Spectromicroscopy of interfacial interactions between thin Ni films and a Au–Si surface

Spectromicroscopy of interfacial interactions betweenthin Ni ®lms and a Au±Si surfaceL.Gregoratti a ,A.Barinov b ,L.Casalis a ,M.Kiskinova a,*aSincrotrone Trieste,Area Science Park Basovizza,34012Trieste,Italy bKurchatov Institute,Kurchatov sq.1,123182Moscow,RussiaReceived 29May 2000;accepted 24August 2000AbstractThe interaction between a (p 3Âp3)R308-Au/Si surface with thin ( 2ML)Ni ®lms with different dimensions deposited at 300K is studied by means of a scanning photoelectron spectromicroscopy.The interfacial processes occurring at different Ni coverages and annealing temperatures are probed with lateral resolution of 0.12m m.The results have revealed that Ni displaces Au from the Si surface and the presence of Au lowers the formation temperature of the Ni disilicide phase.It has been found that the behavior of the Ni/Au±Si interfaces at high temperatures,in particular the changes in the morphology of the interface and the formation of well de®ned Ni x (Au 1Àx )Si 2islands,is strongly in¯uenced by the dimensions of the Ni patches.The observed evolution of the Ni/Au±Si interfaces is interpreted considering the high solubility of Ni in Si,the miscibility between Ni and Au and the Au±Si alloying at elevated temperatures.#2001Elsevier Science B.V .All rights reserved.Keywords:Silicide interfaces;Surface structures;Spectromicroscopy;Surface reactions;Si;Ni;Au1.IntroductionTransition metal silicides have widely been used in microelectronic technology as Schottky barriers,con-tacts,interconnects,etc.In very large scale integration circuits the diminishing widths of the metal silicide lines can lead to undesirable increase of the resistance.The resistance of Ni silicides turned out to be the least affected by the shrinking dimensions of the line widths which makes them attractive for potential applications [1].Among the various nickel silicide phases the NiSi 2is the most desired but its nucleation-controlled for-mation requires a high temperature.However,asshown recently,even after high temperature annealing NiSi still coexists with NiSi 2[2,3].Recent structural and Rutherford backscattering spectroscopy studies reported that the presence of Au could modify the solid state reaction between Ni and Si lowering the nucleation temperature of NiSi 2[4,5].Earlier studies of formation of other transition metal silicides in thepresence of an Au interlayer of thickness >100AÊrelated the effect of Au to enhanced Si out-diffusion [6].However,the mechanism of the Au `promotion'effect is not clear.It is well known that depending on the temperature a Au±Si alloy or ordered 2D phases mixed with agglomerated Au islands can dominate the Au/Si interface [7,8].In the case of bimetallic sys-tems,the evolution of the interface becomes more complicated because it depends on many factors,such as metal±Si af®nity,miscibility of the metals withtheApplied Surface Science 171(2001)265±274*Corresponding author.Tel.: 39-040-375-85-49;fax: 39-040-375-85-65.E-mail address :kiskinova@elettra.trieste.it (M.Kiskinova).0169-4332/01/$±see front matter #2001Elsevier Science B.V .All rights reserved.PII:S 0169-4332(00)00763-7formed phases,etc.[9].Very important also is the surface morphology of thin NiSi 2®lms grown in the presence of Au and the lateral distribution of Au during temperature treatments of the bimetallic interface.The aim of the present study is to get insight of the interactions between very thin Ni ®lms and Si in the presence of Au,using a method that combines surface chemical sensitivity with high spatial resolution.Par-ticular attention is paid to the changes in the chemical state and lateral distribution of Au and Ni.Here,we started with a very well de®ned two-dimensional (2D)(p 3Âp 3)R308-Au ordered phase on Si(111)and studied the interfacial reactions with Ni ®lms,depos-ited through masks of different dimensions.The con-centration pro®le across the edge of such `con®ned'Ni ®lms allows investigation of the effect of the ®lm thickness under the same experimental conditions and of the mass transport of material during temperature treatments.Essential for better understanding the behavior of this bimetallic interface is the possibility of reference measurements of the Ni-free H Au/Si region outside the Ni patch.It should also be