X-ray diffraction imaging of metal-oxide epitaxial tunnel junctions made by optical lithography: use of focused and unfocused X-ray beams.

1.5 MB·English

by  Mocuta, Cristian

research papers
Journalof X-ray diffraction imaging of metal–oxide epitaxial
Synchrotron
tunnel junctions made by optical lithography:
Radiation
use of focused and unfocused X-ray beams
ISSN0909-0495
Received14March2012 Cristian Mocuta,a* Antoine Barbier,b Stefan Stanescu,a Sylvia Matzen,b
Accepted26November2012
Jean-Baptiste Moussyb and Eric Zieglerc
aSynchrotronSOLEIL,L’OrmedesMerisiers,Saint-Aubin,BP-48,F-91192Gif-sur-Yvette,France,
bCEA-Saclay,DSM/IRAMIS/SPCSI,F-91191Gif-sur-Yvette,France,andcEuropeanSynchrotron
RadiationFacility,6rueJulesHorowitz,BP-220,F-38043GrenobleCedex9,France.
E-mail:[email protected]
X-raydiffractiontechniquesareusedinimagingmodeinordertocharacterize
micrometre-sized objects. The samples used as models are metal–oxide tunnel
junctions made by optical lithography, with lateral sizes ranging from 150mm
down to 10mm and various shapes: discs, squares and rectangles. Two
approaches are described and compared, both using diffraction contrast: full-
field imaging (topography) and raster imaging (scanning probe) using a
micrometre-sizedfocusedX-raybeam.Itisshownthatthefull-fieldimagegives
access to macroscopic distortions (e.g. sample bending), while the local
distortions, at the micrometre scale (e.g. tilts of the crystalline planes in the
vicinity of the junction edges), can be accurately characterized only using
focused X-raybeams.These local defects aredependenton thejunction shape
and larger by one order of magnitude than the macroscopic curvature of the
sample.
Keywords:X-rayimagingwith diffraction contrast;microbeam X-raydiffraction;
rastermicroscopy; metal–oxide epitaxial tunneljunctions.
1. Introduction faces or layers). X-ray diffraction (XRD) using hard X-rays
(energies in the 10keV range) has the advantage of probing
Imaging microscopy techniques are valuable tools providing
deeperinsidethesampleandallowsprobingofdeeplyburied
important information about complex systems leading to an
interfaces. Consequently, the minimum sample preparation
improved understanding of their physical properties. Well
and the non-invasive approach1 leaves the sample intact for
established techniques using local probe microscopy (atomic
a possible complementary investigation with the above-
force microscopy, AFM; scanning tunnelling microscopy,
mentioned microscopy. The use of sample environment
STM; scanning electron microscopy, SEM; etc.) allow easy
(liquid,gas,highpressureand/ortemperature)isalsopossible
access of information at the sub-nanometre scale. Various
in an XRD experiment and rather difficult in electron-based
contrast mechanisms (topography, chemical, etc.) can be
microscopies, for which mostly high- and ultra-high-vacuum
exploited in order to obtain images of the samples. In the
is required. Acquisition of data under extreme (pressure,
present paper we consider an approach using X-rays and
temperature)or‘real’conditions(catalysisat1atm.pressure)
diffractioncontrast.Insomecasesitcanbecomethetechnique
thus need the use of X-rays.
of choice, giving access to new information, not otherwise
X-ray spectroscopies do present some of the above-
available. It should thus be considered not as an alternative
mentioned advantages and can be used in combination with
but more likely complementary to the above-mentioned
XRD;thisis,however,beyondthescopeofthepresentpaper.
microscopy techniques.
Inthefollowing,weintroducethebasicprinciplesofXRD
Theadvantagesoflocalprobemicroscopymethods,already
imaging.Fordiffractionexperimentsthematerial(sample)to
established, are undeniable. Very good resolution images of
beinvestigatedisimplicitlysupposedtobecrystalline(atomic
thesamplesurfacecanbeobtainedratherquickly,withdetails
long-range order), since the lattice parameter or lattice
onthenanometrescale.Variouscontrasts,includingchemical,
spacing is probed. Accessing the lattice parameter of the
can be used to generate a sample image. They do remain
mostly surface-sensitive techniques, thus the sample needs to
1In some particular cases, soft condensed materials (e.g. polymers) can be
be mechanically prepared (thinned, cleaved, etched) if infor-
alteredbytheX-rays,butthetechniqueremainsessentiallynon-destructive
mation from below the surface is needed (e.g. buried inter- formostofthematerialssciencesamples.
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crystallinematerial is done via the Bragg law (Als-Nielsen& al.,2001,2002;Poulsen,2004;Martinsetal.,2004;Jakobsenet
McMorrow,2001; Warren, 1990), al., 2006; Winther et al., 2004; Schmidt et al., 2004)]. Similar
effects (for semiconductor samples) were shown in an
2dsinð(cid:2)Þ¼(cid:3); ð1Þ
approachdevelopedbyMurrayetal.(2003,2005,2008,2011):
withdbeingtheprobedlatticespacing,(cid:2)theincident(Bragg) they show not only that lattice spacing selectivity is achieved
angle, (cid:3) the wavelength of the X-ray radiation used and n but also that lattice plane rotations are shown as well and
beinganinteger.Itcanthusprobelatticevariationsof(cid:2)d/d’ quantified with sub-micrometre lateral resolution. This last
10(cid:2)4,that is by far better than microscopy methods. cited method is thus more adapted for the type of systems
Ofcourse,themethodpresentsdrawbacksaswell,themain investigated in this paper, as a complementary technique to
one being the work in the so-called ‘reciprocal space’ or theBraggtopography.
