Backscatter Kikuchi Diffraction Research Articles

Texture Gradient Effects in Tantalum

The effects of inhomogeneities in texture on the mechanical response of a tantalum plate were investigated. Through-thickness compression samples exhibited an hourglass shape after deformation. The compression tests were modeled using a finite-element approach. A polycrystal plasticity model is embedded at each element of the mesh enabling the simulation to account for the spatial distribution of texture and its evolution. The texture was characterized using spatially specific individual lattice orientation measurements. The orientations were measured using automatic analysis of electron backscatter Kikuchi diffraction patterns. The influence of the spatially nonuniform distribution of texture on the experimental and simulated results is discussed. This work demonstrates the capability of currently available tools for characterizing inhomogeneous microstructures and modeling the effects of variations in texture on mechanical behavior.

Local Texture and Electromigration in Fine Line Microelectronic Aluminum Metallization

ABSTRACTAluminum films 1 μm thick were deposited on oxidized silicon by sputtering and partially ionized beam evaporation to vary the crystallographic texture. These films were patterned into lines and subsequently annealed at 400 °C for 1 h. A strong correlation between the electromigration behavior and the blanket film texture (X-ray diffraction (XRD) / pole figures) has been reported previously for these films. In this work, an Electron Backscatter Diffraction (EBSD) a.k.a. Backscatter Kikuchi Diffraction (BKD) technique was employed using a scanning electron microscope (SEM) to interrogate individual grain orientations. BKD pole figures were acquired for lines ≥0.3 μm wide and for blanket (pad) regions. Identical, inverse pole figures were found for blanket films measured using both XRD and BKD (pads). Furthermore, the BKD (111) fiber texture shows a line width dependency, with narrow lines having a slightly improved texture over blanket (pad) regions. Local grain orientations were investigated near and within electromigration testing sites with characteristic void and hillock morphologies. The relationship of neighboring grain orientations to electromigration damage is shown.

BKD in the SEM: Toward a smaller information volume

Upto now, the recording of backscatter Kikuchi diffraction (BKD) patterns (also called electron backscattering patterns, EBSP) for microscale texture analysis is mostly performed at high primary beam voltages U > 20 kV using an SEM with a tungsten or LaB6 electron gun. Under these circumstances, a lateral resolution of 0.2-0.5 μm has been reported. However, the texture analysis of fine-grained bulk materials or vacuum-deposited thin metallic layers as used in the IC-industry requires a higher spatial resolution. A fieldemission electron gun (FEG) with a much higher electron-optical brightness can deliver the 1-nA beam current needed for EBSP recording in a spot of about 10 nm, enabling a lateral resolution of 20×80 nm3 and an information depth of 10 nm at 20 kV. In Ref. 2, the possibility of using lower beam voltages to obtain an even higher resolution is mentioned. Monte Carlo simulations indeed show that the interaction volume decreases at least linearly with decreasing U.

Orientation imaging of microstructures

The complexity of microstructures characteristic of polycrystalline materials presents the serious investigator with many challenges. The materials engineer hopes to associate the important technological properties of these materials with specific (quantifiable) attributes of the microstructure; however, microscopy presents an overwhelming myriad of details over a wide range of scales of inquiry. Thus, the persistent question becomes: What is important in the microstructure relative to a specific property or aspect of material performance? One particular viewpoint, which stems from the modern atomistic interpretation of the structure of solids, is that for polycrystalline materials it is the spatial placement of lattice orientation that is of essential interest.The past decade has seen some remarkable progress in microdiffraction technique in conjunction with the scanning electron microscope. This progress now makes it possible to vigorously pursue the aforementioned lattice-orientational viewpoint. Since 1987 modern SIT (silicon intensified target) vidicon and CCD (charge-coupled device) cameras have been used to capture the backscattered Kikuchi diffraction (BKD) formed in stationary spot mode in the scanning electron microscope.

