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Acta Materialia 59 (2011) 1184–1190

Nanosized induced low-temperature alloying in binary andternary noble alloy systems for micro-interconnect applications

Tzu-Hsuan Kao a, Jenn-Ming Song b, In-Gann Chen a,⇑, Teng-Yuan Dong c,Weng-Sing Hwang a

a Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwanb Department of Materials Science and Engineering, National Dong Hwa University, Hualien 97401, Taiwan

c Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan

Received 1 February 2010; received in revised form 14 October 2010; accepted 22 October 2010Available online 17 November 2010

Abstract

This study introduces and demonstrates the concept of low-temperature alloying due to the nanosize effect. The interactions betweennoble metallic nanoparticle deposits (NPD), including Au and bimetallic Ag3Au, and bulk substrates (Ag, Cu and Ni) upon heating wereinvestigated systematically. According to the experimental results, in the very early stage of heating at temperatures higher than the melt-ing points of nanoparticles, which are substantially lower than those in the bulk state, the supercooled liquid reacted with the substrateand gave rise to binary or ternary alloying. The reaction products under such non-equilibrium conditions reveal a variety of metallurgicalfeatures depending on the nature of the NPD/substrate systems.� 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Nanoparticle deposit; Auger electron microscopy; Low-temperature alloying; Nanoindentation; Depth profiling

1. Introduction

Since the emergence of inkjet printing technology, noblemetallic nanoparticle (NP) suspensions have been widelyused as conductive inks to manufacture fine-pitch electricalline patterns for organic transistors [1] or ULSI intercon-nects [2], not only because of their drastically reduced melt-ing temperatures but also because of their high conductivityand flexibility in process. Recently, Li et al. [3,4] suggestedthat it is viable for metallic NPs to be a candidate for low-temperature lead-free interconnect applications. Their simu-lation results also show that silver NPs (2 nm) could collapseand coalesce with gold substrate at a low-temperature of127 �C [5]. However, the reactions between NPs and bulkmetals upon heating are still unclear.

1359-6454/$36.00 � 2010 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2010.10.051

⇑ Corresponding author. Tel.: +886 6 276 3741.E-mail address: ingann@mail.ncku.edu.tw (I.-G. Chen).

A relevant report shows that after heating at tempera-tures slightly above the low melting point of Au NPs, evidentinterdiffusion occurred between the nanoparticle deposits(NPDs) and commonly used electronic substrates [6]. Thecontinuity, morphology and adhesion to the substrate ofthe cured NPDs vary with different substrate materials. Arecent in situ synchrotron X-ray diffraction study observedthat the nanosize-induced low-temperature melting of AuNPs and thus the formation of an unstable Au–Ni interme-tallic phase with thin-film Ni, which has been regarded as theproduct of non-equilibrium processes [7].

For an in-depth understanding of the interaction of noblemetallic NPDs with bulk materials, this study extended thematerial selections of metallic NPs and bulk substrates.Interdiffusions between NPDs (Au and bimetallic Ag3Au)and Ag, Cu and Ni substrates upon heating were investi-gated by means of Auger electron spectroscopy (AES) andgrazing incidence X-ray diffraction (GIXRD) analyses.Based on the chemical and thermodynamic data as well as

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the phase diagrams, Table 1 synopsizes the salient features ofthe alloy systems involved [8–12]. Regardless of the latticemismatches (�0% for Au–Ag and 12% for Au–Cu), theAu–Ag and Au–Cu systems both exhibit negative enthalpyof mixing and thus form ordered phases. Au–Ag is com-pletely miscible at all compositions, while Au–Cu developssuperlattices at low-temperatures. Ag–Cu and Au–Cu bothpossess the lattice mismatch of 12%; however, the formertends to phase separation and becomes a eutectic systemdue to its highly positive mixing enthalpy. With respect toAg–Ni and Au–Ni, both systems exhibit positive enthalpyof mixing and a 15% difference in lattice constant. TheAu–Ni system shows a miscibility gap under which Au-richand Ni-rich phases coexist, while the Ag–Ni system, whichhas a large positive mixing enthalpy and almost no electro-negativity difference, reveals complete immiscibility even inthe liquid state. How these metallurgical factors affect thereactions of NPD/substrate are the main concern of thisarticle.

