Modification of an EM6G electron microscope for X-ray microanalysis

A dual purpose stage has been constructed for an EM6G 100 kV transmission electron microscope. With this stage the composition of thin films and bulk specimens can be determined by X-ray microanalysis. With thin films a change of specimen cartridge then enables a full analysis of crystal defects in the film to be made using tilt controls incorporated in the stage. Modifications to the stage to reduce background effects in X-ray microanalysis spectra are also described. The alternative use of this system in the bulk analysis of specimens by an X-ray fluorescence technique is also discussed.     

Standardless EDS analysis of bulk and thin specimens

A direct relationship between the x-ray intensity ratio and the concentration ratio for bulk and thin specimens has been established by use of a revised full-diffusion model of electron scattering. The suitable ionization cross section, the most important parameter influencing the accuracy of the calculated Cliff-Lorimer factors, has been found after comparing the experimental intensity ratio I(L)/I(K) of eight elements (from Ge to Sn) and I(M)/I(L) of six elements (from Sm to Bi) with the calculated values. The quantitative standardless EDS analysis of bulk samples obtained by this direct method is more satisfactory than the commercial indirect method which gives the composition through ZAF correction after calculating the intensity factors of pure elements. The quantitative standardless analysis of thin samples has been improved by the suitable cross section significantly. This method has been applied to the analysis of film on substrate either without any common element or with one common element (P-Si glass film on Si). It has also been used to calculate the intensity factors of pure bulk samples and the backscattering correction factor in Auger electron spectroscopy.     

Foreword

The volume of thin foil specimens, which contain precipitate or other particles, viewed in the TEM is needed to determine particle density and spacing. It can be determined from the locations of the particles, measured using stereo pairs. A calculation that determines the volume between planes (not necessarily parallel or horizontal) that enclose the points is described. These planes will systematically underestimate the actual surface spacing and hence the volume, but a simple correction factor based on the number of points used in the fit can be used to estimate the actual volume. The method is tested, and its accuracy is evaluated using simulated data and applied to precipitate particles in creep-tested silicon carbide.     

Evaluation of several common standards for the X-ray microanalysis of thin biological specimens

The question of the best type of standard to use for X-ray microanalysis of thin biological specimens remains unanswered. Standards embedded in an organic matrix have the advantage that they resemble biological specimens, but their composition is generally not known exactly. We compared several standards and, surprisingly, inorganic binary salts sprayed onto a supporting film were the most suitable: they corresponded closely with several other methods using organic matrices; they were easily produced; and their composition is known. Glutaraldehydeurea aminoplastic resin thin sections and thin films containing dissolved salts were problematic. The composition of the polymer appears to be variable, and the thin films did not correspond with any other standard tested. Chelex¹⁰⁰ bio-standard beads and flakes loaded with accurately determined concentrations of ions, embedded in epoxy resin and thin sectioned, tended to correspond to the results obtained with the binary salts. However, the results from some bio-standards were inexplicably aberrant. An epoxy resin standard was used for bromine, and was found to agree closely with the binary standards.     

Quantitative X-ray microanalysis of biological specimens

Qualitative X-ray microanalysis of biological specimens requires an approach that is somewhat different from that used in the materials sciences. The first step is deconvolution and background subtraction on the obtained spectrum. The further treatment depends on the type of specimen: thin, thick, or semithick. For thin sections, the continuum method of quantitation is most often used, but it should be combined with an accurate correction for extraneous background. However, alternative methods to determine local mass should also be considered. In the analysis of biological bulk specimens, the ZAF-correction method appears to be less useful, primarily because of the uneven surface of biological specimens. The peak-to-local background model may be a more adequate method for thick specimens that are not mounted on a thick substrate. Quantitative X-ray microanalysis of biological specimens generally requires the use of standards that preferably should resemble the specimen in chemical and physical properties. Special problems in biological microanalysis include low count rates, specimen instability and mass loss, extraneous contributions to the spectrum, and preparative artifacts affecting quantitation. A relatively recent development in X-ray microanalysis of biological specimens is the quantitative determination of local water content.     

