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A scanning precession electron diffraction system has been integrated with a direct electron detector to allow the collection of improved quality diffraction patterns. This has been used on a two-phase α–β titanium alloy (Timetal® 575) for phase and orientation mapping using an existing pattern-matching algorithm and has been compared to the commonly used detector system, which consisted of a high-speed video-camera imaging the small phosphor focusing screen. Noise is appreciably lower with the direct electron detector, and this is especially noticeable further from the diffraction pattern center where the real electron scattering is reduced and both diffraction spots and inelastic scattering between spots are weaker. The results for orientation mapping are a significant improvement in phase and orientation indexing reliability, especially of fine nanoscale laths of α-Ti, where the weak diffracted signal is rather lost in the noise for the optically coupled camera. This was done at a dose of ~19 e−/Å2, and there is clearly a prospect for reducing the current further while still producing indexable patterns. This opens the way for precession diffraction phase and orientation mapping of radiation-sensitive crystalline materials.

A residual stress depth gradient is characterized in a 1.8 μm thick AlN/Al0.25Ga0.75N/GaN/Al0.22Ga0.78N heteroepitaxial structure grown using metallic-organic chemical vapour deposition on Si(111) substrate. The cross-sectional stress profile is evaluated with a step of 100 nm using ion beam layer removal (ILR) method based (i) on a sequential focused ion beam milling of a microcantilever, (ii) on an evaluation of a cantilever bending after every milling step and (iii) on a stress profile recalculation using finite element simulation. The profile shows tensile stress of ~1.5 GPa in AlN nucleation layer, stress changing from compressive to tensile in Al0.25Ga0.75N and GaN sublayers and relatively small stresses below 100 MPa in the top Al0.22Ga0.72N sublayer. The stress profile is qualitatively correlated with the results from precession electron diffraction which indicates approximately the same stress behavior. The cross-sectional stress magnitude and variation are interpreted by the mismatches of lattice constants and coefficients of thermal expansion as well as by growth mode changes during Al0.25Ga0.75N and GaN sublayer formation. The approach demonstrates the possibility to resolve nanoscale variation of residual stresses in heteroepitaxial structures using ILR method. Keywords: Heterostructure, Residual stress, Ion beam layer removal method, GaN, FIB, Precession electron diffraction

Electron irradiation of maximally-valent transition-metal oxides within the transmission electron microscope leads to a variety of surface reactions which depend primarily upon the energy and local current density of the incident electron beam. When the oxides Nb 2 O 5 , V 2 O 5 , TiO 2 and WO 3 are irradiated at current densities of 5–50 A cm −2 then formation of the respective metallic monoxide is observed. The surface profile imaging technique reveals that the surface monoxide invariably has a well-defined epitaxial relationship with the original oxide. When a small focused probe of extreme current density (⋍10 3 -10 4 A cm −2 ) is used, reduction beyond the monoxide phase for WO 3 , V 2 O 5 and TiO 2 is observed. Extensive pitting, amorphization and some evidence for the based metal is also found. Measurements have also been performed under uhv conditions and show that sputtering from the exit surface is an important damage mechanism.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

The production of secondary electrons by fast (100 keV) electrons is investigated by analyzing the time coincidence between inelastically scattered incident electrons and energy-filtered secondary electrons. Thin conducting and semiconducting films show differences in both the coincidence and generation spectra at energies near the bulk-plasmon excitation, suggesting that plasmon decay does not play a central role in the production of secondary electrons in Si. At primary energy losses greater than 35 eV, the secondary-electron production rate is proportional to the energy deposited by the incident electrons

In the 3-dimensional (3-D) reconstruction of protein crystals with variable thicknesses the electron images and diffraction patterns can only be merged if the crystal thickness is known. Measurement of the thickness using the ratio of the number of inelastically scattered electrons to the number of electrons in the zero loss peak can be accomplished with parallel electron energy loss spectrometry (PEELS). A theoretical analysis of the accuracy of the technique on paraffin crystals of different thicknesses is presented. Our experimental studies with paraffin crystals show the feasibility of measuring a single layer of 47 A with good accuracy under low dose and low temperature conditions. A simple experimental apparatus is proposed to obtain thicknesses from small regions of unstained protein crystals prior to collecting the 3-D data sets from the unexposed area of the same crystal.

