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Highly conductive Cu-Cu joint formation by lowtemperature sintering of formic acid-treated Cu nanoparticles Jingdong Liu, Hongtao Chen, Hongjun Ji, and Mingyu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10280 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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ACS Applied Materials & Interfaces

Highly Conductive Cu-Cu Joint Formation by Low-Temperature Sintering of Formic Acid-Treated Cu Nanoparticles

Jingdong Liu1,2¶, Hongtao Chen2¶, Hongjun Ji2, Mingyu Li1,2*

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State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

2

Department of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen Graduate School, Shenzhen, 518055, China

¶

Equal contribution, * Corresponding author： [email protected]

Abstract

Highly conductive Cu-Cu interconnections of SiC die with Ti/Ni/Cu metallization and direct bonded copper substrate for high power semiconductor devices are achieved by the low-temperature sintering of Cu nanoparticles with a formic acid treatment. The Cu-Cu joints formed via a long-range sintering process exhibited good electrical conductivity and high strength. When sintered at 260 ºC, the Cu nanoparticle layer exhibited a low resistivity of 5.65 µΩ·cm, and the joints displayed a high shear strength of 43.4 MPa. When sintered at 320 ºC, the resistivity decreased to 3.16 µΩ·cm, and the shear strength increased to 51.7 MPa. The microstructure analysis demonstrated that the formation of Cu-Cu joints was realized by metallurgical bonding at the contact interface between the Cu pad and the sintered Cu nanoparticle layer, and the densely sintered layer was composed of poly-crystals with a size of hundreds of nanometers. In addition, high-density twins were found in the interior of the sintered layer, which contributed to the improvement of the performance of the Cu-Cu joints. This bonding

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technology is suitable for high power devices operating under high temperatures.

Keywords: Cu nanoparticles, low-temperature bonding, sintering, power electronic packaging, high conductivity

Introduction

Recently, a substantial of attention has focused on the miniaturization and integration of high power semiconductor devices for harsh environment applications such as aeronautic arena and automotive industries in which the junction temperature during operation is high.1-4 The potential raw material of high power semiconductor devices, such as SiC and GaN, with wide band gaps are expected to operate for an extended period at temperatures above 200 ºC, even 350 ºC,5-7 which are too high for the traditional die attach materials that are currently widely used. In general, tin-based lead-free solders that may suffer from remelting are excluded from these high temperature applications.8 Other plumbum-, bismuth-, gold-, and zinc-based solders with high melting point have their own drawbacks, including environmental damage, high cost, high process temperature and inferior electrical conductivity.9-11 A useful bonding material free of plumbum for high temperature applications has not been established to date. Therefore, the development of a bonding technology that could endure elevated temperatures has become increasingly important. In recent years, nano-joining with metal nanoparticles (NPs), which could realize low temperature joining and high temperature service, has been considered a promising substitute technology for soldering.

Because of the small size effect of NPs, the melting point of metal NPs is greatly depressed compared that of the bulk metal. This property indicates the possibility of the low temperature fabrication of Cu-Cu joints using metal NPs, such as silver NPs and copper NPs, which show 2


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significant advantages over traditional solders, including eco-friendliness, high thermostability, the free of plumbum, and high electrical and thermal conductivity. Some reports have been published on low-temperature bonding with Ag NPs.12-14 However, due to the severe electromigration and high cost, the practical application of Ag NPs as a bonding material is limited.15 Since copper is less expensive than silver and has a good resistance to ion migration, the electrical properties and thermal properties of copper are comparable to that of silver. Consequently, Cu NPs show significant application potential as a type of low-temperature bonding material. However, from the point of view of thermodynamics, the surface copper oxide is more stable than pure copper, and the favorable formation of surface copper oxide would increase the sintering temperature of Cu NPs, which is a serious concern when using Cu NPs for low-temperature interconnections. Although a protective layer of polymers coated on the surface of Cu NPs, such as PVP16-18 and CTAB19-21, was shown to be a good barrier to oxidation, the oxidation was only diminished and copper oxide still existed as a passivation layer. Therefore, a high process temperature over 300 ºC, even 400 ºC, was required to obtain high-strength joints.8,22,23 Obviously, this processing temperature is too high for Cu NPs to be widely used in the field of consumer electronics, and more efforts should focus on lowering the processing temperature. A metal core-shell structure is also used to improve the oxidation resistance of Cu NPs, including Ag-Cu,24,25 and Ni-Cu,26 but its cost is not low enough compared with pure Cu, and a substantial amount of copper is abandoned, moreover, the synthesis process of core-shell structure is more complicated.

Recently, a reductive atmosphere containing formic acid vapor has been considered as an effective reductant for the reduction of surface oxidation of Cu NPs.27-29 It could not only decrease the sintering temperature of Cu NPs but also increase the conductivity of sintered Cu NPs, and it is suitable for the fabrication of highly conductive printed electronics if not considering about the corrosion of 3



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formic acid. Woo et al.30 reported that printed Cu NP films exposed to a reducing atmosphere of formic acid vapor exhibited a resistivity of 2.3 µΩ·cm when sintered at 250 ºC, which is the lowest resistivity that has been reported. It is an effective way to realize the sintering of Cu NPs at temperatures as low as possible. However, there are few reports on Cu-Cu joint formation using Cu NPs exposed to formic acid vapor. One possible reason for the scarcity of these studies is that the sandwich-structured Cu-Cu NPs-Cu joints would hinder sufficient contact with Cu NPs and formic acid vapor. As a result, only the Cu NPs that react sufficiently with the reducing vapor are fused as a whole, while the rest ones are loosely connected. Previously our laboratory found that the sintering temperature of Cu NPs can be lowered by formic acid soaking process,31 however, the thermal sintering process is not clear, and the possibility of its usage as nanopaste to fabricate Cu-Cu joints is still unknown.

