in-situ investigation of the formation of eutectic alloys in the systems silicon–silver and...

LJournal of Alloys and Compounds 335 (2002) 149–156www.elsevier.com/ locate / jallcom

In-situ investigation of the formation of eutectic alloys in the systemssilicon–silver and silicon–copper

*B. Bokhonov , M. KorchaginInstitute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia

Received 7 June 2001; accepted 21 August 2001

Abstract

An in-situ electron microscopic investigation of the formation of eutectic alloys in the systems amorphous Si /particle Ag andamorphous Si /particle Cu showed that the formation of eutectics is preceded by metal diffusion into amorphous silicon with the formationof metastable amorphous metal silicide. Decomposition of the formed metastable amorphous metal silicide results in the evolution ofpolycrystalline silicon. An oriented formation of liquid eutectic alloy is observed in the systems crystalline (100) /particle Ag andSi

(100) /particle Cu. In these cases, at the metal–silicon eutectic / silicon crystal interface the formation of silicide phases and a dislocationSi

structure are observed. Investigations allowed us to propose a scheme for the formation of eutectic alloys in the studied systems. Thescheme involves the diffusion of metal atoms into the silicon crystal structure, leading to the formation of a metastable silicide or solidsolution, the occurrence of strain at the interface and, as a consequence, the formation of defects of different types. The subsequentprocess is accompanied by synchronous dissolution of metal atoms into the liquid eutectics and by diffusion from the liquid eutectics tothe solid crystalline silicon phase, giving rise to metastable silicide, so that an intermediate diffusion zone is always present at theinterface between the liquid eutectics and crystalline silicon. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Alloys; Interfaces; Diffusion; TEM

1. Introduction between the solid particles in contact, which is the reasonfor the decrease in melting points.

At present, no unambiguous understanding of the pro- In addition, there is the problem concerning the direc-cesses leading to the formation of liquid eutectics during tion of the diffusion flux at the moment when the liquidcontact melting in the interaction of solids can be found in phase has already formed at the interface with the solids.the literature. According to the common opinion, the According to the theory developed in Ref. [3], it isformation of the liquid phase in contact melting is initiated assumed that the dissolution of solid metal in the liquidby mutual diffusion of the components [1,2]. This develop- occurs by means of atoms passing from the surface of thement of the process causes the formation of supersaturated solid phase into the liquid phase and their diffusion carry-solid solutions or metastable intermetallic compounds at off into the melt, away from the interface. On the contrary,the contacts between interacting crystals. These intermetal- another model [4] describing the interaction between thelides pass into the liquid phase rather easily, which solid and liquid phases implies the decisive role of thecorresponds to a more profitable energy state of the diffusion of atoms from the liquid phase into the solidsystem. Another viewpoint binds the formation of the phase at the contact between solid and liquid. The diffu-liquid phase at eutectic melting with the notion concerning sion of atoms from liquid to solid leads to the formation ofadsorptive interaction between atoms at the interface, or an intermetallic compound or a solid solution at the liquid /force fields in the contact region of crystals. According to solid interface, which then passes into the liquid phase. Inthis model, eutectic melting is preceded by the formation other words, according to this model, the formation of theof a metastable strained epitaxial layer at the interface liquid alloy in eutectic systems is accompanied by the

consequent formation and dissolution of the metastableintermetallic compound at the interface. In addition, ac-cording to the author of this concept [4], the difference in*Corresponding author.

E-mail address: [email protected] (B. Bokhonov). diffusion coefficients of the components in eutectic melting

0925-8388/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 01 )01822-9

150 B. Bokhonov, M. Korchagin / Journal of Alloys and Compounds 335 (2002) 149 –156

should lead to strain in the diffusion zone because an contrast to the extensive studies of Ag adsorption on the(111) and (001) Si surface.excess amount of vacancies or intercalated atoms is

formed. According to the model under consideration, therelaxation of strain is accompanied by the formation andredistribution of various defects (dislocations, intergrain 2. Experimentalboundaries, etc.).

