National Center For Electron Microscopy Research Articles

Sub-Ångstrom Transmission Electron Microscopy at 300keV

Abstract Sub-Ångstrom TEM to a resolution of 0.78Å has been demonstrated by the one-Ångstrom microscope (OÅM) project at the National Center for Electron Microscopy. The OÅM combines a modified CM300FEG-UT with computer software able to generate sub-Angstrom images from experimental image series. Sub-Ångstrom HREM is gaining in importance as researchers design and build artificially-structured nanomaterials such as semiconductor devices, ceramic coatings, and nanomachines. Commonly, such nanostructures include atoms with bond lengths shorter in projection than the point resolution of a mid-voltage HREM. in addition, image simulations have shown that structure determinations of defects such as dislocation cores require sub-Angstrom resolution, as will hold true for grain boundaries and other interfaces. Sub-Ångstrom microscopy with a transmission electron microscope requires meticulous attention to detail. As resolution is improved, resolution-limiting parameters need to be reduced. in particular, aberrations must be minimized, power supplies must be stabilized, and the microscope environment optimized to reduce acoustic and electromagnetic noise in addition to vibration.

Quantitative In-Situ Nanoindentation of Thin Films in a Transmission Electron Microscope

A unique in situ nanoindentation stage has been built and developed at the National Center for Electron Microscopy in Berkeley, CA. By using piezoceramic actuators to finely position a 3-sided, boron-doped diamond indenter, we are able to image in real time the nanoindentation induced deformation of thin films. Recent work has included the force-calibration of the indenter, using silicon cantilevers to establish a relationship between the voltage applied to the piezoactuators, the displacement of the diamond tip, and the force generated.In this work, we present real time, in situ TEM observations of the plastic deformation of Al thin films grown on top of lithographically-prepared silicon substrates. The in situ nanoindentations require a unique sample geometry (see Figure 1) in which the indenter approaches the specimen normal to the electron beam. in order to meet this requirement, special wedge-shaped silicon samples were designed and microfabricated so that the tip of the wedge is sharp enough to be electron transparent.

The NCEM One-Angstrom Microscope Project Reaches 0.89Å Resolution

Abstract Transmission electron microscopy to a resolution of 0.89Å has been achieved at the National Center for Electron Microscopy and is available to electron microscopists who have a requirement for this level of resolution. Development of this capability commenced in 1993, when the National Center for Electron Microscopy agreed to fund a proposal for a unique facility, a one- Ångstrom microscope (OÅM).2 The OÅM project provides materials scientists with transmission electron microscopy at a resolution better than one Angstrom by exploiting the significantly higher information limit of a FEG-TEM over its Scherzer resolution limit. To turn the misphased information beyond the Scherzer limit into useful resolution, the OÅM requires extensive image reconstruction. One method chosen was reconstruction from off-axis holograms; another was reconstruction from focal series of underfocused images. The OÅM is then properly a combination of a FEG-TEM (a CM300FEG-UT) together with computer software able to generate sub-Ångstrom images from experimental images obtained on the FEG-TEM. Before the advent of the OÅM, NCEM microscopists relied on image simulation to obtain structural information beyond the TEM resolution limit.

Design and Implementation of a Site for a One-Ångstrom TEM

Abstract The National Center for Electron Microscopy has recently acquired a field-emission TEM to form thebasis of a project to achieve a resolution of one Ångstrom. To reach this resolution, both instrumental and environmental factors need to be considered. We have designed and constructed a new building to provide a suitable environment for this instrument, with emphasis on providing isolation from external influences detrimental to the achievement of ultra-high resolution. Such influences include mechanical vibration, temperature fluctuations, acoustic noise, and stray electromagnetic fields. The microscope chosen for the one-Ångstrom project is a Philips CM300 Ultra-Twin equipped with a field-emission gun. Pre-installation specifications provided by Philips for this 1.7Å-resolution TEM specify maximum-allowable values for vibration levels in three mutually-perpendicular directions. In the most critical direction (console left to right), vibration is required to remain below 0.8)μm/sec in the frequency range from 1Hz to 5Hz, although allowed to rise to 6μm/sec above 10Hz (Region I in fig. 1). Even when resolution is not a critical requirement, vibration must be minimized at 2.5Hz (Region II in fig.1).