Electron holography, and high spatial resolution (17Å) computer controlled Parallel Electron Energy Loss Spectrometry (PEELS) were used to probe the structure of and chemical profile across a thin silicon Oxide-Nitride-Oxide (ONO) layered structure of nominal width 10Å-50Å-10Å. We found that the layer widths are on the average 13Å-28Å-18Å, the first oxide layer was discontinuous, and the second oxide layer contained nitrogen. The nitride layer had a silicon to nitrogen concentration ratio of 1.0 ± 0.1. These results show, for the first time, the power of holography in characterizing thin, light element, amorphous layers and the importance of computer controlled parallel energy loss line scans for obtaining analytical information at the highest spatial resolution with minimum dose.

The secondary electron generation process is studied in an ultra-high vacuum scanning transmission electron microscope using electron coincidence spectroscopy. Production pathways for secondary electrons are determined by analyzing coincidences between secondary electrons and individual excitation events. The ultimate spatial resolution available in scanning electron microscopy is limited by the delocalization of the secondary electron generation process. This delocalization is studied using momentum resolved coincidence electron spectroscopy. The fraction of secondary electrons resulting from localized excitations can explain the high spatial resolution observed in secondary electron microscopy images.

The continuing remarkable progress of computer technology is dramatically changing how electron optical instrumentation is being used. Instead of concentrating on taking just one spectrum or one image at a time, it now has become possible to design complete experiments aimed at solving the problems at hand more rapidly and more directly.We have designed a PC-based acquisition system that aims to innovate and simplify both TEM and STEM operation. The first advance this system represents is integrated data acquisition from all detectors commonly found on electron microscopes: TV and slow-scan CCD cameras, different flavors of STEM detectors, and EDX, CL, Auger and EELS spectrometers. The new system allows experiments combining microscope control with the acquisition capabilities of all these detectors. The second advance is the implementation of “live” processing. Most common processing tasks can be executed on-the-fly, even at video-rate acquisition speeds.

Computers were considered to be mainly “add-on” equipment for analytical electron microscopes until only the last several years. Before that time, although most microscopes had been microprocessor-controlled for at least five years, the only use of more conventional computers was for controlling spectrometers connected to the microscope. The fundamental limitation of computers for scanned imaging was mostly one of speed. Spectra could be acquired and displayed in real time even by computers of early 1980’s vintage, whereas only modern computers are capable of acquiring and displaying video rate digital scanned images.The result of this fundamental difference in signal rates between imaging and spectroscopy is that separate computer systems have been developed for imaging and spectroscopy. Even in the cases where spectroscopy systems (most notably EDX systems) have integrated some image acquisition capabilities, or where imaging systems have integrated some spectroscopy, the design of the systems is virtually always centered around either imaging or spectroscopy.

The production or generation of SE by focused, fast electron beams is a multistage process which includes excitation of target electrons by the energetic incident beam, subsequent decay yielding hot SE, transport to the surface, and transmission over the surface potential barrier. The ultimate spatial resolution of a SE image formed by scanning a well focussed probe across a surface is limited by the excitation events' spatial derealization through the transverse momentum transferred to the specimen.The accepted model for SE production is not well characterized since it is extremely difficult to separate experimentally the generation, transport and transmission processes during a given SE creation event. Here, we examine the SE generation pathway by correlating SE of a given energy produced by an initial inelastic excitation using time coincidence detection. This technique can be used, for example, to isolate the role of plasmon decay in the SE generation process. The experiments were performed in a Vacuum Generators HB501-S UHV scanning transmission electron microscope (STEM), operating at base pressures of 5 × 10−11 torr.