Fourier transform space (Warren, 1990; Als-Nielsen &
2. Samples
McMorrow, 2001). Diffraction conditions ((cid:2) angles) for
families of crystalline planes (identified by their HKL Miller Themodelsamplesusedforthisstudyaremetal–oxidetunnel
index, of lattice spacing d ) are translated into Bragg peaks junctionsmadebyopticallithography.Inordertoinvestigate
hkl
(points) in the HKL reciprocal space. This is much less possiblemorphologicalanisotropyeffectswepreparedobjects
straightforward orintuitivetounderstand than a‘real’ image with sizes ranging from 150mm down to 10mm and various
ofthesample.Also,theinformationextractedfromXRDdata shapes: discs, squares and rectangles. Magnetic tunnel junc-
originatesfromalltheilluminatedsample,theresultbeingthe tions (two ferromagnetic electrodes, a hard and a soft
incoherent sum of intensities corresponding to individual magnetic layer, separated by an insulating layer) have been
regionsorobjects.InastandardXRDexperiment(beamsize thoroughly studied over the past years because of their
’0.1–1mm)itisnotalwaysstraightforwardtodisentanglethe applications in spintronic devices at an industrial level. A
contributionscomingfromthedifferentpartsofthesample:if number of fundamental issues regarding these structures, as
distributions can be, in general, extracted, measurements on well as for other multilayered systems, are still unresolved.
more complicated samples (e.g. multi-modal growth modes) For various layer compositions and preparation conditions,
are difficult to interpret. A microscopy-like approach in such the tunnel magneto-resistive response has been extensively
cases becomes highly desirable. investigated and correlated to the (crystalline) structure and
We will show here that XRD can be used as well to yield interfacialfeatures(DeTeresaetal.,1999),bytechniqueslike
contrast, be it in full-field imaging (topography) with a high- transmission electron microscopy (TEM), spectroscopy or
resolution area detector or in a point-probe scanning mode X-ray diffraction. However, the effect of the lithography
approach using a micrometre-sized X-ray beam. We will process itself, especially when the junction size, or more
highlight the advantages and the specificity of each of these generallythesizeofthelithographedobject,decreases,started
methods.Thepaperisorganizedasfollows:afterdetailingthe to be investigated only recently (Murray et al., 2003, 2005,
samples used for this study, the experimental set-up will be 2008, 2011; Mocutaet al.,2007, 2009).
describedforeachtypeofmeasurementandthentheobtained The samples consisted of multilayers made of epitaxial
results are detailed. The first experimental approach uses metal and oxide layers of 15nm Co/1.5nm (cid:4)-Al O /5nm
2 3
X-ray diffraction contrast in a full-field imaging mode at the CoFe O and15nmCo/3nm(cid:4)-Al O /25nmFe O grownon
2 4 2 3 3 4
Bragg angle (topography) in order to obtain information (cid:5)-Al O (0001) substrate. Two different thicknesses (0.3mm
2 3
about the sample and the possible presence of defects. The and 1mm; see further text for details) for the (cid:5)-Al O (0001)
2 3
reader can find details about the technique in the work of substrate were used, the former thickness providing better
Moore (1995, 2009, 2012), Moore et al. (1999), Bowen & thermal conductivity and electrical charge dissipation. A
Tanner (1998), Yacoot et al. (1998), Wierzchowski & Moore 10nm-thick Pt buffer [known to increase the crystalline
(2007) and Hoszowska et al. (2001a,b) (and references quality of the subsequent metal oxide layers (Barbier et al.,
therein).Thesecondapproach(microbeamX-raydiffraction) 2005,2007)]wasepitaxiallydepositedonthesubstratepriorto
isusedtoaccesslocalinformationaboutthecrystallinityofthe the metal–oxide layer growth. The oxide layers were epitaxi-
sampleandevidenceinhomogeneitiesincrystallinestructure, allygrownbyatomic-oxygen-assistedmolecularbeamepitaxy
close to the edges of the lithographed objects at micrometre (AO-MBE)(Gao&Chambers,1997;Moussyetal.,2004;Gota
length scale. et al., 1999, 2000) and had very good crystalline quality, as
Well established methods which could evidence such checkedbyhigh-resolutionTEM(notshownhere;seeMocuta
features exist and are used at a number of synchrotrons et al., 2009) and XRD (see later in this paper). A 15nm Au
worldwide (HASYLAB, APS, ESRF). We can point here capping layer was applied for protection against oxidation.
to the use of polychromatic-focused X-ray beams [Laue The crystalline quality of the layers and the sharpness of the
microbeam diffraction (Budai et al.,2003; Larson et al., 2002; different interfaces was checked during the growth by in situ
Chung & Ice, 1999; Ice & Larson, 2000; MacDowell et al., reflectionhigh-energyelectrondiffraction(RHEED)(Mocuta
2001)]ortrackingprocedures[combining‘themonochromatic etal.,2009)andexsitubycross-sectionalhigh-resolutionTEM
rotation method with X-ray tracing’ in order to access dis- (Moussy et al.,2004; Ramoset al.,2007).
locations, strain and lattice rotation of individual grains The optical lithography process was performed ex situ
(Poulsenetal.,1997,2001;Lauridsenetal.,2001;Margulieset (Bataille,2005;Nassar,1999;Bowenetal.,2003)onaspecially
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modified lithography machine (water-cooled sample holder
and use of neutral Ar atoms instead of Ar+ ions) (Bowen,
2003; Bowen et al., 2003). Owing to the presence of the
different layers, the realisation of a tunnel junction structure
requires severallithography steps:
(i) Definition of the junctions and etching of the upper
(free) layer;
(ii)Definitionofthelowertracks(Ptelectrode)andetching
downtothesubstrate,throughthedifferentlayers(freelayer,
tunnel barrier,pinned layer);
(iii) Encapsulatingthe wholesampleinto Si N insulator;
3 4
(iv) Opening the insulating layer to access and contact the
upper andlower electrodes;
(v)Making the contacts.
The precise alignment of the masks between the different
steps, as well as an accurate control of the etching processes
forthedifferentlayers,ismandatorytoguaranteetheproper
columnarstructureshownintheinsetofFig.1(a).Thelasttwo
steps are necessary for a measurement of the magnetic
behaviourofthedevice(tunneljunction)butwerenotcarried
out in the case of the samples used for the ex situ XRD
measurements.