noted that the morphology of the `con®ned'®lms is similar to interconnect and contacts in the electronic devices,where mass transport phenomena and edge effects may become important.Here,we will show that the processes dominating the thermal evolution of the Ni/Au±Si interface depend on the actual size of the Ni patches with respect to the substrate dimensions.This con®rms that the con®ned metal ®lms can behave differently from discontinuous ®lms,an important issue in characterization of metal/semiconductor interfaces at length scales of interconnects and contacts.2.ExperimentalAll experiments were carried out with the scanning photoelectron microscope (SPEM)at the ELETTRA synchrotron light source,which uses zone plate optics for producing a microprobe with sub-micrometer dimensions [10].Here we used a zone plate,which provided spatial resolution of 0.12m m.The micro-scope works in an imaging mode with the electron analyzer collecting photoelectrons with a speci®c kinetic energy,while scanning the sample with respectto the beam,or in a micro-spectroscopy mode,mea-suring photoelectron (PE)spectra from a feature selected from the image.The measurement station is equipped with facilities for specimen preparation,metal evaporation and sample characterization by low energy electron diffraction (LEED)and Auger elec-tron spectroscopy (AES).The temperature of the samples is controlled using an optical pyrometer with calibrated emissivity factors.The initial Au/Si interface was prepared by anneal-ing of about 1ML Au ®lm deposited on a (7Â7)-Si(111)(n-doped,0.17O cm)sample with dimen-sions 3mm Â7mm.1ML equals the (1Â1)-Si(111)surface atomic density,7X 8Â1014atoms/cm 2.The LEED,AES and PE spectra indicated that the formed (p 3Âp 3)R308structure contains 0X 9Æ0X 1ML Au and consists of uniform H 3-domains and domain walls [11,12].1and 2ML of Ni were post-deposited through masks with dimension 1X 4mm Â7mm and 0X 6mm Â0X 1mm.In the former case,bout 43%of the H 3-Au/Si surface was covered by Ni,whereas the smaller Ni patch covered only 0.3%of the sample.During all evaporation and annealing procedures the pressure was kept in the low 10À10mbar range.All measurements were per-formed with a photon energy of 493eV and energy resolution of 0.3eV.For the used photon energy the effective escape depths of Si 2p,Au 4f and Ni 3p photoelectrons for our geometry are estimated to bearound 4AÊ.The Fermi energy position is determined from the valence band spectrum of the fresh Ni ®lm deposited on the Ta clips.Doniach±Sunjic functions convoluted with gaussians,which simulated the over-all energy spread,are used for ®tting the PE spectra [13].3.ResultsThe Ni 3p and Au 4f maps and the valence band (VB),Si 2p,Au 4f and Ni 3p spectra,measured inside,outside and across the edge of the Ni ®lms,are used as ®ngerprints for the evolution of the interface as a function of the Ni coverage and temperature.Fig.1(a)shows a typical Ni 3p map,where the part covered with Ni appears bright.In the present study,the concentration pro®le measured across the edge of the Ni ®lm varies between 0and 2ML and extends266L.Gregoratti et al./Applied Surface Science 171(2001)265±274over about 15m m,as can be seen in Fig.1.This allows precise monitoring of the Ni coverage effects.As will be demonstrated in the next sections the lateral changes in the contrast of the Ni 3p and Au 4f maps inside the patch re¯ect the temperature-induced changes in the morphology of the interface.3.1.Coadsorption of Ni at 300KFigs.1(b)and 2show selected VB,Ni 3p,Au 4f and Si 2p spectra taken on the Ni-free H 3-Au/Si area and across the edge of a 2ML Ni ®lm deposited at 300K.The VB spectrum of the H 3-Au/Si surface is domi-nated by the Au 5d states,forming the doublet below À4eV in Fig.1(b).The intensity close to the Fermi level increases with increasing Ni coverage with a maximum moving from approx.