A dedicated superbend x-ray microdiffraction beamline for materials, geo-, and environmental sciences at the advanced light source

A dedicated superbend x-ray microdiffraction beamline for materials,geo-, and environmental sciences at the advanced light source

Martin Kunz,1,a� Nobumichi Tamura,1 Kai Chen,1,2 Alastair A. MacDowell,1

Richard S. Celestre,1 Matthew M. Church,1 Sirine Fakra,1 Edward E. Domning,1

James M. Glossinger,1 Jonathan L. Kirschman,1 Gregory Y. Morrison,1 Dave W. Plate,1

Brian V. Smith,1 Tony Warwick,1 Valeriy V. Yashchuk,1 Howard A. Padmore,1 andErsan Ustundag3

1Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA2Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095, USA3Materials Science and Engineering, Iowa State University, 3273 Gilman Hall, Hoover Ames,Iowa 50011, USA

�Received 10 December 2008; accepted 14 February 2009; published online 24 March 2009�

A new facility for microdiffraction strain measurements and microfluorescence mapping has beenbuilt on beamline 12.3.2 at the advanced light source of the Lawrence Berkeley NationalLaboratory. This beamline benefits from the hard x-radiation generated by a 6 T superconductingbending magnet �superbend�. This provides a hard x-ray spectrum from 5 to 22 keV and a fluxwithin a 1 �m spot of �5�109 photons /s �0.1% bandwidth at 8 keV�. The radiation is relayedfrom the superbend source to a focus in the experimental hutch by a toroidal mirror. The focus spotis tailored by two pairs of adjustable slits, which serve as secondary source point. Inside the leadhutch, a pair of Kirkpatrick–Baez �KB� mirrors placed in a vacuum tank refocuses the secondaryslit source onto the sample position. A new KB-bending mechanism with active temperaturestabilization allows for more reproducible and stable mirror bending and thus mirror focusing.Focus spots around 1 �m are routinely achieved and allow a variety of experiments, which have incommon the need of spatial resolution. The effective spatial resolution ��0.2 �m� is limited by aconvolution of beam size, scan-stage resolution, and stage stability. A four-bounce monochromatorconsisting of two channel-cut Si�111� crystals placed between the secondary source and KB-mirrorsallows for easy changes between white-beam and monochromatic experiments while maintaining afixed beam position. High resolution stage scans are performed while recording a fluorescenceemission signal or an x-ray diffraction signal coming from either a monochromatic or a whitefocused beam. The former allows for elemental mapping, whereas the latter is used to producetwo-dimensional maps of crystal-phases, -orientation, -texture, and -strain/stress. Typically achievedstrain resolution is in the order of 5�10−5 strain units. Accurate sample positioning in the x-rayfocus spot is achieved with a commercial laser-triangulation unit. A Si-drift detector serves as ahigh-energy-resolution ��150 eV full width at half maximum� fluorescence detector. Fluorescencescans can be collected in continuous scan mode with up to 300 pixels/s scan speed. A charge coupleddevice area detector is utilized as diffraction detector. Diffraction can be performed in reflecting ortransmitting geometry. Diffraction data are processed using XMAS, an in-house written softwarepackage for Laue and monochromatic microdiffraction analysis. © 2009 American Institute ofPhysics. �DOI: 10.1063/1.3096295�

I. INTRODUCTION

The advanced light source �ALS� is a relatively low en-ergy �1.9 GeV�, third generation synchrotron optimized forthe production of vacuum ultraviolet and soft x-ray lightfrom undulators. However, local demands chiefly from theprotein crystallography community required the developmentof hard x-ray sources at the facility. As a result, three 6 Tsuperconducting bending magnets replaced three 1.2 T regu-lar bending magnets.1,2 This resulted in a shift in the criticalenergy for these three sources from 3 to 12 keV allowing forthe development of various hard x-ray programs. The suc-

cess of the initial x-ray microdiffraction program on the lim-ited energy range provided by the warm bending magnet ofbeamline 7.3.3 motivated the move of this program to one ofthe three superbend sources. The recent emergence of x-raymicrodiffraction techniques is linked to the increased avail-ability of high brilliance synchrotron sources as well asprogress in x-ray focusing optics that nowadays allows ob-taining hard x-ray beams with full width at half maximum�FWHM� size in the order of a few tens of nanometers.3–6

X-ray microdiffraction is performed both in polychromaticand monochromatic modes.

Synchrotron polychromatic scanning x-ray microdiffrac-tion is a technique that proves to be useful for studying me-chanical properties of materials at length scales of the so-a�Electronic mail: [email protected].

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called mesoscale �100 nm–10 �m�, i.e., in the range of thesize of the constitutive grains and defect interactions. Thetechnique is based on raster scanning the sample with ahighly focused �submicron� white synchrotron x-ray beam.At each step a two-dimensional �2D� diffraction pattern�Laue pattern� is recorded. The proper analyses of the result-ing arrays of Laue patterns allow for reconstruction of mapsof grain orientation, strain/stress, and dislocation density.The spatial resolution of such maps is limited by the x-raybeam spot size as well as the step size used during the datacollection.