Development of an In-Situ Nanoindentation Specimen Holder for the High Voltage Electron Microscope

Abstract Progress on the development of an in-situ nanoindentation specimen holder for the Kratos 1.5MeV HVEM located at the National Center for Electron Microscopy, Berkeley, CA, USA, is reported. There is currently considerable work being reported on the mechanical properties (i.e., hardness, delamination, wear, etc.) of single and multicomponent thin films, nanoclusters and fibers by techniques such as nanoindenting, ref. [1] are recent examples. However, with all of these tests there has not been direct, unambiguous observation of the response or evolution of the microstructure. With many of these reports, there has been little “post-mortem” TEM characterization and there has been no real attempts to simulate these dynamically in the TEM. For the case of nano-testing of the materials the interaction volumes are often on the scale of the natural sampling volume of the HVEM. It seems natural that post-mortem and in-situ TEM characterization techniques be applied.

Remote On-Line Control of a High-Voltage in situ Transmission Electron Microscope with A Rational User Interface

Remote on-line electron microscopy is rapidly becoming more available as improvements continue to be developed in the software and hardware of interfaces and networks. Scanning electron microscopes have been driven remotely across both wide and local area networks. Initial implementations with transmission electron microscopes have targeted unique facilities like an advanced analytical electron microscope, a biological 3-D IVEM and a HVEM capable of in situ materials science applications. As implementations of on-line transmission electron microscopy become more widespread, it is essential that suitable standards be developed and followed. Two such standards have been proposed for a high-level protocol language for on-line access, and we have proposed a rational graphical user interface. The user interface we present here is based on experience gained with a full-function materials science application providing users of the National Center for Electron Microscopy with remote on-line access to a 1.5MeV Kratos EM-1500 in situ high-voltage transmission electron microscope via existing wide area networks. We have developed and implemented, and are continuing to refine, a set of tools, protocols, and interfaces to run the Kratos EM-1500 on-line for collaborative research. Computer tools for capturing and manipulating real-time video signals are integrated into a standardized user interface that may be used for remote access to any transmission electron microscope equipped with a suitable control computer.

Applications of Electron Microscopy in Collaborative Industrial Research

The transmission electron microscope (TEM) is one of the most useful tools available to the materials scientist. Yet both the complexity and expense of the equipment, and the huge investment in time necessary to become proficient in specimen preparation and image acquisition and analysis, mean that it is difficult for most industrial institutions to maintain a state-of-the-art TEM facility. How can industry overcome this problem? One solution is to set up a collaboration with a university, an industrial partner, or a government research laboratory. Such collaborations can be extremely valuable to the company, which gains access to microscopes, specimen-preparation equipment and the expertise of professional microscopists, and to the research laboratory, which benefits from the industrial perspective and the private sector's proficiency in materials preparation and processing.Such collaborations exist, and they can produce excellent results. In this article, we present three case studies in which successful collaboration has occurred between industry and one of the Department of Energy's scientific user facilities, the National Center for Electron Microscopy (NCEM-see sidebar). Our aim is not only to describe results that we hope will be of scientific interest but also to encourage industrial researchers to consider collaborations with institutes such as NCEM.

The NCEM public domain software library of extensions to digital micrograph

The National Center for Electron Microscopy (NCEM) develops software for use in image processing and image interpretation as they are needed in ongoing research at the center. Some routines are esoteric and of little use to others, while some are more general and could be considered to be of interest to a large number of scientists. This represents the first announcement of the NCEM's dedication to making software developed at the center available to the general microscopy community.An unfortunate by-product of software development is the often necessary connection of the software to a hardware platform and/or another software platform. Most of the image processing carried out at the NCEM is done under Digital Micrograph running on the Macintosh, either through the built in functionality of the program or through extensions that are written at the center. Some of the functionality that we added were routines that were available under the SEMPER image processing software which was retired not because of limitations of the program, but because of the death of the hardware on which it was running.

3D structure determination from electron-microscope images: electron crystallography of staurolite.

Resolution of better than 2 A has been obtained in many crystals by high-resolution electron microscopy. Although this resolution is sufficient to resolve interatomic spacings, structures are traditionally interpreted by comparing experimental images with contrast calculations. A drawback of this method is that images are 2D projections in which information is invariably obscured by overlap of atoms. 3D electron crystallography, developed by biophysicists to study proteins, has been used to investigate the crystal structure of staurolite. Amplitudes and phases of structure factors are obtained experimentally from high-resolution images (JEOL ARM 1000 at the National Center for Electron Microscopy at LBL), taken in different directions from thin regions where dynamic scattering is minimal. From images in five orientations (containing 59 independent reflections to a resolution of 1.38 A), a 3D electron potential map is constructed which resolves clearly all cations (Al, Si, Fe, including those with partial occupancy) and all O atoms. This method has great potential in crystal structure determinations of small domains in heterogeneous crystals which are inaccessible to X-ray analysis. It is estimated that 3D structure determinations should be possible on regions only about ten unit cells wide and should resolve not only atom positions but also site occupancies. The method is also applicable to space-group determination.