It should be noted that the complex character of the In the present study, we applied in-situ electron micro-problem concerning the mechanism of formation of liquid scopy, which was used earlier to study the formation ofeutectic alloys can be explained, to a substantial extent, by eutectic alloys and silicide phases [2,15].the difficulties in interpreting experimental data, which are In-situ electron microscopic studies were performedindirect in character. In our opinion, this problem can be using a JEM-2000FX II transmission and scanning electronsolved with the help of the proposed in-situ technique of microscope equipped with a special holder to heat theelectron microscopic studies which allows direct observa- samples (EM-SHH4). The temperature was measured by ation of structural and morphological characteristics during Pt–Pt,Rh (13%) thermocouple with an accuracy of 6108C.the interaction between solids, including the processes ˚Single crystal silicon films (1000–2000 A) were pre-accompanied by the formation of liquid eutectics. Even the pared by chemical etching of (100) single crystal siliconfirst results [2] obtained with the help of this in-situ plates by 98% HNO /40% HF in the ratio 1:1.3

˚technique in the eutectic systems gold–silicon and A 1500 A thick amorphous Si layer was deposited onaluminum–silicon showed that the formation of the eutec- the surface of (100) NaCl crystals in a vacuum chamber

27tic alloy is preceded by the diffusion of metal atoms into (pressure ¯2310 Torr during evaporation) by electronsilicon. Moreover, it was observed in the gold–silicon gun evaporation. Transmission electron microscopy sam-system that the formation of the liquid eutectic alloy ples were prepared by floating off the NaCl substrate inoccurs in an oriented manner on the surface of single- de-ionized water and picking the film up on tungsten grids.crystalline silicon and proceeds via the formation of an Before in-situ experiments, in order to create reactionintermediate ordered state of gold in silicon. A question couples, fine particles of silver or copper were depositedarises: how general are the observed features of the onto the surface of silicon thin films or single crystal foils.

In some cases, metal particles were heated locally byformation of liquid eutectics? We believe that, in order tofocusing a high-intensity electron beam (beam currentreveal the mechanism of eutectic melting, it is useful to

21 22 210 –10 A/cm ) on the surface in order to provide aobtain information on simple systems and on more com-tighter contact between the silicon film (foil) and theplex eutectic systems. An example of a complex system isparticle. We used an EF-4 electron microscope (Carl Zeiss,the system in which the liquid eutectic alloy is formed withJena) for this purpose.the participation of an intermetallic phase which is formed

during the interaction between solid components.For this purpose, we selected systems such that, for one

of them (silicon/silver), the phase diagram exhibits eutec- 3. Resultstic character; the melting point of the eutectics is 8308C ata silicon concentration of 15.4 at.% [5]. Another system 3.1. Changes in morphological and structuralinvestigated in the present study was silicon/copper. characteristics during annealing of the amorphousAccording to the phase diagram [5], at least seven silicides silicon film with silver particlesexist in this system: Cu Si (h-phase), Cu Si (h9-phase),3 3

Cu Si (´-phase), Cu Si (d-phase), Cu Si (g- Our investigation of the changes in the morphological15 4 31 8 4.9–4.6

phase), Cu Si (b-phase), Cu Si (k-phase). There are and structural characteristics during in-situ isothermal6.7–5.3 8–6

two eutectic points, at 820 and 8028C. In addition, the annealing of the amorphous silicon film with silver par-25solubility of copper in silicon is insignificant (8.5310 ticles showed that changes in the system are observed at

at.%. at 8008C). On the other hand, silicon is very soluble temperatures much below the eutectic melting pointin copper, the lattice parameter of the solid solution of the (8308C). For example, at a temperature as low as 7008Ca-phase increasing with increasing silicon content; for 10 we observed the formation and propagation of the reaction

˚ ˚at.%, a 5 3.615 A (for pure copper, a 5 3.596 A). front into the silicon amorphous film; a diffusion zone isIt should be noted that the Ag/Si and Cu/Si interfaces formed in the silicon film near silver particles (Fig. 1). The

are among the most intensively studied metal semicon- electron diffraction patterns from this diffusion zone areductor interfaces, but their properties are not completely characterized by the presence of ring reflections fromunderstood because of the strong dependence on the polycrystalline silicon and an amorphous compound with ainterface structural quality [7–14]. Despite the great im- diffuse halo with the maximum intensity corresponding to

