3D high resolution imaging for microelectronics: A multi-technique survey on copper pillars

In microelectronics, recently developed 3D integration offers the possibility to stack the dice or wafers vertically instead of putting their different parts next to one another, in order to save space. As this method becomes of greater interest, the need for 3D imaging techniques becomes higher. We here report a study about different 3D characterization techniques applied to copper pillars, which are used to stack different dice together. Destructive techniques such as FIB/SEM, FIB/FIB, and PFIB/PFIB slice and view protocols have been assessed, as well as non-destructive ones, such as laboratory-based and synchrotron-based computed tomographies. A comparison of those techniques in the specific case of copper pillars is given, taking into account the constraints linked to the microelectronics industry, mainly concerning resolution and sample throughput. Laboratory-based imaging techniques are shown to be relevant in the case of punctual analyses, while synchrotron based tomographies offer highly resolved volumes for larger batches of samples.

2D/3D Microanalysis by Energy Dispersive X-ray Absorption Spectroscopy Tomography

X-ray spectroscopic techniques have proven to be particularly useful in elucidating the molecular and electronic structural information of chemically heterogeneous and complex micro- and nano-structured materials. However, spatially resolved chemical characterization at the micrometre scale remains a challenge. Here, we report the novel hyperspectral technique of micro Energy Dispersive X-ray Absorption Spectroscopy (μED-XAS) tomography which can resolve in both 2D and 3D the spatial distribution of chemical species through the reconstruction of XANES spectra. To document the capability of the technique in resolving chemical species, we first analyse a sample containing 2-30 μm grains of various ferrous- and ferric-iron containing minerals, including hypersthene, magnetite and hematite, distributed in a light matrix of a resin. We accurately obtain the XANES spectra at the Fe K-edge of these four standards, with spatial resolution of 3 μm. Subsequently, a sample of ~1.9 billion-year-old microfossil from the Gunflint Formation in Canada is investigated, and for the first time ever, we are able to locally identify the oxidation state of iron compounds encrusting the 5 to 10 μm microfossils. Our results highlight the potential for attaining new insights into Precambrian ecosystems and the composition of Earth's earliest life forms.

Developments on a SEM-based X-ray tomography system: Stabilization scheme and performance evaluation

Recent improvements in a SEM-based X-ray tomography system are described. In this type of equipment, X-rays are generated through the interaction between a highly focused electron-beam and a geometrically confined anode target. Unwanted long-term drifts of the e-beam can lead to loss of X-ray flux or decrease of spatial resolution in images. To circumvent this issue, a closed-loop control using FFT-based image correlation is integrated to the acquisition routine, in order to provide an in-line drift correction. The X-ray detection system consists of a state-of-the-art scientific CMOS camera (indirect detection), featuring high quantum efficiency (∼60%) and low read-out noise (∼1.2 electrons). The system performance is evaluated in terms of resolution, detectability, and scanning times for applications covering three different scientific fields: microelectronics, technical textile, and material science.

State-of-the-Art Three-Dimensional Chemical Characterization of Solid Oxide Fuel Cell Using Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry Tomography

In this paper the potential of time-of-flight secondary ion mass spectroscopy combined with focused ion beam technology to characterize the composition of a solid oxide fuel cell (SOFC) in three-dimension is demonstrated. The very high sensitivity of this method allows even very small amounts of elements/compounds to be detected and localized. Therefore, interlayer diffusion of elements between porous electrodes and presence of pollutants can be analyzed with a spatial resolution of the order of 100 nm. However, proper element recognition and mass interference still remain important issues. Here, we present a complete elemental analysis of the SOFC as well as techniques that help to validate the reliability of obtained results. A discussion on origins of probable artifacts is provided.

Non-rigid alignment in electron tomography in materials science

Electron tomography is a key technique that enables the visualization of an object in three dimensions with a resolution of about a nanometre. High-quality 3D reconstruction is possible thanks to the latest compressed sensing algorithms and/or better alignment and preprocessing of the 2D projections. Rigid alignment of 2D projections is routine in electron tomography. However, it cannot correct misalignments induced by (i) deformations of the sample due to radiation damage or (ii) drifting of the sample during the acquisition of an image in scanning transmission electron microscope mode. In both cases, those misalignments can give rise to artefacts in the reconstruction. We propose a simple-to-implement non-rigid alignment technique to correct those artefacts. This technique is particularly suited for needle-shaped samples in materials science. It is initiated by a rigid alignment of the projections and it is then followed by several rigid alignments of different parts of the projections. Piecewise linear deformations are applied to each projection to force them to simultaneously satisfy the rigid alignments of the different parts. The efficiency of this technique is demonstrated on three samples, an intermetallic sample with deformation misalignments due to a high electron dose typical to spectroscopic electron tomography, a porous silicon sample with an extremely thin end particularly sensitive to electron beam and another porous silicon sample that was drifting during image acquisitions.

