Intermetallic compounds formed during diffusion soldering of Au/Cu/Al2O3 and Cu/Ti/Si with Sn/In interlayer

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Intermetallic Compounds Formed During Diffusion Soldering

of Au/Cu/Al

₂

O

₃

and Cu/Ti/Si with Sn/In Interlayer

TUNG-HAN CHUANG,1,2HSIU-JEN LIN,1and CHENG-WEN TSAO1

1.—Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan. 2.—E-mail: [email protected]

A Si wafer was sequentially sputter-coated with Ti (20 nm), Cu (6 mm), Sn (4 mm), and In (4 mm). The specimen was then diffusion-soldered at tem-peratures between 150 and 300°C with an alumina substrate deposited with Cu (4 mm) and Au (6 mm). Experimental results showed that a multilayer of intermetallic phases with the compositions of (Cu0.99Au0.01)6(Sn0.52In0.48)5/

(Au0.87Cu0.13)(In0.94Sn0.06)2/(Au0.98Cu0.02) (In0.95Sn0.05) formed at the Au/Cu

interface. Kinetic analyses revealed that the growth of (Cu0.99Au0.01)6

(Sn0.52In0.48)5 and (Au0.87Cu0.13)(In0.94Sn0.06)2/(Au0.98Cu0.02)(In0.95Sn0.05)

in-termetallics were diffusion-controlled with activation energies of 21.5 and 31.3 kJ/mol, respectively. Sound tensile strengths of 42 and 48 kg/cm2 have been obtained under the bonding conditions of 150°C for 40 min. and 200°C for 30 min., respectively.

Key words: Intermetallic compounds, diffusion soldering, Au/In/Sn/Cu interfacial reactions

INTRODUCTION

Diffusion soldering provides novel applications for high density and high power devices in meeting their requirements for heat-resistant joints.1,2This technique makes use of a low-melting metallic thin-ﬁlm interlayer (LT) that reacts rapidly with both of the high-melting metallic layers or substrates (HT1 and HT2) that are to be bonded. The original thick-ness of the LT interlayer is less than those of the HT1 and HT2 layers or substrates, and the liquid/solid reactions contribute to exhaustion of the LT interlayer as well as the formation of inter-facial intermetallic compounds between HT1 and HT2. Because the newly formed intermetallic phases possess melting points much higher than the original LT interlayer, the resulting joints can withstand considerably higher temperatures.3

On the basis of the underlying principle of the diffusion-soldering process, intermetallic reactions that occur at the LT/HT1 and LT/HT2 interfaces apparently play a key role in achieving joining efﬁ-ciency for this technique. Bader et al. have investi-gated the intermetallic compounds along with their growth kinetics in diffusion-soldered Cu/Sn/Cu and Ni/Sn/Ni systems.3Sommadossi et al. further stud-ied intermetallic reactions during the diffusion soldering of Cu/In-48Sn/Cu joints at temperatures

between 180 and 400°C.4 They reported on an e-Cu57In16Sn27 intermetallic phase that formed at

temperatures below 200°C and an e-Cu77In10Sn13/

h-Cu57In16Sn27double layer that formed at

temper-atures above 200°C. The growth kinetics of the e-intermetallic phase between Cu and h-intermetal-lics was interfacially controlled with an activation energy of 121 kJ/mol. The intermetallic compounds formed at the diffusion-soldered interfaces between Cu/Ti/Si and Au/Cu/Al2O3with pure Sn and pure In

interlayers have been investigated by Liang et al.5 and Tsao et al.,6 respectively. In the wake of pub-lished studies on intermetallic reactions of liquid In-49Sn solder with solid Au and Cu substrates,7,8 this present paper is concerned with thin-ﬁlm diffu-sion soldering between Cu/Ti/Si and Au/Cu/Al2O3

using a near-eutectic In/Sn interlayer.