Figure 1
Object sizes from 10 to 150mm lateral size and various
(Colour online) (a) Schematics of the diffraction imaging experiment.
shapes(squares,rectanglesanddiscs)weremadeonthesame The sample is illuminated by a parallel X-ray beam under an incident
sample. The average distance between the different objects angle set to the Bragg angle ((cid:2) ) of the Pt(111) buffer layer. The
Bragg
(0.5mm) is large enough to separate them well as isolated elevationangle(euleriangeometry)is .Thesampleimageisrecorded
bythehigh-resolutionareadetector(FReLoN).Thepartofthesample
objects when using beam sizes of about a few 100mm. The
diffracting (Pt) is the part labelled as ‘electrode’ [the inset shows the
higherlateralresolutionimagingandfocusedX-raybeamsare multi-layered structure of the sample (tunnel junction)]. (b) Optical
then needed toaccesslocaldetails (defects) in the junctions. microscopyimageofthe120mm-sizedsquarejunctions.(c)Radialscan
Interestingly,oftenfortheXRDmeasurements,thereisno (i.e.(cid:2)–2(cid:2)geometry)performedforathicksampleusingapointdetector
(E=15keV;pinkbeam:(cid:2)E/E=10(cid:2)2).(d)Imagesofthetunneljunction
needtoremovefromthesamplesurfacetheresistusedduring
areatakenforanincidentangleofthesamplesetastheBraggangleofthe
the lithography process. Figs. 1(b) and 3 (top panels) show Ptpeak;(1)maskalignmentmarksforlithography;(2)contactelectrode;
opticalimagesofthelithographedsampleswiththeresiststill (3) junction area of the FReLoN chips; (4) square-shaped junctions
(someofthemwithdefectsappearingasscratches).
presentonthesamplessurface(crackleddarkzones).Indeed,
at the rather high energies used ((cid:4)7keV), the X-rays cross
the resist layer and suffer low absorption; the non-crystalline Wierzchowski&Moore(2007)andHoszowskaetal.(2001a,b)
resist will simply not scatter (diffract) in regions close to the (and references therein). The sample (CoFe O oxide layer,
2 4
signal originatingfrom the metallic layers. 1mm-thick (cid:5)-Al O substrate) containing the lithographed
2 3
tunnel junctions is placed on the diffractometer; a parallel
3. Experimental set-up X-ray beam illuminates its whole surface (or at least the big
junctions of interest, of sizes (cid:5)100mm). The sample is
Two experimental approaches using the diffraction signal in
mountedwithoneofitsedgesparalleltotheX-raybeam.An
order to obtain acontrastare described in this section.Their
optical microscope points to the centre of rotation of all the
complementarity will be discussed in x4. The experiments
circles of the diffractometer (centre of the confusion sphere)
reportedherewereperformedatbeamlinesBM-05andID-01
andallowsapre-positioningofthesample(andoftheregion
at the European Synchrotron Radiation Facility (ESRF) in
of interest) in the X-ray beam. A Fast REadout LOw Noise
Grenoble, France. On beamline BM-05, both pink-unfocused
(FReLoN 2000) CCD camera (http://www.esrf.eu/Users
andmonochromatic(parallelandfocusedtomicrometresize)
AndScience/Experiments/Imaging/ID22/BeamlineManual/
beams were used, while the ID-01 measurements were
Detectors/Cccd/Frelon)withapixelsizeof0.7mmandatwo-
performed only in monochromatic non-focused (parallel)
pixel point-spread function was used2 (Labiche et al., 2007;
beam.
Coan et al., 2006). A pink beam (multilayer monochromator,
energyresolution(cid:2)E/E=10(cid:2)2)atanenergyE=15keVwas
3.1. Full-field imaging with diffraction contrast
used in order to increase the available flux (Ziegler et al.,
The experimental set-up is depicted in Fig. 1(a). For a 2004). Although a poorer angular resolution is expected as a
detailed description of the technique and the experimental
approach,thereaderisreferredtoMoore(1995,2009),Moore
2The pixel size was also checked by a calibration procedure during the
et al. (1999), Bowen & Tanner (1998), Yacoot et al. (1998), experiment.Avalueof0.68mmwasfound.
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consequenceofthedegradedenergyresolution,itstillremains corresponding to the various present crystalline structures
good enough for the investigated system, for which very thin (layers).Inthisway,diffractioncontrastofindividuallayersis
layers (in the several 10nm range) are measured (see obtained. Then, without changing the diffractometer angles,
Appendices AandB). the sample lateral position is scanned in the X-ray focused
In a first approach an X-ray reference alignment is beam. The scattered intensity measured by the detector3 is
performed; this canbe realised usingapointdetector aswell recorded for each lateral point position on the sample. The
asanareadetector.ThesamplesurfaceisalignedintheX-ray result is a raster image of the sample corresponding to the
beam, then specular reflectivity and radial scans are chosen layer.Bytuning the diffraction angle tobe character-
performed [the momentum transfer vector q is kept all the isticofthecrystallinestructureofeachlayer,adifferentimage
time perpendicular to the sample surface, i.e. the incident is obtained [see,forexample,Fig.4(left panel)].
angle is kept equal to half of the detector angle, (cid:2)–2(cid:2) Since the Be-CRLs are mounted on a translation stage
geometry, Fig. 1(c)]. The small (cid:2) angle regime of the reflec- operatingperpendicularly totheX-raybeam,itispossibleto
tivity curve presents characteristic Kiessig fringes; these easilyshifttheminandoutofthebeam,thusswitchingfroma
oscillations have as origin the well defined thickness of the microbeam configuration to one in which a flat and parallel
layers (and interference between them) and are easily iden- monochromatic beam is illuminating the full sample. The
tifiedevenwiththeincreasedwidthduetotheenergyspread accuracy of the translation stage ensures a good reproduci-
mentionedabove.Thevaluesobtainedareinagreementwith bility in position and size ofthefocused X-raybeam.
the thickness ofthe layers deposited. It is thus rather easy to switch from a full-field to a raster
Atlarger(cid:2)anglestheBraggdiffractionregimeisaccessed. microbeamimagingconfiguration.Iftheformerallowsaquick
Wenotethe presenceof the intense (cid:5)-Al O (0006) substrate identification and ensemble view of the sample (or of large
2 3
peak((cid:2)’22.5(cid:6))ofthicknessoscillationsandashoulder((cid:2)’ regions of it), the latter will allow more detailed character-
21.6(cid:6)) attributed to the CoFe O (111) and Pt(111) Bragg ization of the defects at the micrometre scale. This combina-
2 4
peaks, which can hardly be separated due to the degraded tionof the two imaging modes isshown hereafter in x4.
energyresolution.Theeffectofthispoorerenergyresolution
also has the effect of broadening the substrate peak and
smearing the signal (oscillations) (see discussion in 4. Results and discussion
Appendix A).