This technique has been applied to study the interactionsbetween grains during a tensile loading of a free-standing Alfilm.7 It was also used to show that grains deform plasticallyat a very early stage of electromigration in Al and Cuinterconnects.8,9 A third example for the successful applica-tion of microdiffraction is the demonstration of grain rotationeffects in Sn lines subject to electromigration, which in turnexplains their sudden resistivity drop.10 Polychromatic x-raymicrodiffraction is complementary to monochromatic x-raymicrodiffraction. Monochromatic scanning x-ray microdif-fraction is mainly used for phase identification through pow-der diffraction patterns in highly heterogeneous samples withsubmicron grain size. Most of these applications are found ingeological, environmental, or archeological studies.11–13

X-ray microdiffraction offers important advantages overcharged particles techniques such as electron microscopy andfocused ion beam microscopy in that the radiation used ismore penetrating and probes bulk and buried materials atdepths of 10 �m to 1 mm, depending on chemical compo-sition of the sample investigated and the available x-ray en-ergy range. Also, the strain and grain orientation measure-ments with this technique provide for better resolution thanthose obtained by charged particle techniques.

There are a few drawbacks to this technique. The firstone is its low availability. This is addressed by the ongoingdevelopment of new dedicated microdiffraction beamlines atsynchrotron sources around the world. Beamlines offeringmicron to submicron beam sizes are now available at severalsynchrotron facilities, including the ALS �Berkeley, USA�,APS �Chicago, USA�, ESRF �Grenoble, France�, PLS �Po-hang, South-Korea�, and SLS �Villigen, Switzerland� andsome are in construction as at the CLS �Saskatoon, Canada�and in the planning phase as at SOLEIL �Paris, France� andthe Australian Light Source �Melbourne, Australia�.

A second drawback is a possible limitation in flux, whichvaries between facilities. Undulators are perfectly suited formonochromatic experiments but are not ideal for white beamexperiments due to their highly structured energy spectrum.While white beam experiments are feasible to some extentwith undulators by tapering the gap or aligning them off-axis, they are most easily performed on wiggler or bendingmagnet beamlines. The advent of superconducting bendingmagnets, as will be described in this paper, represents thusimportant progress for this technique.

A third drawback in the case of white beam Laue experi-ments is the complexity of the analysis required to extractmeaningful information, such as crystal orientation, strainand stress tensors, or defect densities. Deformed samples of-

ten exhibit diffraction peaks with shapes far from idealLorentzian profiles. Instead, they are streaked in various di-rections, which make them substantially more difficult toanalyze than e.g., Kikuchi patterns from electron backscatter-ing diffraction. Also, several grains can simultaneously sat-isfy the Bragg condition in thicker samples with high pen-etration depth or in the vicinity of grain boundaries. Theseproblems can be overcome if depth resolved Laue patternsare available.

Another issue being addressed is spatial resolution, sinceit is much harder to focus hard x-rays than it is for chargedparticles. However, continued progress in the design and fab-rication of x-ray optics such as Kirkpatrick–Baez �KB� mir-rors is improving the resolution into the nanometer range. Infact, currently achievable x-ray focus spots are in the rangethat resolution is not so much limited by the dimension ofthe focus spot but much more by residual vibrations in the100 nm range of the sample stage assemblies.

In this work we describe a new facility for x-ray micro-diffraction and x-ray microfluorescence, which has been builton beamline 12.3.2 at the ALS of the Lawrence BerkeleyNational Laboratory. Section II describes the characteristicsof the source. The details of the beamline design and thecontrol system are presented in Secs. III and IV, respectively.Beamline performance and future plans are discussed inSecs. V and VI.

II. SOURCE

The three 6 T superconducting bending magnets havebeen operating continuously since their installation in thering lattice in 2002. They have been transparent to the usersand can be viewed as a technical success that allows the ALSto have inexpensive hard x-ray bending magnet sources andthe development of the associated hard x-ray programs. Thedesign allows for four beamlines per magnet, with inboardand outboard pairs of tangent points at field strengths of 4.37and 5.29 T, respectively. These fields increased the criticalenergy from 3 keV for a 1.27 T normal conducting magnet to11.5 and 12.7 keV, respectively. The microdiffraction beam-line described in this work has a source critical energy of11.5 keV. The beam size in the ALS is small, due to the smallemittance of the electron beam �6.3 nm rad �h�; 0.13 nm rad�v�� and the small beta functions �0.95 m �h�; 1.5 m �v��.Together with the small dispersion at the dipole position�dispersion=0.57 m; slope of dispersion=−0.04�, this yieldselectron beam sizes of 230 �m �h� by 35 �m �v� �FWHM�,which is the source size for this beamline.

III. BEAMLINE DESIGN

The design of the beamline had to satisfy the followingrequirements. �1� Easy switch from white beam conditions toa monochromatic setup while accurately maintaining thebeam position. �2� The monochromator needs to be able toscan in energy with a constant beam position without drop-ping off the rocking curve. �3� The sample needs to bescanned through the beam in two directions with an accuracyof at least 100 nm.

