Field Ion Micrographs Research Articles

PULSED LASER ATOM PROBE CHARACTERIZATION OF SILICON CARBIDE

Field-jon microscopy and field evaporation of a-SiC and /3-SiC materials have been successfully demonstrated. Since field-ion micrographs could be taken immediately after pulsed field evaporation, the effects of preferential field evaporation of silicon that occurred under ramped field evaporation were eliminated. However, due to the high electrical resistivity of silicon carbide, conventional voltage pulsed field evaporation was not practical and pulsed laser field evaporation was required. No significant variations in performance were found as a function of laser wavelength but the full-width-at-half maximum mass resolution of 0.4 amu was inferior to voltage pulsed field evaporation of metals. In addition, the background noise level was considerably higher. Within the statistical accuracy of the analyses, the compositions were found to be slightly deficient in silicon. These experiments also revealed that the laser power required to field evaporate SiC was greater than for other semiconducting materials.

STRUCTURAL INVESTIGATIONS ON Al7(Mn0.7Fe0.3)2 AND Al4 Pd DECAGONAL QUASI CRYSTALS BY FIM

Two types of metastable alloys, Al7(Mn0.7Fe0.3)2 and Al4Pd, with decagonal symmetry were prepared by the melt spinning technique. The solidification morphology of the melt spun ribbons of Al7(Mn0.7Fe0.3)2 appeared to contain 5 µm sized pentagonal facets which in fact showed to be constituted of many lobes (SEM). The decagonal symmetry was revealed by transmission electron diffraction, the tenfold axis being always normal to the ribbon plane, as is expected for splat cooling on a cold surface. Bright field imaging (TEM) of quasi-crystalline grains while continuously tilting the specimen revealed a rapid, fine structured contrast variation unlike the usual appearance of crystalline materials. Bright field transmission electron micrographs of an Al4Pd tip revealed decoration with 5 nm sized patches. Both Al7(Mn0.7Fe0.3)2 and Al4Pd had similar field ion micrographs (Ne, 80 K) in the sense that globally disordered images had local ordering on the 5 nm scale. This is in contrast to simulations, which show ordering throughout the image field. This result, combined with the transmission electron micrographs, leads to the conclusion that the stacked Penrose model cannot universally describe the decagonal phase in the investigated alloys. A high defect density (like phasons) could be responsible for the observed phenomena, however the model in which the material is built of a large number of crystallites should not be ruled out either

Atomic Configuration of Carbon Fibers Studied by Field Ion Microscopy

A field ion microscope has been used to investigate the atomic configurations of carbon fibers. A stable field ion micrograph was obtained by wing Ne as an image gas, and the aligned fibers were imaged as small net plane rings. As-received high-modulus fibers made from polyacrylonitrile showed the notable fibril alignment only at the periphery, while repyrolyzed fibers at 3000°C indicated the formation of aligned fibril at the central area as well as at the periphery. The pitch carbon fibers repyrolyzed at 3000°C showed a uniform distribution of aligned large fibrils, which correlated to the high Young's modulus. A model based upon FIM observations was proposed in relating microscopic structures of fibrils to tribological design of carbon fiber composites.

Atom probe analysis of grain boundaries in rapidly-solidified Ni 3Al

The distribution of boron in a rapidly solidified Ni-24 at.% Al alloy containing 0.24 at.% B was investigated by atom probe field-ion microscopy. Boron was found to segregate to both APBs and grain boundaries. Field-ion micrographs revealed that some of the APBs and most of the grain boundaries were decorated with bright spots identified by single atom analysis as boron atoms. A 0.4–1.2 nm thick boron-enriched phase was observed on most of the grain boundaries. The distribution of this phase was not uniform either along a boundary or from boundary to boundary. The degree of long-range order as measured by the atom probe was greater than 0.97. This state of order continued to the interface between the matrix and grain boundary phase.