Self-adapting denoising, alignment and reconstruction in electron tomography in materials science

An automatic procedure for electron tomography is presented. This procedure is adapted for specimens that can be fashioned into a needle-shaped sample and has been evaluated on inorganic samples. It consists of self-adapting denoising, automatic and accurate alignment including detection and correction of tilt axis, and 3D reconstruction. We propose the exploitation of a large amount of information of an electron tomography acquisition to achieve robust and automatic mixed Poisson-Gaussian noise parameter estimation and denoising using undecimated wavelet transforms. The alignment is made by mixing three techniques, namely (i) cross-correlations between neighboring projections, (ii) common line algorithm to get a precise shift correction in the direction of the tilt axis and (iii) intermediate reconstructions to precisely determine the tilt axis and shift correction in the direction perpendicular to that axis. Mixing alignment techniques turns out to be very efficient and fast. Significant improvements are highlighted in both simulations and real data reconstructions of porous silicon in high angle annular dark field mode and agglomerated silver nanoparticles in incoherent bright field mode. 3D reconstructions obtained with minimal user-intervention present fewer artefacts and less noise, which permits easier and more reliable segmentation and quantitative analysis. After careful sample preparation and data acquisition, the denoising procedure, alignment and reconstruction can be achieved within an hour for a 3D volume of about a hundred million voxels, which is a step toward a more routine use of electron tomography.

X-ray micro Laue diffraction tomography analysis of a solid oxide fuel cell

The relevance of micro Laue diffraction tomography (µ-LT) to investigate heterogeneous polycrystalline materials has been studied. For this purpose, a multiphase solid oxide fuel cell (SOFC) electrode composite made of yttria-stabilized zirconia and nickel oxide phases, with grains of about a few micrometres in size, has been analyzed. In order to calibrate the Laue data and to test the technique's sensitivity limits, a monocrystalline germanium sample of about 8 × 4 µm in cross-section size has also been studied through µ-LT. The SOFC and germanium Laue diffraction pattern analyses are compared and discussed. The indexing procedure has been successfully applied for the analysis of the germanium Laue data, and the depth-resolved two-dimensional cartographies of the full deviatoric strain tensor components were obtained. The development and application of an original geometrical approach to analyze the SOFC Laue data allowed the authors to resolve grains with sizes of about 3 µm and to identify their individual Laue patterns; by indexing those Laue patterns, the crystalline phases and orientations of most of the grains identified through the geometrical approach could be resolved.

Correction of absorption-edge artifacts in polychromatic X-ray tomography in a scanning electron microscope for 3D microelectronics

X-ray tomography is widely used in materials science. However, X-ray scanners are often based on polychromatic radiation that creates artifacts such as dark streaks. We show this artifact is not always due to beam hardening. It may appear when scanning samples with high-Z elements inside a low-Z matrix because of the high-Z element absorption edge: X-rays whose energy is above this edge are strongly absorbed, violating the exponential decay assumption for reconstruction algorithms and generating dark streaks. A method is proposed to limit the absorption edge effect and is applied on a microelectronic case to suppress dark streaks between interconnections.