The effect of optical lithography on the different epitaxial
After this preliminary alignment procedure, the full-field
layerswill beaddressed hereusing localprobeXRD;wewill
imaging approach can be started. The incident angle for the
discusstheeffectsofsubstratethickness,sizeandshapeofthe
sample is set to the Bragg angle of the Pt(111) and the area
junctions.
detectorispositionedatthecorrespondingBraggangle(2(cid:2)=
43.2(cid:6)).Full-fieldimagesofthesamplearerecordedbythearea
detector.Thecontrastwhichwillbeusedlaterisextractedby 4.1. Full-field imaging in pink beam
recording the diffracted beam.
A 15nm Co/1.5nm (cid:4)-Al O /5nm CoFe O sample depos-
2 3 2 4
Thesameapproachwasalsousedtoinvestigatethe0.3mm-
ited on a 1mm-thick substrate is considered first. The
thick substrate sample.The results are detailedin x4.1.
measurementsin(cid:2)–2(cid:2) geometry(Fig.1c)allowtoextractthe
thicknessofthelayerfromreflectivitymeasurementsbutalso
to determine the angular positions corresponding to the
3.2.RasterimagingusingfocusedX-raybeamsaslocalprobe
different Bragg peaks characteristic for the various layers of
with diffraction contrast
the sample. The most intense one seems to be the Pt(111) +
The second approach we will show here makes use of a CoFe O (111).4
2 4
focusedX-raybeam(micrometresize)andrasterscanningthe
Let us now look at the full-field image. The results
sample in this beam, while recording diffraction patterns
measuredforthePt(111)BraggpositionareshowninFig.1(d),
(diffraction contrast). The approach has been detailed by
and are compared with an optical image of the sample. The
Mocutaetal.(2008)andStangletal.(2009).Thefollowingisa
sameregion isshown inFig.1(b).16imagesof30sexposure
brief review ofthe technique.
each were summed to produce the resulting image shown in
AmonochromaticX-raybeam((cid:2)E/E’10(cid:2)4)ofenergyE=
Fig.1(d).Theresultingimage(topography)iscorrectedbythe
7keV is used. The X-rays are focused using 39 beryllium
projectionduetotheincidentangle(cid:2).Indeed,itcanbeseenin
compoundrefractivelenses(Be-CRLs)(Lengeleretal.,1999;
Fig.1(a)thatthesizeoftheobject(redarrow)imagedonthe
Snigirev et al., 1996). The resulting spot size was 3.2mm (cid:7)
areadetectorhastobedividedbythesin((cid:2))factorinorderto
7mm (vertical (cid:7) horizontal, full width at half-maximum,
obtainits realsize.With only this correction andbyknowing
FWHM) with a measured photon flux in the spot of
the pixel size on the camera (from calibration or camera
108photonss(cid:2)1.Themeasurementsareperformedtypicallyat
anincidentangle(cid:5)30(cid:6);thusthebeamfootprintonthesample 3Apointorareadetectorcanbeused;inthislastcasetheintegratedintensity
can be considered as disc-shaped with a6–7mmdiameter. overthewholedetectororonlypartofit(aregionofinterest)canbeusedasa
probeforthecontrast.
The diffractometer angles (sample incidence and detector
4Infact,thetwopeakscannotbedistinguished,especiallyinthelow-energy
angles) are set to fulfil Bragg equation (1) for lattice planes resolutionpinkbeam;seealsolaterinthepaper.
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characteristics), the image obtained
matches perfectly with the corre-
sponding optical imageof the sample.
We should note here the presence of
some scratch-like features in Fig. 1(d),
features labelled as (4), although no
corresponding defect is seen in the
optical image (Fig. 1b). This can be
understood in terms of the probed
signal in the two images. In the optical
imagethemorphologyofthesurfaceof Figure 3
(Colouronline)Two-dimensionalimagesofanarrayofsquarejunctions[from(a)to(c),120,30and
the sample is essentially probed by the
10mm size, respectively] (top = optical image, bottom = area detector image). The images were
amount of reflected light. It is indeed recordedfarfromthefocalpoint.Thedifferentpartsofthesampleareidentifiedasfollows:(1)
expected that the surface quality of the externalcontacts;(2)squarejunctions;(3)alignmentmarksusedinlithography;(4)partsofthefilm
deposited layers (and consequently of not removed during the lithography. Images were corrected by the projection due to the Bragg
angleandfocusingeffect(samplecurvature).
the objects after lithography) is very
good.IntheXRDimage,thediffracted
intensity, coming from micrometre-deep layers, is recorded. perpendiculartoit.Byknowingpreciselythesample–detector
Mostprobablythedarkerareacorrespondstoazoneinwhich distances,thesizeofeachimage(calibratedpixelsize)andthe
a crystalline defect appeared (e.g. grain boundary). This size of the illuminated junctions, one can deduce that the
behaviourissimilartothatseenbyHoszowskaetal.(2001a,b). shapeofthesampleisconcave,withradiusofcurvaturevalues
A similar sample (0.3mm-thick substrate) was then investi- of (cid:5)1.8 (cid:8) 0.3m and 2.7 (cid:8) 0.4m in the sagittal and long-
gated using the full-field XRD contrast. Surprisingly,the first itudinal directions, respectively. These values compare well
obtained images could barely be correlated with the corre- withtheopticalmeasurementsofthesamplecurvature,which
sponding optical images of the sample surface (Fig. 2). By yield values of1.4(cid:8)0.1mand 3.0(cid:8)0.5m,respectively.The
taking images with the area detector placed at different optical measurement confirmed as well that the curvature
distancesfromthesample,amagnificationeffectoftheimage directions (astigmatism) were only 3(cid:6) off the edges of the
could be identified. rectangular sample (i.e. from the X-ray beam direction illu-
We understood the different images as the result of X-ray minating the samplein the XRD experiments).
