Electro-migration Behavior in Eutectic Sn-Bi Flip Chip

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研究論文

　＊新光電気工業㈱開発統括部（〒 381-0014 長野県長野市大字北尾張部 36）　　Research & Development Div. Shiko Electric Industries Co., Ltd (36 Kita-Owaribe, Nagano, Nagano, 381-0014 Japan)＊＊㈱富士通研究所基盤技術研究所（243-0197神奈川県厚木市森の里若宮 10-1）　　Devices & Materials Laboratories, Fujitsu Laboratories Ltd.　　(10-1 Morinosato, Wakamiya, Atsugi, 243-0197 Japan)

Electro-migration Behavior in Eutectic Sn-Bi Flip Chip Solder Joints with Cu-Pillar Electrodes

Kei MURAYAMA*, Takashi KURIHARA*, Taiji SAKAI**, Nobuhiro IMAIZUMI**,Kozo SHIMIZU**, Seiki SAKUYAMA** and Mitsutoshi HIGASHI*

(Received May 8, 2013)

This paper discusses electro-migration behavior of eutectic Sn-Bi solder with Cu-pillar bumps. Two types of under bump metal (UBM) of organic substrate were studied, that is, Cu pad and electroless Ni/Au plated on Cu pad. The current density was 4x104A/cm2 at 125 and 150 degree C. Bi quickly migrated to accumulate on the anode side(Cu –pillar) and Sn migrated to the cathode side (Substrate pad). Both the case of surface finishes, although the resistivity was increased to approximately 80 % during approximately 80 hours, it was stabilized more than 2800 hours and there were no electrically break failure. From the cross-sectional analyses of eutectic Sn- Bi solder joints after the test, it was found that Bi layer and intermetallic compound (IMC) behaved as the barriers of the Cu atom migration into Sn solder.

Key Words: Electro-migration, Cu-pillar, Sn-Bi Solder, Flip Chip, Low Temperature Bonding, IMC

1. Introduction

The assembly technique for large size flip chip packages and 3D packages requires a low stress and a low temperature process to achieve high reliability of the micro joining. And Si interposer using Cu through-si-vias (TSV) or high-end logic chips using high performance insulation layer such as a pours low-k are sensitive to stress that occur during flip chip assembly. The CTE mismatch between a large size chip or a Si-interposer and an organic substrate induces the chip insulation layer crack during solder reflow 1). The micro joint using eutectic Sn-Bi solder has been noted as a low temperature bonding method. The melting point of eutectic Sn-Bi solder is 138 degree C.

And electro-migration phenomenon is the most significant problem for flip chip packages. The critical current density at flip chip joint is lower than that in LSI pattern which is generally recognized as a case of electro-migration failure. The current density at the micro solder joint is expected to be in the order of 1x104A/cm2 2-4).

We previously reported the results in electro-migration behavior of Sn 3.0 wt%Ag 0.5 wt%Cu (SAC305) with electroless Ni/Au on Cu pad 5). But SAC305 solder required higher temperature to interconnect. In this paper we will discuss electro-migration phenomenon using eutectic Sn-Bi for two types of under bump metal (UBM) of organic substrate.

2. Experimental

Overview of the electro-migration test sample and the schematic view of the cross-section of the bump used in this study are shown in Fig. 1 and 2, respectively.

The evaluation TEG chip (Phase5) fabricated by Hitachi ULSI systems co., Ltd. was adopted as evaluation.

The size of the test chip for evaluation was 9 mm × 9 mm × 0.55 (t) mm and the minimum bump pitch was 175 m. The daisy chain lines were routed using a wafer-level-packaging redistribution process, that formed 5μm thick Cu. Cu-pillar with 100 μm diameter and 50 μm height was fabricated by electroplating on the pads of Si chip. The size of organic substrate was 37.5 mm × 37.5 mm × 1.1 (t) mm.

37.5mm

Fig. 1 Overview of electro-migration test sample.