EXPERIMENTAL PROCEDURES

The alumina ceramic substrate used for this study was sputter-coated with Cu (4 mm)/Au (6 mm) layers. Sn (4 mm) and In (4 mm) were then deposited on a Ti/Cu-coated Si wafer. The In/Sn/Cu/Ti/Si wafer and Au/Cu/Al2O3 substrate were cut in dimensions of

43 4 mm and assembled as shown in Fig. 1a. The specimens were heated at various temperatures ranging from 150 to 300°C in a vacuum furnace of 5.33 10ÿ4Pa for 10 to 40 min. In this case, Sn and In melted to form a near-eutectic Sn-In liquid layer

(Received December 22, 2005; accepted March 2, 2006)

Journal of ELECTRONIC MATERIALS, Vol. 35, No. 7, 2006 Regular Issue Paper

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that reacted with the Cu and Au ﬁlms (Fig. 1b). Diffusion soldering of the Sn-In layer with Cu and Au ﬁlms resulted in the appearance of intermetallic compounds at the Sn-In(l)/Cu(s) and Sn-In(l)/Au(s) interfaces (Fig. 1c). After bonding, the specimens were cut along the cross section, ground with SiC paper, and polished with 1.0-mm and 0.3-mm alumina powders. The interfacial intermetallics were observed by scanning electron microscopy (SEM), and their chemical compositions were analyzed by an electron probe microanalyzer (EPMA). To evaluate the bond-ing strengths of the diffusion-soldered joints, both sides of the sandwiched specimens were gripped as shown in Fig. 2 and tensile-tested using an MTS-(Minneapolis, USA) Tytron 250 Microforce tester at a crosshead speed of 0.01 mm/s. The load and dis-placement ranges that can be measured with this equipment are 0.01–250 N and 0.1 mm–100 mm, res-pectively, which are suitable for this testing procedure.

RESULTS AND DISCUSSION

During the diffusion soldering between Au/Cu/ Al2O3 and Cu/Ti/Si with In/Sn interlayers, the

liquid Sn ﬁlm (4 mm) reacted rapidly with the In ﬁlm (4 mm) to form a liquid In-Sn alloy with a near-eutectic composition. The liquid In-Sn solder

reacted further with the Au/Cu/Al2O3 and Cu/Ti/

Si, which accounts for the appearance of a multi-layer of intermetallic compounds at the interface, as shown in Figs. 3 and 4. Sound joints were obtained after diffusion soldering at 150°C for 40 min. (Fig. 3a) and at 200°C for 30 min. (Fig. 3b). The intermetallic layers broke into coarse particles with the increase in temperature or heating time, as revealed in Fig. 4a and b. EPMA analyses iden-tiﬁed the Au, Sn, In, and Cu compositions across the multilayers of the diffusion-soldered specimens, which are plotted in Fig. 5. The composition (at.%) of the intermetallic layer adjacent to the Cu/ Ti/Si is Cu:Au:Sn:In 5 57.63:0.32:21.69:20.09, i.e., (Cu0.99Au0.01)6(Sn0.52In0.48)5, which corresponds to

the h-Cu6Sn5 phase in the Cu-Sn equilibrium

dia-gram. A similar intermetallic compound (Cu0.99

Au0.01)6Sn5was found in the diffusion-soldered joint

of Cu/Ti/Si and Au/Cu/Al2O3with a Sn interlayer.5

However, the change of the interlayer to pure In generated a different kind of intermetallic com-pound (Cu0.99Au0.01)In.

6

For the soldering reactions between liquid In-49Sn and Cu substrates, Chuang et al. reported the formation of an intermetallic compound Cu6(Sn0.54In0.46)5at the In-49Sn(l)/Cu(s)

interfaces.8

The intermetallic compounds adjacent to the Au/Cu/Al2O3 substrate were composed of two

lay-ers, (Au0.87Cu0.13)(In0.94Sn0.06)2 and (Au0.98Cu0.02)

(In0.95Sn0.05), which correspond to the AuIn2 and

AuIn phases in the Au-In equilibrium diagram. For diffusion soldering between Cu/Ti/Si and Au/Cu/Al2O3 Fig. 1. Scheme of diffusion soldering for Cu/Ti/Si and Au/Cu/Al2O3

with In/Sn interlayers.

Fig. 2. Tensile test for the bonding strengths of the diffusion-soldered joints.

Intermetallic Compounds Formed During Diffusion Soldering

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using pure Sn5and pure In6 interlayers, the resul-tant interfacial intermetallics were d-(Au0.87Cu0.13)Sn

and (Au0.67Cu0.33)In2, respectively. On the other

hand, soldering reactions between liquid In-49Sn

and Au thick ﬁlm, as reported by Liu,7 resulted in the formation of a continuous (Au0.96Cu0.04)

(In0.92Sn0.08)2 intermetallic layer at the In-49Sn(l)/

Cu(s) interfaces along with a great number of (Au0.98Cu0.02)(In0.94Sn0.06) intermetallic blocks

ﬂoat-ing into the In-49Sn solder matrix.