Beam current stability from localized emission sites in a field ion source

The production of stable ion beam currents using localized, high brightness ion emission from a gas fed, field ion source depends upon the stability of the field emitter surface in the region of the ionization. Research has focused primarily on developing a means of generating a single microstructure on the emitter surface which locally enhances the electrostatic field and thereby serves as an ion beam source. The relative ease of processing and the reliability of such a technique to contour the emitter surface are also factors determining the usefulness of the ion source which is to be used in high resolution microprobe systems. A procedure involving field emission through hydrogen films has been found whereby such a single ion, or electron, emission structure can be formed in situ on 〈110〉 oriented tungsten field emitters. These sites are capable of producing at least 10 to 15 nA beams of H+2 in half angle apertures of 16 mrad. Because the basic physical process involved in the field ionization of H2 is known to yield energy spreads of 1 eV (FWHM), good ion-optical properties are inherently available. This technique is practical in that one may readily and reproducibly localize the emission, but it remains to address the question of whether or not the localized emission structures provide stable beam currents for extended periods of time. Field ion micrographs of these sites indicate that they are protruberances 200 Å in diameter with field enhancement factors of 1.5. The current from these relatively large structures is not affected by surface rearrangement on the atomic scale caused by field induced chemical reactions or molecular contamination. The ability of these sites to provide a stable ion current then depends solely on the structural integrity of the protruberance. As ion bombardment is a factor in microboss growth, the first few surface layers of the field emitter and the microboss itself are radiation damaged tungsten and hence subject to electromechanical detachment. Heating the embossed field emitter to 800–1100 K allows greater ion currents to be drawn from the microboss before detachment occurs. However, these investigations indicate that to provide long term ion current stability a complete anneal (1100 K) of the embossed field emitter, most likely in the presence of an electric field such that the emitter is the anode, thereby holding the microboss in place and simultaneously halting the ion bombardment, is required. The fundamental field ionization mechanism generates ion currents stable to within 0.1% indicating the stability achievable after recrystallization of the surface.

Image Processing for Electron Microscope Investigations of Materials

ABSTRACTA time shared television digital image processing system has been developed for on-line electron microscopy and uses a large mainframe computer. The main component of the system is a digital television frame store which has many standard features for digital analysis such as: digitization, zoom and pan, arithmetic and Boolean processors, alphanumeric generators and so on. Images can be acquired at atomic resolution from a TEM, analyzed in real time and hard copy slides made under full computer control. A full range of computer software has been developed or modified from existing software and is generally compatible with IBM Fortran compilers. Some of the areas where extensive menu driven software has been developed are: particle size and feature analysis, algebraic and geometric image manipulations, Fourier analysis, digitization and process control, image contrast correction, text processing, etc. A number of applications areas have been explored which include: the structure of Si/SiO2interfaces; nucleation of Au on rocksalt; the formation of hexatic structures from amorphous phases under shear, tension and compression; analysis of atomic surface structure and image motion and the analysis of field ion micrographs of amorphous structures. Several of these areas will be discussed in the context of image processing and materials characterization.

Field ion microscopy of alpha uranium

Abstract Field ion micrographs of alpha uranium show atomic details of many orthorhombic crystal planes. Images of the (010) and (001) plane edges are examined and discussed in particular. Micrographs indicate that a surface hydride phase is formed readily under hydrogen imaging conditions. Some micrographs indicate possible hydride particle precipitation at a major crystal defect boundary. No evidence was found, however, of hydrogen/stress induced surface cracks. Procedures used to prepare alpha uranium for field ion microscopy are described.

Field ion microscopy of alpha uranium

Field ion micrographs of alpha uranium show atomic details of many orthorhombic crystal planes. Images of the (010) and (001) plane edges are examined and discussed in particular. Micrographs indicate that a surface hydride phase is formed readily under hydrogen imaging conditions. Some micrographs indicate possible hydride particle precipitation at a major crystal defect boundary. No evidence was found, however, of hydrogen/stress induced surface cracks. Procedures used to prepare alpha uranium for field ion microscopy are described.

液体急冷Ni-10 at%Ti変調構造合金の時効

The aging behavior of supersaturated Ni-10 at%Ti crystals, quenched rapidly from liquid state and solid state, has been studied. The phase transformation is investigated by means of micro-Vicker’s, magnetic balance and field ion microscopy (F.I.M.). The rate of age-hardening for the liquid-quenched (L.Q.) specimen is larger than that for the solid-quenched (S.Q.) specimen. For both the L.Q. and the S.Q. specimen the hardness decreases at about 7 ks, and then increases. The transformation kinetics is discussed by means of generalized Johnson-Mehl equation, where the ratio of transformation is estimated from the hardness and the Curie temperature. The values of time exponential n are approximately equal for the L.Q. and the S.Q. specimen, and are 0.279 (about 1/3) and 0.284, respectively. These results show that the mechanism of transformation on aging for the L.Q. specimen is as the same as that for the S.Q. specimen. Furthermore, the field ion micrographs for both the transformed L.Q. and the transformed S.Q. specimens show a modulated structure. However, the kinetic constant (k) of transformation for the L.Q. specimen is about four times larger than that for the S.Q. specimen on 873 K aging.