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スマートプロセス学会誌　第 2巻　第 4号（2013年 7月）

Two types of under barrier metal (UBM) of organic substrate were fabricated, that is Cu pad and electroless Ni(P) (6 μm )/Au (0.5 μm) plated on Cu pad. Sn57Bi (eutectic) solder ball with diameter 75μm was used to fabricate the bump for connecting to Si chip. The solder balls were reflowed at 160 degree C for 5minutes with flux. After reflow, flux cleaning and underfill resin was supplied and was cured at 165 degree C for 30minutes.

Fig. 3 shows cross-sectional view of Cu-pillar bump after underfill resin curing.

The schematic diagram of electron charge bump circuit used in this study is shown in Fig. 4. The electro-migration test condition was determined based on the JEDEC standard JEP 154 6). For electro-migration test, mainly failure occurred at cathode side 5). Using Cu-pillar bumps, the electro-migration failures occur at substrate pads with electron flow from the substrate side (Cu or Ni/Au pad) to the chip side (Cu-pillar) 5). To accelerate failures only one polarity of bump, three bumps were applied and electron was focused to the bump of subject. A daisy chain of ten subject bumps was built in a test sample. Resistance change of the daisy chain was measured by four-wire method during electro-migration test. Therefore electron flow to induce the electro-

migration was from substrate side to chip side with current of 1 A at 125 and 150 degree C. In these cases the current density was determined by smallest cross-section area of the Cu pad or Ni pad on the solder mask opening diameter. This current corresponds to current density of 4.0×104 A/cm2. In order to minimize the effect of Joule heating, heat sink was attached on the back side of TEG chip. The chip temperature was confirmed to be in the range of 2 to 4 degree C higher than the oven temperature of 150 degree C by measuring the resistance patterned on the chip.

After the electro-migration test, the change of solder composition and microstructure were examined. The subject bumps were cross-sectioned and were observed by Scanning Electron Microscope (SEM). After that, the bumps were pre-treated with focused ion beam (FIB) and were analyzed by Electron probe micro analyzer (EPMA). We investigated phase analysis from X-ray maps and scatter diagrams that was used to identify the composition of the IMCs.

3. Results and Discussion

3.1 Electro-migration test results (Cu pad)Fig. 5 shows typical resistance change of the Sn57Bi on Cu pad

with the current density of 4.0×104 A/cm2 at 125 and 150 degree C.In the case of 125 degree C, the resistivity was gradually

increased to 46 %, during 100 hours. And after, the resistivity was slowly increased to 83 % during 2200 hours. And then, the

Si Chip Cu-pillar Sn57Bisolder

Au(0.5μm)

Ni(6μm)

Cu Pad

Si Chip

Sn Bi Cu

Ni Cu

(a) (b)

Fig. 2 Schematic view of the cross-section of the bump (a) with Cu pad and (b) with Ni/Au pad.

(a) Cu pad (b) Ni/Au pad

Fig. 3 Cross-sectional view of bumps of after underfill resin curing.

Electron flow

Silicon Chip

Bump of subject

Fig. 4 Schematic diagram of bump circuit and electron flow.

125 degree C

150 degree C

Fig. 5 Typical resistance change of Sn57Bi on Cu pad with 4.0×104 A/cm2.

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Electro-migration Behavior in Eutectic Sn-Bi Flip Chip Solder Joints with Cu-Pillar Electrodes (MURAYAMA, KURIHARA, SAKAI, IMAIZUMI, SHIMIZU, SAKUYAMA and HIGASHI)

resistivity was very slowly decreased to 72 % during 4840 hours.On the other hand, in the case of 150 degree C, the resistivity

was quickly increased to 80 % during 80 hours. And after, the resistivity was fast decreased to 55 % during 990 hours. And then, the resistivity was stabilized until at least 2800 hours.

Notice that both case, electrically open failures were not observed.

It was 10 times longer life compared with the SAC305 solder joint 7).

