A strength of the modern electron microprobe is its ability to provide 2D compositional information about materials. These images give the ability to observe features that might otherwise pass unseen. Elements at the trace element level are generally ignored due to the high detection limits imposed by mapping under “standard” EMP conditions.

Trace element mapping requires beam regulation at high (e.g. 300 nA) to very high (e.g. 3 μA) faraday cup currents, reliable beam and stage control, and suitable samples and mounting media. The ability to operate at high accelerating voltage to maximize Pk2/Bkg is desirable (Robinson and Graham, 1992), although we have encountered column difficulties above 25-30 kV.

We are mapping trace and minor elements including Y, Sc, P, Cr, Mn, Ca, in garnets. Fig. 1 shows Y, Sc and Cr maps (Spear and Kohn, 1996; Kohn, Spear and Valley, 1997), and Fig. 2 Y and Sc maps (Cameron, unpub. data), produced with a Cameca SX51.

Electron energy loss spectroscopy is the only direct technique that can semi-quantitatively determine the nature of the bonding in carbon thin films. To quantify the sp2/sp3 bonding fraction, the spectrum taken from the film must be compared to that of a suitable known standard. The bonding fraction can be analysed by studying the K ionisation edge in the electron energy loss spectrum. A method for quantifying the sp2 bonding fraction in an amorphous carbon film has been described by Berger et al (1988), where the area of peak of the film is compared with that of graphite. The principle of quantifying the edge structure is to obtain a ratio of the two peak areas using the following formula, (1), where fπis the ratio between the two π* peaks, Iπ. is the integral of the transition, and ΔE is the integrated counts for the normalising energy window. The superscripts s and u denote the standard and unknown spectra respectively.

Elemental mapping of Co-Cr-X based magnetic recording media at resolutions approaching 1 nm by energy-filtered transmission electron microscopy (EFTEM) can provide quantitative measurements of intergranular Cr segregation for correlation with magnetic properties and materials processing. The thin-film media present many challenges for EFTEM methods, such as diffraction contrast and closelyspaced edges. The goal of this work was to provide robust methods for mapping quantitative compositions in such materials. Results presented here are for a model material of 60 nm of Co84Cr12Ta4 on a 75 nm Cr underlayer; both films were d.c. magnetron sputtered onto a NiP-plated Al substrate pre-heated to 250°C. Other compositions and thinner layers (∼30 nm) have also been studied. EFTEM was performed on back-thinned, plan-view specimens with a Gatan Imaging Filter (GIF) interfaced to a 300 kV LaB6 Philips CM30. Optimized acquisition conditions have been detailed elsewhere. Besides core-loss image series, zero-loss I0 (slit width Δ=10eV), low-loss Ik (Δ=30eV), and unfiltered IT images were recorded, and maps of t/λ. = ln(IT / I0), where t is specimen thickness and λ. is the total inelastic mean free path, were produced.

The development of high-efficiency, low-price photovoltaic devices relies on epitaxially grown layers of ternary chalcopyrites. The question addressed in this study is to which extent the near stoichimetric composition of CuInS2 grown on hydrogen terminated Silicon, Si (111), varies cross the sample area. The method usually employed [1] for such analysis is Argon-Rutherford Backscattering (RBS), whose spatial resolution, however, is limited to the order of millimeters. This limit can readily be overcome by electron microscopic and electron spectroscopic means.

For observation and recording of the energy loss spectra in the transmission electron microscope, samples were carefully prepared by mechanic polishing and ion-milling. The thinning began with polishing the Si substrate on ALLED diamond lapping films, down to 10 μm and a final ion-milling in a Gatan ion-thinning equipment from the Si-side (Ar+ 3 kV, the incident angle less than 7°).Spectra were taken by means of a Gatan image filter (GIF - 100) in a Philips CM200 FEG electron microscope operated at 200 kV.

Combustion synthesis has led to many advances in materials science, in part via the synthesis of powders consisting of particles of nanometer dimensions. Particle morphology is a key concern regarding the powders produced, but also of comparable importance is particle composition. Electron energy loss spectroscopy (EELS) and energy-filtering imaging (EFI) can be used to interrogate the gas-phase combustion synthesis environment for elemental particle composition information. Once established, this diagnostic approach can be used to address control of particle composition and other issues associated with particle formation and growth in flames. The evolution of the particle morphology in a laboratory scale combustion synthesis facility can be examined by passing TEM grids directly through the combustion synthesis flame at various heights above the burner surface, as shown in Fig. la. For the current work, SiOx particle samples are obtained from a SilL/^/FL/Ar flame using a rapid probe insertion technique.