Interfacial structure is a controlling parameter in the behavior of many materials. Electron microscopy methods are widely used for characterizing such features as interface abruptness and chemical segregation at interfaces. The problem for high resolution microscopy is to establish optimum imaging conditions for extracting this information. We have found that off-axis electron holography can provide useful information for the study of interfaces that is not easily obtained by other techniques.Electron holography permits the recovery of both the amplitude and the phase of the image wave. Recent studies have applied the information obtained from electron holograms to characterizing magnetic and electric fields in materials and also to atomic-scale resolution enhancement. The phase of an electron wave passing through a specimen is shifted by an amount which is proportional to the product of the specimen thickness and the projected electrostatic potential (ignoring magnetic fields and diffraction effects). If atomic-scale variations are ignored, the potential in the specimen is described by the mean inner potential, a bulk property sensitive to both composition and structure. For the study of interfaces, the specimen thickness is assumed to be approximately constant across the interface, so that the phase of the image wave will give a picture of mean inner potential across the interface.

Further advances in resolution enhancement of transmission electron microscopes can be expected from digital processing of image data recorded with slow-scan CCD cameras. Image recording with these new cameras is essential because of their high sensitivity, extreme linearity and negligible geometric distortion. Furthermore, digital image acquisition allows for on-line processing which yields virtually immediate reconstruction results. At present, the most promising techniques for exit-surface wave reconstruction are electron holography and the recently proposed focal variation method. The latter method is based on image processing applied to a series of images recorded at equally spaced defocus.Exit-surface wave reconstruction using the focal variation method as proposed by Van Dyck and Op de Beeck proceeds in two stages. First, the complex image wave is retrieved by data extraction from a parabola situated in three-dimensional Fourier space. Then the objective lens spherical aberration, astigmatism and defocus are corrected by simply dividing the image wave by the wave aberration function calculated with the appropriate objective lens aberration coefficients which yields the exit-surface wave.

Reflection high energy electron diffraction (RHEED) has been used extensively to observe changes in surface reconstructions by analyzing the geometry of the RHEED pattern and to monitor growth of layers in MBE systems by measuring the changes of the intensity of the specular spot with time. RHEED is also capable of yielding the structure of the surface by using dynamical diffraction theory to analyze experimental reflection rocking curves. These rocking curves trace the change in the intensity of the RHEED spots as a function of the angle of incident illumination. They are equivalent to the intensity vs. voltage curves (I-V) obtained in low energy electron diffraction (LEED) which have been used to determine most of the known surface structures. The LEED I-V curves are energy filtered and the theoretical calculations consider only the elastically-scattered electrons. The RHEED theory is also developed only for elastic scattering, but the experimental measurements so far have not been energy filtered.

Electron holography has received renewed interest recently as a means of resolution enhancement in high resolution phase contrast imaging and as a technique for imaging microscopic magnetic domain structures. Some of the original applications of the technique involved measurement of the absolute mean inner potential in films of known thickness. This approach has now been extended in order to gain information about changes in elemental concentrations at interfaces in multilayer films.The abruptness of interfaces in multilayer films used for X-ray mirrors strongly affects their reflective properties. Previous studies using high resolution phase contrast imaging have shown that this interface diffuseness can be imaged, but typically with low contrast and an unknown contribution from amplitude contrast effects. The differences in mean inner potential between successive layers suggests that the variation of the mean inner potential across the interface should be a good measurement of the change in elemental concentration.

The interfacial structure and composition of SiC(w)/Si3N4interfaces in ceramic composite are expected to influence the mechanical properties and may depend on processing variables. Whisker/matrix interfaces and matrix grain boundaries are beginning to be intensively investigated in these composites mainly, by HREM to determine the existence of glassy phase that may result from sintering aid reactions. We have shown that the glassy phase volume fraction appears to depend on the sintering aid added for full densification,and that oxygen could not be detected at fully crystalline interfaces by serial EELS (1). Other HREM observations led to emphasis on the continuous character of glassy thin films at all SiC/Si3N4interfaces (2). SiC(w) and SiC powders synthesized by different processes also contain different amounts of oxygen that may contribute to amorphous phase formation (3). Thus there are at least two important variables that can contribute to the amount and distribution of glassy phase in composites. Fully crystalline interfaces in these materials often exhibit a superimposed diffuse background intensity that may result from a very thin amorphous layer that cannot be fully resolved; this problem is further complicated by the requirement that the interfacial plane be atomically flat and tilted to contain the beam. In this paper we report first results of a high spatial resolution parallel EELS study of oxygen in these interfaces, and correlate the EELS results with HREM observations.