Backscattered Electron (BSE) images and phase maps determined from EPMA X-ray maps of Sn57Bi bump on Cu pad after current stressed by 4.0×104 A/cm2 at 125 degree C for 4840 hours and 150 degree C for 2800 hours are shown in Fig. 6 and Fig. 7, respectively. In the case of 125 degree C (Fig. 6), focusing on the bump with thermal stressing 6(a), thick Cu3Sn/Cu6Sn5 IMCs were formed at both of the Cu-pillar side and Cu pad side. Bi layer was formed between the IMCs of Cu-pillar and Cu pad. Focusing on the bump with current stressing 6(b), all of Bi atoms migrated to the anode side (Cu-pillar) and accumulated on it. Only thick Cu3Sn inter-metallic compounds (IMCs) layer were formed at the Cu-pillar side. And under the Bi layer Cu6Sn5 / Cu3Sn IMCs were formed. Sn atoms diffused from the Cu-pillar

side to the cathode side and large void was formed at the interface between Cu6Sn5 and Cu3Sn of cathode side.

On the other hand, in the case of 150 degree C (Fig. 7), although all of Bi atoms migrated to the anode side (Cu-pillar), Bi atoms did not form an uniform layer. Cu3Sn IMCs were formed at near a bump center line connecting anode side and cathode side. Cu6Sn5 IMCs were slightly formed at the Cu pad side. Both cases, all of Sn atoms were consumed to form Cu-Sn IMCs, these IMCs layers were formed at Cu-pillar side and Cu pad side.

3.2 Electro-migration test results (Ni/Au on Cu pad)Fig. 8 shows typical resistance change of the Sn57Bi on Ni/

Au pad with the current density of 4.0×104 A/cm2 at 125 and 150 degree C.

In the case of 125 degree C, the resistivity was gradually increased to 25 %, during 100 hours. And after, the resistivity was slowly increased to 82 % during 3300 hours. On the other hand, in the case of 150 degree C, the resistivity was quickly increased to 80 %, during 80 hours. And after, the resistivity was very slowly decreased to 77 % during 2800 hours.

Same as using Cu pad, electrically open failures were not

Fig. 6(a) BSE image and phase map of the bump after thermal stressed at 125 degree C for 4840 hours (Sn57Bi/Cu pad).

Fig. 6(b) BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 125 degree C for 4840 hours (Sn57Bi/Cu pad).

Fig. 7 BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 150 degree C for 2800 hours (Sn57Bi/Cu pad).

Fig. 8 Typical resistance change of Sn57Bi on Ni/Au pad with 4.0×104 A/cm2.

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observed.BSE images and phase maps determined from EPMA X-ray

maps of Sn57Bi bump on Ni/Au pad after current stressed by 4.0×104 A/cm2 at 125 degree C for 3300 hours and 150 degree C for 2800 hours are shown in Fig. 9 and Fig. 10, respectively.

In the case of 125 degree C (Fig. 9), focusing on the bump with thermal stressing 9(a), Sn-Bi solder grain growth was observed and IMCs layers were formed at Cu-pillar side and Ni pad side.

Focusing on the bump with current stressing 9(b), all of Bi atoms migrated to the anode side (Cu-pillar) and accumulated on it. Sn migrated to the cathode side (Ni pad).

Phase map showed that thin Cu3Sn/Cu6Sn5 IMCs were formed at the Cu-pillar side. And under the Bi layer (Cu,Ni)6Sn5/(Ni,Cu)3Sn4 IMCs and Sn were formed.

On the other hand, in the case of 150 degree C (Fig. 10), similar to the results of the case of 125 degree C all of Bi atoms migrate to the anode side (Cu-pillar), Bi atoms formed an uniform layer. Cu3Sn was thicker than that of the case of 125 degree C. Thin Cu6Sn5 IMCs were formed at the Cu-pillar side. And under the Bi layer thick (Cu,Ni)6Sn5/thin (Ni,Cu)3Sn4 IMCs were formed.

Both cases, all of Sn atoms were consumed to form Cu-Sn

IMCs. BSE images showed that Ni-P (phosphorus) rich layer were formed. However, remarkable cracks or voids were not observed.

3.3 Apparent activation energy for electro-migrationIt is well known that electro-migration lifetime is given by

Black’s equation as below 8).