Anionic modification of silicate-based glasses with carbon has recently attracted the attention of many research groups due to the beneficial effect on mechanical properties and both thermal and chemical stability. These carbon containing glasses, termed silicon oxycarbide glasses, can be synthesized from various organically modified silica gels or from preceramic polymers after a pyrolysis process in an inert atmosphere. The oxycarbide network is often mixed with a free carbon phase, and so the local environment around C atoms exhibits a complex distribution . In the ideal silicon oxycarbide phase, carbon atoms would be present only in carbidic units (forming only Si-C bonds). The carbon phases and the chemical bonding in the silicon oxycarbide have been investigated by high resolution electron microscopy (HREM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Electron Energy Loss Spectroscopy (EELS) is an important analytical technique that can be used to further characterize the local chemistry and electronic structure of these glasses.

GaN-based materials hold promise for electronic and optical applications. Lacking a suitable lattice matched substrate, microstructural investigation by electron microscopy has become an important player in the development of these materials. Electron microscopy investigations vary from standard defect analysis, to polarity determination, to evaluation of electronic structure. To date, no evidence of electron beam damage has been reported.

For this experiment, the beam current from the Cornell University VG501 lOOkV STEM was approximately 0.3nA. During acquisition of multiple PEEL spectra of the N K-edge, the intensity of the core loss decreased and eventually disappeared due to electron beam induced damage. Damage is a function of dose. Thus observation of damage is dependent upon magnification and thickness, i.e., electron dose and fractional change in signal.

The preparation of frozen amorphous thin films of solutions containing crystals, and imaging with cryo energy filtered TEM, opens for the first time the possibility to image the chemistry of solids interacting with liquids. The future possible areas of research include: dynamic of crystal growth processes, adsorption mechanisms, ion exchange, structure determination of solids in equilibrium with solutions, etc.

The sample preparation uses a controlled-environment vitrification system1. Vitrification of the liquid phase was achieved by plunging the grid into liquid ethane. The vitrified specimen was transferred into the microscope by an Oxford CT3500 cryo-holder. The specimen temperature in the microscope column can be kept at -−183 °C. The thickness of the vitrified films, including the crystals, should be about 250 nm or preferably less. The Philips CM120 BioTWIN Cryo, here used for cryo-TEM, has a focal length of 6 mm and a structural resolution of 0.4 nm.

The energy resolution which can be attained in electron energy loss spectroscopy (EELS) is determined by the energy spread of the electron source. The energy width of a high brightness electron gun (typically 0.4 to 0.8 eV) blurs the energy spectrum. A pre-specimen energy filter or monochromator must be used to reduce the energy width of the beam below 0.1 eV to allow detailed EELS analysis of the electronic band structures in materials. The monochromator can not only improve EELS, but it is also capable of improving the spatial resolution in low voltage SEM, which is limited by the chromatic blur of the objective lens. A new type of monochromator the Fringe Field Monochromator has been designed and experiments in an ultra high vacuum setup show the monochromatisation of a Schottky Field Emission Gun.

It would be very desirable to obtain EELS at 0. l-0.2eV resolution for bandstructure studies at interfaces and defects. [1,2] It has been possible to build spectrometers that perform in this energy range, but it has been difficult to utilize this resolution, because the energy width of the cold field emission source is about 0.3-0.4 eV for the currents needed to do practical microscopy. [3] Numerical methods have been used to sharpen the spectra, showing unambiguously that EELS spectra would benefit greatly if a higher resolution source were available. [4,5]

This situation has led one of us (H.W. Mook) to investigate electron monochromator optical designs which would be compatible with the high brightness and current required by the STEM. [6] This work produced a 4cm long monochromator that can be integrated into the field emission source. As shown in Fig. 1, the monochromator optics includes a field lens formed by deceleration of the electron beam from the 3 KeV extraction energy to about 350 eV inside the monochromator, a short Wien Filter velocity dispersive field, 150nm wide Si energy selecting slits, and an acceleration field lens at the monochromator exit.