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The layout for this beamline �beamline 12.3.2 in ALSnomenclature� is depicted schematically in Fig. 1. It consistsof the source, a horizontally deflecting toroidal M1-mirror�grazing angle 3.5 mrad�, a pair of roll slits defining the sizeof the virtual secondary source, a four-bounce two-channel-cut Si�111� monochromator, a second pair of slits �from JJ-x-ray, Denmark� serving as aperture, a set of KB mirrors, thesample stage, and two detectors. The acceptance of the M1-mirror is limited to 0.2 mrad by a water-cooled aperture. Thedistances from the source for this beamline are 13, 22.4, and24.8 m for M1, secondary source slits, and sample, respec-tively. Limitations by the shield wall and the short distanceof the beamline prevent the placement of the M1-mirror atthe usual unity magnification position, where aberrations areat a minimum. Instead, its actual placement causes a demag-nification of the source by a factor of 0.723 at the secondarysource position. The sagittal acceptance of M1 is small, suchthat aberrations due to the M1 demagnification are small thusreducing the brightness by only 10% from the 1:1 condition.The horizontal and vertical focusing KB mirrors are posi-tioned at 0.27 and 0.135 m upstream of the sample, respec-tively. This results in demagnification factors of 7.89 �h� and16.78 �v�, respectively.

All elements downstream from and including the sec-ondary source are enclosed in an interlocked lead hutch �2mm lead between two 1.9 mm steel plates�. Monochromator,aperture slits, and KB mirrors are further contained in analuminum box, which is kept at a vacuum of about10−5 mbar. The vacuum is maintained by combining a dryroughing pump with a turbo pump. To avoid temperaturerelated shifts of the sample with respect to the beam due tothe thermal expansion of the various beamline components,the temperature inside the hutch is maintained constantwithin 0.2 °C.

A. Optics

1. Primary focusing mirror

The silicon M1 mirror has a length of 700 mm, a widthof 75 mm, and a thickness of 75 mm and is coated with25 nm of Pt under 8 nm of Rh. The additional Rh layersuppresses the Pt absorption edges between 10 and 13 keVbut maintains high reflectivity at higher energies as thesex-rays reflect from the Pt layer. The mirror can be adjusted inpitch, yaw, and two bends �upstream and downstream�. Ther-mal load calculations indicate that with such a configurationno cooling of the mirror is required as the total power ab-sorbed by the mirror will only be 1.77 W. In practice we findthe M1 to be stable with a beam position stability �10% ofthe source size. The size of the primary focus point �second-ary virtual source� is controlled by two pairs of water-cooledtungsten rods orientated in the horizontal and vertical, re-spectively. Each pair of rods is separated by a 2 mm gapthrough which the beam passes. The rods are mounted on arotary stage, which, when turned closes down this 2 mm gapto micron dimensions required to define the secondarysource. This is the first element within the lead hutch. Thesize of the secondary source size has been calculated to be175 �m �h� by 30 �m �v�. Measurements show slightlylarger beam spot of 190 �m �h� by 36 �m �v� at the rollslits. In order to achieve a final focus spot of 1�1 �m withthe present geometry, the secondary source has to be slitteddown to 8 �m �h� to 16 �m �v�. This causes a loss in fluxby a factor of �50 relative to the flux accepted by the M1mirror.

2. Kirkpatrick-Baez mirrors

A pair of bendable mirrors in KB configuration providesthe microfocus spot. The mirror substrates are made of asuper polished plane single crystal of silicon of 102 mm�length��8–13 mm �width��4 mm �thickness�. The sub-strates are coated with 25 nm Pt under 8 nm Rh. The sub-strate width has been carefully profiled wider in the middleto correct for higher order errors between the desired ellipti-cal shape and that achievable by simple beam bendingalone.14 Each mirror has 4 degrees of freedom: one lineartranslation to position the mirror relative to the beam, onepitch to adjust the incident angle and thus focal length, andtwo bends, one at each end, to set the elliptical curvature ofthe mirrors. Each degree of freedom can be remotely drivenwith picomotors. Absolute position feedback is provided by alinear variable differential transformer on each of the pico-motors. An additional picomotor applied to the vertical KBmirror is used to correct the orthogonality between the twomirrors. In order to stabilize the mirror temperature, the tem-perature of the mirror bases is controlled with a Peltier mod-ule. The heat flows from the ends of the mirror substratethrough the flexural assembly to the Peltier thus stabilizingthe substrate temperature and minimizing undesirable strainsthat heat straps might induce. Temperature of the mirrorsinside the vacuum tank is maintained constant within�0.1 °C. The mirrors are mounted on partially constrainedflexural supports allowing one end to float longitudinally toadjust for mirror length changes and the other end to twist

FIG. 1. Schematic layout of the ALS beamline 12.3.2 with a superbenddipole magnet source. The beamline acceptance is 0.2 mrad �horizontal andvertical�. The toroidal M2 mirror focuses the source onto a virtual object.This intermediate image is demagnified by a ratio of 8:1 and 16:1 in hori-zontal and vertical, respectively, by a pair of KB mirrors. A four-bounceSi�111� monochromator can optionally be brought into the beam path formonochromatic microdiffraction on nanosized grains.