The thicknesses (DX) of the intermetallic layers formed at the interfaces were measured and plotted vs. the square root of reaction time (t1/2). Figures 6 and 7 show that the growth of the h-(Cu0.99Au0.01)6

(Sn0.52In0.48)5 intermetallic layer and (Au0.87Cu0.13)

(In0.94Sn0.06)2/(Au0.98Cu0.02)(In0.95Sn0.05) double layer

exhibits a parabolic relation, implying that the interfacial reactions during the diffusion soldering of Cu/Ti/Si and Au/Cu/Al2O3 with the In/Sn

inter-layer were diffusion-controlled. The growth rate constants (K5 DX/t1/2), as calculated from Figures 6 and 7, are listed in Table I. From the Arrhe-nius plots of growth rate constants in Fig. 8, the activation energies (Q) for the growth kinetics of h-(Cu0.99Au0.01)6(Sn0.52In0.48)5 and (Au0.87Cu0.13)

(In0.94Sn0.06)2/(Au0.98Cu0.02)(In0.95Sn0.05)

interme-tallics are determined to be 21.5 and 31.3 kJ/mol, respectively. The former value (21.5 kJ/mol) is quite consistent with the activation energy for the diffusion

Fig. 3. Morphology of intermetallic compounds formed after diffusion soldering between Cu/Ti/Si and Au/Cu/Al2O3using In/Sn interlayers:

(a) 150°C, 40 min; (b) 200°C, 30 min.

Fig. 4. Morphology of intermetallic compounds formed after diffusion soldering between Cu/Ti/Si and Au/Cu/Al2O3 using In/Sn interlayers:

(a) 200_{°C, 40 min.; (b) 300°C, 30 min.}

Fig. 5. Au, Sn, and Cu concentrations across the multilayers of a diffusion-soldered specimen (from Fig. 3b).

Chuang, Lin and Tsao 1568

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of Cu in liquid Sn (19.5 kJ/mol), as reported by Ma and Swalin.9The rate-limiting step in the growth of the (Cu0.99Au0.01)6Sn5 intermetallics should be the

diffusion of Cu dissolved near the intermetallic reac-tion front into the surrounding liquid Sn thin ﬁlm. Chuang et al.8 calculated the activation energy for

the growth of h-Cu6(Sn0.54In0.46)5 intermetallics

during the soldering reactions between liquid In-49Sn and Cu substrates and found it to be 28.9 kJ/mol, which, by comparison, was also nearly the same for the growth of h-(Cu0.99Au0.01)6(Sn0.52In0.48)5

adja-cent to the Cu/Ti/Si during the thin-ﬁlm diffusion-soldering reactions in this present study. The result implied that the growth mechanism of the h-Cu6

(Sn,In)5 intermetallic phase for thin-ﬁlm diffusion

soldering was not different from that for bulk solder-ing reactions. From the work of Liu,7the activation energy for the growth of the continuous (Au0.96Cu0.04)

(In0.92Sn0.08)2intermetallic layer was 51 kJ/mol, while

the block-shaped (Au0.98Cu0.02)(In0.94Sn0.06)

inter-metallic compounds were observed to ﬂoat into the 49Sn solder matrix. In this present study, the In-49Sn solder was exhausted throughout the diffusion-soldering process, with a greater number of AuIn2

-phase intermetallics reacting with Au to form the AuIn phase by a solid/solid diffusion mechanism. The calculated activation energy for the growth of (AuIn2

1 AuIn) during the thin-ﬁlm diffusion-soldering reac-tions is therefore lower than that for the growth of AuIn2 during the soldering reactions between liquid

In-49Sn and Cu substrates.

The tensile strengths of Si/Ti/Cu/Sn/In/Au/Cu/ Al2O3specimens diffusion-soldered under various

con-ditions are listed in Table II. At lower temperatures (,200°C) and for shorter times (,20 min.), the

Fig. 6. Growth thickness (Dx) of (Cu0.99Au0.01)6(Sn0.52In0.48)5

inter-metallic compounds formed during diffusion soldering between Cu/Ti/Si and Au/Cu/Al2O3with In/Sn interlayers.

Fig. 7. Growth thickness (Dx) of (Au0.87Cu0.13)(In0.94Sn0.06)2/

(Au0.98Cu0.02) (In0.95Sn0.05) double layer formed during diffusion

sol-dering between Cu/Ti/Si and Au/Cu/Al2O3with In/Sn interlayers.

