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JOURNAL OF APPLIED PHYSICS VOLUME 90, NUMBER 1 1 JULY 2001

In situ transmission electron microscopy study of Ni silicide phasesformed on „001… Si active lines

V. Teodorescu and L. NistorNational Institute of Materials Physics, P.O. Box MG-7 Magurele, 76900 Bucharest, Romania

H. Bender, A. Steegen,a) A. Lauwers, and K. Maexa)

IMEC, Kapeldreef 75, B-3001 Leuven, Belgium

J. Van Landuytb)

Universiteit Antwerpen, EMAT, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

~Received 2 January 2001; accepted for publication 12 April 2001!

The formation of Ni silicides is studied by transmission electron microscopy duringin situ heatingexperiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thinchemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially inpyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicidelayer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grainsshow different epitaxial relationships with the Si substrate. Ni2Si is never observed. ©2001American Institute of Physics.@DOI: 10.1063/1.1378812#

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I. INTRODUCTION

Metal silicide thin films grown in a controlled manner osilicon wafers play an important role in device fabrication fthe microelectronic industry. Due to their low resistivity theare widely used in very large scale integrated technoloAmong these materials, nickel monosilicide has attractespecial interest in the fabrication of advanced submicronvices because it can be produced at lower temperatures,limiting diffusion in the process of high density integratecircuits. It also results in less Si consumption than neededthe formation of CoSi2, allowing more shallow junctionsFor these devices, the microstructure of silicides playsimportant role. Understanding the formation and growth elution of nickel silicides in patterned structures with finlines is not only technologically important but it is also iteresting from a pure scientific point of view.

Nickel silicide formation and growth were quite extesively studied in the past.1–7 They are produced by deposiing a rather thick~.100 nm! Ni film on ~111! or ~001! Siwafers followed by annealing at different temperatureshas been found that the first silicide to grow is Ni2Si. It startsforming at 200~Ref. 2! or 250 °C. It takes approximately 1to grow 100 nm of Ni2Si on~001! Si at 300 °C.3 When all theNi is consumed for the formation of Ni2Si, NiSi starts grow-ing at approximately the same temperature. As it is poinout in Ref. 3, it also takes 1 h to grow 100 nm NiSi at350 °C.

NiSi has the lowest resistivity among the Ni silicides atherefore it is desirable for device fabrication. It hasorthorhombic structure with lattice parametersa50.5233 nm,b50.3258 nm, andc50.5659 nm, and spac

a!Also at: E.E. Department, K. U. Leuven, Leuven, Belgium.b!Author to whom all correspondence should be addressed; electronic

[email protected]

1670021-8979/2001/90(1)/167/8/$18.00

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group Pnma. Both Ni2Si and NiSi growth are diffusioncontrolled.6

NiSi2 is the last silicide to be formed at much hightemperatures, i.e., around 800 °C.5,6 It can grow epitaxiallyon ~001! and ~111! Si and the reaction is controlled bynucleation phenomena. According to marker experiment3,5

Ni is the only moving species for the formation of all thresilicides. NiSi2 has a cubic CaF2 structure with a lattice pa-rametera50.5395 nm. There is only a small lattice mimatch with Si at room temperature~20.4%!. Therefore, ep-itaxial growth of NiSi2 on Si is the easiest among asilicides.8 Thin layers of epitaxial NiSi2 are grown on~111!Si even at room temperature.9 More recently, it has beenshown that NiSi2 can grow epitaxially on~001! Si at tem-peratures as low as 220~Ref. 7! or 400 °C,8 if the annealedNi layer is very thin@55 ~Ref. 7! and,10 nm,8 respectively#.

The in situ production of the silicidation reaction in thelectron microscope permits the direct structural investition of the formation and transformation of the different sicide phases. The growth of Ti10 and Co11 silicide phases inactive lines has been studied in this way. In this articlepresent anin situ transmission electron microscopy~TEM!study of nickel silicidation processes in submicron actlines on patterned~001! silicon substrates.