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axially to remove twist from the longitudinal axis. Picomo-tors apply moments to the ends of the mirror via a weak leafspring pulled by a thin flexible wire to demagnify actuatormotion and minimize the effects of thermal expansion andoff axis motions of the linkage assembly. The mirrors werebent to desired shape to an accuracy of �0.5 �rad with theaid of a long-trace-profiler in the optical metrologylaboratory15 prior to installation at the beamline.

In addition, a simple system for quick online-optimization of bend and pitch based on the Hartmann16 testhas been developed. A Hartmann test applied to the reflectivefocusing optics of the KB mirror works such that differentparts of the KB mirror are successively illuminated and thechanges of the spot position at the focal point are recorded asa function of the illuminated part. We expect an error in themirror pitch to be expressed by a linear dependence of thespot position on the illuminated path. An erroneous shape ofthe mirror stemming from nonoptimal bend setting, on theother hand, should lead to a curvature in the plot. We use theaforementioned high precision aperture slits �JJ X-Ray�closed to a size of 5 �m and placed �10 cm upstream theKB assembly to act as a pinhole that can be translated verti-cally and horizontally within the acceptance area of the KBfocusing system. The position of the doubly reflected beamat the focal point is recorded using a device consisting of ascintillator placed at the focal point and a high magnificationcharge coupled device �CCD� camera. The scintillator thatconverts x-ray into visible light is a 1 mm thick piece ofsingle crystal CdWO4. The CCD camera capturing the beamimage on the scintillator is equipped with a zoom lens �Ed-mund Optics VZM-450�. Aluminum foils need to be placedupstream of the scintillator to attenuate the x-ray beam inorder to prevent saturation of the signal on the scintillator.We found that it is necessary to precisely focus the lens ontothe scintillator to minimize errors on the beam position. Thecamera is therefore mounted on a remotely controlled stage.With this technique, mirror figure errors of 0.2 �rad can bemeasured and if they stem from wrong bend settings can becorrected for. A software code developed in-house ALS �Ref.17� is used to optimize the bends with only a few iterations.Figure 2 shows the result of an online Hartmann scan afterpitch and bend optimization for different bend configura-

tions. The horizontal line indicates that the focused spot doesnot move as the mirror surface is probed by means of track-ing the beam across the mirror surface. Deviation from ahorizontal line is a slope error plot of the mirror surface.

3. Monochromator

In order to be able to easily switch between white andmonochromatic modes, a four-bounce Si�111� monochro-mator was installed upstream of the aperture slits and KBmirror pair. Figure 3 gives basic dimensions of the mono-chromator. The monochromator consists of two channel cutSi�111� crystals mounted on separate rotation stagesequipped with Reinshaw optical linear encoders. The stagesare rotated through a sine-bar mechanism with two linearactuators pushing against a steel bar attached tangentially tothe rotary stages. The crystals are mounted on the rotarystages, such that the rotation axes lie in the diffracting planeof the second and third crystal, respectively �Fig. 3�. In thisgeometry, the coordinated but opposite rotation of the mono-chromator crystals allows for energy change while keepingthe beam at a constant position, which is identical to thewhite beam position. The first crystal is cooled through aPeltier element, which in turn is water cooled. Measurementsshow a slight energy dependence of the angular settings forthe two crystals, which is different for both crystals. Thisresults in a linear shift of the energy calibration with energy,as well as a linear shift of the angular offset of crystal-2relative to crystal-1 �rocking curve offset�. Both effects arerelated to the nonplanarity of the diffracting surface. This iscaused by nonhomogeneous etching when the �recycled�crystals were prepared for installation in beamline 12.3.2.Since both effects are linear with angle and seem to be con-stant over time, they are corrected for by calibrating the rel-evant correction factors and implementing them into the con-trol software. The nonplanarity of the monochromatorsurfaces also leads to a reproducible small vertical offset ofthe monochromatic beam, relative to the white beam, of2 �m. This error is compensated for by the precision samplestage described next.

B. Sample stage

The sample is positioned on a high-precision stage con-sisting of eight different motorized stages as indicated in Fig.

FIG. 2. �Color online� Results of an online Hartmann test on the horizontalKB mirror. The variously colored lines refer to different bend settings. Thered line corresponds to the optimized setting and is depicted with the verti-cal scale in microrad slope error in the inset.

FIG. 3. Schematic of monochromator with basic measures, elevation, andcross section. The channel cut Si crystal has an additional groove cut intothe lower crystal to allow for passing the direct white beam when the crys-tals are set to 0°.