MTTF = exp A EaJ n kT

(1)

where MTTF is the mean time to failure at accelerated test conditions, A is the constant, J is the current density, n is the current density exponent, Ea is activation energy to electro-migration failure, k is the Boltzmann’s constant and T is the absolute temperature. The MTTF was defined by log normal distribution model. In the solder joining industry, 10 % resistance increase is widely adopted as the failure criteria. Because usually a resistance rapidly increase, and electrically open failure is detected. However, in case of using SnBi solder, electrically open failure was not observed.

Therefore electro-migration lifetimes were defined by several resistance increase rates from initial.

Typical Arrhenius plot for the Sn57Bi on Ni/Au pad between ln (TTF) and 1/T is shown in Fig. 11. The TTF were defined 10 %, 40 % and 70 % resistance increase from initial value.

The slope of the line will be equal to Ea/k.By solving for the activation energy, the following equation is

obtained.

Ea = kx1T1

ln(TTF1) - ln(TTF2)

1T2

-( ) (2)

An apparent activation energy is plotted in Fig. 12 as a function of resistance increase rate from initial. A resistance increase from initial at 125 degree C and 150 degree C are used to determine

Fig. 9(a) BSE image and phase map of the bump after thermal stressed at 125 degree C for 3300 hours (Sn57Bi/Ni/Au pad).

Fig. 9(b) BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 125 degree C for 3300 hours (Sn57Bi/Ni/Au pad).

Fig. 10 BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 150 degree C for 2200 hours (Sn57Bi/Ni/Au pad).

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activation energy.Using the Ni/Au pad, apparent activation energy of 10 % and

70 % resistance increase were found to be 0.2eV and 2.03eV, respectively. In contrast, using the Cu pad, apparent activation energy of 10 % and 70 % resistance increase were found to be -0.55eV and 0.76eV, respectively. Apparent activation energy showed negative value when it is defined as lifetime of resistance increase rate up to 45 %.

3.4 Electro-migration phenomenon of Sn-Bi eutectic solder using Ni/Au pad

In order to explore the cause of the significant resistance increase at initial stage, we performed the cross-sectional observation of the samples 10 % resistivity changed at 125 degree C.

BSE image and phase map determined from EPMA X-ray map of Sn57Bi bump on Ni/Au pad after current stressed by 4.0×104

A/cm2 at 125 degree C for 10 % resistivity changes is shown in Fig.13.

Almost the amount of Bi atoms migrated to the anode side (Cu-pillar) by the electron flow and accumulated on it. At the same time, Sn atoms diffused to the cathode side (Ni/Au pad). And

almost the amount of Sn atoms formed a Sn layer. A small amount of Ni atoms diffused into Cu6Sn5 and (Cu,Ni)6Sn5 were formed. In this time there were hardly Ni3Sn4 and (Ni,Cu)3Sn4 layer in this bump.

Table 1 shows electrical resistivity of solders and IMCs 9,10).It was reported that an increment in electrical resistance

strongly depended on the growth of Bi-layer at anode side from the results of Sn58Bi solder / Cu micro joints sample experiment by Ohtake et al. 9,11). The electrical resistivity of Bi is higher than eutectic Sn-Bi solder (Sn58Bi), they are 156 μ ohm cm and 58μ ohm cm, respectively. Therefore, growth of the Bi-layer is related to the increment in electrical resistance in this study.

In the case of 150 degree C, the resistivity was very slowly decreased to 3 % from 80 hours to 2800 hours.

Fig. 9 and Fig. 10 show that thickness of Cu6Sn5 at anode side of the case of 150 degree C is thinner than that of 125 degree C.

In the case of Cu-Sn IMCs system, it is well known that Cu3Sn / Cu6Sn5 are formed at Cu side and growth rate of Cu6Sn5 is faster than that of Cu3Sn 12-13).

In this study, it can be presumed that Cu3Sn exist in the initial reaction stage. But the Bi layer has effect as barrier to diffuse Sn atoms from cathode side. It is induced that Cu6Sn5 of Cu-pillar side transformed to Cu3Sn. It is further supported by the results of any (Cu,Ni)6Sn5 was not observed at Cu-pillar side.