˚portance of metal / silicon interfaces, the chemical inter- the interplanar distance d 5 2.40 A (Fig. 2).action of metal with Si has received very little attention in An increase in the annealing temperature of the amor-

B. Bokhonov, M. Korchagin / Journal of Alloys and Compounds 335 (2002) 149 –156 151

phous Si /particle Ag system above the eutectic meltingpoint is characterized by an increase in the rate ofcrystallization zone propagation in amorphous silicon andthe formation of an amorphous compound. In addition,after the formation of the diffusion zone, the formation of aeutectic alloy near silver particles is observed. Annealingand the formation of a eutectic alloy are accompanied by asubstantial decrease in the size of the silver particles and asimultaneous increase in the size of the liquid eutecticalloy (Fig. 3). The diffraction pattern of the liquid eutecticalloy shows a set of diffuse rings with the maximum in the

˚region of the interplanar distance d 5 2.40 and 1.47 A(Fig. 4). Cooling of the eutectic melt leads to the crys-tallization of silicon and silver metal. The diffractionpatterns of the crystallized eutectic alloy consist of a set ofring reflections of polycrystalline phases of silicon andsilver.

3.2. Changes in morphological and structuralcharacteristics during annealing of a crystalline (100)silicon foil with silver particles

In-situ studies of the changes in morphological andFig. 1. Electron micrograph of the diffusion zone formed during iso- structural characteristics during the isothermal annealing ofthermal annealing (T 5 7008C) of silver particles on the surface of an

crystalline (100) silicon foil with silver particles showedamorphous silicon film. The formation of a diffusion zone in thethat the formation of a eutectic alloy started at 8308C andamorphous silicon film near a silver particle is observed.was accompanied by the propagation of a flat reactionfront between the forming liquid eutectic alloy and single-

Fig. 2. SAD pattern from the diffusion zone (Fig. 1) of the products ofthe interaction between a silver particle and an amorphous silicon film.The diffraction characteristics of the products are characterized by thepresence of ring-like reflections of crystalline silicon and a diffuse halo, Fig. 3. Electron microphotograph of the diffusion and liquid eutecticwith the maximum intensity corresponding to the interplanar distance zones formed during the interaction between silver particles and an

˚d 5 2.40 A. amorphous silicon film during isothermal heating (T 5 8508C).


Fig. 4. SAD pattern from a liquid eutectic alloy formed during theinteraction between silver particles and an amorphous silicon film duringisothermal heating (T 5 8508C). The diffraction patterns of the liquideutectic alloy consist of a set of diffuse rings with maximum interplanar Fig. 5. HREM micrograph of the oriented formation of a eutectic melt in

˚distances d 5 2.40 and 1.47 A. the system silver particle–crystal (100) silicon (T 5 8508C).

crystalline (100) silicon. The reaction front formed during eutectic alloy. The interaction is observed to start at aheating between the liquid eutectics, and the crystalline noticeable rate at temperatures as low as 6508C. Undersilicon matrix had a strict orientational relationship to the these conditions, the reaction front is observed to becrystallographic axes of single crystal (100) . The direc-Si

tion of front movement was strictly coincident with thedirections of type [100] . Cooling of the liquid eutecticSi

alloy leads to the crystallization of liquid eutectics with theevolution of silicon crystals and metallic silver. Moreover,after crystallization of the liquid eutectics the formation ofmetastable silver silicide at the interface between thecrystalline eutectic alloy and the initial crystalline siliconsubstrate is observed (Fig. 5). Electron diffraction patternsfrom the interface between the crystalline eutectic andsingle-crystal Si exhibit superstructural reflections, alongwith the reflections of (100) , which are evidence of theSi

formation of the metastable silver silicide (or ordered solidsolution) of silver atoms in the silicon crystalline matrix(Fig. 6).