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4. The lower xyz stage allows for aligning the rotation axes�� and �� onto the x-ray focal point. The upper xyz stagesallow to place the sample onto the center of rotation and toscan the sample through the x-ray beam. Since the beamlineis designed for high spatial resolution with an x-ray focusspot below 1 �m, the scanning stages need to be more pre-cise than this. We use two linear microstepping stages �Mi-cos PLS-85� with 50 nm resolution Heidenhain linear encod-ers to fulfill these requirements. The � partial arc allows foreasy change between reflection ��=45°, CCD center=90°�and transmission mode ��=0°, 0° �CCD center�90°�. Thesample is viewed through an Infinity K2 long working dis-tance microscope with variable magnification and a videocamera. The maximum magnification of the viewing systemis 1.6 �m /CCD-pixel.

The sample is accurately positioned on the previouslycalibrated �knife edge scans� x-ray focus point by means of acommercial laser triangulation unit �Keyence LK-G152�.The system works such that the diffuse image of the laserspot �0.95 mW 650 nm class II diode laser� is imaged onto alinear CCD unit. The relative positions of the CCD unit andthe laser-emitting diode are fixed and known. The position ofthe laser image on the CCD unit thus measures the anglebetween the incident laser beam and the diffusely reflectedimage. This in turn determines the distance from the unit tothe image forming surface. The system allows for position-ing with an accuracy of below �1 �m, i.e., about two or-ders of magnitude better than required by the convergence ofthe x-ray beam �2.6 mrad vertical, 1.3 mrad horizontal�.

The absolute distance between sample and diffractiondetector is additionally calibrated using a Si-diffraction stan-dard single crystal. The precisely known cell parameters al-low for calibration of sample to detector distance togetherwith other instrument parameters such as detector tilt and thecenter of the CCD detector. Since the calibration is doneusing a quasiperfect crystal of Si �thickness �500 �m�, thepenetration of the x-rays into the crystal and its effect on thecalibration are negligible. Tests on crystals of different thick-nesses and with or without penetration correction did notshow any significant difference in the calibrated parameters.

Vibration control of the experimental stage is critical.This is always a problem for experimental stations at syn-chrotron sources as the source is fixed and the usual vibra-

tion isolating pneumatic supports lack adequate positionalstability to be used for vibration isolation. As a result thisexperimental station is mounted hard from the floor. Theoptical layout within the hutch, sample stage, and CCD go-niometer are mounted on a regular optical bench, which inturn is mounted hard off the floor. The usual design consid-erations are employed whereby items are designed to be stiffand of low mass to drive the resonant frequencies to highvalues where amplitude excursions are less. Considering this,the vibrational modes of the present stage configuration weredetermined using a commercial accelerometer �SignalCalcMobilyzer� as well as a laser Doppler shift vibrometer �Poly-tec Inc.; OFV-552 Fiber-Optic Interferometer�. Both, accel-erometer and vibrometer, can only detect vibrations betweenthe different components of the beamline but not vibrationsof the beam itself. In addition, the intensity variation of thex-ray beam with a knife-edge �positioned on the samplestage� shadowing 50% of the beam profile was frequencyanalyzed. This gives us the frequency distribution of thestage-vibration relative to the x-ray beam. Vibrations mea-sured with this arrangement show strong horizontal variationwith main frequencies at 15–30, 40, 45, and 90 Hz �Fig. 5�.These measurements are the most meaningful as they inte-grate all the vibrations as seen by the sample with respect tothe beam. Of these modes, the 40 and 45 Hz mode seem tobe connected to vibrations on the sample stage as they arealso present in the accelerometer measurements performedon the sample stage but absent or weak on measurementsmade on the KB box or the roll slits stand. The 25 Hz modeis present in all measurements made on the stages, KB box,and roll slits indicating that it is most likely associated withvibrations of the ALS experimental floor. The 90 Hz contri-butions seem to be linked to movements of the roll slits,which translates into beam motion at sample stage and isonly present in measurements performed on the roll slits andthe knife edge. Of all the frequencies, the largest amplitudescome from the 40 and 45 Hz associated with the samplestage in the horizontal direction, meaning that the stage isswinging in the x-direction. Mechanical coupling of samplestage to the KB box considerably reduces the magnitude ofthe vibration, but the FWHM of the measured spot size re-mained unchanged.

The amplitude of the vibration was measured using afast camera. These results show a FWHM of the vibrational

FIG. 4. Schematic sketch of the sample stage. Two stacked xyz-stages allowfor centering the sample onto the rotation axes �� ,�� and the rotation axesonto the focus spot.

FIG. 5. Horizontal vibrational spectrum as measured from the intensityvariation induced by a knife edge placed on the sample stage in the middleof the beam. Note the prominent modes at 40 and 45 Hz mainly caused byhorizontal vibrations of the sample stage.

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amplitude around 0.5 �m. The spot size limit is presentlyaround 1 �m and thus not limited by the vibrations of thesystem.