II. EXPERIMENT

~001! silicon wafers are patterned with a field oxidmask by means of the polyencapsulated local oxidatprocess,12 yielding active lines with different widths andspacings in the range of 0.25–10mm isolated by a 150 nmthick oxide. Also, larger unpatterned regions are presentthe wafers. After cleaning, a thin chemical oxide is leftil:

© 2001 American Institute of Physics

IP license or copyright, see http://ojps.aip.org/japo/japcr.jsp

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the silicon surface. Subsequently, these wafers are covwith a metal layer consisting of 12 nm Ni or 12 nm Ncapped with a thin Ti layer.

Specimens for thein situ TEM analysis are prepared iplan view by cutting 3 mm diameter discs from both tblanket and patterned regions with an ultrasonic cutter.discs are polished from the back side of the Si waferdimpling and subsequent ion milling from the same sidea hole appeared in the center region.In situ heating experi-ments are performed in the Philips CM20 electron micscope equipped with a double tilt Gatan heating holder whworks up to 1000 °C. The specimen for thein situ TEManalysis is fixed on its rim with a Pt washer and screw ismall furnace mounted in the tip of the heating holder.thermocouple welded to the furnace measures the tempture during the experiments. The furnace is heated by splying current at a manually controlled rate. The stagewater cooled so that the temperature can be maintaineddesired value. Therefore, a firm contact to the furnacenecessary. As this is realized, the difference between thetual temperature of the specimen and the measured onepends on the thermal conductivity of the specimen. Oncetemperature on the specimen is stabilized this differetends to vanish. It has been observed that at high temptures~higher than 800 °C! the contact between the specimand furnace can become looser because of dilatation diences between specimen, washer, and screw. This can inslight differences between the measured temperature anactual temperature on the specimen of a few degrees cgrade.

III. RESULTS

A. In situ heating of blanket specimens

In order to determine the temperature ranges whereferent forms of Ni silicides are produced, severalin situ heat-ing experiments are performed on blanket specimens. Fig1 shows the evolution of the electron diffraction patterwith temperature for the 12 nm thin Ni layer deposited on~001! Si substrate. At room temperature,@Fig. 1~a!#, the elec-tron diffraction pattern shows rings corresponding topolycrystalline Ni film and spots of the@001# oriented Sisubstrate. Note that the 200 and 020 reflections, forbidfor the Si structure, do not appear in the diffraction patteBy heating the specimen in the electron microscope300 °C for 5 min, the diffraction pattern changes as shownFig. 1~b!. Faint diffraction spots at the 200 and 020 positioappear, showing that epitaxial NiSi2 starts growing as thefirst silicide to be formed in these experiments. By continuheating, in a temperature interval of approximately 100for 10 min, NiSi2 is completely formed@Figs. 1~c! and 1~d!#.It grows fully epitaxial on the~001! Si substrate. The crystallographic relationships are:@001#NiSi2i@001#Si and(200)NiSi2i(400)Si.

The TEM images~Fig. 2! of the blanket specimen adifferent temperatures corresponding to the diffraction pterns of Fig. 1 illustrate the formation and growth of NiS2.The room temperature image of Fig. 2~a! reveals, of coursethe morphology of the polycrystalline Ni thin film. The N


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layer is continuous and the crystallites are rather small:the average less than 10 nm. When NiSi2 starts forming, thecontrast of the TEM image, Fig. 2~b!, changes. A graymottled ‘‘flower’’-like contrast appears at 300 °C showinthe formation of the NiSi2 phase. The possible epitaxial Nsilicide formation at these low temperatures is consistwith previous results.7 As the temperature is slightly increased@Figs. 2~c! and 2~d!#, the NiSi2 crystallites doubletheir size and many of them reveal square shapes. In2~d!, which corresponds to the diffraction pattern of Fi1~c!, besides the Ni and NiSi2, some denuded zones of brighcontrast can be observed. Hence, the thin film is no loncontinuous. These denuded zones start appearing at 33@Fig. 2~c!# and develop with increasing temperature. Nothat approximately 5 min are spent on each heating step

FIG. 1. The evolution with temperature of the electron diffraction patteof Ni/~001!Si blanket specimen:~a! room temperature~RT!, ~b! 300, ~c!350, and~d! 400 °C. Note the apparition of NiSi2, as first silicide to beformed.