focusingbythesampleitself,whichisconfirmedbythe‘zoom Oncethis‘zoom’effectisknown,itcanbeusedinorderto
effect’visiblewhengoingwiththeareadetectorfurtheraway enhance the resolution achieved during the measurement,
from the sample. The measurements made at different similar to imaging modes with high-divergent X-ray beams
distances allowed the macroscopic curvature of the sample (see,forexample,Rauetal.,2006;Rau&Liu,2007).Knowing
(sagittaland longitudinal directions) tobe determined. the sample radius of curvature, the full-field images can be
We can note that the images are distorted both along the corrected. In order to explore and test the achievable reso-
impinging X-ray beam direction as well as in the direction lution, images of various areas of the 0.3mm-thick sample
wererecordedwiththeareadetectorplacedfarawayfromthe
sample(twotothreetimesthefocusingdistancegivenbythe
bendsample,asfoundinFig.3).Fig.3showsacomparisonof
optical5 and full-field images for square junctions of lateral
sizesof120mm, 30mmand 10mm.The full-field imageswere
corrected bythe incident angle (cid:2) andthe measuredradius of
curvature; the results compare well with the corresponding
opticalimages.Variouspartsofthesamplearehighlighted:the
squarejunctions(2),thealignmentmarks(3)andthecontact
pads (1). All the sizes down to 10mm can be identified.
Moreover the presence of defects in lithography are high-
lighted in Fig.3(c): thebrightspots seenin the optical image
[Fig.3(c),top]aremetallicstructures.Bytuningthediffraction
angle to be sensitive to various layers, one can determine
whether the layer structure is intact or whether some of the
Figure2
layers were etchedaway.
(Colouronline)Schematicsofthefull-fieldimagingexperiment(thecase
ofthebendsample,0.3mm-thicksubstrate).TheparallelincidentX-ray Onemaynoteherethattheresolutionoftheareadetector
beam is focused by the sample, both in the longitudinal and sagittal andofthewholeset-upisgoodenoughtoseedetailslikethe
directions.Theresultisadistortedimageontheareadetector.Imagesat
severaldistancesaretaken:(1)closetofocalpoint;(2)outofthefocal
pointand(3)farawayfromthefocalpoint.Thestructureofthesampleis 5Thepresenceoftheresistusedforlithographycanbeseenasdarkregionson
similartotheoneshowninFig.1(b). thesurfaceofthesample.
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alignmentmarksordefectsontheobjects.Thismethodcanbe
used also as a first approach in finding the objects of interest
before switching to the focused beam experiment (see x3.2,
x4.2 and x4.3).
4.2. Raster imaging in monochromatic focused X-ray beam
ThepowerofX-raydiffractionusingmicrobeams(mXRD)
inatwo-dimensionalscanningapproach(rastermaps)canbe
illustratedinthefollowingexample.Aparticularlithographed
sample containing several junctions of various sizes was
considered.Thelayerstructureofthesample(15nmCo/3nm
(cid:4)-Al O /25nm Fe O ) was slightly different from the struc-
2 3 3 4
ture of the other samples used for this study: the hard
magnetic oxide layer ‘layer 2’ (typically CoFe O ) was
2 4
replaced by Fe O . This particular sample, which from the
3 4
point of view of lithography can be considered as a ‘bad’
sample, was in fact obtained during checking and validating
alignment procedures between the various steps in the litho-
graphyprocess.Theresultisthatthedifferentetchingregions
are shifted, resulting in a shift of the mask corresponding to
thedifferentlayers/objects,ascanalreadybeseeninanoptical
image of the sample. For XRD, the result is a heterogeneous
samplewithlaterallyshiftedareashavingdifferentdiffraction
signals, aswill be detailed in the following.
In investigating this sample, the first XRD measurement
was performed on the whole sample with a large (0.1–1mm-
sized)X-raybeam.ThisallowedthepresenceofBraggpeaks
originatingfromthe different materials (Co,Pt,Fe O )tobe
3 4
identified and confirmed. Once the diffraction angles are
known, raster mapping of the sample by using diffraction
contrast characteristic of these layers was performed. The
result is shown in Figs.4(a)–4(c).
The image exhibits various contrast mechanisms. The Figure 4
(Colour online) A poorly lithographed sample is used. (a, b and c)
patterns and contours observed in diffraction contrast maps
SeveralrastermapswithdiffractioncontrastcorrespondingtoFeO,Pt
3 4
canbeeasilyfoundinagreementwiththeopticalimageofthe andCo,respectively;thecolouroftheframecorrespondstotheposition
sample(andthetemplateofthemask).Letusdiscussindetail ontheradialscan(d)indicatedbythecorrespondingarrow(seetextfor
these contrasts and how we can understandthem. more details). (d) Radial scan in the vicinity of the Bragg peaks
characteristicofthedifferentlayersperformedwithapointdetectorin
(i) The image in Fig. 4(a) was recorded at the position
a monochromatic (7keV) focused X-ray beam, at different lateral
characteristicofthePt(111)(lowerelectrode)andFe3O4(111) positions on the sample (see inset legend). Positions in the reciprocal
Bragg peaks (in fact these two peaks are rather close and space(angularvalues)atwhichrastermaps(a)to(c)wereperformedare
markedbycolouredarrows.
cannot be resolved easily within the present approach). In
rasterimagingwhenusingthiscontrast,itshouldthusmainly
render the full mask, including the lower electrode track junctions.However,duringtheexperimentacontrastisfound
connecting the junctions. Apart from the misalignment, the withalower intensity at the junction positions.