À2.0to À1.6eV.TheNi 3p panel in Fig.1(b)shows that the Ni 3p intensity gradually increases with Ni coverage and shifts by 0.6eV in the coverage range 0.8±2.0ML,reaching the energy position of the metallic Ni state (À67.0eV).The Au 4f spectra of the H 3-Au/Si phase,shown in Fig.2,are shifted by À1.3eV with respect to the metallic Au energy position and require a larger gaussian width,most likely due to the different bond-ing con®gurations within the domains and the domain walls.The Si 2p spectra of the H 3-Au/Si phase are ®tted using a bulk component,B-Si,and two `reacted'Si components,R1and R2.The B-Si 2p 3/2energy,À99.4eV ,coincides with the B-Si position of the pinned (7Â7)-Si(111)surface,indicating negligible band-bending effects.The R1-Si and R2-Si compo-nents are chemically shifted by À0.3and À0.5eV with respect to the B-Si component.They are assigned to the Si atoms bonded to Au within the H 3-domains (R1)and at the Au-rich domain walls (R2).The R1/R2ratio depends on the density of the domain walls,which can cover up to 34%of the surface [14].The Si 2p and Au 4f spectra undergo intensity and lineshape changes with increasing Ni coverage,which are man-ifested by the spectra in Fig.2and the plots in Fig.3(a).The Au 4f spectra become broader in the Ni coverage range 0.9±1.5ML and sharpen with further increase of the Ni coverage.Close to 2ML the Au 4f lineshape and energy position (À84.1eV)become practically identical with that of the metallic Au state.The metallic Au component appears and grows at the expense of the `reacted'Au component at Ni coverage !0.8ML,which causes the broadening of the Au 4f peaks up to 1.5ML.The change of the Au chemical state at high Ni coverages also leads to a shift of the Au 5d states (Fig.1)by 0.3eV .In the Ni coverage range 0±1.0ML,the intensity of the Si 2p signal decreases due to attenuation of the B-Si and R1components.The ®ts of the Si 2p spectra in Fig.2reveal that 0.25ML a new R3-Si component grows rapidly.From here on we will use the assignment R3-Si for the Si atoms with a mixed Ni Au coordination.It should be noted that the binding energy of the growing R3-Si at 300K is almost identical to the R2component assigned to the Si coordinated with Au at the domain walls of the H 3-Au structure,where the local Au coverage is far beyond 1ML.This assigns the R2-Si and R3-Si as components corresponding to Si atoms in a metal-rich environment.As can be seen in Fig.3(a)theR3-SiFig.1.(a)Ni 3p 64Â8m m 2map centered close to the edge of a 2ML Ni patch at 300K and the corresponding Ni concentration pro®le across the edge;(b)valence band (left)and Ni 3p (right)spectra as a function of the local Ni coverage.The spectra are taken in spots across the patch edge,as indicated by the dashed arrow in (a).L.Gregoratti et al./Applied Surface Science 171(2001)265±274267component shifts to a lower binding energy (BE)with increasing Ni coverage.Up to about 0.8ML this energy shift is accompanied by an increase of the gaussian width.Above 0.8ML the ®t of the Si 2p spectrum requires only the R3-Si component.With further increasing Ni coverage the Si 2p spectrum becomes narrower and loses intensity.3.2.Evolution of the Ni/Au±Si interface after annealingThe evolution of the interfaces for annealing tem-peratures below 770K is independent on the size of the Ni patches and is illustrated by the plots in Fig.3(b)and the selected PE spectra in Fig.4.The ®rst anneal-ing at $480K has already leads to dramatic changes in the PE spectra indicating vigorous reactions and formation of silicide phases.The major changes at 480K can be summarized as follows:(1)an appear-ance of a feature at approx.À3.1eV in the VB spectra;(2)a decrease of the Ni 3p intensity accompanied by a chemical shift with respect to the metallic state,which varies weakly (between À0.9and À1.1eV)with Ni coverage;(3)an increase of the Si 2p intensity,accompanied by a shift of about À0.45eV and (4)an energy shift of the Au 4f spectra by approx.À1.0eV with respect to the metallic state.A distinct feature is that after annealing to 480K the shapes and energy positions of the Si 2p and Au 4f spectra change negligibly with Ni coverage.After the second anneal-ing to 620K the bimetallic interface is characterized by Si 2p spectra shifted by À0.55eV with respect to the B-Si,Ni 3p levels shifted by À1.2eV with respect to the metallic state,a prominent feature at about À3.1eV in the VB spectra and negligible changes of the Au 4f region.The signi®cant lineshape changes of the Si 2p spectra after annealing to 770K are due to the growth of a B-Si component,leading also to a further increase of the total Si 2p signal (see Fig.3(b)).The Au 4f spectra become narrower and shift towards the energy position of the reacted Au in the H 3-Au/Si phase.The ®rst effect of the patch size observed after annealing to 770K is related