C. Detectors

The beamline is equipped with two detectors, namely, aMAR �now Rayonix� 133 x-ray CCD and a Si-drift detector.The MAR133 is used to record the polychromatic and mono-chromatic diffraction patterns. It has a 133 mm diameter ac-tive area, which is tapered down to a 2k�2k chip with asingle fiber-optic system. By default, the CCD is used in a2�2 rebinned 1k�1k mode as a best compromise betweenreadout time and resolution. The CCD detector is mountedon a Huber 440 stage with its rotation axis mounted horizon-tally. This allows placing the detector at various vertical two-theta angles between 0° and 90°. Careful commissioningmeasurements of refined sample-to-detector distances as afunction of two-theta angle of the CCD detector found thecenter of the two-theta arm to be about 3.5 mm upstream and1.3 mm below the calibrated x-ray focus spot. This results ina change of the sample to detector distance. Since this offsetis constant it can be calibrated and variations in sample todetector distance with changing detector angle can be cor-rected for without the need to record a separate calibrationpattern for each individual detector setting.

The Si-drift detector �Vortex-EM by SII NanotechnologyInc.� is used for x-ray fluorescence elemental mapping,which in turn can also be used to precisely locate the focusedx-ray on the sample, provided a marker with appropriate di-mension and fluorescence line is present on the sample sur-face. The detector has a 50 mm2 single element active area,which is capable to handle count rates up to 600 kcps at aresolution of �150 eV at 6 keV. It is mounted on a transla-tion stage, which allows for remote distance adjustment andthus additional increase in the dynamic range.

IV. CONTROL SYSTEM AND SOFTWARE

Beamline components �including the fluorescence detec-tor� and MAR133 CCD are controlled from different comput-ers and thus control systems. The beamline control is basedon LABVIEW running on a Windows platform. This system isused on most ALS beamlines, which allows quickly transfer-ring software developments on a given beamline to otherstations. At the same time, beamline specific applications canbe added to the base software in a modular way. For themicrodiffraction beamline a fast fluorescence mapping soft-ware as well as a 2D video scanning routine has been added.The former allows for very rapid collection of fluorescencemaps with several 105 map points. The latter is essential for2D scanning of video captured images of the x-ray beam fore.g. mirror bend optimization and Hartmann tests.

The x-ray CCD is controlled with MAR proprietary soft-ware running on a Linux station. For diffraction scans, anIDL and Fortran based, in-house written control software�XMAS� interfacing the CCD control with the LABVIEW motorcontrol has been added.18 Besides acting as interface in thediffraction scan process, XMAS is also the primary data re-duction and analysis software used on the beamline. It allows

for system calibration �sample-to-detector distance, detectortilt angles, detector center�, background correction, indexing�provided relative cell parameters are known�, as well asstrain and stress tensor refinements. Depending on data qual-ity and size and symmetry of unit cells in question, up to 100individual grains can be differentiated and indexed in a givenpattern. XMAS can be used in an interactive file-by-file modeor in an automatic mode where several thousands of diffrac-tion files scan can be sequentially analyzed automatically.While XMAS is designed for Laue pattern analysis, it can alsobe used for monochromatic powder diffraction data, where itserves to compare recorded powder diffraction patterns withknown phases. A cluster-based version of XMAS has alsobeen developed running on a 24 nodes dual processors Linuxcluster.

V. BEAMLINE PERFORMANCE

Beamline 12.3.2 is fully operational since summer 2007and accepts user proposals since August 2007. While a mini-mal spot size of 0.6 �m �h� �0.5 �m �v� �FWHM� wasachieved in the initial commissioning period, user experi-ments are presently run with a �1�1 �m2 beam. This spotsize can be reproduced on a routine basis and seems not todegrade over the time-period observed. The initial decline ofthe mirror performance is probably due to the degradation ofthe mirror coatings due to the fact that for practical reasons,many of the test runs during the initial commissioning periodwere performed with nitrogen flow instead of vacuum insidethe enclosure. Spot sizes are measured by scanning atungsten-knife edge through the beam at the focal point andmeasuring the change in recorded x-ray intensities with aPIN diode. Figure 6 shows the results of spot size measure-ment obtained with a 150 �m diameter tungsten wires usedas a knife edge. Beam intensities were collected via a cali-brated PIN diode.

The calculated and measured fluxes arriving at thesample are shown in Fig. 7. The electron source used has anenergy of 1.9 GeV within a field of 4.37 T at the tangentpoint. The absolute flux was measured by using an ion cham-ber �IC�. The IC has a total length of 65 mm, an active length

FIG. 6. Measured spot shape on beamline 12.3.2. The spot size is measuredby scanning a 0.15 mm thick W-wire through the beam and recording theabsorption. The derivative of this measurement gives the spot shape. TheFWHM of the horizontal and vertical focus spots are 0.63 and 0.50 �m,respectively.