Focused ion beam sample preparation of continuous fibre-reinforced ceramic composite specimens for transmission electron microscopy

The microanalysis of interfaces in fibre-reinforced composite materials is dependent on the successful preparation of specimens suitable for transmission electron microscope (TEM) inspection. Ideal samples should possess large amounts of structurally intact and uniform thin area in the fibre/matrix interface regions of the samples. Because fibre/matrix interfaces in this class of materials are often designed to fail under mechanical stress, conventionally prepared samples are prone to interfacial failure and differential thinning, both of which preclude detailed TEM microanalysis. These effects were seen in a conventionally dimpled and ion-beam-thinned specimen prepared from a continuous fibre reinforced ceramic composite composed of CaWO₄-coated Nextel 610^TM fibres in an alumina matrix. The dimpled specimen showed large amounts of interfacial failure, with only thick regions of the specimen left intact. To overcome these limitations, a focused ion beam (FIB) technique was applied to this same material. The superiority of the FIB-produced sample is evident in both the morphology and scanning transmission electron microscopy analyses of the sample.     

Static-charging mitigation and contamination avoidance by selective carbon coating of TEM samples

Prior to transmission electron microscopy (TEM) analyses, insulating specimens need to become coated with a charge-draining layer. Rather than coating the entire TEM foil with a thin film of homogeneous thickness, selective coating is proposed. Using a novel preparation tool, peripheral parts of the sample are coated with a relatively thick (4-8 nm) carbon film while the central, electron-transparent part of the sample is hidden behind a shape-adopted mask and thus not directly exposed to carbon deposition. Beneath the mask, an ultrathin (3-7 A) carbon film is formed that is (i) thick enough to drain charges evolving upon electron irradiation in the electron microscope and (ii) thin enough to avoid typical contamination effects caused by superficial carbon diffusion. Consequently, image quality is becoming enhanced in high-resolution imaging and sensitivity is significantly increased in all nano-beam related techniques including elemental analytics, convergent-beam and nano-beam electron diffraction, and spectral imaging.     

X-ray microanalysis of organic thin sections in TEM using an UTW Si(Li) detector: comparison of quantification methods

We compared Hall's peak to continuum ratio method with a peak ratio method in order to quantify light elements (C, N, and O) in organic specimens as a model for biological thin sections. X-ray spectra were recorded by an energy dispersive X-ray spectrometer equipped with an ultra thin window detector. Spectra were processed by means of a top-hat filter adapted to peak full-width half maximum. The peak intensities were measured by multiple least square fitting to reference spectra. For most elements of biological interest, theoretical and experimental k-factors were determined. Absorption correction was found to be important for quantitation of carbon, nitrogen, and oxygen. Boron was efficiently detected; however, quantitative analysis was not possible. We conclude from our experiments that the peak ratio method is more suitable for quantitation of elemental concentrations in biological thin sections than the peak to continuum method.     

Wave field restoration using three-dimensional Fourier filtering method.

A wave field restoration method in transmission electron microscopy (TEM) was mathematically derived based on a three-dimensional (3D) image formation theory. Wave field restoration using this method together with spherical aberration correction was experimentally confirmed in through-focus images of amorphous tungsten thin film, and the resolution of the reconstructed phase image was successfully improved from the Scherzer resolution limit to the information limit. In an application of this method to a crystalline sample, the surface structure of Au(110) was observed in a profile-imaging mode. The processed phase image showed quantitatively the atomic relaxation of the topmost layer.     

Retrieving overlapping crystals information from TEM nano‐beam electron diffraction patterns

The diffraction patterns acquired with a transmission electron microscope (TEM) contain Bragg reflections related to all the crystals superimposed in the thin foil and crossed by the electron beam. Regarding TEM‐based orientation and phase characterisation techniques, the nondissociation of these signals is usually considered as the main limitation for the indexation of diffraction patterns. A new method to identify the information related to the distinct but overlapped grains is presented. It consists in subtracting the signature of the dominant crystal before reindexing the diffraction pattern. The method is coupled to the template matching algorithm used in a standard automated crystal orientation mapping tool (ACOM‐TEM). The capabilities of the approach are illustrated with the characterisation of a NiSi thin film stacked on a monocrystalline Si layer. Then, a subtracting‐indexing cycle applied to a 70 nm thick thin foil containing polycrystalline tungsten electrical contacts shows the capability of the technique to recognise small nondominant grains.     