only to the thickness of the formed NiSi 2phase (see the VB spectra in Fig.4).The annealing temperature at which the Ni 3p signal disappears depends on the size of the Ni patch.In the case of a small 1ML patch the Ni signal disappears and the H 3-Au/Si interface seems restored after annealing to 920K (see the plots in Fig.3(b)and the Ni 3p spectra in Fig.4).However,a close inspec-tion of the data in Figs.3(b)and 4shows certain differences between the Si 2p and the Au 4f spectra taken inside and outside the Ni-covered area.TheAuFig.2.Au 4f (left)and Si 2p (right)spectra as a function of the local Ni coverage.The spectra are taken in spots across the patch edge,indicated by the dashed arrow in 1(a).The ®tting components of Si 2p spectra are:dotted line ÐB-Si,dashed line ÐR1-Si;full line R2-Si,which converts to R3-Si with increasing Ni coverage.The vertical dotted lines indicate the positions of B-Si,metallic Au and reacted Au in the initial H 3-Au/Si(111)structure.268L.Gregoratti et al./Applied Surface Science 171(2001)265±2744f spectra are more intense from the area where the Ni patch was deposited.Accordingly,the Si 2p ®ts reveal an enhanced weight of the R2-Si component in the Si 2p spectra from the area where the Ni patch was.In the case of a small patch further annealing to 1070K removes completely the difference between the spec-tra taken from the area of the Ni patch and the H 3-Au/Si surface is re-established,which appears uniform at our length scale of 0.12m m.In the case of the 1or 2ML thick Ni ®lms,which cover more than 40%of the surface the Ni 3p signal diminishes with temperature but does not disappear completely even after anneal-ing to $1070K.A distinct feature in the case of the large Ni patches is the lateral heterogeneity in the interface composi-tion,that induced by annealing above 1070K.It is manifested by the Ni and Au maps and concentration pro®les in Fig.5(a)and the selected PE spectra in Fig.6.Further annealing to 1170K,a temperature above the Ni solubility line and the onset of Au desorption,leads to more dramatic changes in the surface morphology.The Ni 3p and Au 4f maps in Fig.5(b),taken inside the large 2ML Ni patch,reveal formation of Ni-rich islands separated by Au-rich areas.The Ni and Au concentration pro®les and PE spectra in Figs.5(b)and 6show that Ni is present only in the islands,where small amounts of Au are incor-porated as well.As can be judged from the pro®les in Fig.5(b)and the selected spectra in Fig.6the com-position of the islands varies with respect to the Ni/Au content.The Au 4f and Si 2p PE spectra out of the islands are similar to the spectra of the initial H 3-Au/Si interface but the Au coverage has diminished by $20%,which is due to partial desorption of Au and incorporation of some Au in the islands.4.Discussion4.1.Evolution of the bimetallic interface at room temperatureThe data in Figs.1and 3clearly show that the evolution of the Ni/Au±Si interface with increasing Ni coverage can be divided in two stages.The ®rst stage is the formation of a dense Au±Ni adlayer,completed after coadsorption of about 0.8ML of Ni and the second is the displacement and segregation of AuonFig.3.(a)Plots of the coverage dependence of (top)the Au 4f,Si 2p photoelectron yields and the relative weight of the R3-Si component;(bottom)Au 4f 7/2,Ni 3p 3/2and R3-Si energy shifts,D E .The Au 4f and Si 2p yields are normalized against the Au 4f and Si 2p yields corresponding to the Ni-free H 3-Au/Si structure outside the Ni patch.The energy shifts are with respect to the energy position of the B-Si 2p 3/2,metallic Au 4f 7/2and metallic Ni 3p 3/2.(b)The Au 4f,Si 2p and Ni 3p photoelectron yields as a function of annealing temperature,measured for a 0.3m m 21ML Ni patch.The Si 2p and Au 4f intensities are normalized against the intensities of the Si 2p and Au 4f spectra measured outside the patch after each annealing step.The Ni 3p intensity is normalized against the Ni 3p intensity at 300K.L.Gregoratti et al./Applied Surface Science 171(2001)265±274269top of the Ni ®lm,which occurs at Ni coverages above 0.8ML.In more details,the scenario can be described with the following elementary steps.Initially the deposited Ni can adsorb on the one of the following three possible sites within the H 3-Au domains:on top of the Au trimers,which are in the ®rst layer,on top of the Si