FIG. 2. TEM images of Ni/~001!Si blanket specimen showing the formatioof NiSi2: ~a! RT, ~b! 300 °C NiSi2 appears, at~c! 330 °C and~d! 350 °Cdevelopment of NiSi2.


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169J. Appl. Phys., Vol. 90, No. 1, 1 July 2001 Teodorescu et al.

that, until Fig. 2~d! is taken, the specimen has been anneafor about 15 min.

By keeping the specimen temperature around 400 °C10 min, another important modification appeared in the TEimages. A rather fast transformation front traverses theage, with a velocity which is estimated at approximately 2nm/s, leaving the image as in Fig. 3~a!. The correspondingdiffraction pattern of this region of the film is given in Fig3~b!. By comparison with Fig. 1~d!, which shows the diffrac-tion pattern of the same region of the specimen, just bethe transformation front has passed, one can see thanewly appeared diffraction spots reveal the formation ofother compound. Note that the lapse of time between thediffraction patterns is approximately 1 min. As we shall sfurther the fainter diffraction reflections in Fig. 3~b! can beindexed as epitaxial NiSi along the@001# NiSi zone axis.

The TEM image in Fig. 3 also shows that the squashaped NiSi2 crystallites grow in size and join each otheThe monosilicide layer tends to grow continuously coverthe NiSi2 crystallites and the empty spaces. The ‘‘wrinkledlike contrast which appears might suggest the presencinterfacial dislocations between the two silicides. This cotrast is present in the TEM images as long as NiSi is presin the specimen. At temperatures higher than 800 °C, Ntransforms to NiSi2. This is obvious in the diffraction patterns ~not shown here!, but also in the TEM image of Fig4~b!. This image shows an increased sharpness of the N2

grains. It reveals the pyramidal shape of NiSi2 crystallites, as

FIG. 3. TEM image~a! and corresponding diffraction pattern~b! at 400 °Cshowing the formation of NiSi on the blanket specimen.


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it was already observed in cross section TEM imagesdifferent authors.13,14 They showed that the NiSi2 pyramidsgrow with the tip inside the Si substrate and the~111! facesparallel to the~111! planes of silicon. The square base of tpyramids is parallel to the Si wafer surface. The NiSi2 pyra-mids are oriented on the Si cube faces with the edges ofsquare base parallel with the^110& directions of the Si. Whenthese pyramids grow or join they form truncated pyram@Fig. 4~b!# with a rectangular base. It is also worth mentioing that the NiSi2 crystallites did not grow in size by heatinin the temperature range of 400–800 °C, where the Nphase still exists~compare Figs. 3 and 4!, showing that theNiSi2 phase has been formed at low temperatures~300–400 °C!.

Figure 5 reveals the diffraction patterns of differerather large regions~5 mm in diameter! of the silicide filmgrown on the~001! Si blanket. These diffraction patterns ataken at temperatures of 470 or 500 °C where the NiSi2 andNiSi coexist and are fully formed. They show that both sicides grow epitaxially. Note the similarities of the diffractiopattern from Fig. 5~a! with that from Fig. 3~b!, which istaken just after the NiSi is formed. Because of the laymorphology of the specimen, double diffraction effects apresent which give rise to a large number of diffraction spin positions forbidden by the space group. By indexing tdiffraction patterns of Figs. 5~a!–5~c!, the following crystal-lographic relations for the epitaxy of NiSi can be deduce

~1! From Fig. 5~a!: @001# NiSi//@001# Si and ~020! NiSi//~040! Si.

FIG. 4. TEM images of the blanket specimen at different temperatures~a!at 500 °C both NiSi and NiSi2 coexists, ~b! at 860 °C only the NiSi2

pyramids are present.