Table I. Growth Rate Constants (K) for the Intermetallic Compounds Formed during Diffusion Soldering between Cu/Ti/Si and

Au/Cu/Al2O3with In/Sn Interlayers

Temperature (°C) (Cu0.99Au0.01)6 (Sn0.52In0.48)5 (mm/min1/2 ) (Au0.87Cu0.13) (In0.94Sn0.06)21 (Au0.98Cu0.02) (In0.95Sn0.05) (mm/min1/2 ) 150 0.124 0.147 200 0.253 0.392 250 0.273 1.022 300 0.641 1.371

Fig. 8. Arrhenius plots of growth rate constants (k) for intermetallic compounds formed during diffusion soldering between Cu/Ti/Si and Au/Cu/Al2O3with In/Sn interlayers.

Table II. Tensile Strength of Cu/Ti/Si Wafer

Diffusion-Soldered with Au/Cu/Al2O3Substrates

at Various Temperatures and Times Using In/Sn Interlayers

Temperature (°C)

Bonding Strength (kg/cm2₎

10 min 20 min 30 min 40 min

150 — — 126 5 426 9

200 — 296 6 486 8 216 7

250 — 236 6 166 3 76 4

300 — 86 3 96 4 46 2

Intermetallic Compounds Formed During Diffusion Soldering

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intermetallic reactions at the interfaces of Cu/In-Sn solder/Au were insufﬁcient, and many cavities or even crevices appeared at the interfaces. In these cases, the specimens failed marginally to join. How-ever, at temperatures above 250°C, the intermetallic layers broke into coarse particles, as shown in Fig. 4, which resulted in a drastic reduction of the bonding strengths, even to the values lower than 10 kg/cm2. Sound tensile strengths of 42 and 48 kg/cm2 have been obtained under bonding conditions at 150°C for 40 min. and at 200°C for 30 min., respectively.

CONCLUSIONS

Multilayer thin-ﬁlm systems of Cu/Ti/Si and Au/ Cu/Al2O3were diffusion-soldered at various

temper-atures ranging from 150 to 300°C with an In/Sn dou-ble layer inserted between them. The In/Sn doudou-ble layer melted at the outset of heating, forming a near-eutectic In-49Sn solder ﬁlm, which reacted fur-ther with the Cu/Ti/Si and Au/Cu/Al2O3substrates to

form a trilayer of intermetallic compounds. The com-positions of these intermetallics as determined by EPMA analyses were (Cu0.99Au0.01)6(Sn0.52In0.48)5/

(Au0.87Cu0.13)(In0.94Sn0.06)2/(Au0.98Cu0.02)(In0.95Sn0.05),

which can be simpliﬁed as Cu6(Sn,In)5/AuIn2/AuIn.

Kinetic analyses showed that the growth of inter-metallics was diffusion-controlled. However, the growth rate of the h-(Cu0.99Au0.01)6(Sn0.52In0.48)5

was much lower than those of the Au-In inter-metallics (Au0.87Cu0.13)(In0.94Sn0.06)/(Au0.98Cu0.02)

(In0.95Sn0.05). The activation energies as calculated

from the Arrhenius plots of the growth rate constants for h-(Cu0.99Au0.01)6(Sn0.52In0.48)5and (Au0.87Cu0.13)

(In0.94Sn0.06)/(Au0.98Cu0.02)(In0.95Sn0.05) were 21.5

and 31.3 kJ/mol, respectively. Sound tensile strengths of 42 and 48 kg/cm2 have been obtained under bonding conditions at 150°C for 40 min. and at 200°C for 30 min., respectively. Heating at the temperatures above 250°C caused the AuIn2

inter-metallic compounds in front of the h-(Cu0.99Au0.01)6

(Sn0.52In0.48)5 layer to break into coarse particles,

and their bonding strength decreased drastically to values below 10 kg/cm2.

ACKNOWLEDGEMENT

The authors gratefully acknowledge National Science Council, Taiwan, for their ﬁnancial support of this research under Grant No. NSC-93-2216-E002-024.

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3. S. Bader, W. Gust, and H. Hieber, Acta Metall. Mater. 43, 329 (1995).

4. S. Sommadossi, W. Gust, and E.J. Mittemeijer, Mater. Chem. Phys. 77, 924 (2002).

5. M.W. Liang, T.E. Hsieh, S.Y. Chang, and T.H. Chuang, J. Electron. Mater. 32, 952 (2003).

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