trimers in the second layer or in the hollow site [7,15].In the last two sites Ni actually sits above a Si atom from the fourth layer but the difference is in the distance to the second layer Si-trimer atoms.The fast decrease of the B-Si component (see Fig.3(a))and the À0.6eV shift of the Ni 3p line (similar to that reported for Ni atoms adsorbed on Si [16])suggest preferential Ni adsorption on top of the Si-trimers up to Ni coverage of approx.0.2ML.This is also consistent with the observed very small energy shifts of the Au 4f and 5d levels.Since in this locationthe Ni±Si interlayer distance (!1AÊ)is larger than the Au±Si distance ($0.5AÊ)in the H 3-Au/Si structure the observed attenuation in the Au 4f intensity can be ascribed to a screening effect,which is enhanced by our very grazing acceptance geometry [7].Further increase of the Ni coverage leads to occupation of the hollow and on-top Au trimer sites.The extinction of the B-Si component,the fast growth of the R3-Si and the evolution of the Au 4f and Ni 3p spectra (see Figs.1and 3(a))provide strong evidence that a rather dense Ni Au adlayer can be formed.The broadening and the chemical shift of the Au 4f spectra when the Ni coverage exceeds 0.2ML suggest conversion of the Au-trimers into a mixture of differently coordinated,less strongly coupled to the substrate,Au adatoms.Previous studies have shown that a dense,disordered Au (or Au Ag)layer of 1.7ML can reside on the Si(111)surface [15,17].Indeed,according to the Ni 3p and Au 4f energy positions in the present study both Ni and Au atoms should have remained in contact with Si with incorporation of up to about 0.8ML of Ni into the H 3-Au/Si surface.The growth of the metallic-like Au 4f component and the small variation of the Au 4f intensity (see Figs.2and 3(a))indicate that when the Ni coverage exceeds 0.8ML the Au atoms are gra-dually expelled from the Si surface and ¯oat on top of the Ni adlayer.The driving force for the displacement of Au should be the energy gain from the formation of the stronger Ni±Si bond.The intermixing between Ni and Si,i.e.the amount of Si incorporated in the Ni layer diminishes with increasing the Ni ®lm thickness.This is evidenced by the negligible energy shifts (À0.05eV)of the R3-Si component and the Ni 3p level,the metallic-like lineshape of the VB spectra and the fast attenuation of the Si 2p signal,observed when the thickness of the Ni ®lm exceeds 1.5ML.The VB spectrum of the 2MLFig.4.Evolution of the valence band (VB)(from left to right),Ni 3p,Au 4f and Si 2p spectra with annealing temperature measured for a 0.3m m 21ML Ni patch.In the VB panel with dotted line the spectra measured after annealing of a large 1ML Ni patch to 770and 920K are shown for the sake of comparison.In the Au panel the metallic Au 4f spectra measured for 2ML Ni patch is shown as well.The Au 4f spectra taken out of the patch after annealing to 920K are plotted with a dotted line.The ®tting components in the Si panel are:dotted line ÐB-Si,dashed line ÐR1-Si;full line R3-Si or R2-Si.The vertical dotted lines indicate the positions of NiSi 2derived states in VB,metallic Ni 3p 3/2,B-Si,metallic Au 4f 7/2and reacted H 3-Au 4f 7/2and B-Si.270L.Gregoratti et al./Applied Surface Science 171(2001)265±274thick Ni ®lm deposited on a H 3-Au/Si interface resembles the VB spectrum of an amorphous Ni 3Si interface layer with some pure Ni on top,obtained after deposition of much thicker Ni ®lms of 7ML on an atomically clean Si surface [18].This suggests that the Ni±Si intermixing at 300K is inhibited by the presence of Au.In fact our previous studies on the interaction of a 2ML Ni ®lm with a 7Â7-Si(111)surface have shown stronger Ni±Si intermixing at 300K leading to a Ni 2Si-like phase [16].From the attenuation of the Au 4f intensity,we estimated that the Au incorporated in the Ni-rich interface layer formed after deposition of 2ML Ni is about 0.15ML.This amount of Au appears to be suf®cient to suppress the Ni±Si intermixing at 300K.The rest $0.8ML of Au segregates on top and wets the Ni-rich phase,which is consistent with the much smaller surface free energy of Au compared to Ni [19].This behavior also is in accordance with the low miscibility between Au and Ni at room temperature,where only formation of a surface alloy is reported [20,21].The effect of Au on the degree of the Ni±Si intermixing is