2. Experimental

2.1. Synthesis

Octanethiol-protected Au NPs were prepared by themethod developed by Hostetler et al. [13]. Briefly, aqueousAuCl4� was dissolved in toluene using TOABr as the phasetransfer agent. The toluene solutions, containing a 2:1 Mratio of octanethiol to HAuCl4sx, were reduced with NaBH4

at room temperature. The organic layer was concentrated,poured into methanol, centrifuged and rinsed with methanolto give the desired Au nanoparticles. The particles were thenwashed, dried and suspended in toluene solvent at a propor-tion of 20 wt.%.

The preparation of the octanethiol-protected Ag3Au NPsfollowed a modified two-phase method [14], which was car-ried out by separately transferring the AgBr2� sol and theAuCl4� from the aqueous phase to the organic phase usingTOABr as the phase transfer agent. By utilizing octanethiolas the stabilizing agent, the above two toluene phases werecombined at a molar ratio of 3:1 (Ag:Au) and co-reducedby NaBH4 to yield octanethiolate-encapsulated bimetallicAg3Au NPs. The particles were then washed, dried and sus-pended in toluene solvent at a proportion of 20 wt.%.

2.2. Sample preparation

The metallic substrates, including Ag, Cu and Ni(1 cm � 1 cm in size), were sequentially polished using the0.03 lm Al2O3 suspension, degreased in an alkaline solutionof 5 wt.% NaOH at 70 �C for 5 min, rinsed in deionized (DI)water for 5 s, cleaned in 10 vol.% HNO3 solution for 5 s,rinsed in DI water for 5 s and finally blow dried with nitro-gen. Each NPDs/substrate combination was prepared bypipetting NPs suspensions (approx. 30 ± 0.9 ll) onto thesubstrates with subsequent spin-coating at 500 rpm for 3 sand then 2000 rpm for 15 s. The estimated particle densities

were estimated as 6.11 � 1015 and 1.93 � 1015 particlescm�2 for Au and Ag3Au NPDs, respectively. The curing pro-cess was then performed in an infrared furnace under a pro-tective atmosphere of 90%N2–10%H2, and consisted ofrapid heating, followed by an isothermal stage at 300 �Cfor 60 min and finally cooling to room temperature.

2.3. Characterization

A transmission electron microscope (TEM) and ScionImage 4.0.2, the software for image analysis, were used tomeasure and estimate the NPs’ size. Differential scanningcalorimetry (DSC) and thermogravimetry analysis (TGA)were used to determine the melting point of the NPs andmonitor the desorption of surfactants, respectively, whichwere both conducted at a heating rate of 5 �C min�1 innitrogen. The NPD coated samples after curing were thenanalyzed with a scanning electron microscope. Throughthe elemental depth profiling analyses performed by anAES, the elemental distribution of the reaction productsalong the through-thickness direction was examined. Phaseidentification was performed using a GIXRD spectrometerwith an 18 kW Cu Ka X-ray source at a voltage of 40 kVand a current of 100 mA. The data were collected by a stepscan from 2h = 30� to 2h = 80�, with a 0.02� step size.

The mechanical properties of the production layers ofthe NPDs reacted with the substrates were investigatedby nanoindentation using an MTS Nano Indenter XP witha Berkovich tip (tip radius 20 nm). A continuous stiffnessmeasurement technique was used during indentation. Theindenter was pressed into the specimen up to 1 lm andthe data for the penetration depth of 80 nm were collectedto prevent the substrate effect. The load–displacement dataobtained were analyzed using the method of Oliver andPharr [15] to determine the hardness and the elastic modu-lus as functions of the displacement of the indenter. Eachdatum was the average of at least 10 tests.

3. Results and discussion

3.1. Characterization of NPs and morphology of the cured

NPDs

The insets in Fig. 1a show TEM micrographs of the Auand bimetallic Ag3Au NPs prepared, which exhibit averagecore diameters of 2.5 ± 0.7 and 3.67 ± 0.32 nm, respectively.The TGA thermograms in Fig. 1a indicate apparent weightlosses of 23% at 215 �C for Au NPs and 30% at 245 �C forAg3Au NPs, which mean that most of the surfactants des-orbed at those temperatures. These can be referred to theexothermic peaks of the DSC curves upon heating shownin Fig. 1b. Remarkably, the endothermic peaks, representingthe melting of NPs, ranged from 230 to 270 �C for Au NPsand from 250 to 270 �C for Ag3Au NPs.