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~2! From Fig. 5~b!: @100# NiSi//@001# Si and ~020! NiSi//(2̄20) Si.

~3! From Fig. 5~c!: @101̄# NiSi//@001# Si and ~020! NiSi//(22̄0) Si.

For NiSi2 the epitaxial relations remain unchanged as mtioned before. From the above epitaxial relations,~1! and~3!are most often encountered. Random NiSi orientationsnot observed on blanket specimens. The epitaxy of NiSi

FIG. 5. Diffraction patterns of large grains on the blanket specimen showthree different epitaxial orientations for NiSi~a!, ~b!, and~c!. The NiSi andepitaxial NiSi2 coexist at temperatures around 500 °C.


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~001! Si is realized in such a way that one of the faces ofNiSi rectangular prism is parallel with the Si cube face. TNiSi rectangular prism is oriented with one of its edges pallel to the face diagonal of the Si cube. No importachanges occur in the TEM images and in the diffraction pterns upon heating the specimen in the temperature inte500–800 °C. At temperatures higher than 800 °C, the diffrtion reflections characteristic for the nickel monosilicide dappear. The disappearance of NiSi at high temperatureindeed also observed in the TEM image from Fig. 4~b!, as ithas been shown previously.

B. In situ heating of narrow structures

The next step for thisin situ study is to find out how thesilicidation reaction takes place in narrow structures~submi-cron lines!, which are used in semiconductor devices.14–16Inour particular case the narrow structures consist of groupten lines with equal nominal width and spacing, or of islated lines. The thinning of the plan view specimensTEM is done in such a way that a group of active lincrosses the region transparent to the electron beam moless parallel with the slope direction of the specimens.

Different specimens for TEM analysis have been ppared for different heating treatments in the electron micscope. From these experiments some general observacan be outlined. The growth of Ni silicides in active linefollows the regime of growth as observed for the blankfilm. This is shown in Fig. 6 which illustrates in~a! a planview TEM image of a 800 nm wide active line. The linitself can be recognized from the bend contours producedthe Si substrate. A 12 nm Ni layer has been deposited online and on the amorphous SiO2 spacings. By heating to approximately 300 °C firstly NiSi2 is formed. The flower-likecontrast is evident in the line@Fig. 6~b!#, while the contraston the oxide spacings remains unchanged. Also, the~200!reflections appear in the diffraction pattern correspondingthe line, as in Fig. 1~b!. At 370 °C the NiSi2 pyramids arewell formed. They tend to align themselves at the borders

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FIG. 6. Successive plan view TEM images showing the formation ofsilicides at different temperatures, in an 800 nm thick active line:~b!–~c!NiSi2, ~d! NiSi and NiSi2. Note the formation of railways at the border othe lines.


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the active line, while in the center of the line they grow morandomly, as in the blanket~Fig. 2!. After approximately 10min at 400 °C@Fig. 6~d!# the transformation front passes anNiSi is formed and the whole active line is covered wsilicides. The NiSi2 pyramids join and form, at the edges,kind of ‘‘railway’’ which will exist no matter how high andhow long the specimen is heated. As in the blanket spmens, in the active lines NiSi2 grows epitaxially by follow-ing the same crystallographic relations, i.@001#NiS2//@001#Si and (200)NiSi2 //~400!Si. It is worthmentioning that the active lines on the~001! Si wafer arealong @110# directions of Si. Because, as it has been preously mentioned, the edges of the NiSi2 pyramids are parallewith the ^110& direction of Si, it is possible that this favorthe alignment of NiSi2 pyramids along the borders of thactive lines.

Also, in the active lines NiSi grows in an oriented waduring further heating. Large 300–400 nm crystallitesformed in crystallographic relationships with Si suppoMost often the NiSi crystallites grow in such a way that oface of the NiSi prism is parallel with the Si cube facDiffraction patterns like those from Figs. 5~a! or 5~b! onblanket specimens are common for the active lines too.