contrast is expected at first to be uniform, possibly more (iii)TheimageinFig.4(c)wasrecordedtohavediffraction
intense on the junctions (owing to the extra contribution of contrastattheCo(0002)Braggpeak.Itspresenceonlyonthe
the oxide layer). It can indeed be seen that the junctions do junctionsisindeedconfirmedbythecontrastmeasuredinthe
exhibit a higher-intensity signal. The slightly lower intensity diffraction raster scanning experiment. A faint and diffuse
on the left side of Fig. 4(a) is most probably due to a small signalfromthetrackisalsopresentandcanbeused,aswell,
misalignment of the sample angles when this one is laterally for imaging. The origin of this faint signal can be multiple:
translated (over several millimetres) in order to perform the eithersomeverysmallquantityofComightstillbepresenton
raster map. the track after the lithography, or scattered signal can be
(ii)TheimageinFig.4(b)wasrecordedataslighterlower(cid:2) detected,thesignaloriginatingfromthethicknessoscillations
angle, thus closer to the theoretical position of the Pt(111) (Kiessig fringes) extending further away in angular range,
Braggpeak.ThePtisexpectedtoexistonlyonthetrackand closetothevaluecorrespondingtothecharacteristicposition
thejunctions,thusnocontrastisexpectedfromthetrackand of the Co(0002) Bragg peak. Although the first hypothesis
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seems less probable (the lithography has to be performed andFe O layers(35nm)whichisaboutfourtimeslargerthan
3 4
downtothePtlayer,throughtheFe O layerbelowaswell), thethicknessof the Ptlayer (10nm).
3 4
ourdatadonotallow ittobecompletelyexcluded.Wepoint
outherethatthelowerthresholdoftheintensitycontrastscale
4.3. mXRD experiments: local defects
used in Fig. 4(c) was set very close to the background level
[7–8counts s(cid:2)1,see Fig.4(d)]. WithanX-raymicrobeamscanningapproach,itispossible
If the third case above (and possibly the first) is rather toshow(crystalline)defectsinsidethejunctions(eitherbeing
logical,howdoweunderstandthepeculiarbehaviourcloseto sensitivetothecrystallinityviathelatticeparameterortothe
the Pt(111) peak? In order to obtain a clearer image, (cid:2)–2(cid:2) layer thickness using thickness oscillations). In the following,
XRDmeasurementswereperformedusingtheX-rayfocused the crystalline defects close to the objects edges were
beam at different positions on the sample: on the square measured.
junction, on the track connecting two junctions and on the Wehaveshownintheprevioussectionhowthemicrometre-
baresubstrate(Fig.4b).Letusdiscussthesecasesonebyone. sized X-ray beam can be used for local probe imaging with
(i) On the bare substrate, only the (cid:5)-Al O (0006) peak of diffraction contrast. Besides this imaging approach, a high-
2 3
thesubstratecanbemeasured.Somesmallcontributionclose resolution XRD study with lateral resolution (essentially
tothePt(111)positioncanbedetected(butseveralordersof givenbythespotsize) isalsopossible.Someresultscovering
magnitudelowerthanothersituations).Itcouldbeattributed this topic were already shown above (Fig. 4d), in which the
either to traces of Pt after the lithography process or, more crystalline structure and thickness of various areas of the
probably,tothepresence ofavery lowintensityhalo around sample were accessed.
the microbeam. We will show here a second example of measuring locally
(ii) On the track between junctions, in addition to the crystalline defects in these samples. The approach was intro-
substrate peaks, only the Pt layer should exist. Its Bragg ducedbyMurrayet al.(2003,2005, 2008,2011;Mocutaetal.,
characteristic peak exhibits thickness oscillations showing its 2007, 2009). The sample used in this case is one having
good quality. The thickness obtained through this measure- CoFe O as a pinned layer and with the lithography process
2 4
ment is in good agreement with the expected thickness of completed correctly. High-resolution XRD data are acquired
10nm. at various positions across tunnel junctions of different sizes
(iii) On the junction, all the layers should exist. In the and shapes. They allow one to determine the tilts of the
probed angular region, the 10nm Pt(111) and 25nm crystalline planes corresponding to the different layers. If in
Fe O (111) peaks interfere due to the presence of only one the previously reported results only cross-section data along
3 4
high intense peak [slightly shifted towards the Fe O (111) onedirectionwerereported,withthisexperimentalset-upwe
3 4
expected position]. Again,thickness oscillations appear. can also probe both directions. This is done by tuning the
With this information we can now understand the contrast angular resolution of the detector (by using detector slits)
inFigs.4(a)–4(c).Owing totheproximityeffectoftheBragg eitherintheverticalorhorizontaldirection.Thenthesample
peaks of Pt and Fe O , measuring at this position yields an angles have to be scanned, either in (i) the incident angle (cid:2),
3 4
enhancementofthesignalonthejunction.ThebroadPtpeak thus sensitive to crystalline tilts in the direction along the
alsoproducesacontrastwhenthetrackbetweenthejunctions X-ray beam (longitudinal), or in (ii) the elevation angle ,
is measured. Owing to the presence of thickness oscillations thussensitivetocrystallinetiltsinthedirectionperpendicular
(Kiessigfringes)asmentionedinx3.1andFig.4(d),whenthe tothe X-ray beam(transverseor sagittal).
XRD contrast is tuned by shifting towards the Pt position In fact, in both cases, the tilts of the crystalline planes are
[pinkarrowinFig.4(d)],itmighthappenthatweprobeanode probed inside the plane described by the scanning angle,
of intensity on the (cid:2)–2(cid:2) curve. The detected signal is thus whichisinonecasealongtheX-raybeam,andperpendicular
lower than the corresponding intensity measured at the toit in the second case.
maximum of the Pt(111) + Fe O (111) peaks. This is exactly The results for these two directions are shown for two
3 4
whathappenswhentheX-raybeamislaterallypositionedon samples of different shapes: 120mm (cid:7) 40mm rectangular-
a tunnel junction, and it explains well the reversed contrast shapemagnetictunneljunction(Fig.5)anda50mm-diameter
obtained in Fig. 4(b). disc-shaped junction (Fig. 6). By setting the 2(cid:2) angle to be
Thisreversedcontrastis,inthiscase,asimpleinterference sensitivetothedifferentlayers,eachofthemcanbemeasured
effect.Theseremarksdoshowthatwearenotonlysensitiveto individually.WeshowheretheresultsobtainedfortheCoand
latticeparametersbut,inamoregeneralmanner,todiffracted CoFe O layers.