Automatic idexing of electron-backscatter diffraction patterns

A typical Backscatter Kikuchi Diffraction pattern (BKD, also referred to in the literature as an EBSP or a BEKP) is shown in figure 1. Since the bands in the pattern represent planes in the diffracting volume, the lattice orientation can be determined from their geometrical arrangement. The task of correctly orienting a BKD can be broken into two parts: 1) finding the salient features in the pattern (either the diffraction bands or the intersections of the bands) and 2) using these features to determine the lattice orientation. Recent advances in feature detection in BKDs along with methods for digital image enhancement will be described in some detail. The determination of orientation from a set of detected bands (or intersections of bands) will also be discussed.Dingley has demonstrated that lattice orientation can be practically obtained from BKDs by imaging the diffraction patterns using a low light level video camera and indexing the patterns with the aid of an online computer.

Orientation Imaging of the Microstructure of Polycrystalline Materials

ABSTRACTRecent developments, coupling the scanning electron microscope with image processing and crystallographic analysis, now make it possible to automatically index many thousands of lattice orientations exposed on section planes in polycrystalline materials. Backscattered Kikuchi diffraction patterns obtained from high-gain SIT and CCD cameras are analyzed using the Hough transformation (Cartesian coordinates - polar coordinates) in order to identify diffraction band widths and interplanar angles. From this basic information local lattice orientation can be determined. Information from raw data sets (in excess of 100,000 single orientations in some examples) can be used to construct orientation imaging micrographs which emphasize certain aspects of the exposed field of lattice orientations. Thus, features of the spatial placement of lattice orientation (including grain boundary misorientation, microtexture, and connectivity of the microstructure) are readily studied. In this paper these techniques are reviewed, and recent explorations of the connectivity of grain boundary misorientation structure are presented for an interesting nickel-chromium-iron alloy.

Automatic analysis of Kikuchi diffraction patterns

Recent advances in the automatic indexing of backscatter Kikuchi diffraction patterns on the scanning electron microscope (SEM) has resulted in the development of a new type of microscopy. The ability to obtain statistically relevant information on the spatial distribution of crystallite orientations is giving rise to new insight into polycrystalline microstructures and their relation to materials properties. A limitation of the technique in the SEM is that the spatial resolution of the measurement is restricted by the relatively large size of the electron beam in relation to various microstructural features. Typically the spatial resolution in the SEM is limited to about half a micron or greater. Heavily worked structures exhibit microstructural features much finer than this and require resolution on the order of nanometers for accurate characterization. Transmission electron microscope (TEM) techniques offer sufficient resolution to investigate heavily worked crystalline materials.Crystal lattice orientation determination from Kikuchi diffraction patterns in the TEM (Figure 1) requires knowledge of the relative positions of at least three non-parallel Kikuchi line pairs in relation to the crystallite and the electron beam.

Assessment of implantation damage by backscatter Kikuchi diffraction in the scanning electron microscope

Surface damage caused by ion implantation is assessed by backscatter Kikuchi diffraction in the scanning electron microscope. Backscatter Kikuchi diffraction patterns from Si(100) specimens implanted with different doses of 80-keV Ge ions between 3×1012 and 3×1014 ions cm−2 are obtained. The pattern contrast is quantified in a straightforward procedure employing standard image-processing operations. A comparison between the contrast in backscattering Kikuchi diffraction patterns and the channeling minimum in energy spectra by Rutherford backscattering spectrometry as a measure for surface implantation damage is made. As it turns out, both measures are about equally sensitive to the level of the surface implantation damage. The lateral resolution (<100 nm) and the high surface sensitivity (of the order of 10 nm) of backscatter Kikuchi diffraction may be exploited to study lateral implantation profiles with sub-μm resolution.

Orientation imaging: The emergence of a new microscopy

A new microscopy, called orientation imaging microscopy, is described. Imaging results from precise measurements of local lattice orientation facilitated by backscattered Kikuchi diffraction. The hardware configuration of the microscope is described, and a formal description of image formation is developed. Application of the method to several cubic materials and material conditions is described. Emphasis is given to those areas of application where new insight into polycrystalline microstructures has begun to emerge.

Microscale elastic-strain determination by backscatter Kikuchi diffraction in the scanning electron microscope

It is shown that backscatter Kikuchi diffraction in the scanning electron microscope can be used for the determination of elastic strain with μm resolution. From the shift of Kikuchi bands in backscatter Kikuchi diffraction patterns of epitaxial Si1−xGex layers on Si(100) the perpendicular elastic strain was determined to be 2.5% for x=0.34 and at 1.0% for x=0.16 with an accuracy of about 0.1%. The values found on a μm scale were in good agreement with high-resolution x-ray diffraction measurements averaging over mm distances.