Diamond-like carbon (DLC) has attracted considerable interest with a variety of potential applications [1]. A new class of amorphous films has been produced recently by incorporating metals within the carbon network. The electrical conductivity of these films varies a wide range, however nonlinear with the metal concentration. It is believed it is because metals may form clusters. Such nano-phase metal cluster encapsulated by an inert matrix can offer interesting applications, for example as nano-electrodes used in electrochemistry[2]. It is therefore fundamentally important to observe the metal cluster formation. By using Z-contrast imaging [3], the metal distribution can be directly determined on the atomic level. The local metal concentration and other electronic properties cafi also be analyzed by spatial resolved electron energy loss spectroscopy (EELS)

Good quality polycrystalline diamond is transparent and has an optical band-gap very close to that of single crystal diamond, 5.5 eV. Tetrahedral amorphous carbon (ta-C) is a form of amorphous carbon which can have up to 90 % sp3 type bonds. It is the form of amorphous carbon, which is closest to having a true diamond-like structure. The optical band-gap of ta-C is however very different from that of polycrystalline diamond, typically in the range 2-3 eV. This difference in the optical bandgap has been ascribed to the electronic states formed by the residual graphitic bonding. The π − π* states at the graphitic sites effectively form tail states on the valance and conduction band edges created by the diamond-like bonding. These tail states extend deep into the band-gap, which would exist if there were only (100 %) diamond-like bonds. The poor electronic properties obtained thus far for ta-C are now accepted as being associated with localised conduction in the π − π* tail states.

Extinction distances for two beam conditions are readily available through many sources, but under more complex zone axis conditions extinction distances are not well known. The use of thickness fringes as an approximate thickness estimate under zone axis conditions, such as in high resolution Annular Dark Field imaging, can be a useful tool. We have measured thickness vs. fringe number under zone axis conditions using low loss Electron Energy Loss Spectroscopy (EELS). Work was done on the Cornell UHV HB501 STEM.

Several orientations of silicon were glued together and thinned in cross section by the wedge technique. Additional thinning was done by low angle ion milling. Surface oxide was removed with an HF etch before loading into the microscope.Each piece of silicon was tilted to the zone axis condition and low loss EELS spectra was taken at selected points. The thickness fringes were numbered with the following convention: the center of the first black fringes close to the edge is 1 and the center of the second black fringe is 2, etc.

The microstructure and mechanical properties of alloy systems are known to be influenced by the size and distribution of sub-micron dispersoids through boundary pinning and Orowan strengthening. Explaining and accurately predicting the behaviour of these systems relies in part on the measurement of the volume fraction of the second phase precipitates. The transmission electron microscope (TEM) provides the necessary resolving power to measure the dispersoids of interest, and when coupled to Electron Energy Loss Spectroscopy (EELS) based filtering systems, the thickness of the observed volumes can be determined using the log-ratio technique.

A series of nominally AA5754 alloys with increasing Manganese content (0%, 0.2%, 0.5%, 0.8%) was used to evaluate the method. The data was collected using a Philips FEGSTEM equipped with a Gatan Imaging Filter (GIF). A series of images were collected from each of a number of contiguous fields in each specimen. These included an unfiltered image, an image collected from the zero-loss region of the transmitted spectrum, and a filtered image generating sufficient precipitate contrast to allow subsequent spatial measurement. Using a filtered image, the contribution of diffuse scattered electrons is minimized, thus enabling the analysis of thicker regions.

The first MSA Special Symposium honors Professor Archie Howie CBE FRS for his pioneering contributions to the physics and applications of electron microscopy, initially in TEM and more recently also for SEM; and his leadership, outstanding teaching and wide ranging collaborations in these and related fields.

Professor Howie’s important contributions have included fundamental studies of diffraction contrast effects in the TEM which have been extensively leveraged in numerous applications and collaborations, by Howie, his coworkers and many others. They have had a major impact in column approximations, an early discovery of the weak beam effect in many beam computer simulations of coherency strain contrast and subsequent advances in dislocation weak beam imaging, and the first identification of 311 planar defects in irradiated Ge. Inelastic scattering and particularly valence losses, electron channeling, optical potential and x-ray production, amorphous materials were all major contributions, as were the series of projects centered on novel STEM methods.