Optimized preparation of cross-sectional TEM specimens of organic thin films

We present a route for the preparation of cross-sectional TEM specimens of crystalline organic thin films which minimizes the mechanical, chemical and thermal load of the organic film during preparation and allows to take TEM images with molecular resolution. A typical example of a thin film of diindenoperylene capped with a thin layer of gold is shown to demonstrate the application of the technique for the investigation of metal-organic interfaces.     

Micrometallurgy: a technique for examining the structure of binary-element thin films over a wide range of composition

We describe a procedure for producing thin-film TEM specimens containing a thickness and/or composition gradient, utilizing an out-of-contact mask at an appropriate distance from the substrate. Imaging and diffraction capabilities of the TEM are used to examine the local structure of the film; EELS or EDX analysis provides the local elemental composition. The procedure is illustrated by results obtained from two binary-alloy systems: Se–Te (which displays a complete range of solid solubility) and Sn–Ge (where mutual solubility is very low).     

Theory analysis of asymmetrical electric double layer effects on thin film lubrication

This paper presents a theoretical research concerning the effects of the asymmetrical electric double layers on thin film lubrication. A mathematical model of electro-viscosity considering the asymmetrical electric double layers is developed, which acts as an extensional study of the symmetrical cases. Then, the numerical analyses based on this model are performed to evaluate the influence of electric double layers on the thin water lubricating film with and without consideration of the elastic deformation of the opposing surfaces. The results show that the electric double layers lead to significant variations in the properties both of hydrodynamic and elasto-hydrodynamic lubricating film.     

Optimizing thin film X-ray spectra for quantitative analysis

Energy dispersive X-ray detectors are frequently attached to electron microscopes to enable microanalysis to be performed, but because such detectors accept X-rays generated within an appreciable solid angle, the recorded spectra usually include some spurious contributions from the instrument. This paper describes instrumental modifications firstly to reduce and secondly to permit the subtraction of the residual extraneous contributions. The probable accuracy of this subtraction procedure is examined. Results are presented showing the effects of various instrumental modifications on spectra from thin specimens and demonstrate that by careful attention to experimental details it is possible to separate the spectrum due to the thin specimen alone from all other extraneous signals. Two test specimens and a number of test procedures for investigating the analytical performance of (scanning) transmission electron microscopes are presented. Sources of error and the method of their correction when the thin specimen Bremsstrahlung is used for quantitative analysis are discussed.     

一些硫属化合物非晶材料相变温度的测定

本文研究了一些硫属化合物非晶半导体薄膜的表面电阻随热处理温度的变化.有些薄膜加热到一定温度时表面电阻发生突降.一般降低4个数量级左右.表面电阻是否发生突降以及发生突降的温度随薄膜成分和制备方式不同而变.X射线衍射和透射电镜分析结果表明.薄膜在表面电阻突降温度发生了非晶态的相转变.此时温度称为相变温度.发生相变前后薄膜的反射率也发生较大变化.说明这些薄膜材料可作为光存储介质使用.     

Assessment of adhesion between thin film and silicon based on a scratch test

Thin film coatings are commonly utilized to prevent wear, modify surface properties, and manipulate the frictional behavior of various mechanical systems. The behavior of a coating has a direct effect on the life as well as performance of the system. However, the coating itself is subject to damage, and the quality of the coating is related to the adhesion characteristics between the coating and the substrate. Therefore, a quantitative assessment of the adhesion properties of thin film is important to guarantee the reliability of not only the thin film but also the mechanical system. In this study, ramp loading scratch tests were performed to assess the adhesion characteristics of Ag and ZnO thin films coated on a silicon wafer. Silver thin film, deposited by sputtering, and ZnO thin film, fabricated by a sol-gel method, were used as scratch specimens. Scratch tests using a diamond tip were performed with a continuously increasing normal force. During the scratch test, the normal and frictional forces were monitored to assess the integrity of the film. The Benjamin and Weaver model commonly used for obtaining the horizontal force during the scratching of films coated on a substrate showed large discrepancies with the experimental results. In this work, the model was modified with a plowing term to minimize the difference between the experimental and theoretical results. Using the modified model, the experimental results could be predicted with an accuracy of about 10%.     