3.3. Changes in morphological and structuralcharacteristics during annealing of an amorphous siliconfilm with copper particles

As already mentioned, several silicide phases exist inFig. 6. SAD pattern from the interface (Ag/Si eutectic)–single crystalthe system copper / silicon, unlike the system silver / silicon.(100) Si after crystallization. The SAD pattern from the interface is

Our in-situ investigations of the annealing of an amor- evidence of the topotaxial formation of the silver silicide phase, which isphous film with copper particles on its surface showed that exhibited in the patterns by the presence of ordered superstructuralthe interaction started below the melting point of the reflections.


formed in the silicon film near the copper particles and tomove inside the amorphous silicon film (Fig. 7). Propaga-tion of the reaction front is accompanied by crystallizationof the silicon film and by the formation of an amorphouscompound, which was confirmed by the electron diffrac-tion patterns exhibiting the reflections of polycrystallinesilicon and a diffuse halo, the maximum intensity of which

˚corresponds to the interplanar distance d 5 2.12 A (Fig. 8).This value is somewhat higher than the interplanar distance

˚d 5 2.08 A for crystalline copper. In our opinion, this(111)

is due to the formation of a metastable amorphous solidsolution of silicon in copper or copper silicide. Cooling toroom temperature leads to the disappearance of the reflec-tions of the amorphous phase, while electron diffractionpatterns exhibit ring reflections of crystal phases — siliconand copper silicide Cu Si (h-phase) (Fig. 9).3

An increase in the annealing temperature in the systemamorphous Si /Cu particle above the melting point of theeutectics (T 5 8028C) is characterized by the formation ofa diffusion zone and crystalline Cu Si. The formation of3

the liquid eutectic alloy is observed in the vicinity of thecopper particles (Fig. 10). The increase in size of the zone Fig. 8. SAD pattern from the diffusion zone in the system copper

particle–amorphous silicon under annealing at T 5 6508C. The reflectionswhere the liquid eutectic alloy is formed is accompaniedof polycrystalline silicon and the amorphous compound are present in theby a decrease in the size of the copper particles. A smallpatterns.

zone of the crystallization of amorphous silicon alwaysexists before the front of the liquid eutectic alloy (Fig. 10).Reducing the annealing temperature below the melting

point of the eutectics results in crystallization of theeutectic alloy. Crystallization of the Si /Cu Si liquid3

eutectics leads to the evolution of dendrite silicon crystalsinside the crystalline matrix, which is crystalline copper

Fig. 9. SAD pattern from the diffusion zone in the system copperFig. 7. Electron microphotograph of the diffusion reaction zones in the particle–amorphous silicon cooled to room temperature. The reflectionssystem copper particle–amorphous silicon which formed during heating of polycrystalline silicon and copper silicide Cu Si (h-phase) are present3

at 6508C. in the patterns.


Fig. 11. SEM microphotograph of the oriented formation of a eutecticmelt in the system copper–single-crystal (100) silicon (T 5 8508C). Thereaction front formed during heating between the liquid eutectic alloy andsingle-crystal silicon has a strict orientational relationship to the crystallo-graphic axes of single crystal (100)Si.

Fig. 10. Electron microphotograph of the diffusion zone and eutecticsilicon and the eutectic alloy, which is evidence that aalloy formed during the interaction of copper particles with the amor-large number of dislocations is formed at the interfacephous silicon film under isothermal conditions (T 5 8108C).between single crystalline silicon and the crystallizedeutectic alloy (Fig. 13). According to our estimates based

silicide Cu Si (h-phase) according to the electron diffrac- on electron micrographs, the size of the defect-containing3

tion patterns. dislocation zone is 0.5 mm.

3.4. Changes in morphological and structuralcharacteristics during annealing of a crystalline (100) 4. Discussionsilicon film with copper particles

The experimental data obtained during the in-situ in-In the in-situ studies of the changes in the morphological vestigation of the formation of eutectic alloys in the

and structural characteristics during annealing of crys- systems amorphous Si /Ag particles, crystalline Si /Agtalline (100) silicon foil with copper particles on its particles, amorphous Si /Cu particles, crystalline Si /Cusurface, it was observed that the formation of a liquideutectic alloy between the crystal components is observedwhen the temperature rises to the melting point of thereadily melting eutectics (8028C). Similarly to the systeminvestigated earlier (single crystal silicon–silver particles),the propagation of a reaction front between the formedliquid eutectic alloy and single crystalline silicon followsselected directions of type [100] with respect to theSi

crystallographic axes of crystalline silicon (Fig. 11).Cooling of the eutectic alloy results in crystallization,which is accompanied by the formation of a silicon phaseand copper silicide Cu Si (h-phase).3