Aluminum nitride is known to have a large affinity for oxygen as an impurity. At high levels (>∼4 wt/o) the oxygen is incorporated in the form of planar stacking faults where “pure” 2H AIN is regularly interspersed with a layer of oxygen at the faults. At oxygen levels lower than ∼ 4 wt/o the structure shows an expanded c-axis. The present authors have not observed this effect, rather a random distribution of stacking faults is observed along with another, more prevalent, extended defect identified as an inversion domain boundary (IDB). The IDBs are significantly aplanar (indicating a low interface energy), and often have precipitates and other, faceted defects associated with them. The role of these defects in oxygen accommodation in AIN has been investigated both structurally and chemically by electron optical methods (SEM, TEM, STEM, HREM, CBED, EDS, EELS, and CL-TEM). The structural nature of the boundaries, in the absence of oxygen, requires Al-Al or N-N bonding to occur with some frequency across the boundary. Such bonding is unlikely due to the excess energy required. Chemical analysis (EELS) and luminescence studies (CL-TEM) reveal that oxygen is often associated with the boundaries and may mediate the bonding at the boundary. A model is proposed for the IDB which includes structural aspects combined with considerations of stoichiometry in an effort to understand the origin and energetics of this defect.

It is well-known that electron-beam irradiation within the electron microscope can induce a variety of surface reactions. In the particular case of maximally-valent transition-metal oxides (TMO), which are susceptible to electron-stimulated desorption (ESD) of oxygen, it is apparent that the final reduced product depends, amongst other things, upon the ionicity of the original oxide, the energy and current density of the incident electrons, and the residual microscope vacuum. For example, when TMO are irradiated in a high-resolution electron microscope (HREM) at current densities of 5-50 A/cm2, epitaxial layers of the monoxide phase are found. In contrast, when these oxides are exposed to the extreme current density probe of an EM equipped with a field emission gun (FEG), the irradiated area has been reported to develop either holes or regions almost completely depleted of oxygen. ’ In this paper, we describe the responses of three TMO (WO3, V2O5 and TiO2) when irradiated by the focussed probe of a Philips 400ST FEG TEM, also equipped with a Gatan 666 Parallel Electron Energy Loss Spectrometer (P-EELS). The multi-channel analyzer of the spectrometer was modified to take advantage of the extremely rapid acquisition capabilities of the P-EELS to obtain time-resolved spectra of the oxides during the irradiation period. After irradiation, the specimens were immediately removed to a JEM-4000EX HREM for imaging of the damaged regions.

The new parallel detection EELS systems have opened up the possibility of performing time-resolved studies of compositional changes and the evolution of near-edge structure caused by radiation damage or positional changes across heterogeneous interfaces. Zaluzec has described a system using a 2-D CCD array in which one axis corresponds to time. Such a system can acquire 100 512-channel spectra in a time limited by the CCD readout and digitization time. Our system is an extension of an existing data acquisition system using modified software and a Gatan 666 parallel EELS equipped with a 1024-element 1-D photodiode array. We have taken advantage of the computer’s memory architecture to stack a time series of multiple spectra (or any portion of these spectra) in the extended memory space above 256 K-bytes. The 4 M-bytes of system memory enables storage of up to 1920 spectra 1024 channels long. For inner-shell edges, a 100-500 msec signal integration time per spectrum is typical. After acquisition the series of spectra are recovered from extended memory and are stored on disk. They can then be retrieved for post processing or can be played back through the display, from which VCR recordings can be directly made.