Two other situations recorded at different temperatuand in different parts of a 500 nm wide active line are illutrated in detail in Figs. 7 and 8. When the rail joins tmiddle part of the line, the NiSi and NiSi2 crystallites join.The tendency is to create a continuous layer of silicides. Toccurs around 400 °C when the transformation front pasThe diffraction pattern of Fig. 7~b! corresponds to the zonmarked with an arrow on Fig. 7~a!, with a large NiSi crys-tallite and two smaller grains near the rail. The diffractispots corresponding to this large NiSi crystallite are marwith white lines on Fig. 7~b!. Si and NiSi2 diffraction reflec-tions are encircled. From this pattern one can deduce

following crystallographic relation: @011̄#NiSi//@001#Si.There is a rotation of 2° between the two structures. Insimilar way, Fig. 8~b! shows the diffraction pattern mostlcorresponding to the large NiSi crystallite from Fig. 8~a!. Inthis case the crystallographic relation i

@11̄0#NiSi//@001#Si, with the NiSi and Si rotated by 3°These last two crystallographic relations imply that for ctain NiSi crystallites in the active lines the diagonal of tNiSi rectangular prism faces is parallel with the edges ofSi cube. On the borders of the active lines, i.e., whereNiSi2 rails are, the NiSi crystallites frequently showed diffeent orientations. The most observed orientations are a

the @13̄1# or @ 2̄3̄1# zone axes.The evolution of the silicidation reaction in lines whe

12 nm Ni capped with Ti are deposited on~001! Si is shownin Fig. 9. Again NiSi2 is the first silicide to form at temperatures around 300 °C, as rails on the borders of the active land in the lines as well@Fig. 9~b!#. At 400 °C NiSi is also

formed@Fig. 9~c!#. The dark field image with the (101)̄ dif-fraction spot of NiSi @Fig. 9~d!# reveals the presence onickel monosilicide in the active line and on its borders.further temperature increase results in an increase in sizthe silicide crystallites which start to agglomerate.


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Finally, we have to point out that Ni2Si is not found inany of the heating experiments, either on blanket specimor on specimens with active lines.

IV. DISCUSSION

In the literature it is reported that when a rather thi~100–200 nm! Ni film is deposited on Si and annealethe silicidation reaction follows the sequence Ni2Si ~ortho-rhombic!→NiSi ~orthorhombic!→NiSi2 ~cubic!, in a tem-perature range of 250–800 °C.6 NiSi forms only after thecomplete growth of Ni2Si with complete consumption of thNi.3 It is observed that this sequence is very dependenthe thickness of the deposited Ni film.8,17,18 Very thin ~,3nm! Ni films deposited on~001! Si transform into NiSi2 atannealing temperatures in the range 350–400 °C.8,19 It isshown that NiSi2 nucleates directly from the deposited NNiSi2 being the only silicide formed. The NiSi2 crystallitesgrow laterally across the film and epitaxial with the Si sustrate. In the initial nucleation stage the very thin NiSi2 filmsconsist only of islands limited by~111! facets, called facetbars, without~100! interfaces. As the islands grow by facprogression flat~100! interfaces are left behind.

FIG. 7. TEM image~a! and corresponding diffraction pattern~b! of Nisilicides in an active line. In~b! the NiSi2 and Si reflections are encircledwhile the NiSi reflections are marked with black points and gratings.