Diffraction/Scattering Tomography on multi-phase crystalline/amorphous materials

By suitably combining diffraction/scattering and tomography (DSCT), it is possible to access to selective submicron 2D/3D structural and micro-structural information, which cannot be obtained from separate, independent diffraction and tomography experiments. DSCT is used to discriminate between multi-phase crystalline and amorphous materials, especially when the similarities in densities limit the use of other methods. In addition, this method is sensitive to local variation of the crystalline state, texture, grain size or strains inside the object and can allow simultaneous 3D mappings of such properties. The DSCT phase-selectivity can be easily combined with fluorescence and absorption for added chemical and density resolution allowing multi-modal analyses. As samples can be used in their original state, this method can be applied without cutting or polishing them. Moreover the setup can be adapted with specific sample environments in order to monitor phase and microstructure evolution as a function of an externally controlled parameter with a non-invasive approach. After a first report on in 1998 [1], since 2008 capabilities of DSCT have been demonstrated using x-rays on complex materials as diverse as biological tissue, pigments, Portland cements, Carbon-based materials, Uranium-based nuclear fuel, Ni/Al2O3 catalysts or amorphous systems [2]. More recently, the technique has evolved towards quantitative characterization of the microstructure and stress/strain through either Rietveld or Peak Profile analyses and also pair distribution function techniques (PDF) and their application to nanostructured materials [3]. In this poster contribution, we briefly review the principle and methodology of pencil-beam based x-ray DSCT which is two-fold: (i) selective structural imaging and (ii) extraction of selective scattered patterns of ultra-minor phases.

Deep sub micrometer imaging of defects in copper pillars by X-ray tomography in a SEM

The potential of X-ray nanotomography hosted in a SEM in presented in this paper. In order to improve the detail detectability of this system, which is directly related to the X-ray source size, thin metal layers have been studied and installed in the equipment. A 3D resolution pattern has been created in order to determine the smallest detectable features by this setup. This sample is a 25 μm diameter copper pillar in which size-controlled holes have been milled using a plasma-focused ion beam. This pattern has then been scanned and the resulting 3D reconstruction demonstrates that the instrument is able to detect 500 nm diameter voids in a copper interconnection, as used in 3D integration.

3D spatial resolution improvement by dual-axis electron tomography: application to tri-gate transistors

The performance of semiconductor devices can be linked to geometry and variations of the structure. For transistors in particular, the geometry of the gate stack is essential. In this work we investigate the gate stack of a tri-gate transistor using dual-axis electron tomography. This allows the reconstruction of all surfaces of the gate of the transistor with high resolution and measurement of the local thickness of the gate oxide. While previously, dual-axis electron tomography was employed for reducing missing wedge artifacts, our work demonstrates the potential of dual-axis tomography for improving the resolution of a tomographic reconstruction, even for structures not affected by missing wedge artifacts. By simulations and experiments we show the value of dual-axis tomography for characterization of nanoscale devices as an approach that requires no prior information and that can be easily extended even to more than two tilt axes.

Selenium segregation in femtosecond-laser hyperdoped silicon revealed by electron tomography

Doping of silicon with chalcogens (S, Se, Te) by femtosecond laser irradiation to concentrations well above the solubility limit leads to near-unity optical absorptance in the visible and infrared (IR) range and is a promising route toward silicon-based IR optoelectronics. However, open questions remain about the nature of the IR absorptance and in particular about the impact of the dopant distribution and possible role of dopant diffusion. Here we use electron tomography using a high-angle annular dark-field (HAADF) detector in a scanning transmission electron microscope (STEM) to extract information about the three-dimensional distribution of selenium dopants in silicon and correlate these findings with the optical properties of selenium-doped silicon. We quantify the tomography results to extract information about the size distribution and density of selenium precipitates. Our results show correlation between nanoscale distribution of dopants and the observed sub-band gap optical absorptance and demonstrate the feasibility of HAADF-STEM tomography for the investigation of dopant distribution in highly-doped semiconductors.

Specifications for hard condensed matter specimens for three-dimensional high-resolution tomographies

Tomography is a standard and invaluable technique that covers a large range of length scales. It gives access to the inner morphology of specimens and to the three-dimensional (3D) distribution of physical quantities such as elemental composition, crystalline phases, oxidation state, or strain. These data are necessary to determine the effective properties of investigated heterogeneous media. However, each tomographic technique relies on severe sampling conditions and physical principles that require the sample to be adequately shaped. For that purpose, a wide range of sample preparation techniques is used, including mechanical machining, polishing, sawing, ion milling, or chemical techniques. Here, we focus on the basics of tomography that justify such advanced sample preparation, before reviewing and illustrating the main techniques. Performances and limits are highlighted, and we identify the best preparation technique for a particular tomographic scale and application. The targeted tomography techniques include hard X-ray micro- and nanotomography, electron nanotomography, and atom probe tomography. The article mainly focuses on hard condensed matter, including porous materials, alloys, and microelectronics applications, but also includes, to a lesser extent, biological considerations.