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of 50 mm, and is filled with 1 bar of Ar gas. The currentmeasured in the IC is converted into number of x-ray pho-tons, using a conversion factor of 26 eV of energy perelectron—ion pair.19 The result of the flux measurements iscompared against the expected values in Fig. 7. The calcu-lated total flux from the Si�111� crystals is about an order ofmagnitude higher than the one measured. The reason for thisdiscrepancy is still under investigation. We suspect the afore-mentioned etching induced irregularities on the monochro-mator planes to induce walk-off effects on the individualplanes thus affecting the monochromator efficiency. For thetime being, the two individual channel-cut crystals cannot betuned onto each other online with respect to their position.Subtle alignment errors can have significant effects in com-bined reflectivity, especially at high energies, i.e., low angles.

With the achieved spot size in combination with thehigh-resolution x-y scanning stage and the energy spectrumof the superconducting bending magnet, strain resolutions of5�10−5 strain units at a spatial resolution of 0.2 �m areroutinely achieved. Such strain resolution is achieved by av-eraging out the errors on peak positions over the whole Lauediffraction patterns. The beam convergence on the sample is1.3 �h� and 2.6 �v� mrad, respectively, so that each diffractionpeak spreads over several pixels on the detector, allowing toobtain subpixel resolution on peak position by 2D peak fit-ting and/or centroiding. The spatial distortion is correctedwithin the MAR acquisition software and shown to be insig-nificant after correction �as tested on a piece of perfect sili-con crystal�. Note that scanning a 1 �m beam in submi-crometer steps allows for higher spatial resolution of thestrain maps than given by the beam size. When switchingfrom white light to monochromatic beam, an intensity loss of

�4 orders of magnitude is observed. For experiments in-volving weakly diffracting samples �environmental science�,this loss of intensity is detrimental but can be compensatedby opening the virtual source and thus using a larger spotsize ��5 �m �h� �2 �m �v� up to �20 �m �h� �2 �m�v��. In those cases monochromatic scans are thus spatiallyless resolved than white-beam scans. The upcoming upgradeof the ALS storage ring to top-off mode and its operation ata higher electron current of 500 mA helps alleviate some ofthese drawbacks of the monochromatic beam operationalmode. Scans in white beam mode are presently limited bythe readout time of the x-ray CCD detector �5 s�. The over-head time contributes significantly more to the total scan-time in white-beam mode, where typical exposure times arepresently in the order of tenths of seconds, whereas in mono-chromatic mode, exposure times in the order of tens of sec-onds to minutes dominate the total experimental time. Weexpect exposure times to be reduced up to a factor of 0.5when operating in top-off mode.

The beamline is presently mainly used for material sci-ence studies related to the micromechanics and reliabilityissues of technologically interesting manufactured samples.One such example is the study of electromigration in Pb-freeSn-based solder joints used in flip-chip technologies. Currentcrowding20 leads to localized high current densities at thecorner where the electrons enter into or exit from the solderbumps, so that Sn whiskers and pancake-type voids areformed at the anode and cathode ends, respectively.21 TheSn-based solder joints have body-centered tetragonal crystalstructure. Its thermal and electric conductivity, followingNeumann’s principle, are therefore anisotropic properties:the electric conductivity in the crystallographic c-direction issignificantly lower than in a /b-direction.10 Grain orientationhas been monitored during an accelerated in situ electromi-gration test using white-beam x-ray microdiffraction. The re-sults showed that the grains in the current crowding regionwere reoriented after exposure to high current densities�Fig. 8�. The sense of rotation is such that it realigns thecrystallographic a-direction along the electron flow direction,which lowers the effective resistance of the grains. Figure 9shows the evolution of the effective conductivity as a func-tion of time. Here the conductivity has been derived from theorientation of the Sn-grains.22

Another example using white beam microdiffraction isthe study of the orientation of aragonite grains at the inter-face between mother-of-pearl and prismatic calcite layers inan abalone shell.23 Mother of pearl �Nacre� can be viewed asa natural composite material built of stacks of aragonite crys-tals linked with organic matrix �mostly �-chitin�. It is re-markable in that its strength surpasses that of its primarybuilding material �Aragonite� by a factor of 3000, due to itsaccurately self-assembled architecture. The goal of this studywas to better quantify the architecture of the nacre compositeon the length scale of the individual crystallites. White beamx-ray microdiffraction measurements on beamline 12.3.2confirm and add quantitative information to linear dichroismmeasurements under a photoemission electron microscope�PEEM� microscope. They show that the orientational order-ing of the aragonite crystals increase with increasing distance

FIG. 7. �a� Calculated and observed flux curve for the monochromaticbeam. Both curves have been determined with 0. 15 mm �h� �0.1 mm �v�source slits and 1�1 mm aperture slits. �b� Calculated flux curve for whitebeam configuration assuming an 8 �m �h� �16 �m �v� source size, i.e.,1�1 �m2 spot size. A 0.1% bandwidth window is assumed for the calcu-lations.

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from the calcite to mother-of-pearl interface with an initialspread of the orthorhombic c-axis of �30° to less than 10° at35 �m distance from the interface �Fig. 10�. These data areconsistent with a theoretical model in which aragonite crystallayers are nucleated sequentially in the presence of confiningmatrix sheets and grow epitaxially on aragonite crystals inlayers below, with the aragonite tablets with its crystallo-graphic c-axes oriented normal to the layers growing fasterthan misoriented tablets.