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In the in situ TEM heating experiments described hethe Ni layer is thicker~12 nm!, but yet much thinner than inthe earlier annealing experiments~.100 nm!.1–6 Conse-quently, it becomes less surprising that NiSi2 is the first sil-icide to be formed. Since, as determined by marexperiments,5,6 Ni diffusion coefficients are 10 times largethat Si diffusion coefficients in the initial stage of NiSi2 for-mation Ni diffuses in Si and forms NiSi2. NiSi2 nucleationon ~111! Si occurs even at room temperature9 and the epitaxyis easiest on the$111% Si planes. For this reason the growinNiSi2 crystallites take pyramidal shapes, oriented withpyramid tip protruding into the Si substrate@the ~111! planesof NiSi2 being parallel to the~111! planes of Si#. The pyra-mid bases are parallel with the wafer plane, since, asvealed in the diffraction patterns@Fig. 1~d!# the ~200! NiSi2plane is parallel to the~400! Si plane. If the Ni source issufficient, these pyramids grow and neighboring crystallijoin forming truncated pyramids~Fig. 4!. This process oc-curs in our case in the temperature interval of 300–400 °Cis obvious from this description that the NiSi2 /Si interfacehas to be very rough. The atomic structure of this interfacanalyzed by Chernset al.20 and Chen and Chen.21 Theyshowed that the junctions between the~001! and~111! facets

FIG. 8. TEM image~a! and corresponding diffraction pattern~b! of Nisilicides in an active line. In~b! the NiSi2 and Si reflections are marked witbullets and the NiSi reflections are marked with crosses and gratings.


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are made through dislocations with Burgers vectora/4^111&.In the in situ TEM experiments, in the temperature ran

400–800 °C NiSi is found to coexist with NiSi2. This is thetemperature range where NiSi is found as the stable silicin all the previous experiments. NiSi can only be formedunreacted Ni still existed. This can happen only if the Ni filis thick enough not to be completely consumed by the Ni2

pyramids. Ni traces are indeed still observed in many ofdiffraction patterns taken at temperatures around 400 °C,fore the transformation front passed. After NiSi is formedtraces of Ni are observed in the diffraction patterns anymoIt is worth mentioning in this respect that CoSi2, which isisostructural to NiSi2, can transform by annealing to CoSiadditional Co is available for the reactioCoSi21Co52 CoSi2. This would imply a shrinkage or annihilation of the pyramids which is not observed in the presexperiments.

The NiSi film grown in ourin situ heating experimentsshould be rather thin since only a slight increase of the Ni2

crystallite size is observed at 860 °C, Fig. 4, where NiSifinally completely transformed to NiSi2.

One more problem is the presence of the railway strture at the border of the active lines in all the specimeinvestigated. In the attempt to understand this phenomecross section TEM specimens are prepared from theheated samples. One set of cross section specimens istained in the standard procedure by slicing, gluing the pieface to face, curing the glue at 150 °C for 1 h, polishing, afinally ion milling ~in a Balzers RES 010, 5 kV, 6° incidencfrom the sample surface! to get transparency to the electrobeam. The temperature of the specimen during the ion ming is not well known, but from our experience it is nohigher than 150 °C.

Figure 10 a shows a TEM image of an active linecross section. Several interesting features are revealedFirst, a very thin amorphous layer~of approximately 1 nm!exists between the metallic layers and the silicon substr

FIG. 9. Successive plan view TEM images showing the formation ofsilicides in a 400 nm thick active line with Ti cap:~b! NiSi2, ~c!–~f! NiSi2and NiSi. Note the presence of railways at all temperatures where the N2

crystallites exist and the dark field image in~d! revealing the NiSicrystallites.


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This is the chemical oxide left on the silicon surface aftercleaning step done before the metal deposition. This layenot visible at the borders of the active line, where, onother hand, the Ni layer becomes thinner. NiSi2 pyramids areformed at the borders and in the middle of the lines eventemperatures as low as 150 °C necessary for the cure oglue. At higher magnifications~Figs. 11 and 12! more detailsare revealed. Figure 11 is a high resolution cross secimage of a middle of an active line. The three-layer perioicity in the contrast is typical for twinned material22 andindicates that the NiSi2 is at least partially formed in thetwinned ~‘‘ B-type’’ 19,21,23! orientation.