It should be noted that, according to the electrondiffraction patterns, the formation of an ordered solidsolution takes place at the interface between single crys-talline silicon and the crystallized eutectic alloy; this isrevealed through superstructural reflections (Fig. 12). Theobserved interplanar distances can be identified as thereflections of single crystalline silicon oriented as (100)with weak superstructural reflections. In addition, a typical Fig. 12. SAD pattern from metastable crystal copper silicide formed aftercontrast is observed in the image of the interface between cooling of the eutectic Cu–Si alloy in the Cu–(100)Si system.


we can outline the following general features characteriz-ing the formation of eutectic alloys in the studied systems.

In all the systems under investigation Me(Ag, Cu, Au,Al)–single crystal (100) silicon, the formation of a eutecticalloy is preceded by the diffusion of metal into silicon withthe formation of an intermediate diffusion zone (even attemperatures below the melting point of the eutectics).Metal diffusion proceeds from the formed eutectic phasesinto solid silicon. We came to this conclusion on the basisof the following experimental facts.

(i) There is always a diffusion zone of the crystalliza-tion of amorphous silicon in front of the forming liquideutectic alloy.(ii) The formation of ordered solid solutions and (or)silicides is always observed at the interface eutectics /silicon single crystal.

In the present study we observed the topotaxial evolu-tion of metastable silver silicide and copper silicide at theinterface between crystalline eutectic alloy and single

Fig. 13. Morphological characteristics of the defect-containing disloca- crystal silicon. The epitaxial formation of silver nuclei,tion zone near the interface copper–silicon eutectic and single crystal islands and silver silicide has been observed under silver(100) Si.

adsorption on Si(001)(231) and Si(111) [6–14] and forthe ion beam mixed Ag/Si(111) system [16].

particles allows us to reveal the following features of the In our opinion, the oriented formation of the liquideutectic alloy formation process in these systems. eutectic is also evidence of the important role played by

First, in the systems amorphous Si /Ag particles, amor- diffusion processes in the formation of liquid eutectics,phous Si /Cu particles we observe the formation of an since the observed directions of oriented growth of liquidintermediate diffusion zone between the metal particles eutectics agree well with the directions of the prevailingand amorphous silicon; the crystallization of amorphous diffusion in the crystalline silicon structure. Thus, thesilicon and the formation of a compound of silicon with silicon structure is known to contain channels in directionsthe diffusing metal take place in this zone. It should be such as [110] along which the diffusion of impurities takesnoted that the formation of the diffusion zone of amor- place easily. In our opinion, in the case where the frontphous silicon crystallization in the systems metal–amor- moves parallel to the [100] direction and the highestphous silicon is observed both at temperatures below the propagation rate is observed along directions such aspoint of eutectic alloy formation and above the melting [110], the observed ‘facing’ of liquid eutectics in thepoint of the eutectics. crystalline silicon matrix is confirmation of the important

Second, the formation of liquid eutectic alloys during part played by the diffusion of metal atoms (silver, copper,the annealing of metal particles on silicon single crystalline and gold) in the crystalline structure of silicon before the(100)Si foil occurs along definite crystallographic direc- formation of liquid eutectics.tions, thus exhibiting an orientational correspondence. We have already stated that the formation of liquid

Third, in-situ investigation of eutectics formation in the eutectics is preceded, and then accompanied, by thesystems crystalline Si /Ag particles and crystalline Si /Cu diffusion of metal atoms from the liquid phase to the solidparticles demonstrates that the formation of an inter- phase. The insertion of metal atoms into the siliconmediate zone is observed at the interface between the crystalline structure should cause strain in the diffusioneutectic alloy and monocrystalline silicon. One of the basic zone at the interface. An increase in the concentration ofcharacteristics of this intermediate zone is the formation of metal atoms in the crystalline structure of silicon causes anmetastable ordered solid solutions (metal silicides) and the increase in the strain. Relaxation of the strain results in thepresence of a defect dislocation structure. formation of dislocation structures at the interface in the