Monochromatic beam experiments are typically used forphase identifications and mineralogical mapping. One suchexample is the identification of secondary nanocrystallinephases during the passivation process of acid mine water.24

The standard treatment of acid mine water involves an an-oxic limestone drainage through which the acid water perco-lates. Dissolution of calcite leads to a buffering of the acidityand thus significant increase of the pH. Reprecipitation ofsecondary phases over time, however, leads to a passivationof these systems. The goal of this particular study was toquantify the kinetics of this process and to understand the

nature of the secondary precipitates, which coat the calcitegrains and thus passivate the buffer zone. Monochromaticx-ray microdiffraction measurements along the calcite grainsshowed unambiguously that gypsum �CaSO4 H2O� acts asthe first coating material on the surfaces of the calcite grains.Fe-bearing phases �mostly goethite �FeOOH�� fill interstitialsbetween the grains but are not in direct contact with thecalcite grains.

VI. FUTURE DEVELOPMENTS

In its present configuration, beamline 12.3.2 produces2D strain/stress maps and phase identification on a routinebasis. This, however, exploits only a part of the potentialprovided by a microfocused white and/or monochromaticsynchrotron beam. In particular we envisage the expansionof the 2D strain mapping into three dimensions by includingdepth resolution. We are adopting a two-pronged approach.On the one hand we intend to implement the wire scanmethod as developed by Ref. 25. This requires a largeamount of data-collection and analysis time. In cases wherethe requirements for depth resolution are less stringent, analternative method based on depth triangulation using thevariable position of an x-ray reflection on the area detector asa function of the detector-distance is being implemented.While this does not give the depth resolution achieved by thewire scan method, it has the advantage that it is much fasterand computationally less demanding, thus allowing for largerareas to be covered. Efforts in these directions had been ini-tiated on beamline 7.3.326 before the shutdown of its micro-diffraction program in 2006 and are being continued on12.3.2, the goal being to implement these techniques in afully user-friendly fashion. One of the problems inherent toeither method will be the relatively limited high-energyrange of 12.3.2, which will pose limits on the penetrationdepth, especially for high-Z material.

2D as well as three-dimensional strain/stress scanningonly makes use of the peak positions. The information pro-vided by the peak intensities is largely ignored. Interpretingintensities may be useful in cases where traditional mono-chromatic diffraction methods �powder or single crystal� arenot applicable.27,28 This could occur in the case of rare andvery small crystals, which cannot be isolated and mounted or

FIG. 8. �Color� �a� Out-of-plane orientation map of the solder joint in whichelectrons flow from the bottom to the top and the top left corner is thecurrent flow corner. �b� Angular variation between the a-axis of the grainsmarked in the orientation map and the electron flow direction as a functionof electromigration time. Label numbers refer to the grains numbered in �a�.

FIG. 9. Effective resistivity of selected grains as a function of electromigra-tion time. Label numbers refer to the grain numbers in Fig. 8�a�.

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where bulky and immobile ancillary equipment prohibit thethree-dimensional rotations required for a monochromaticsingle crystal data set. The application of a white x-ray beamon a static sample compensates for the lack of mobility with-out losing the three dimensional information about the recip-rocal lattice, as it happens in the case of monochromaticpowder diffraction. The problems to be solved are the deter-mination of the various wavelength dependent correctionfactors and the absolute indexing of a substance with un-known unit cell. First steps to solve the former challengewere undertaken by using well-known standard crystals �Sili-con, Ylid� and comparing its expected intensities �based ontheir known structure as well as analytical polarization andabsorption corrections� with the observed intensities. To ad-dress the absolute indexing problem we have started devel-oping procedures and software involving monochromatorscans. These first efforts produced encouraging results. Firstexperiments on Zeolite microcrystals revealed a series of ex-perimental issues, such as improved background reductionand optical imaging, which have to be addressed to enhanceLaue intensity collection.

VII. CONCLUSIONS

Beamline 12.3.2 of the ALS provides for a versatile mi-crofocus beamline allowing for strain/stress mapping as well

as phase identification with high spatial resolution. The in-creased energy range up to 22 keV delivered by the super-conducting magnet provides strain resolution as small as5�10−5 strain units. The beamline can be operated in white-light mode as well as in monochromatic mode with an ex-tremely easy switch in less than 2 min between the twosetups. This allows for experiments combining phase identi-fication of nanocrystalline material and strain/stress and ori-entation mapping of microcrystalline samples.

ACKNOWLEDGMENTS

The ALS is supported by the Director, Office of Science,Office of Basic Energy Sciences, Materials Sciences Divi-sion, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231 at Lawrence Berkeley National Labo-ratory and University of California, Berkeley, California. Themove of the microdiffraction program from ALS beamline7.3.3 onto the ALS superbend source 12.3.2 was enabledthrough the NSF Grant No. 0416243.

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