The formation of the NiSi2 pyramids by diffusion of Nithrough the thin SiOx layer is obvious. Figure 11 revealspartial depletion~consumption! of the Ni thin film in theregion facing the pyramid base. Such a process of NiSi2 for-

FIG. 10. Cross section TEM images prepared in two different ways showthe profile of the active lines. In~a! the sample is heated at 150 °C for 1while preparing the cross section specimen. Note the presence of N2

pyramids in the line and on the borders. In~b! the cross section specimenprepared without heating. Note the borders of the line where the Nitouches the Si substrate.

FIG. 11. Cross section high resolution TEM images taken at the middlan active line. The specimen is prepared in cross section by heatin150 °C for 1 h. Note the amorphous SiOx layer at the interface between thmetallic film and silicon and the NiSi2 pyramids protruding in the siliconsubstrate.


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mation might be similar to the oxide mediated epitaxy of tisostructural CoSi2 on Si, recently reported.24 Figure 12 is ahigh resolution image of the border of the active line.actually shows how the railways are formed by diffusionNi in Si at the line border, with subsequent formationNiSi2. This indeed confirms the results obtained by tin situ experiments performed on plan view specimens~Figs.6–9!. The preferential growth of NiSi2 on the edges might berelated to a smaller thickness of the chemical oxide in thregions or to the higher stress which might favor the epitial growth.

In an attempt to study the initial condition of the linecross section TEM specimens are also prepared avoidingheating. A single strip is glued on a TEM grid with cyanorylate which does not need a curing step. The thinningdone by focused ion beam~FIB! milling with the trenchprocedure.25 The temperature rise during FIB milling is estmated to be only on the order of 10 °C.25,26 Figure 10~b!shows a cross section TEM image of an active line inspecimen prepared in this way. The amorphous SiOx layerbetween the Ni film and Si substrate is, again, less visiblethe borders of the line where the Ni film ‘‘touches’’ the Ssubstrate. This is better evidenced at higher magnificationFig. 13, which is a high resolution image of the active liedge. Figure 13 also shows that the Ni layer near the edgthe line is thinner, but to a lesser extent than in Fig. 1Hence, although the minimized heating procedure followfor the FIB specimen preparation, the reaction still occurat the edges of the active lines. Whether this reaction tplace during the specimen preparation or before cannoconcluded. It is however clear that the configuration atedges of the active lines, i.e., a possibly thinner chemoxide and high stress are the origin of the railway formatduring thein situ heating experiments.

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FIG. 12. Cross section high resolution TEM image taken at the edge oactive line. Note the disappearance of the amorphous SiOx layer and thesignificant decrease of the Ni film thickness at the line edge. A Ni2

pyramid is also revealed at the line edge in the Si substrate.


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V. CONCLUSIONS

It is shown that during thein situ heating of thin Nilayers on silicon epitaxial NiSi2 is already formed at lowtemperatures. The formation of the disilicide as the fiphase is related to the presence of a thin chemical obetween the Ni and silicon substrate. The disilicide coexwith unreacted Ni until a temperature of 400 °C when a ftransformation of the unreacted Ni to NiSi occurs. This laytends to cover the whole surface, i.e., over the disilicpyramids as well as over the silicon in between. It shodifferent epitaxial relationships with the silicon. Up t800 °C the NiSi and NiSi2 coexist and show no noticeabgrain growth. At higher temperatures the monosilicide atransforms to the disilicide. In the whole temperature ranused in this study, epitaxial NiSi2 grains are present. However, Ni2Si, reported as the first phase during the reactionthick Ni layers on Si, is not observed during thesein situexperiments.

FIG. 13. Cross section high resolution TEM image at the edge of an acline. The specimen is prepared in cross section by FIB without heating. Nthat at the line edge the Ni film touches the Si substrate. The Ni lathickness slightly decreases while the amorphous SiOx layer vanishes.


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In line structures, a preferential growth of the epitaxgrains near the edges of the lines is observed, resultingrailway structure. This is related to the specific shape ofopened oxide windows. Also, the stress at the line bordmight favor the epitaxial growth by minimizing the mismatch.

ACKNOWLEDGMENT

This work was performed in the framework of the Bilaeral Program BIL 97/93 between Flanders and Romania.

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