In addition, it was also reported that Cu6Sn5 transform to Cu3Sn induced to decrease in electrical resistance from the results of Cu-

Fig. 11 Arrhenius plot for the Sn57Bi on Ni/Au pad between ln(TTF) and 1000/T(K-1) at resistance increase.

Fig.12 Apparent activation energy determined by resistance increase from initial.

Fig. 13 BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 125 degree C for 10% resistivity cuchanges (Sn57Bi/Ni/Au pad).

Table 1 Electrical resistivity of Solder and IMCs

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Sn micro bump sample experiment by Riet et al. 10).According to this report, the electrical resistivity of Cu3Sn and

Cu6Sn5 are 8.8 μ ohm cm and 17.5μ ohm cm, respectively.It follows from these arguments that assuming the reaction

at 150 degree C, it lead us to presume that slowly decrement of electrical resistance after 80 hours indicate Cu6Sn5 transform to Cu3Sn.

Fig. 9(b) shows (Cu,Ni)6Sn5 /(Ni,Cu)3Sn4 formed at 125 degree C. But there were hardly Ni3Sn4 and (Ni,Cu)3Sn4 layer in this bump at 150 degree C (Fig. 10).

It was reported that the kind of IMC formed in Sn, Ni, Cu system was determined by the Cu concentration, that is, when the Cu content was low (x = 0.2 wt%), the reaction product was (Ni1–

xCux)3Sn4, at high Cu content (x = 0.7 and 1 wt%), the reaction product was (Cu1–yNiy)6Sn5, when the Cu content was in-between (x = 0.4 wt%), both (Ni1–xCux)3Sn4 and (Cu1–yNiy)6Sn5, were formed 14-16).

Therefore in a case of Cu content is higher than 0.4 wt%, only (Cu,Ni)6Sn5 is formed.

The diffusion rate of Cu atoms at 150 degree C is higher than that at 125 degree C. It seems right to presume that Cu atoms value in solder at 150 degree C is higher than that at 125 degree C.

It follows from these argument that assuming the reaction mode at 150 degree C, it lead us to presume that Cu content is sufficient higher than 0.4 wt%. Therefore (Ni,Cu)3Sn4 IMCs was not formed, but (Cu,Ni)6Sn5 IMCs was formed.

3.5 Electro-migration phenomenon of eutectic Sn-Bi solder using Cu pad

BSE images and phase maps determined from EPMA X-ray maps of Sn57Bi bump on Cu pad after current stressed by 4.0×104

A/cm2 at 125 degree C for 10 % resistivity changes is shown in Fig. 14.

Two types of reaction mode were observed. Regarding Fig. 14(a), almost all of Bi atoms migrated to the anode side (Cu-pillar) by the electron flow and accumulated on it. At the same time, Sn atoms diffused to the cathode side (Cu pad). Almost all of Sn atoms formed Sn layer. Thin Cu3Sn/ Cu6Sn5 IMCs were formed at both of the Cu-pillar side and Cu pad side. In contrast as shown in Fig. 14(b), Bi atoms did not form a layer and a portion of Sn was in contact with the Cu-pillar. The growth of the Bi-layer is related to increment in electrical resistance at initial stage.

Although the uniform shape of Bi layer was observed at the anode side (Cu-pillar) at Ni/Au pad, substantially uniform shape of Bi layer (Fig. 14(a)) and non-uniform shape of Bi layer (Fig. 14(b)) were observed at the anode side (Cu-pillar) at Cu pad. Fig. 14(b) show that Bi atoms did not form a layer and a portion of Sn was in contact with the Cu-pillar of bump was observed.

Using the Cu pad, apparent activation energy showed negative value if it is defined as lifetime of resistance increase rate up to 45 %. This reason is that a resistance increase from initial was defined as a lifetime. It is assumed that the reaction accompanied by resistance reduction occurs at 150 degree C.

Fig. 15 shows Arrhenius plot between ln (TTF) and 1/T. The TTF were defined 10 % resistance increase from initial

Fig. 14(a) BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 125 degree C for 10% resistivity changes (Sn57Bi/Cu pad).

Fig. 14(b) BSE image and phase map of the bump after current stressed by 4.0×104 A/cm2 at 125 degree C for 10% resistivity changes (Sn57Bi/Cu pad).