Figs. 2 and 3 illustrate the planar and cross-sectionalviews of the Au NPD and Ag3Au NPD on different sub-strates after curing at 300 �C for 60 min. It can be observed

Table 1Properties of the noble alloy systems involved.

System Latticemismatch (%)a

Hmix (x = 1/2)(meV atom�1)

Electronegativitydifferenced

Type of nearest–neighborpair interactionc

Low-temperaturephasesb

Characteristic of phasediagrame

Au–Ag 0 �48b 0.61 Ordering-type L12, L10, L12 Completely miscible at allcompositions

Au–Cu 12 �91b 0.64 Ordering-type L12, L10, L12 Formation of superlattice atlow-temperature

Au–Ni 15 +176 0.63 Ordering-type Phase separation Immiscible at low-temperature

Ag–Cu 12 +104c 0.03 Clustering-type Phase separation Simple eutectic systemAg–Ni 15 +238 0.02 Clustering-type Phase separation Immiscible in solid and

liquid states

a Ref. [8].b Ref. [9].c Ref. [10].d Ref. [11].e Ref. [12].

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that both the cured Au and Ag3Au NPDs on Ag and Cusubstrates were continuous. On the other hand, discretefilms were found on the Ni substrates (Figs. 2c and 3c),which might stem from large lattice mismatch of Au–Niand Ag–Ni systems (�15%). It is noticeable that in the caseof the samples with the Ag substrate the brightness of thebackscattered electron images degraded gradually fromNPDs to the substrate. However, for those with Cu andNi substrates, the boundaries between the NPDs and thesubstrate were distinct.

3.2. Characterizations of the reaction products in the vicinity

of the interface

AES elemental depth profiles revealing the composi-tional variations along the through-thickness direction ofthe NPDs and GIXRD patterns for phase identificationwere illustrated in Fig. 4. The signals of oxygen were notplotted in the depth profiles as the concentration of oxygenin the sub-subsurface was quite low and negligible.

3.2.1. NPDs/Ag substratesFig. 4a illustrates that the composition of the reaction

layers for both Au and Ag3Au NPDs varied over the entire

Fig. 1. (a) TGA thermal curves and insets of TEM images and (

interdiffusion region due to the feature of complete misci-bility, which is in accord with the GIXRD pattern inFig. 4d showing coexistent diffractions of Au and Ag. Asfor the Au signals, it is apparent that with a higher initialconcentration of Au the diffusion distance of Au wasgreater. The most important finding is that, compared witha recent report using a bilayer Ag/Au couple under similarheat treatment conditions (350 �C for 40 min), the diffusiondistance in our case was almost five times larger [16]. It canthus be deduced that the reaction occurring at NPD/sub-strate interface was not just a simple solid-state diffusion,but also a liquid–solid interaction at the very initial stagebecause of the transient melting of the NPDs.

3.2.2. NPDs/Cu substrate

As shown in Fig. 4b, a region with a constant ratio ofAu/Cu appeared adjacent to the surface. The XRD patternin Fig. 4e suggests that this region consisted of a stoichiom-etric phase, the Cu3Au superlattice. Regarding the morecomplex case of NPD Ag3Au/Cu, each of the binary sys-tems in the ternary Au–Ag–Cu exhibits diverse metallurgi-cal characteristics, which cause Au–Ag to form a solidsolution, Au–Cu to form superlattices and Ag–Cu to formeutectics. Interestingly, Fig. 4b also shows that, besides the

b) DSC curves of the Au and Ag3Au NPs used in this study.

Fig. 2. Surface and cross-sectional morphologies of the cured Au NPD on different substrates: (a, d) Ag substrate; (b, e) Cu substrate; and (c, f) Nisubstrate. The curing was performed at 300 �C for 60 min.