Use of Kikuchi line intersections in crystal symmetry determination: application to chalcopyrite structure

The crystallographic point group 4 2m of chalcopyrite has been determined using Kikuchi diffraction patterns obtained in both the TEM and the SEM. Backscatter Kikuchi diffraction patterns (BKDPs) recorded in the SEM revealed evidence for the breakdown of Friedel's law, thus confirming the non-centrosymmetric structure of chalcopyrite. In addition, shifted Kikuchi line intersections were observed in BKDPs within the 0 hh systematic row of reflections, indicating that the [100] axis has Laue group 2, in agreement with 4 2m. Diffraction patterns obtained from chalcopyrite in the TEM also revealed fine displacements at the intersections of various Kikuchi lines including 41 3 and 4 3 1, in agreement with the point group symmetry of chalcopyrite. It was observed that Kikuchi line intersections were sensitive to small differences in interplanar spacings and were unaffected by variations in specimen thickness, diffraction intensities and off-axis electron beams.

A study of the breakdown of Friedel's law in electron backscatter Kikuchi diffraction patterns: application to zincblende-type structures

The breakdown of Friedel's law has been observed in backscatter Kikuchi diffraction patterns (BKDP) obtained in the scanning electron microscope (SEM) from a series of zincblende structures including GaAs, InP, GaSb, CdHgTe and the minerals sphalerite (ZnS), chalcopyrite (CuFeS2) and tetrahedrite (Cu12Sb4S13). Differences in intensities were observed between the reflections 51{\bar 1} and 5{\bar 1}{\bar 1} in InP, GaSb, CdHgTe and sphalerite, thus allowing the non-centrosymmetric point group {\bar 4}3m to be determined. In GaAs, differences in intensities were noted between {\bar 5}11 and {\bar 4}{\bar 1}. In chalcopyrite and tetrahedrite, non-equivalent intensities were observed between {\bar 2}15 and 2{\bar 1}{\bar 5} and between 3{\bar 1}{\bar 2} and 31{\bar 2}, respectively. In addition, BKDPs obtained from chalcopyrite revealed a small displacement at the point where the pair of equivalent reflections {\bar 4}06 and 460 intersect within the Kikuchi band 02{\bar 2}. The presence of this displacement together with observation of the breakdown of Friedel's law confirmed the tetragonal point group {\bar 4}2m for chalcopyrite. Although the point groups of GaAs, chalcopyrite and tetrahedrite were derived successfully using BKDPs, determination of their space groups proved unsuccessful. The superstructure reflections were invisible because the structure factors are very small. The behaviour of the invisible 200 reflection in GaAs is investigated using many-beam dynamical intensity profiles calculated across the h00 systematic row of reflections. Dynamical intensity profiles calculated across the h00 systematic rows of reflections for Ge, InP and sphalerite are also discussed.

Automated Determination of LatticeOrientation From ElectronBackscattered KikuchiDiffraction Patterns

As described in a previous paper (Wright and Adams, 1990), with the advent of new techniques it has become practical to consider using single orientation measurements for texture investigations.Advantages of using single orientations measurements over the traditional bulk measurement using pole figures are: direct access to odd‐l terms in the series expansion of the Orientation Distribution Function (ODF) (Bunge, 1982) and the capability to measure local textures and to obtain spatial orientation correlation information. The primary obstacle to the use of single orientation measurements is the large investment of direct operator time required to obtain a statistically reliable data set. This study examines a technique that has been developed to automate the orientation identification process. The technique essentially compares an electron backscattered Kikuchi diffraction (BKD) pattern with a set of idealized patterns and finds the best match. The set of idealized patterns is created from a set of orientations which represent a tessellation of the asymmetric region of Euler space. The technique shows promising preliminary results, BKD patterns for silicon and stainless steel were correctly identified by the computer.