Three-dimensional imaging of copper pillars using x-ray tomography within a scanning electron microscope: a simulation study based on synchrotron data

While microelectronic devices are frequently characterized with surface-sensitive techniques having nanometer resolution, interconnections used in 3D integration require 3D imaging with high penetration depth and deep sub-micrometer spatial resolution. X-ray tomography is well adapted to this situation. In this context, the purpose of this study is to assess a versatile and turn-key tomographic system allowing for 3D x-ray nanotomography of copper pillars. The tomography tool uses the thin electron beam of a scanning electron microscope (SEM) to provoke x-ray emission from specific metallic targets. Then, radiographs are recorded while the sample rotates in a conventional cone beam tomography scheme that ends up with 3D reconstructions of the pillar. Starting from copper pillars data, collected at the European Synchrotron Radiation Facility, we build a 3D numerical model of a copper pillar, paying particular attention to intermetallics. This model is then used to simulate physical radiographs of the pillar using the geometry of the SEM-hosted x-ray tomography system. Eventually, data are reconstructed and it is shown that the system makes it possible the quantification of 3D intermetallics volume in copper pillars. The paper also includes a prospective discussion about resolution issues.

Diffraction/scattering computed tomography for three-dimensional characterization of multi-phase crystalline and amorphous materials

The three-dimensional characterization method described herein is based on diffraction and scattering techniques combined with tomography and uses the variation of these signals to reconstruct a two-dimensional/three-dimensional structural image. To emphasize the capability of the method in discriminating between different poorly ordered phases, it is named diffraction/scattering computed tomography (DSCT). This combination not only allows structural imaging but also yields an enhancement of the weak signals coming from minor phases, thereby increasing the sensitivity of structural probes. This article reports the suitability of the method for discrimination of polycrystalline and amorphous phases and for extraction of their selective local patterns with a contrast sensitivity of about 0.1% in weight of minor phases relative to the matrix. The required background in tomography is given and then the selectivity of scattering signal, the efficiency of the method, reconstruction artefacts and limitations are addressed. The approach is illustrated through different examples covering a large range of applications based on recent literature, showing the potential of DSCT in crystallography and materials science, particularly when functional and/or precious samples with sub-micrometre features have to be investigated in a nondestructive way.

Multiscale measurements of residual strains in a stabilized zirconia layer

Residual stresses in a polycrystalline material have been determined experimentally at different length scales using three different techniques, with the aim of obtaining quantitative values. The polycrystalline material used is the electrolyte of solid oxide fuel cells, made of yttria-stabilized zirconia and submitted to a high biaxial compression stress state. Macroscopic measurements were performed using traditional X-ray diffraction with the sin2ψ method. Residual stresses within the grains were determined by the X-ray microdiffraction technique using synchrotron radiation. The variation in the strain within each grain was analysed by high-resolution electron backscatter diffraction. The results are self-consistent and give further information on the relation between strain/stress values and grain orientation, and on intragranular strain variations. These results are very important for the validation of mechanical microscopic constitutive equations.

"Compressed graphite" formed during C60 to diamond transformation as revealed by scattering computed tomography

The collapsing of C60 into polycrystalline diamond has been studied after nonhydrostatic pressurization at ambient temperature using x-ray scattering computed tomography. Using this selective structural probe we provide evidence of concentric coexistence of "compressed graphite" (d(00l)∼3.09-3.11  Å), sp2-graphitelike phase (d(00l)∼3.35-3.42  Å), and sp3-like amorphous carbon surrounding polycrystalline diamond (a∼3.56-3.59  Å). The so-called "compressed graphite" exhibits a collapsed c axis and is textured with disordered layers. This latter phase is better described as a short interlayered carbon phase with buckled sp2-sp3 layers with possible interlayer bonding. Additionally, our 3D maps of phase distribution and of the residual stress retained in the polycrystalline diamond phase support the importance of stressed synthesis conditions for diamond formation.

Status of the hard X-ray microprobe beamline ID22 of the European Synchrotron Radiation Facility

The ESRF synchrotron beamline ID22, dedicated to hard X-ray microanalysis and consisting of the combination of X-ray fluorescence, X-ray absorption spectroscopy, diffraction and 2D/3D X-ray imaging techniques, is one of the most versatile instruments in hard X-ray microscopy science. This paper describes the present beamline characteristics, recent technical developments, as well as a few scientific examples from recent years of the beamline operation. The upgrade plans to adapt the beamline to the growing needs of the user community are briefly discussed.