Fig. 3. Surface and cross-sectional morphologies of the cured Ag3Au NPD on different substrates: (a, d) Ag substrate; (b, e) Cu substrate; and (c, f) Nisubstrate. The curing was performed at 300 �C for 60 min.

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subsurface area, two plateaus with constant compositioncould be distinguished. As indicated, the composition ofthe plateau adjacent to the subsurface (stage I) was about50 at.% Ag–35 at.% Au–15 at.% Cu and that for the pla-teau next to the substrate (stage II) is 30 at.% Ag–30 at.%Au–40 at.% Cu. By superimposing the measured composi-tions on the isothermal section of the Ag–Au–Cu ternaryphase diagram at 300 �C [17,18], it can be seen that stageI corresponds to a mixture of two face-centered cubic solid

solutions (a1 and a2), and that stage II corresponds toanother two-phase region comprising a1 and an orderedphase, Cu3Au, the existence of which can be verified bythe XRD pattern given in Fig. 4e.

From basic thermodynamic considerations, only single-phase regions can be formed in binary diffusion couples fol-lowing the phase rule, because three degrees of freedomwould be necessary to fix the temperature and pressureand vary the concentration. This means two-phase regions

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cannot be formed through solid-state diffusion [19,20].Notably, for the interdiffusion in a ternary system at a fixedtemperature and pressure, the extra degree of freedommakes the occurrence of the two-phase layers possible[20]. According to the Ag–Au–Cu isothermal section at300 �C [18], diffusion paths through two two-phase regions,a1 + a2 and a1 + Cu3Au, can be predicted for the diffusioncouple of Ag–25% Au vs. Cu (to simulate the case of NPDAg3Au/Cu). This implies that the two successive plateausdetected with constant composition might be obtained

Fig. 4. Atomic concentrations via AES depth profiling from the surface of the cray diffraction patterns of the NPDs: (a, d) Ag substrate; (b, e) Cu substrate;

through solid-state diffusion. One should note, however,generally the prediction of diffusion paths from the phasediagram and subsequent phase formation is based on theassumption that the concentrations at either end of the dif-fusion couple are unaffected by the diffusion process. Forvery short annealing times or in thin-film experiments,deviations from equilibrium conditions certainly occur[20]. In our case, the Ag3Au NPDs were apparently con-sumed and transformed. Therefore, we still believe that thisintricate alloying behavior was likely the consequence of

ured Au and Ag3Au NPDs toward the substrate, and the corresponding X-and (c, f) Ni substrate.

Fig. 5. Hardness and Young’s modulus of the reaction layers of Au andAg3Au NPDs reacted with different substrates.

Table 2Metallurgical features of the different NPD/substrate systems.

Alloy system Case Feature

Miscible solidsolution

Au NPD/Ag Ag3AuNPD/Ag

Wide diffusion distance

Stoichiometricphases

Au NPD/NiAuNPD/Cu

Intermetallic compounds(Au3Ni and Au3Cu)

Immisciblealloys

Ag3Au NPD/Ni Segregation at surface

Complex system Ag3Au NPD/Cu Two-phase regions

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the liquid–solid reaction between molten NPs and the Cusubstrate followed by solidification, rather than simplesolid-state diffusion.

3.2.3. NPDs/Ni substrate

By AES depth profiling, Fig. 4c shows a reaction layerwith constant composition of 75 at.% Au–25 at.% Ni forthe NPD Au/Ni samples. This can be referred to our recentfinding about the emergence of Au3Ni superlatticesthrough a non-equilibrium process, i.e. the interfacial reac-tion between supercooled liquid Au and Ni [7]. Althoughthe miscibility gap appears at low-temperatures in theAu–Ni system due to the large lattice mismatch and highlypositive mixing enthalpy, there is still the likelihood forordering-type nearest–neighbor pair interactions to occurin theory due to the large difference in electronegativity[10,21,22]. Experimentally, Reichert et al. [23] proved theformation of Au3Ni and Au3Ni2 through diffuse scatteringof Au–Ni alloys with specific compositions quenched froma temperature above the miscibility gap. Unlike the afore-mentioned Cu3Au superlattices, the Au3Ni was unstableand decomposed when heated to 260 �C and above [7,24].The GIXRD patterns in Fig. 4f confirm the absence ofAu3Ni; however, the AES results indicate that the stoichi-ometric ratio of Au to Ni still remains.