similar to the observed in¯uence of the impurities on the reaction rates and/or the mobility of the reacting species during formation of metal silicides [9].Let us compare the behavior of the present bime-tallic interface at Ni coverages close-to and above 1ML with the changes observed for Ni ®lms deposited on H 3-Ag/Si and H 3-Pd/Si surfaces [22,23].In the ®rst case the Ag is expelled from the surface,whereas in the second case no displacement of Pd occurs and the Ni ®lm grows on top of the H 3-Pd/Ni surface.The corresponding metal±Si bond strengths are 3.3eV for Ni±Si,3.16eV for Au±Si,2.7eV for Pd±Si and 1.8eV for Ag±Si.It is obvious that the different behavior of these three bimetallic systems is not simply governed by the thermodynamic force related to the energy gain of forming the Ni±Si bond [24].Apparently in the case of Ni±Pd/Si the activation energy barrier and/or the change in the interfacial energy play an important role as well.4.2.Temperature-induced transformations of the Ni±Au/Si interfaceThe evolution of the Ni±Au/Si interface upon step annealing up to 770K can simply be described as formation of a NiSi 2-like phase.It is evidenced bytheFig.5.(a)64Â4m m 2Ni 3p and Au 4f maps close to the edge of a large 2ML Ni patch taken after annealing to 1070K and the corresponding Au and Ni concentration pro®les across the edge.(b)64Â32m m 2Ni 3p and Au 4f maps after annealing of a large 2ML Ni patch to 1170K and the Au and Ni concentration pro®les taken in the three zones indicated in the maps.L.Gregoratti et al./Applied Surface Science 171(2001)265±274271appearance of the characteristic NiSi 2features:a 3.1eV peak in the VB due to the Ni 3d-derived states in the NiSi 2phase,the Ni 3p energy shift by À1.2eV and the reacted Si 2p component shifted by approx.À0.5eV [2,3,25].The attenuation of the Ni 3p signalalso is in accordance with the fact that 1ML ($1.4AÊ)of Ni ®lm forms about 5AÊthick disilicide layer.The completion of the NiSi 2phase in the presence of Au occurs at about 150K lower temperature compared to the Au-free Ni/Si interface [16].Similar `promotion'effect,i.e.lower temperature of formation,is also observed during annealing of the Ni ®lms deposited on H 3-Pd/Si and H 3-Ag/Si surfaces [22,23,26].The NiSi 2formation is a nucleation-controlled process and its activation energy is proportional to D s 3/D G 2(where D s and D G are the changes in the interface and free energy,respectively)[25].Thus,the promot-ing effect of Ag,Pd and Au atoms can be attributed to a decrease of the interfacial energy.In the case of Au,an affect on the free energy term also cannot be excluded because the incorporation of Au may slightly increase the bulk lattice parameters [4,5].The onset of NiSi 2formation at elevated tempera-tures also affects the chemical state of the segregated Au,which changes from metallic to `reacted'.Since the intensity of the Au 4f signal remains practically the same up to 620K (see Figs.3(b)and 4)this indicates that the segregated Au does not dissolve into the disilicide lattice.In fact the NiSi 2already contains 0.15ML Au,`trapped'during the Ni ®lm growth at 300K and as reported in [3]maximum 12%of Ni can be substituted by Au in the NiSi 2lattice.Hence,the change in the chemical state of Au should be attributed to interaction with the top epitaxial Si layer that terminates the NiSi 2phase [2±3,25].A rather peculiar observation is the different evolu-tion of the Ni patches of different dimensions at temperatures above 770K.In the case of a Ni ®lm with very small dimensions with respect to the Au/Si surface the Ni coverage decreases very fast and the patch practically disappears at 920K (see Fig.3(b)).In the case of large patches,which cover more than 20%of the surface,the Ni(Au)Si 2phase is still present at 920K and further annealing leads to dramatic changes in the morphology without a complete loss of Ni.This different behavior we attribute to the high Ni mobility in Si.Ni transport via subsurface inter-stitial paths in Si and Ni diffusion in thin NiSi 2®lms at elevated temperatures are well documented events occurring below 1070K,the temperature at which Ni dissolves into the bulk of Si [16,25,27].Apparently in the case of the small Ni patch,which covers only 0.3%of the surface,the Ni transport out of the patch leads practically to extinction of the patch already at 920K.The