Hard X-ray diffraction scanning tomography with sub-micrometre spatial resolution: application to an annealed γ-U0.85Mo0.15particle

It is demonstrated that scanning X-ray diffraction tomography of heterogeneous and polycrystalline samples can provide real-space semi-quantitative three-dimensional structural information at a submicrometre spatial resolution. The capabilities of this technique are illustrated by the study of a slice of a spherical particle consisting of a UMo core (about 37 µm in diameter) surrounded by a UMoAl shell (5 µm thick). The technique allows precise characterization of the embedded UMo/UMoAl interface where the phases α-U (in the core), UAl2and U6Mo4Al43(in the shell) are found. Moreover, an unexpected phase (UC) is detected at a trace level. It is shown that the thickness of the UMoAl shell is locally anticorrelated with the amount of UC, suggesting that this phase plays a protective role in inhibiting thermally activated Al diffusion in UMo.

Monitoring microbial redox transformations of metal and metalloid elements under high pressure using in situ X-ray absorption spectroscopy

X-ray absorption spectroscopy is a well-established method for probing local structural and electronic atomic environments in a variety of systems. We used X-ray absorption near-edge structure (XANES) spectroscopy for monitoring in real-time conditions selenium reduction in situ in live cultures of Shewanella oneidensis MR-1 under high hydrostatic pressure. High-quality XANES data show that Shewanella oneidensis MR-1 reduces selenite Se(IV) to red elemental selenium Se(0) up to 150 MPa without any intermediate redox state. MR-1 reduces all selenite provided (5-10 mM) between 0.1 and 60 MPa. Above 60 MPa the selenite reduction yield decreases linearly with pressure and the activity is calculated to stop at 155 ± 5 MPa. The analysis of cultures recovered after in situ measurements showed that the decrease in activity is linked to a decrease in viability. This study emphasizes the promising potential of XANES spectroscopy for real-time probing in situ microbial redox transformations of a broad range of metal and metalloid elements in live samples, including under high hydrostatic pressure.

A new white beam x-ray microdiffraction setup on the BM32 beamline at the European Synchrotron Radiation Facility

A white beam microdiffraction setup has been developed on the bending magnet source BM32 at the European Synchrotron Radiation Facility. The instrument allows routine submicrometer beam diffraction to perform orientation and strain mapping of polycrystalline samples. The setup features large source to optics distances allowing large demagnification ratios and small beam sizes. The optics of the beamline is used for beam conditioning upstream a secondary source, suppressing any possible interference of beam conditioning on beam size and position. The setup has been designed for an easy and efficient operation with position control tools embedded on the sample stage, a high magnification large aperture optical microscope, and fast readout detectors. Switching from the white beam mode to the monochromatic mode is made easy by an automatic procedure and allows the determination of both the deviatoric and hydrostatic strain tensors.

Microstructural mapping of C60 phase transformation into disordered graphite at high pressure, using X-ray diffraction microtomography

An extended use of synchrotron-based X-ray diffraction microtomography (XRD-µCT) to study simultaneously the phase distribution and microstructure in phase-transformation processes is proposed. This three-dimensional non-invasive imaging approach has been applied to understand the phase transformation of C60 rhombohedral polymer (C60R) into disordered graphite (DG) at high pressure and high temperature. The heterogeneous sample was synthesized (5 GPa, 1100 K) using a Paris–Edinburgh cell and selective image reconstructions were achieved for all different phases present in this sample. The XRD-µCT analysis evidences elongated DG domains with a fiber texture where nested (002)DGplanes show ±70° preferential orientation relative to the compression axis. In contrast C60R domains are found to be small and spotty, preferentially in the middle of the sample. The parent and product phases are mutually interpenetrative and exhibit a crystallographic relationship. This study evidences that formation of (002)DGplanes occurs parallel to {111}C60Cpseudo-cubic planes. Among these four possible alignments, uniaxial pressure favors one [111]C60Cdirection. Transmission electron microscopy observations validate these nondestructive XRD-µCT results.