The preparation of cross-section specimens for transmission electron microscopy

The structure and chemistry of thin solid films are best studied by transmission electron microscopy (TEM) when they are viewed in cross-section—that is, when the surface normal of the film is made perpendicular to the electron beam. In this orientation, the substrate, the thin film layers, and the interfaces between them can be imaged either simultaneously or individually. Further, information from each of these regions remains distinct from that obtained from the others, eliminating the problems of superimposition that are a consequence of viewing a layered structure in the conventional manner (i.e., parallel to the surface normal). A technique for fabricating TEM specimens that can be viewed in cross-section is described here. Although the majority of our work is with silicon-based materials, the technique can be readily adapted to the study of other systems.     

A method for preparing cross sections of films on wear surfaces for transmission electron microscopy study

A method for preparing cross sections of surface layers which exist on bulk metal substrates for transmission electron microscopy (TEM) study is described. The surface layer or film is protected by a vacuum-deposited or sputtered coating of a suitable metal. A mask is placed over the surface and non-masked areas are subjected to ion beam etching until the substrate is exposed. A thick electroplated layer is then applied to the surface. This layer adheres well to the ion-etched substrate and seals the coated surface film against damage during the usual slicing and grinding steps which are required for the preparation from bulk materials of thin foils for TEM study. The method was developed specifically for the analysis of boundary and extreme pressure lubrication films on wear surfaces together with the near-surface region of the substrate. However, it is also applicable to the investigation of oxide, corrosion and other surface films.     

Microstructural analysis of a FeAl/quasicrystal-based composite prepared using a focused ion beam miller

A composite consisting of a brittle multiphase matrix containing both an Al-based quasicrystalline phase (ψ) and an ordered body centred cubic phase (β) and a relatively ductile ordered body centred cubic intermetallic FeAl phase has been developed as an abrasive wear-resistant coating material. It is applied as a 500 μm thick layer onto stainless steel substrates through plasma spray processing. The microstructure of such materials can be readily examined by optical and scanning electron microscopy, but the inherent difficulty of preparing transmission electron microscope (TEM) samples has inhibited higher resolution studies. However, the relatively recent development of the focused ion beam (FIB) miller as a tool in materials science provides a method ideal for the preparation of TEM specimens of these materials. In this study a coating consisting of a mixture of an Al–Cu–Fe based quasicrystal and FeAl+Cr was deposited on to a 304 stainless steel substrate. TEM specimens were prepared using a FIB and subjected to detailed microstructural characterization. The structure consisted of elongated bands of a FeAl phase about 100 nm in width and several micrometres in length, which enclosed more equiaxed regions about 1 μm in diameter that consisted of fine mixtures of quasicrystal and two Al-Fe-Cu phases isostructurally related to FeAl.     

CTF determination and correction in electron cryotomography

Electron cryotomography (cryoET) has the potential to elucidate the structure of complex biological specimens at molecular resolution but technical and computational improvements are still needed. This work addresses the determination and correction of the contrast transfer function (CTF) of the electron microscope in cryoET. Our approach to CTF detection and defocus determination depends on strip-based periodogram averaging, extended throughout the tilt series to overcome the low contrast conditions found in cryoET. A method for CTF correction that deals with the defocus gradient in images of tilted specimens is also proposed. These approaches to CTF determination and correction have been applied here to several examples of cryoET of pleomorphic specimens and of single particles. CTF correction is essential for improving the resolution, particularly in those studies that combine cryoET with single particle averaging techniques.