With the participation of Ag, the AES depth profile ofNPD Ag3Au/Ni in Fig. 4c illustrates that the concentrationof Ni is quite low at the subsurface region, implying thatAu–Ag tended to segregation at the surface in the formof solid solutions rather than diffused into the Ni substrate.For the same reason, the outward diffusion of Ni wasretarded. It is noteworthy that the concentration of Agdeclined rapidly, whereas that of Au decreased graduallyand penetrated further into the Ni substrate. This isbecause the Ag–Ni system is more positive in mixingenthalpy and has a smaller electronegativity differencecompared with Au–Ni, although they exhibit similar latticemismatches (�15%). The GIXRD results in Fig. 4f demon-strate the coexistence of Au, Ag and Ni.

3.2.4. Mechanical properties of the reaction products

Fig. 5 lists the hardness and Young’s modulus of thereaction layers obtained by nanoindentation. All the datawere collected by averaging the acquired values within80 nm from the subsurface. For both the NPDs Au andAg3Au, the hardness and modulus of the samples withthe Ni substrate were greater than those with Cu, whichin turn were greater than those with the Ag substrate. Incombination with the Ag substrate, Ag3Au samples had aslightly higher Young’s modulus than Au NPDs, althoughtheir hardness values were very similar. This can beascribed to the larger Young’s modulus of Ag (83 GPa)compared with that of Au (78 GPa) [25]. As for the sampleswith the Cu substrate, the observed stronger intensity ofthe Cu3Au superlattices for NPD Au/Cu in Fig. 4d contrib-uted to its greater hardness and modulus compared toNPD Ag3Au/Cu. As mentioned, the Au3Ni superlattices

might have emerged in the NPD Au/Ni samples but soondecomposed. The harder and stiffer Au/Ni reaction prod-ucts compared to other immiscible alloys or complex sys-tems could be ascribed to their high Ni content. Niexhibits much greater hardness and Young’s modulus thanthe other elements involved [25].

3.3. The nanosize-induced low-temperature alloying behavior

Based on the above observations on the interdiffusionbetween the NPDs and substrates, the systems examinedand the corresponding metallurgical phenomena are sum-marized in Table 2. As listed, phase separation, solid solu-tion and the formation of stoichiometric intermetallicphases, together with complex two-phase constitutionalstructures, occurred, depending on the characteristics ofthe alloy systems. Interestingly, the much longer diffusiondistance of Au in the Au NPDs/Ag samples, the presenceof the unstable phase Au3Ni in the Au NPDs/Ni and theformation of the two-phase regions in the Ag3Au NPD/Cu samples are all evidence of nanosize-induced liquid–solid reactions. Accordingly, the following non-equilibriumlow-temperature alloying process can be proposed. First,the surfactant of NPD desorbs gradually upon heating(Fig. 6a). As the melting point is reached, partial or volumemelting of NPs occurs (Fig. 6b). Even though the durationis very short, the substrate can dissolve in the supercooledliquid to achieve a binary or ternary liquid alloy (Fig. 6c),then diverse phase transformations of phase separationand/or ordering take place in the subsequent solidification(Fig. 6d).

Fig. 6. Schematic illustrations of the alloying behavior between the NPDand the bulk substrate at elevated temperatures.

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4. Conclusions

Successive low-temperature melting, alloying and solid-ification processes have been proposed and verified via theinvestigation of interactions between noble metallic NPDsof Au and Ag3Au with low melting points (230–270 �C forAu NPs and 250–270 �C for Ag3Au) and bulk metallic sub-strates of Ag, Cu and Ni upon heating from the viewpointof metallurgy and thermodynamics. Subjected to curing at300 �C, the reaction layers formed through the non-equilib-rium liquid–solid reaction exhibited diverse microstructuralfeatures, which are predominately dependent on the ther-modynamic characteristics of the alloy systems involved.

Acknowledgements

This work was supported primarily by the NationalScience Council of ROC through Contract No. NSC 99-

2120-M-006-009 and the NCKU Project of PromotingAcademic Excellence & Developing World Class ResearchCenter: D98-R048, for which the authors are grateful.

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