2 4
(or scattered) signals including interference effects. The The objects of interest (junctions) can be identified using
informationthatcanbeobtainedcanbemuchmorecomplex the methods presented previously in the paper. The micro-
than just a lattice parameter (obtained locally, with lateral metre-sized X-ray beam is moved laterally across the tunnel
resolution). junction.Ateachlateralpositionthesampleangles((cid:2)and )
WecanalsoseeinFig.4(d)thatthethicknessmeasuredon are scanned while recording with the detector the X-ray
thejunctionisapproximatelyfourtimeslargerthanthatfound scattered signal. The angular resolution of the detector is
onthetrack(theperiodoftheoscillationsisaboutfourtimes adapted in consequence to the scanned direction ((cid:2) or ) by
smaller).This isin agreement with atotal thickness ofthePt closingthedetectorslits[withthegeometryinFig.1(a),inthe
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J.SynchrotronRad.(2013).20,355–365 CristianMocutaetal. (cid:3) X-raydiffractionimagingbyopticallithography
research papers
vertical and horizontal directions,respectively].Theresulting (iii)Thewidthofthecurves.Apossibleenlargementcould
curves are then fitted in order toextract the following. giveanindicationaboutthedegradedqualityofthecrystalline
(i) The integrated area beneath the curve, which corre- layer.
spondstothetotalscatteredsignalfromtheprobedlayer.This Letusnowdiscussthesituationoftherectangularjunction,
area is representative of the quantity of layer in the X-ray shown in Fig. 5. The top curves (red) in each panel [(a)–(d)]
beam and is used to determine when, or when not, the beam show the integrated area (in arbitrary units) of the peak at
illuminates the junction. differentpoints,correspondingtothedifferentpositionsofthe
(ii) The (cid:2) position of the peak. The shift between the X-ray beam across the junction. We have also reported the
measured value ((cid:2) ) and the value (2(cid:2)/2) measured in the theoreticalsizeofthejunctioninthecorrespondingdirection
exp
centreofthejunctionisalsoreported((cid:2)(cid:2)=2(cid:2)/2(cid:2)(cid:2) ).This bytheshadedrectangularshape.Thissizeistheoneusedfor
exp
valuecorrespondstotheoneexpectedforaCo/Fe-oxidefilm, the design of the lithography mask and confirmed by optical
as measured in the centre of large lithographed features microscopy. It can be easily seen that the agreement is very
(alignment marks and contacts), and thus without, or with good for both Co and CoFe O layers. We can also note the
2 4
minimal influence from, the edges. The results are also presenceofsomefluctuationsintheintegratedintensityvalue;
supported by studying a Co film for a similar system (Co/ if most of the time they are within the statistics error bar,
Fe O ;Bezencenetetal.,2010).Weshowthepresenceoftilts sometimes theyaresignificantoflocaldefects onthesample,
2 3
of the crystalline planes with respect to the theoretical posi- as the ones seen in Fig.1(d) and labelled(4).
tionexpectedforanepitaxiallayer,apositionwhichisinfact The (cid:2)(cid:2) shift angle is also reported, for lateral positions of
found in the centre ofthe junctions. thebeamonthejunction,inordertohaveawelldefinedpeak
Figure 5
(Colouronline)Crystallineplanetiltangles((cid:2)(cid:2))measuredinamXRDexperimentforarectangulartunneljunction(Co/FeO sample,120mm(cid:7)40mm)
3 4
fortheColayer(a,b)andCoFeO layer(c,d).AteachlateralpositionoftheX-raybeamonthejunctiontheintegratedintensityofthepeak(rocking
2 4
scan)(toppanels)andthepositionofthecentreofmassofthepeak(bottompanels)arereported.Thelateralpositionisdisplayedonthehorizontalaxis
withrespect tothecentreofthejunction.Thereported errorbar represents theangularstepused whenscanning (typically 0.1–0.2(cid:6)).Thehatched
rectangular area is the corresponding width of the junction, as from lithography parameters (120mm and 40mm, respectively). The insets show
schematicallytheexperimentalgeometryconsidered:thediffractionplane(containingtheincidentandscatteredX-raybeam)isperpendiculartothe
surfaceofthesampleandcontainsthearrowlabelled‘X-rays’.ThethinlinedescribesthelateralpositionoftheX-rayfocusedbeamonthesample;its
curvatureschematicallyillustratesthetiltoftheCo(0001)crystallineplanes.
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Again a convex tilting is found for the Co planes but the
amplitudeoftheeffectisnowslightlysmaller,withamplitudes
inthe0.5–1(cid:6)range.InthecaseoftheCoFe O layertheeffect
2 4
seems to be present as well, again with a smaller amplitude,
which is now close to the error bar of the measurement. We
canunderstandthisloweramplitudeinthefollowingway:itis
expected that the layers will relax close to the object edges.
Thisresultsinthetiltsshownhere.Inthecaseofasquareor
rectangular-shaped object, the whole edge can deform, with
approximatelythesameamplitude.Foradiscshape,foreach
point, the nearby regions will have the tendency to deform
along the radial direction, thus limiting mechanically the
amplitude of this tilt. This kind of effect and similar approa-
ches can also be found by Murray et al. (2003, 2005, 2008,
2011).
We should point out that the amplitude effect measured
here using focused X-ray beams is not the macroscopic
curvature,forthe followingreasons.
(i)Itappearsclosetothejunctionedgesandisnotfoundin
theircentres.Amacroscopiccurvature willtilt thecrystalline
planes continuouslyfrom one edge toanother.
(ii) Its amplitude ((cid:5)1(cid:6) for the Co layer) is one order of
magnitudelargerthantheoneresultingfromthemacroscopic
curvature. This last one was estimated (from the measure-
ments)to(cid:5)0.02–0.03(cid:6)fora200mmjunctionfromitscentreto
its edge(andten times less forthe 1mm-thick substrate).
The crystalline quality of the epitaxial layers was found to
beunchangedafterlithography.Thepeakwidths(FWHM)of
theCoandCoFe O layersamountedto1.3(cid:6)(cid:8)0.3(cid:6)and0.4(cid:6)(cid:8)
2 4
0.1(cid:6), respectively. These values were found for all junctions
andforallmeasuredlateralpositions,andarecompatiblewith
the initial crystalline quality of the layers before the litho-
graphy process.