HREM study of heteroepitaxial interfaces in the Ni.50Co.50O/Al2MgO4(100) system

Epitaxial CoO/NiO multilayers and alloys have been produced which show interesting structural and magnetic properties. Typically, epitaxial films are grown either by chemical vapor deposition or evaporation. However, sputtering can also yield high quality epitaxial films. The structure of the substrate and the interface of the film/substrate are critical factors to determine the quality of the epitaxial film. The CoO/NiO epitaxial films on the α-Al2O3 had been studied extensively in our previous work. In this paper, the heteroepitaxial interface in the Ni.50Co.50O/Al2MgO4 is studied by using high resolution electron microscopy. Transmission electron microscopy was performed using the JEOL-200CX and JEOL-ARM1000 microscopes at the National Center for Electron Microscopy, Berkeley.

A comparative HVEM study of ICB-deposited aluminum films

The Ionized Cluster Beam (ICB) technique pioneered by Takagi and colleagues in Kyoto is an exciting new method for depositing thin films possessing novel microstructures and unusual properties, (see for example Ref. 1 for a review of recent work). These materials are of interest not only for their potential use in electronic applications but also because of their eminent suitability for fundamental high resolution studies of grain boundary structureThe HVEM's at the National Center for Electron Microscopy are employed in a complementary fashion to characterize fully the microstructure of ICB deposited Al films. In-situ annealing studies of the films are conducted in the 1.5 MeV Kratos HVEM taking advantage of its heating stage, excellent specimen chamber vacuum (10-8 torr), and high resolution video camera. The increased penetration at 1.5 MeV allows different film thicknesses of Al film to be examined as well as the regions of the foil where the silicon substrate remains backing the Al film. High resolution studies of the atomic structure of grain boundaries are performed on the 1 MeV JEOL ARM using its unique tilting stage and ability to image structures at the 0.15 nm level.

δ2-Y2Si2O7 Structure Confirmed by Processing and Simulation of Atomic-Resolution Images

High resolution electron micrographs have been obtained in various orientations for δ 2 -Y 2 Si 2 O 7 using the Atomic Resolution Microscope (ARM) at the National Center for Electron Microscopy (NCEM). These ARM images were processed by masking the diffractogram of the digitized image in Fourier space and applying an inverse transform (using the SEMPER program at the NCEM Image Analysis Facility) in order to reveal details obscured by amorphous contrast originating in the glassy matrix. Processed experimental images from very thin regions of the crystalline phase were compared with images simulated from postulated models (SHRLI images); close agreement was obtained.

Up Close: Advanced Light Source at Lawrence Berkeley Laboratory

A marriage of the new and the old is under way at the Lawrence Berkeley Laboratory (LBL), Berkeley, California. The new is the Advanced Light Source (ALS), a facility for generating laser-like, partially coherent beams of x-ray and ultraviolet (collectively, the XUV) synchrotron radiation of unprecedented spectral brightness. (See Figure 1.) The old is the domed hall that has sheltered the historic 184-inch cyclotron, a nuclear accelerator built 46 years ago by the laboratory's founder E.O. Lawrence. The ALS is to reside in an enlarged version of the old hall, a familiar landmark in the Berkeley-San Francisco area whose architectural features are to be preserved.Another sign of the marriage is an increased emphasis on materials research. Though by no means forsaking its nuclear science origins, LBL has seen its budget for materials research grow steadily in recent years, especially with the establishment of the Center for Advanced Materials and the construction of two new laboratories to house its activities. [See the April 1988 MRS BULLETIN, p. 54, for an Up Close article on LBL's National Center for Electron Microscopy.] With its focus on the use of electromagnetic radiation for structural and spectroscopic studies of physical, chemical, and biological systems, including those investigated by materials researchers, the ALS fits neatly into this new emphasis. In fact, funding for the $98.7 million ALS comes to LBL through the Division of Materials Sciences of the U.S. Department of Energy (DOE).