Automated Lattice Orientation Determination From Electron Backscatter Kikuchi Diffraction Patterns

The ability to measure individual orientations from crystallites enables a more complete characterization of the microstructure. A primary obstacle to the use of single orientation measurements is the large investment of direct operator time necessary to obtain a statistically reliable data set. The most viable technique for obtaining individual orientation measurements is electron backscatter Kikuchi diffraction (BKD). Current technology requires an operator to identify a zone axis pair in a BKD pattern. This paper describes research in progress to automate the identification of lattice orientation from BKD patterns by identifying the planes associated with the bands that appear in the patterns. This work considered FCC crystal symmetry.

A study of directly recorded RHEED and BKD patterns in the SEM

A new method of recording reflection high-energy electron diffraction (RHEED) patterns in the scanning electron microscope (SEM) is discussed. The method involves direct recording of RHEED patterns on photographic film positioned in the vacuum of the chamber at ≈10 -7 Torr, using a probe diameter ≈20 nm and a working distance ≈12 mm. RHEED patterns obtained from bulk as-received silicon and GaAs surfaces were seen to exhibit Kikuchi bands, continuous circles of intensity and, in some cases, specular reflections. The technique was used to investigate thin GaAs capping layers deposited on GaAlAs. An account is also given of the application of backscatter Kikuchi diffraction patterns (BKDP) in the SEM to point group and space group determinations. Using the technique, a space group Fd 3m was determined for silicon. Theoretically calculated and experimentally obtained intensity profiles were used to investigate contrast variations across the hh0 systematic row of reflections in BKDPs from silicon. The geometrical differences between BKDPs and RHEED patterns are also discussed.

Measurement of boundary plane inclination in a scanning electron microscope

Recently, a method has been devised for measuring the boundary orientations using backscattered Kikuchi diffraction (BKD, otherwise known as electron backscattering, EBS). The work reported demonstrates that BKD can be efficiently used to measure both the misorientation across grain boundaries and also the orientation of boundary planes. In nickel it has been shown that the boundaries of grains which are situated along the corner of a rectangular specimen rotate so as to minimize their interfacial energy. For non-coincidence site lattices related grains, boundaries tend to align normal to the edge of the specimen, while {Sigma} = 3 and {Sigma} = 9 CSLs tend to rotate to tilt configuration, particularly asymmetric tilts such as {l brace}111{r brace}/ {l brace}115{r brace} or {l brace}110{r brace}/{l brace}114{r brace}.

Application of backscatter Kikuchi diffraction in the scanning electron microscope to the study of NiS2

Backscatter Kikuchi diffraction has been applied to an investigation of a selection of nickel sulfide crystals. In the first instance the technique itself was investigated to see whether it could be used successfully to distinguish between the space groups P21/a{\bar 3} (Pa{\bar 3}) of pyrite and Pnnm of marcasite. Subsequently, it was used to investigate the structure of a thin epilayer which was observed to have grown on selected facets of a certain synthetic nickel pyrite crystal. The epilayer was shown to be rhombohedral with possible space groups R{\bar 3}2/m (R3m) and lattice parameters ar = 6.4 Å, αr = 36°. In an attempt to positively identify this structure, the technique was applied to a number of related compounds including the mineral millerite (NiS, R3m), α-NiS – a high temperature polymorph of millerite (P63/mmc and P63mc), and the layer compound NiI2 (R{\bar 3}m). It was concluded that the epilayer was most probably NiCl2 formed by reaction of the NiS2 crystal with chlorine contained in the carrier gas used during the chemical vapour deposition growth process.

Backscatter Kikuchi diffraction in the SEM for identification of crystallographic point groups

AbstractA preliminary account is given of the application of backscatter Kikuchi diffraction patterns (BKP) to point group determination in the SEM. Studies carried out on bulk specimens selected from the 32‐point groups indicate that 27 of the 32‐point groups can be determined unambiguously using BKPs, though distinction between 6,6,4,4 and 3 and 3 is difficult and requires special procedures. Evidence for breakdown of Friedel's law was observed in BKPs from GaSb, thus confirming the noncentrosymmetric point group 43m. Differences in intensities observed between the reflections hkl and hk̄l̄ reflections in GaSb which arise from the sensitivity of dynamical interactions to phases of the structure factors, could not be found in GaAs, illustrating that point group determination using BKPs should be considered with caution.