculations of structure images and calculations of diffracted beam intensities in electron diffraction patterns. In carrying out these calculations, the high energy approximation is normally made and it is usually assumed that the crystal is in a symmetrical Laue orientation. In practice, however, a specimen in the electron microscope will generally be oriented so that the non-symmetrical Laue case is obtained. Even in those special cases where the symmetrical Laue case is obtained for the zero order Laue zone reflections, non-symmetrical Laue effects may occur if reflections in higher order zones are important. It has been shown in the literature that the Bloch functions are not orthogonal in more general forms of the dynamical theory in which the high energy approximation is not made and the nonsymmetrical Laue case is considered,.

Interpretation of transmission electron microscope images of amorphous silicon/germanium and silicon/iron multilayers

We show that the observed contrast in transmission electron microscope images of multilayer specimens composed of amorphous materials can be understood in terms of the scattering of electrons passing through the material. This leads us to develop a simple method of analyzing the image contrast to give quantitative information about the thicknesses of individual layers, which cannot be obtained by any other technique. The results of this analysis for images of silicon/germanium and silicon/iron multilayers are in good agreement with the results of small-angle x-ray scattering and the readings of quartz thickness monitors during the deposition of the layers. The accuracy of the thickness measurements we obtain from transmission electron microscope images is 7% for the average layer thickness of a single component and 10% for the thickness of individual layers when the layer thickness is about 4 nm, which is comparable to the accuracy which can be attained for crystalline layers.

Magnetic constrast in reflection electron microscopy

The possibility of obtaining magnetic contrast in reflection electron microscope images is considered. We consider the problem of observing two-dimensional magnetic domains in Fe monolayer films on a GaAs substrate. By introducing the vector potential into multiple scattering RHEED calculations based on the multi-slice algorithm, the effect of the Lorentz force on diffraction conditions is evaluated at the specular condition for 120 kV electrons. This provides a contrast mechanism. For Fe(110)/GaAs(110) the contrast which results in the (440) specular beam from a reversal of B normal to the beam in the plane of the surface is found to be about 28%, suggesting that useful contrast might be obtained from a single monolayer. The penetration depth of the wavefield into the crystal is studied as a function of incident beam direction The distance along the beam path needed to re-establish the steady-state condition following a reversal of B is used to estimate the resolution of the method. The method, which would allow real-time image recording at high sample temperatures, is compared briefly with other magnetically sensitive surface imaging techniques.

Transmission Electron Microscopy Study on the Annealing of YBa2Cu3O7‐δ

Annealing in oxygen‐rich atmosphere at temperatures between 400° and 600°C is an important step in the manufacture of superconducting YBa2Cu3O7‐δ. The symmetry of the orthorhombic phase requires that if more than one type of twin plane is present within a grain, a distorted region should exist inside the multiple twinned grain. This distorted region hinders the tetragonal‐to‐orthorhombic transformation and may account for some retained tetragonal phase inside an otherwise orthorhombic grain. A physical model is presented describing the formation of such regions and their eventual transformation into low‐angle grain boundaries after long annealing. Extended annealing at intermediate temperatures apparently leads to the formation of planar faults in off‐stoichiometric samples. Transmission electron microscope image contrast and energy dispersive X‐ray analyses of highly defective regions suggest these defects are CuOx (x= 1, 2) extra layers. These extra layers tend to form near grain boundaries or free surfaces, where oxygen is readily available.

The many beam dynamical theory of contrast in electron microscope images of microtwins for the non-symmetrical Laue and non-column cases

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Composition determination in the GaAs/(Al, Ga)As system using contrast in dark-field transmission electron microscope images

Abstract The sensitivity to composition of transmission electron microscope images of (Al, Ga)As obtained using the 002 reflection is well known. Here we assess the potential quantitative use of such images for the compositional analysis of heterostructures and find that typical values of the composition parameter (x in AlxGa1−xAs) can in principle be determined to ±0·02, with unit-cell spatial resolution. A treatment is described which is simple to apply and which, although based on a kinematical approach, nonetheless takes account of the effects of dynamical scattering, inelastic scattering and a variety of other sources of error. The method and its limitations are demonstrated for a variety of technologically interesting structures. We note the further improvement in accuracy that is potentially achievable by the use of energy filtering.

A model for the superstructure of Bi2Sr2CaCu2O8.2

Abstract A model is proposed for the superstructure of Bi 2 Sr 2 CaCu 2 O 8.2 . It involves the addition of one in ten oxygen atoms in the BiO planes and a displacement of the surrounding atoms. This model is consistent with electron microscope, neutron and X-ray diffraction data. In particular it is consistent with the symmetry determined by convergent beam electron diffraction and with details of the high resolution electron microscope image contrast.

Electron microscope image contrast formed by electrons from elastic–inelastic and inelastic–elastic scattering processes

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