Introduction

There is considerable interest in CVD diamond layers for a wide variety of applications. Growth of high quality and phase pure material is now achieved in a number of laboratories but it remains to optimize the growth process for particular applications so that physical properties approaching or even exceeding those of the best natural diamond can be achieved. We have found that the combined use of a wide range of different microscopies is effective in the process of refinement.

Diamond quality and phase purity

Raman spectroscopy is widely used to investigate the phase purity of diamond layers. Infra-red Raman is particularly sensitive to the presence of traces of non-diamond carbon in diamond while ultra-violet Raman reveals the presence of traces of diamond carbon in layers that largely are not sp bonded. For thick rather perfect layers confocal micro-Raman mapping is a convenient method for revealing the locations of non-diamond regions.

The work function ϕ, the bandgap Eg, the threshold energy level Et, for the inelastic scattering of excited electrons and the threshold energy transfer Ed for the onset of structural ionisation damage are clearly of major significance in various actively developing forms of hot carrier imaging. Two exciting examples here are the ability to image small and dynamic local changes in work function by PEEM and as well as mapping the variations in electronic structure between p and n-type regions of a semiconductor by SE imaging in the SEM. More recently still there have been indications that the SE signal in the ESEM might be sensitive to local changes in bandgap and suggestions that it might be even possible to image spatial variations in pH. It is increasingly clear that if these attractive opportunities are to be efficiently explored and developed, systematic and preferably quantitative observations are needed. Such work requires specimens whose atomic and electronic structure is either fully known beforehand or can be deduced from other signals available in a situation where the physical processes in the microscope are well understood.

The performance of many materials systems depends on our ability to control the distribution of atoms on a nanometre or sub-nanometre scale within those systems. This is as true for steels as it is for semiconductors. A key requirement for improving their performance is the ability to determine the distribution of the elements resulting from processing the material under a given set of conditions. Analytical electron microscopy (AEM) provides a range of powerful techniques with which to investigate this distribution. By combining information from different techniques, many of the ambiguities of interpretation of the data from an individual technique can be eliminated. The electron energy loss near edge structure (ELNES) present on an ionisation edge in the electron energy loss spectrum reflects the local structural and chemical environments in which the particular atomic species occurs. Thus it is a useful contribution to the information available. Since a similar local environment frequently results in a similar shape, ELNES is useful as a “fingerprint”.

The electronic structure of ceramics can be extracted quantitatively from the valence electron energy-loss spectroscopy (VEELS) of transitions between the valence and conduction bands. We obtained VEEL spectra of several ceramics (FIG. 1) with a VG HB501 dedicated STEM equipped with Gatan PEELS. Improved data acquisition and new methods of data analysis allowed us to treat the data fully quantitatively. The reliable and accurate removal of the zero loss peak was crucial because intensities at energy losses just above the band gap of the ceramic material have a large influence on the results. An asymmetric Pearson VII function was fitted into the zero loss peak up to an energy loss for which no transitions are expected (an energy smaller than the band gap of the ceramic) and then extrapolated to higher energies. This limits the analysis to non-metallic materials, exhibiting non-zero band gap energies. We are currently developing methods to perform the analysis on metallic materials, using ellipsometric data in the visible and extrapolate the energy-loss function to 0 eV and thereby remove the need for the no transition energy.

A series of recent experiments, using electron as well as photon probes [1-3], have attracted considerable interest on the importance of surface plasmon modes in nanostructured media such as nanospheres, nanotubes, nanowires, nanopores or nanoholes.

The basic principle for STEM operation had early been recognized to be well suited to the investigation of cases where non penetrating electrons propagate at a given impact parameter from an external surface (along "aloof7 trajectories, using a terminology proposed by Warmack et al. [4]). One of the most spectacular effect is that signals attributed to the excitation of-surface plasmon modes could be detected in vacuum at distances as large as a few tens of nanometers from the outside surface of a specimen, see for instance [5] for the planar geometry and [6], for the spherical one. Most interpretations accounting for the spatial dependence of EELS spectra in the "aloof geometries, rely on models using the bulk dielectric coefficients of the material to describe the induced charges and polarization responsible for the specimen electric field acting on the probe electron and consequently for the measured energy loss.