loss of Ni is accompanied by a release of the Au incorporated in the Ni(Au)Si 2®lm,asFig.6.Selected valence band (VB)(from left to right)Ni 3p,Au 4f and Si 2p spectra taken from two areas of the large 2ML patch illustrating the induced compositional inhomogeneity after annealing to 1070K (top panel)and from two different islands formed after annealing to 1170K (bottom panel).The ®tting components of the Si 2p spectra at 1070K correspond to R3-Si and B-Si (dotted line).The additional small lowest energy component in the Si 2p spectra from the islands is related to the structural arrangement of the segregated Si atoms on top of the islands [2,3].272L.Gregoratti et al./Applied Surface Science 171(2001)265±274evidenced by the increase of the Au4f signal within the patch area at temperatures!620K.Additional source for Au might be the transport of Au from the Ni-free areas by the mobile`liquid'droplets of the Au±Si alloy,formed above the eutectic melting tem-perature($620K)of the Au±Si alloy[8,28].We cannot also exclude that some of the metastable Au x 1À3Si phases are stabilized by the presence of Ni. Finally,let us discuss the temperature-induced morphological changes in the case of the large Ni patches,which actually can be considered as almost continuous bimetallic interfaces.Similar Au-free con-tinuous Ni thin®lms form a morphologically complex interface when Ni precipitates from the bulk during cooling from1150K to room temperature,which consists of a`(1Â1)'-Ni surface phase and randomly distributed micron and submicron-sized NiSi and NiSi2islands[2,3].In the present case,a heteroge-neous interface is already developed after annealing to $1070K(just below the Ni solvus line),when still the Ni(Au)Si2-like phase is present everywhere.Anneal-ing to1170K(above the Ni solvus line)leads to clear formation of Ni(Au)Si2islands divided by a Ni-free Au/Si surface.Apparently the presence of Au favors only the formation of disilicide islands.The differ-ences in the composition of the islands,revealed by the Au and Ni images and PE spectra in Figs.5and6, are most likely due random lateral redistribution of Au,when the highly mobile Au±Si liquid eutectic droplets decompose during cooling.It is possible that the`freezing'Au x Si y droplets act as nucleation centers for the islands.This interpretation is consistent with the Si2p spectra showing that the Ni(Au)Si2islands, which contain more Au,are also covered with a thicker Si layer.Apparently this Si layer segregates during the decomposition of Au x Si y droplets,when the temperature falls below the eutectic temperature.5.ConclusionThe observed changes in the composition and mor-phology of the interface formed after deposition of Ni ®lms with varying lateral dimensions on a H3-Au/Si surface lead to the following speci®c conclusions:1.At300K Ni coadsorption leads to gradual displacement of Au from the Si interface,whichis completed when the thickness of the Ni®lm approaches2ML.The major fraction of Au segregates on top of the®lm.The amount of Au trapped inside the®lm does not exceed$10%of the Ni coverage,but exerts a strong in¯uence on the evolution of the pared to the interface formed between a pure Ni®lm and Si the Ni±Si intermixing at300K is suppressed by the presence of Au.2.At elevated temperatures Au acts as a`promoter' reducing by150K the formation temperature of the NiSi2-like phase,where the trapped Au is incorporated as well.3.Above620K the temperature transformation of the bimetallic interface is strongly in¯uenced by the actual size of the Ni®lm.This dependence is related to the high mobility of Ni,which leads to a complete extinction of very small Ni patches below the dissolution temperature of Ni in Si.4.The Ni®lms with large dimensions develop complex morphology after annealing to tempera-tures close-to and above the dissolution tempera-ture of Ni in Si.The lateral variations of the Au content in the patch and in the formed Ni(Au)Si2 islands is controlled by the local concentration of the rapidly diffusing Au x Si y droplets during nucleation and growth of the islands.AcknowledgementsThe work was supported by Sincrotrone Trieste SCpA.The technical support of D.Lonza is greatly acknowledged.References[1]D.X.Xu,S.R.Das,J.P.McCaffrey, C.T.Peters,L.E.Erickson,Mat.Res.Soc.Symp.Proc.402(1996)59. 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