Cartilage collagen matrix reorientation and displacement in response to surface loading

An investigation of collagen fiber reorientation, as well as fluid and matrix movement of equine articular cartilage and subchondral bone under compressive mechanical loads, was undertaken using small angle X-ray scattering measurements and optical microscopy. Small angle X-ray scattering measurements were made on healthy and diseased samples of equine articular cartilage and subchondral bone mounted in a mechanical testing apparatus on station ID18F of ESRF, Grenoble, together with fiber orientation analysis using polarized light and displacement measurements of the cartilage matrix and fluid using tracers. At surface pressures of up to approximately 1.5 MPa, there was reversible compression of the tangential surface fibers and immediately subjacent zone. As load increased, deformation in these zones reached a maximum and then reorientation propagated to the radial deep zone. Between surface pressures of 4.8 MPa and 6.0 MPa, fiber orientation above the tide mark rotated 10 deg from the radial direction, with an overall loss of alignment. With further increase in load, the fibers "crimped" as shown by the appearance of subsidiary peaks approximately +/-10 deg either side of the principal fiber orientation direction. Failure at higher loads was characterized by a radial split in the deep cartilage, which propagated along the tide mark while the surface zone remained intact. In lesions, the fiber organization was disrupted and the initial response to load was consistent with early rupture of fibers, but the matrix relaxed to an organization very similar to that of the unloaded tissue. Tracer measurements revealed anisotropic solid and fluid displacement, which depended strongly on depth within the tissue.

A hard x-ray nanoprobe for scanning and projection nanotomography

To fabricate and qualify nanodevices, characterization tools must be developed to provide a large panel of information over spatial scales spanning from the millimeter down to the nanometer. Synchrotron x-ray-based tomography techniques are getting increasing interest since they can provide fully three-dimensional (3D) images of morphology, elemental distribution, and crystallinity of a sample. Here we show that by combining suitable scanning schemes together with high brilliance x-ray nanobeams, such multispectral 3D volumes can be obtained during a single analysis in a very efficient and nondestructive way. We also show that, unlike other techniques, hard x-ray nanotomography allows reconstructing the elemental distribution over a wide range of atomic number and offers truly depth resolution capabilities. The sensitivity, 3D resolution, and complementarity of our approach make hard x-ray nanotomography an essential characterization tool for a large panel of scientific domains.

High-resolution angular beam stability monitoring at a nanofocusing beamline

Two semi-transparent imaging beam-position monitors developed at the ESRF have been installed at the micro-analysis beamline ID22 for monitoring the angular stability of the X-ray beam. This system allows low-frequency (10 Hz) angular beam stability measurements at a submicroradian range. It is demonstrated that the incoming macro-beam angular fluctuations are one of the major sources of focal spot instabilities downstream of the Kirkpatrick-Baez mirrors. It is also shown that scanning the energy by rotating the so-called fixed-exit monochromator induces some unexpected angular beam shifts that are, to a large extent, deterministic.

Probing the structure of heterogeneous diluted materials by diffraction tomography

The advent of nanosciences calls for the development of local structural probes, in particular to characterize ill-ordered or heterogeneous materials. Furthermore, because materials properties are often related to their heterogeneity and the hierarchical arrangement of their structure, different structural probes covering a wide range of scales are required. X-ray diffraction is one of the prime structural methods but suffers from a relatively poor detection limit, whereas transmission electron analysis involves destructive sample preparation. Here we show the potential of coupling pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The demonstration is carried out on a high-pressure pellet containing several carbon phases and on a heterogeneous powder containing chalcedony and iron pigments. The present method enables a non-invasive structural refinement with a weight sensitivity of one part per thousand. It enables the extraction of the scattering patterns of amorphous and crystalline compounds with similar atomic densities and compositions. Furthermore, such a diffraction-tomography experiment can be carried out simultaneously with X-ray fluorescence, Compton and absorption tomographies, enabling a multimodal analysis of prime importance in materials science, chemistry, geology, environmental science, medical science, palaeontology and cultural heritage.

Mechanical and structural characterisation of completely degradable polylactic acid/calcium phosphate glass scaffolds

This study involves the mechanical and structural characterisation of completely degradable scaffolds for tissue engineering applications. The scaffolds are a composite of polylactic acid (PLA) and a soluble calcium phosphate glass, and are thus completely degradable. A factorial experimental design was applied to optimise scaffold composition prior to simultaneous microtomography and micromechanical testing. Synchrotron X-ray microtomography combined with in situ micromechanical testing was performed to obtain three-dimensional (3D) images of the scaffolds under compression. The 3D reconstruction was converted into a finite element mesh which was validated by simulating a compression test and comparing it with experimental results. The experimental design reveals that larger glass particle and pore sizes reduce the stiffness of the scaffolds, and that the porosity is largely unaffected by changes in pore sizes or glass weight content. The porosity ranges between 93% and 96.5%, and the stiffness ranges between 50 and 200 kPa. X-ray projections show a homogeneous distribution of the glass particles within the PLA matrix, and illustrate pore-wall breakage under strain. The 3D reconstructions are used qualitatively to visualise the distribution of the phases of the composite material, and to follow pore deformation under compression. Quantitatively, scaffold porosity, pore interconnectivity and surface/volume ratios have been calculated. Finite element analysis revealed the stress and strain distribution in the scaffold under compression, and could be used in the future to characterise the mechanical properties of the scaffolds.