Atomic resolution microscopy of carbonates. Interpretation of contrast

The atomic resolution microscopy (ARM) at the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, California has been used to image structural features in rhombohedral carbonates. The resolution of the microscope is better than 1.6 A, but beam damage presently limits the resolution of some of our images to slightly better than 2.6 A. More details can be extracted through image processing. We were able to interpret contrast in through focus series of “ideal” dolomite by comparing processed images with multibeam contrast calculations. Fair agreement was obtained for focus and thickness variations both of which display great changes. Even for ideal dolomite, the matching is not straight-forward, due to minor orientation variations, the presence of and amorphous overlayer, and surface roughness induced by ion beam thinning, etc. We also find good agreement for calcian δ-dolomite with a cation distribution model which assumes a periodic substitution of alternating Mg layers by Ca. Some atomic resolution examples are shown for coherent calcite-dolomite intergrowths and δ-dolomite domains in dolomite.

Up Close: The National Center for Electron Microscopy at Lawrence Berkeley Laboratory

Up Close: The National Center for Electron Microscopy at Lawrence Berkeley Laboratory

The National Center for Electron Microscopy at Berkeley: An Update

Since its establishment in 1984 the National Center for Electron Microscopy (NCEM) has provided a wide variety of users with advanced electron microscopy facilities for microstructural characterization. It has, in addition continued to develop and improve its resources to expand the range of features available to users.A major effort has been expended in building up a comprehensive computing facility to assist microscopists in optimizing instrument performance and extracting the maximum information from experimental data. The best currently available hardware has been assembled and a long menu of user-friendly software programs have been developed for general use. Sophisticated image simulation and image enhancement programs provide users of the Atomic Resolution Microscope all the necessary information for the correct interpretation of high resolution images. Direct links to both the ARM and the 1.5 MeV Kratos HVEM allow Fast Fourier Transforms to be performed, producing on-line diffractograms and greatly facilitating the effective and efficient use of the microscopes.

A computer system for HREM image processing and simulation

A facility for electron microscope image analysis and interpretation has been established at the National Center for Electron Microscopy (NCEM) as a support for the electron microscopes. The main component of the facility is a computer system designed to assist the experimenter in both “real time” and post-recording image analysis. Real-time includes operator-controlled enhancement of the video-rate TV signal coming from the microscope, and the presentation of diffractograms as required by the microscope operator. Post-recording analysis includes post-processing of the experimental image as well as image simulation from model structures. Our chosen system is based on two host computers (Fig. 1): one host controls the post-recording workstations and the other runs the real-time stations. MicroVAX II minicomputers running the VMS operating system were chosen for both the main and real time hosts because the VMS system is multi-user and has virtual memory capability; in addition, it is the operating system most familiar to outside users of the facility.

Interactive simulation of high-resolution electron micrographs

Only a few years ago image simulation of HRTEM images were mostly carried out as low priority jobs on large mainframe computers with images available the next day as overprinted characters on computer paper. Today the same calculation can be carried out on a dedicated workstation in a matter of minutes with output on high resolution video monitors. This improvement in computer hardware has produced a shift from software primarily designed to run as batch jobs to interactive software that allows instant changes to both the atomic model and microscope parameters. In these near “real time” calculations the user interface becomes an important part of the software, since setting up the conditions for the calculation may take longer than the calculation itself.In an attempt to create a truly interactive environment for simulation of HRTEM images, a new set of programs has been written at the National Center for Electron Microscopy (NCEM) at Berkeley.

Atomic structure of the GaAs/Si interface

The atomic structure of the GaAs/Si(100) interface is studied using high resolution transmission electron microscopy. The samples studied were grown by molecular beam epitaxy using a conventional two-step growth process of a relatively low-temperature (405 °C) GaAs buffer layer followed by a higher temperature (575 °C) device layer. Following this growth procedure, the interface is found to be atomically rough with nonuniform distributions of steps, and to contain regions of disrupted crystallinity. These regions are found to be more prevalent in material which has undergone only low-temperature (buffer layer) GaAs deposition. By using the atomic resolution microscope at the National Center for Electron Microscopy, Lawrence Berkeley Laboratories, we are also able to atomically image the same area of interface along orthogonal 〈011〉 directions, observing qualitatively similar structures.

Recent TEM Applications

This paper describes the conceptual basis for the systems design and new equipment integration at Lawrence Berkeley Laboratory’s National Center for Electron Microscopy. Computer aided image processing helps scientists develop better structural models which, in turn, help define new directions for improved image processing. Several examples are cited to demonstrate how improvements in experimental techniques, high voltage, high resolution and microanalytical capabilities, combined with computer processing impact on materials characterization.