1

1.5

2

2.5

3

3.5

2.35 2.4 2.45 2.5 2.55

ln(T

TF)

/a.u

.

1000/T /1/K

Ni/AuCu

Fig. 15 Arrhenius plot between ln(TTF) and 1000/T(K-1) at resistance increase.

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value. Using the Cu pads, the TTF at 150 degree C (1000/T=2.364) is longer than that at 125 degree C (1000/T=2.513). This phenomenon indicates the resistance increase is slow at 150 degree C.

Fig. 15 show typical resistance change of the Sn57Bi with the current density of 4.0×104 A/cm2 at initial stage.

Focusing on using the Ni/Au pads, Fig. 15(a) shows that resistance increment at 150 degree C is higher than that at 125 degree C. But using the Cu pad, resistance decrement was observed at 150 degree C immediately after the start of the test (Fig. 15(b)). This phenomenon is a cause of a resistance decrease.

The simplified model diagram of difference in distribution of Bi and Sn phase layer of Sn-Bi bump on Cu pad for 10 % resistivity changes is shown in Fig. 16.

Focusing on Fig. 17(a), in the case of layered Bi atoms to Cu-pillar, combined resistance model of Bi and Sn are arranged in series. The electrical resistivity of Bi is higher than that of eutectic Sn-Bi solder (Table 1). It can be presumed that this model induces quickly increase of electrical resistance.

In contrast, focusing on Fig. 17(b), the case of a potion of Sn

atoms connected with Cu-pillar, combined resistance model of Bi and Sn are arranged in parallels. The electrical resistivity of Sn, Sn58Bi, and Bi are 15.8, 58 and 156μ ohm cm, respectively (Table 1).

It follows from these argument that assuming the reaction mode at 150 degree C, it lead us to presume that a resistance decrement is caused.

Same as using the Ni/Au pad, remarkable growth of Cu3Sn was observed at Cu-pillar side. Specifically, in case of at 150 degree C, remarkable growth of Cu3Sn was observed at Cu pad side. As shown in Fig. 5, the rapid resistance decrease indicates supportive evidence that is Cu6Sn5 of Cu-pillar side and Cu pad side transformed to Cu3Sn.

4. Conclusion

The electro-migration test is performed on Sn57Bi with Cu-pillar at 125 and 150 degree C.

Although the resistivity was increased to 80 % during 80 hours, it is stabilized more than 2800 hours and there were no electrically break failure.

Initial reaction stage, Bi migrated to the anode side (Cu-pillar) and accumulated on it. The Bi layer has effect as barrier to diffuse Sn atoms from cathode side.

Next reaction stage, Cu6Sn5 of Cu-pillar side transformed to Cu3Sn.

Using the Ni/Au pad, not (Ni,Cu)3Sn4 but rather selectively (Cu,Ni)6Sn5 was formed.

Using the Cu pad, apparent activation energy for electro-migration lifetime seems natural to conclude that full-layered Bi atoms was not formed but a portion of Sn atoms and Bi atoms in parallels were formed. And it induces resistance decrease.

References

1) Po-Jen Cheng, C. M. Chug, T.M. Pai and D.Y. Chen, “A Challenge of 45 nm Extreme Low-K Chip Using Cu Pillar Bump as 1st Interconnection”, Proceedings of 60th ECTC, Las Vegas, NV USA,

Fig. 16(a) Resistance change of Sn57Bi on Ni/Au pad with 4.0×104 A/cm2 at initial stage.

Fig. 16(b) Resistance change of Sn57Bi on Cu pad with 4.0×104 A/cm2 at initial stage.

(a) (b)

Fig. 17 Schematic diagram of combined resistance model for 10% resistivity changes (Sn57Bi/Cu pad).

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June 2010, pp.1618-1622.2) Fei Ren, et al., “In-situ Study of the Effect of Electromigration on

Strain Evolution and Mechanical Property Change in Lead-free Solder Joints” Proceedings of 56th ECTC, San Diego USA (2006), pp.1160-1163.

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Electro-migration Behavior in Eutectic Sn-Bi Flip Chip