Taking into account our earlier results obtained during system copper / single-crystal silicon.an in-situ investigation of the formation of eutectic alloys Thus, our studies, along with data from the literaturein the systems gold /silicon and aluminum/silicon [2], in [1,2], allow us to propose the following scheme describingwhich we also observed the formation of intermediate the formation of eutectic alloys in the systems Me/silicon:diffusion zones, metastable ordered solutions and metalsilicides, as well as oriented formation of liquid eutectics, MeuSi → Meu(Me–Si) uSi (1)MS


[2] B.B. Bokhonov, M.A. Korchagin, J. Alloys Comp. 312 (2000) 238.Meu(Me–Si) uSi → Meu(Me–Si) u(Me–Si) uSi (2)MS LE MS[3] W.A. Kaysser, G. Petzow, Powder Met. 28 (3) (1985) 145.[4] A.P. Savickii, Interaction of Components During Liquid Phase

The first stage of the process (1) involves the diffusion Sintering, Nauka, Novosibirsk, 1991, in Russian.of metal atoms into the silicon lattice, which results in the [5] M. Hansen, K. Anderko, Composition of Binary Alloys, McGraw-formation of metastable silicide or solid solution Hill, New York, 1958.

[6] A. Endo, S. Ito, Surf. Sci. 293 (1993) 165.(MeSi) , the occurrence of strain at the interface, and theMS¨[7] S. Gunther, A. Kolmakov, J. Kovac, L. Casalis, L. Gregoratti, M.formation of various defects. This defines the formation of

Marsi, M. Kiskinova, Surf. Sci. 377–379 (1997) 145.the liquid eutectic melt (Me–Si) (2). In our opinion,LE [8] V.G. Lifshits, A.A. Saranin, A.V. Zotov (Eds.), Surface Phases onstage (2) is accompanied by the synchronous dissolution of Silicon, Wiley, New York, 1994, p. 52.metal atoms in the liquid eutectics, as well as by their [9] G. Le Lay, Surf. Sci. 132 (1983) 169.

[10] S.M. Shiviaprasad, T. Abukawa, H.W. Yeom, N. Nakamyra, S.diffusion from the liquid eutectics to the solid phase,Suzuki, S. Sato, K. Sakamoto, T. Sakamoto, S. Kono, Surf. Sci. 344giving rise to metastable silicide, so that an intermediate(1995) L1245.

diffusion zone always exists at the interface liquid eutec- [11] S.M. Shiviaprasad, C. Anandan, S.G. Ayatzan, Y.L. Gavriljuk, V.G.tics /crystalline silicon. Lifshits, Surf. Sci. 382 (1997) 258.

We understand that the scheme describing the formation [12] S.M. Shiviaprasad, Y. Aparna, S. Singh, Solid State Commun. 107(6) (1998) 257.of liquid eutectic alloys presented in this paper is some-

[13] W.S. Cho, J.Y. Kim, N.G. Park, K.H. Chae, Y.W. Kim, I.W. Lyo, S.S.what simplified. Nevertheless, even this simplified schemeKim, D.S. Choi, C.N. Whang, Surf. Sci. 439 (1999) L792.

of the formation of eutectic alloys, composed on the [14] Y. Ohba, I. Katayama, T. Numata, H. Ohnishi, M. Watamori, K.grounds of experimental data, allows us to conclude that Oura, Appl. Surf. Sci. 121/122 (1997) 191.the role of diffusion and orientation processes in the [15] B.B. Bokhonov, M.A. Korchagin, J. Alloys Comp. 319 (1 /2) (2001)

187.formation of the liquid phase in the systems under consid-[16] D.K. Sarkar, S. Dhara, K.G.M. Nair, S. Chowdhury, Studies oferation is important.

phase formation and chemical states of ion beam mixed Ag/Si(111)system, Nucl. Instrum. Meth. Phys. Res. B 168 (2000) 215.

References

[1] V.M. Zalkin, The Nature of Eutectic Alloys and the Contact MeltingEffect, Metallurgy, Moscow, 1987, in Russian.

in-situ investigation of the formation of eutectic alloys in the systems silicon–silver and...

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