Figure6
(Colour online) Similar to Fig. 5, crystalline plane tilt angles ((cid:2)(cid:2))
measured in a mXRD experiment for a 50mm-diameter disc-shaped 5. Conclusion
junction:(a)Colayerand(b)CoFeO layer.
2 4 We have shown here how diffraction contrast imaging of
epitaxially magnetic tunnel junctions based on metal–oxide
(i.e. situations for which the integrated area of the peak is interfaces and made by optical lithography can be used to
>10% of its maximum value, in the centre of the junction). characterize the different individual objects and their indivi-
(cid:5)
For the Co layer, we found a tilt of the Co(0001) crystalline dual constitutive layers, in evidencing the presence of local
planesclosetotheedgesofthejunction(Mocutaetal.,2007) defects. Two imaging approaches were detailed: one in full-
with an amplitude up to 1–1.5(cid:6). The inset in Fig. 5 shows fieldmode,usingaparallelX-raybeamandanareadetector,
schematically, for each measurement geometry, the convex and the other in raster scanning mode, using a micrometre-
tilting of the measured crystalline planes. Then the CoFe O sized focused X-ray beam.
2 4
layer is characterized in the same way. Similar features are Moreover,accesstocrystallinedefectslocally,withalateral
found,buttheamplitudeoftheeffectismuchsmaller(’0.2(cid:6)) resolutionofafewmicrometres(essentiallygivenbythespot
asreported in Figs.5(c) and 5(d). size), is shown. We observe the presence of defects on some
Forthisrectangular-shapedjunction,itsratherlargesizein tunneljunctionsonthefull-fieldimages,defectswhichcanbe
bothdirections(>10mm)hasasaconsequencearelaxationof attributed to the eventual presence of dislocation lines. The
thelayerswithapproximatelythesameamplitudealongboth examples detailed in this paper concern mostly the tilt of the
the long and short side. A lower tilt amplitude as a conse- crystalline planes of the layers composing the oxide/metal
quence ofareduced lateral size of the object was only noted junction.Thiswasprobedforadisc-andarectangular-shaped
forjunctions sizes (cid:9)10mm(Mocuta et al.,2007). junction, in both directions (across the long and the short
Inordertocheckwhethertheshapeofthetunneljunction edge).
has any effect on this tilt of the crystalline planes in regions The morphology, crystallinity and magnetic properties for
close to the edges, a disc-like shape was measured (Fig. 6). such samples are intimately related. The lithography process
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research papers
canmodifythecrystalline structure oftheobjects (junctions) APPENDIX B
asawholeoronlyclosetoedges.Consequently,accessingthe Diffraction signal broadening in divergent (focused)
localcrystallinestructureofamagnetictunneljunctionsample monochromatic beam
opensupthewaytocorrelatethese‘defects’withitsmagnetic
Whenusingmonochromaticbeam,wehaveshownbeforethat
properties, like its magneto-resistive response (De Teresa et
the expected peak broadening for the substrate is in the few
al.,1999).
0.001(cid:6) range. This is valid only with the assumption of a
parallel (zero divergent) incident beam. The results reported
in this paper for the monochromatic beam (7keV X-ray
energy) correspond to a set-up with a focused beam. Conse-
quently, the beam impinging the sample surface is divergent.
APPENDIX A
Thebeamdivergencecanbegeometricallycalculatedfromthe
Diffraction signal broadening in parallel pink beam
aperture of the focusing element (Be-CRL) and the working
Whenusingapinkbeam(degradedenergyresolutionbyusing distance (distance from lensestosample),
a multilayer monochromator, and not a single-crystal one, in
(cid:2) (cid:3)
thiscase(cid:2)E/E’10(cid:2)2),itisexpectedtodegradetheangular div¼arctan aperture : ð4Þ
resolution in diffraction measurements. Let us discuss the working distance
visibility of the thickness oscillation (be it at small or wide
In this particular case, div ’ 0.05(cid:6). This divergent incident
angles)byconsideringthecaseofa10nm-thickfilmmeasured
beam will cause a broadening of all diffraction signals
at X-ray energy E = 15keV in pink beam. The period of
measured with this geometry. In the case of the thickness
the thickness oscillations for such a 10nm film at 15keV
oscillations originating from the layers (typical periodicity in
(monochromatic) can be estimated as
the0.1–0.2(cid:6) range),thiseffectisbarelyvisible,butitbecomes
importantforthemeasuringofthe(cid:5)-Al O (0006)Braggpeak:
2 3
(cid:3)
ð(cid:2)(cid:2)Þ ¼ ; ð2Þ the single-crystalline sapphire sample should exhibit a
0 2cosð(cid:2)ÞD mosaicity (peak width) in the few 0.001(cid:6) range.
ThelithographywasperformedatUnite´ MixtedePhysique
where (cid:3) is the wavelength of the X-ray radiation, (cid:2) is the
CNRS-Thales, Palaiseau, France. F. Petroff, R. Mattana and
incident angle and D is the thickness of the layer. In the
C.Deranlotareacknowledgedforhelpduringthelithography
reflectivity regime (cid:2) is small, thus cos((cid:2)) ’ 1. The obtained
process. The diffraction experiments reported here were
angular period value is thus ((cid:2)(cid:2)) = 0.25(cid:6) [see, for example,
0 performedattheEuropeanSynchrotronRadiationFacilityin
Figs. 1(c) and Fig. 4(d)]. The peak width spread (when using
Grenoble, France. The authors acknowledge the staff of
thepinkbeam)canbecalculated,alongthespeculardirection,
beamlines BM-05 and ID-01 for excellent technical support
as
during the measurement campaigns.
@ð(cid:2)(cid:2)Þ¼ð(cid:2)(cid:2)Þ (cid:8)ð(cid:2)E=EÞtanð(cid:2)Þ; ð3Þ References
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2 3 Bowen, M., Bibes, M., Barthe´le´my, A., Contour, J.-P., Anane, A.,
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Coan,P.,Peterzol,A.,Fiedler,S.,Ponchut,C.,Labiche,J.C.&Bravin,
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