Thorium field ion microscopy

Thorium metal (fee) is potentially useful in nuclear reactor technology [I], and is the object of increasing scientific interest [ 1,2]. Thorium has not previously been imaged with either field ion or field electron microscopy. Since field ion microscopy (FIM) is capable of resolving structures such as lattice steps, vacancies, impurities, dislocations, etc. on an atomic scale [3], the application of FIM to thorium provides an important investigative technique. We describe here a method of preparing and imaging thorium specimens. Wires 0.25 mm in diameter were prepared [4] from 99.9% thorium rods, 1 cm diameter. These were electrolytically thinned and tapered using either of the following polishing conditions: (i) 1 : 1 mixture of phosphoric and acetic acids, -50 V (ac), (ii) 1 : 10 mixture of 70% perchloric acid diluted in methanol, 20-10 V (ac). A carbon rod was used as the counter-electrode. Final tip sharpening was accomplished with either fresh, concentrated hydrochloric acid, 4 V (dc), or a 5102, mixture of perchloric acid (70%) in glycerin, 30-12 V (dc). Specimens were rinsed with methanol. A small platinum wire loop (0.127 mm diameter wire) containing a drop of electrolyte was moved about the thorium tip-end by a remote-control micromanipulator and the procedure observed at 250 magnification. The unbaked field ion microscope was equipped with a 90 mm diameter microchannel plate. The specimen temperature was controlled by adjusting the flow of cold gaseous helium from a liquid helium dewar over a specimen-mount feedthrough pin. During field ion microscopy reagent grade imaging gases flowed through the microscope. Imaging gas mixtures were made by simultaneously leaking the appropriate gases, and mixture compositions were estimated from ion gage pressure measurements. Fig. 1A is a helium promoted neon field ion micrograph of thorium showing the fee lattice symmetry. There are also extra bright spots distributed randomly with no crystal plane preference, indicating the presence of another species on the thorium surface. Both species, distinguished as normal and bright spots, field evaporated coincidently, indicating a bulk-wise distribution of the less abundant species. No obvious structural defects in the lattice were found in the bright spots’ vicinity.

Field ion microscopy of polycrystalline iron whiskers

Field ion micrographs of polycrystalline iron (steel) whiskers are presented. These micrographs indicate that the whiskers examined here possess a two-phase structure consisting of an ordered iron phase and a randomly imaging phase interpreted to be the iron carbide, Fe3C. The iron image indicates not only a high defect density but also the presence of twins.

A gnomonic method for the indexing of field ion micrographs

A simplified and accurate method was found for the indexing of field ion micrographs. The procedure makes use of a gnomonic projection that is modified to fit the particular microscope and thus describes the ionic paths. A unique modification was incorporated into the microscope by allowance of a ±15° vertical rotation of the image screen. Calculations of ionic paths at several screen rotations proved that the ionic paths were accurately defined.

Quantitative analysis of field-ion micrographs using moiré techniques

It has previously been demonstrated that the configuration of imaged atoms on the surface of a field-ion emitter may be interpreted as a moiré pattern. As a consequence it becomes possible to relate the structure of each plane in a field-ion image to that of a number of other planes, by means of ring counting procedures, and hence to obtain precise quantitative information concerning the emitter geometry. Here it is shown how the analysis may be applied to an understanding of how the various planar facets develop, to the determination of the relative field evaporation rates of different planes, and to the measurement of more accurate values of local radius.