Regional variations of collagen orientation in normal and diseased articular cartilage and subchondral bone determined using small angle X-ray scattering (SAXS)

OBJECTIVE

To determine regional differences in the orientation of collagen in the articular cartilage of the equine metacarpophalangeal joint as well as describing cartilage orientation in lesions using small angle X-ray scattering (SAXS).

DESIGN

SAXS diffraction patterns were taken at the European Synchrotron Radiation Facility (ESRF), with increasing depth into cartilage and bone cross sections. Results for healthy samples were taken at different regions along the joint which receive different loads and differences in collagen orientation were determined. Results were also taken from diseased samples and the collagen orientation changes from that of healthy samples observed.

RESULTS

Regions subject to low loads show a lower degree of orientation and regions exposed to the highest loads possess oriented collagen fibres especially in the radial layer. In early lesions the orientations of the collagen fibres are disrupted. Subchondral bone fibres are twisted in regions where the joint receives shear forces. Changes in fibre orientation are also observed in the calcified cartilage even in regions where the cartilage is intact. In more advanced lesions where there is loss of cartilage the fibres in the calcified layer are realigned tangential to the surface.

CONCLUSIONS

Regional variations in collagen arrangement show that the highly ordered layers of the articular cartilage are the most important elements in supporting high variable loads. In lesions changes occur in the deep tissue whilst the overlying cartilage appeared normal. We therefore suggest that the interface region is a key element in the early stages of the disease.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed ( approximately 180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Mineralogy and petrology of comet 81P/Wild 2 nucleus samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Comet 81P/Wild 2 Under a Microscope

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

ID22: a multitechnique hard X-ray microprobe beamline at the European Synchrotron Radiation Facility

The ID22 beamline is dedicated to hard X-ray microanalysis allowing the combination of fluorescence, spectroscopy, diffraction and tomography techniques in a wide energy range from 6 to 70 keV. The recent installation of an in-vacuum undulator, a new sample stage and the adaptation of various focusing optics has contributed to a great improvement in the capabilities of the beamline, which is now accessed by a wide user community issued from medical, earth and environmental science, archaeology and material science. Many applications requiring low detection limits for localization/speciation of trace elements together with structural analysis have been developed at the beamline on the (sub)micrometer scale. The possibility of combining simultaneously different analytical probes offers the opportunity of a thorough study of a given sample or scientific problem. This paper presents a review of the recent developments of the beamline and a detailed description of its capabilities through examples from different fields of applications.

Quantification of Single Fluid Inclusions by Combining Synchrotron Radiation-Induced μ-X-ray Fluorescence and Transmission

Fluid inclusions represent the only direct samples of ancient fluids in many crustal rocks; precise knowledge of their chemical composition provides crucial information to model paleofluid-rock interactions and hydrothermal transport processes. Owing to its nondestructive character, micrometer-scale spatial resolution, and high sensitivity, synchrotron radiation-induced micro-X-ray fluorescence has received great interest for the in situ multielement analysis of individual fluid inclusions. Major uncertainties associated with the quantitative analysis of single fluid inclusions arise from the inclusion depth and the volume of fluid sampled by the incident beam. While the depth can be extracted directly from the fluorescence spectrum, its volume remains a major source of uncertainty. The present study performed on natural and synthetic inclusions shows that the inclusion thickness can be accurately evaluated from transmission line scans. Experimental data matched numerical simulations based on an elliptical inclusion geometry. However, for one nonelliptical inclusion, the experimental data were confirmed using a computed absorption tomography reconstruction. Good agreement between the imaging and scanning techniques implies that the latter provides reliable fluid thickness values independent of the shape of the inclusion. Taking into consideration the incident angle, the incident beam energy, the inclusion fluid salinity, and the transmission measurement stability resulted in errors of 0.3-2 microm on calculated fluid inclusion thicknesses.