Multilayer field evaporation patterns

Multilayer field evaporation patterns of the fee metals, Ir, Pt, Rh, and the bcc metals, W, Ta, Mo, exhibit sharp networks of dark zones. These patterns are explained as ion-optical effects resulting from deviations of ion-trajectories from an exactly radial projection, as the emitter is a polyhedron rather than a sphere. Step widths on vicinals of major zone lines are retained during progressing evaporation, thereby projecting the ions into fixed directions while keeping the zone image itself dark. The bright [110] zones on fee metals reflect a focusing effect of long atom chains across the zones. Circular intensity features around the basal planes depend upon the step-wise shift of local magnification each time a residual central plane collapses. If as we surmise multilayer desorption image features are predominantly due to tip morphology, they should also be contained in the conventional field ion image, although partially hidden by brightness effects due to varying local gas supply and the large scattering disc of a single atom image. Using multilayer sequences of field ion micrographs we map the location of each atom before it evaporates as a small dot to simultate the sharpness of a single ion spot of a field desorption image. We also use another less subjective photographic superposition method. As these FIM techniques display the three essential features of the multilayer FDM image, the hypothesis of general pre-evaporation local surface migration needs not to be invoked for explaining the FDM image.

On the Shape of Field-Ion Emitters

The shape of the emitter is of cardinal importance to field-ion microscopy. First, the field evaporation process itself is closely related to the initial tip shape. Secondly, the imaging stress, which is near the theoretical strength of the material and intrinsic to the imaging process, cannot be characterized without knowledge of the emitter shape. Finally, the problem of obtaining quantitative geometric information from the micrograph cannot be solved without knowing the shape. Previously published grain-boundary topographies were obtained employing an assumption of a spherical shape (1). The present investigation shows that the true shape deviates as much as 100 Å from sphericity and boundary reconstructions contain considerable error as a result.Our present procedures for obtaining tip shape may be summarized as follows. An empirical projection, D=f(θ), is obtained by digitizing the positions of poles on a field-ion micrograph.

Determination of field ion tip shapes

An empirical method has been developed for determining the tip shapes from field ion micrographs utilizing digitization and computer graphics techniques. Application of this method to tungsten and iridium specimens indicates that the tip profiles deviate significantly from a hemisphere, and on a fine scale the shape varies from region to region over the specimen surface. However, for practical purposes the entire end form can be represented by a polynomial of degree 4 or 5.

Field-ion microscopic observation of a twin boundary in iridium

Twin boundaries in field-ion micrographs are expected to show matching of rings of prominent planes in the matrix crystal with rings of other prominent planes in the twin crystal. This paper offers additional experimental verification of this contrast feature and shows the usefulness of this concept in identifying orientation relationships in bicrystals without the uncertainty involved in the assumption on that the field-ion projection corresponds to any of the standard projections.

On the quantitative analysis of field-ion micrographs

Semiempirical methods for extracting quantitative information from field-ion micrographs are described. The distinction between average and local magnification is quantified, and a number of new geometrical relationships are derived. New methods for indexing micrographs, for assessing image magnification and for determining tip shapes are presented. These methods improve the accuracy with which the parameters describing the angular misorientation of a grain boundary can be determined. Tip profiles reconstructed using these methods are in excellent agreement with those-derived from electron microscopy. In addition it is shown how grain boundary topography can be investigated, to an accuracy of 5–10 Å, over the whole of the imaging region.

A field ion study of the adsorption of oxygen on ruthenium at low temperatures

The effect of low coverage adsorption of oxygen on ruthenium surfaces at low temperatures in the FIM is comparable with results already reported for tungsten under similar conditions, namely that most extensive corrosion can be correlated with the presence of deep adsorption sites in the proximity of low coordination surface metal atoms. Such experiments have also permitted of the correct assignment of structures to the bright and dim sequence of planes observed on field ion micrographs at certain poles and which are the result of the ABAB… stacking arrangement of hexagonal metals.

A Field Ion Microscope Study of Thin Films of Molybdenum on Tungsten, Iridium and Rhenium

Thin films of Mo on W, Ir and Re were grown from vapor phase in a demountable field ion microscope whose attainable vacuum was about 1×10-7 Torr or better. It was found that when specimens were treated thermally at adequate temperatures during and after deposition thin films were grown epitaxially with its own structure for all three systems. However, the interface between the films and their underlying substrates, one or possibly two atomic layers of Mo, repeated substrate structures with a certain amount of atomic disorder. Orientation relationships could be directly determined from field ion micrographs.