Herein, we report the formation of highly conductive Cu-Cu joints via the low-temperature annealing process of Cu NP paste (Fig. 1). A simple and rapid approach that eliminates surface oxide of Cu NPs by using a mixture of formic acid and absolute ethyl alcohol is presented, and its effect on the thermal sintering behavior of Cu NPs is explored. In addition, the bonding properties of Cu NP paste are evaluated, and the bonding mechanism of Cu-Cu NPs-Cu joints is analyzed in detail.

Experimental section

Materials

Poly(N-vinylpyrrolidone) (PVP, Mw=58.000), copper sulfate pentahydrate (CuSO4·5H2O, 98%), sodium hypophosphite (NaH2·PO2·H2O), diethylene glycol (DEG, 99%) and formic acid were supplied by Sigma Aldrich. Absolute ethyl alcohol (99.7%) was obtained from Sinopharm Chemical Reagent Co., Ltd. All of the raw materials were used as received. Deionized water (18.2 MΩ·cm−1) was 4


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supplied in the washing stage. In addition, direct bonding copper (DBC) substrate and SiC components were provided by Delta Electronics.

Preparation of Cu nanoparticles

The Cu NPs were prepared with a polyol method modified from Park’s group.32 Briefly, a mixture of 0.04 mol of copper sulfate pentahydrate and 0.086 mmol of PVP was dissolved in 300 ml of DEG that had been heated to 100 ºC in the atmosphere. Next, 0.45 mol of sodium hypophosphite was added to another 300 ml of DEG, which had also been heated to 100 ºC, and then, the second hot DEG solution was quickly added to the first solution under vigorous mechanical stirring. After 5 min of the chemical reaction, the hot reaction solution containing synthesized Cu NPs was naturally cooled to ambient temperature. After that, the prepared Cu NPs were subsided with the aid of centrifugal separation for 5 min at 4000 rpm, and then, the washing process with deionized water and absolute ethyl alcohol were carried out to wash the collected Cu NPs for two times to remove impurities.

Formic acid treatment of Cu nanoparticles

For the formic acid treatment, Cu NPs were dispersed in a mixture of absolute ethyl alcohol (vol: 97.5%) and formic acid (vol: 2.5%). The Cu NPs were separated by centrifugation after being soaked for different holding time varied from 10 min to 30 min. The mixed colorless solution shifted to light blue one, implying the generation of Cu2+. Finally, the formic acid treated Cu NPs were washed with absolute ethyl alcohol for two times to wipe off residual formic acid.

Characterization

The average size and surface morphology of the Cu NPs were investigated by Transmission Electron Microscope (TEM, Tecnai G2 F20, FEI) and Scanning Electron Microscope (SEM, S-4700, 5



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Hitachi). The phase composition was detected with X-ray Diffractometer (XRD, DMAX2500, Rigaku). And the surface chemistry composition of the Cu NPs was detected by X-ray Photoelectron Spectroscopy (XPS, PHI 1800, ULVAC-PHI). TG-DSC was carried out in the shielding gas of N2 by Thermogravimetric Differential Scanning Calorimeter (TG-DSC, STA449, Netzsch), the heating rate was 10 ºC·min-1. The heat treatment was performed in heating furnace (VBF-1200X, MTI).

Evaluation of shear strength of the Cu-Cu joint

For bonding experiment, the DBC substrate was used as the Cu pad (additional structural details are shown in supplementary Fig. S1), and the SiC components were metallized with 50 nm (Ti)/50 nm (Ni)/10 µm (Cu) and cut into 2×2 mm2 pieces (additional structural details are shown in supplementary Fig. S2). For the preparation of Cu NP paste, Cu NPs were mixed with ethylene glycol with a solid content of 70%, and then, with the help of a specially-made metal mask, Cu NP paste was printed onto the Cu pad; the area of printed Cu NP paste was 3×3 mm2 and the thickness was 0.10 mm. Then, the printed Cu NP paste was dried in the atmosphere at 60 ºC for 10 min; subsequently, the bonding experiments were performed in an atmosphere of 5% H2 + 95% N2 (The purity of the gas mixture is 99.99%) for 5 min from 160 ºC to 320 ºC, and the assist pressure was 10 MPa, the heating rate was 5 ºC·min-1. The shear strengths test was carried out at ambient temperature using a bond tester (DAGE series 4000 HS) (a schematic diagram of the shear test details is shown in supplementary Fig. S3). The shear height was 100 µm, and the shear speed was set to 200 µm·s-1 (Standard JIS Z 3198-7). A minimum of 10 samples was adopted in each group for the shear tests, and the average value was reported as the shear strength. The microstructure evolution of the interfaces and the fracture surfaces after the shear strengths test of the Cu-Cu joints was explored using SEM and TEM.

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Evaluation of electrical resistivity of the Cu-Cu joints

It is difficult to evaluate the electrical properties of sandwich-structured Cu-Cu joints. Instead, to assess the electrical resistivity of these joints, the resistivity of the sintered Cu NP film was measured by four probe method with a resistivity measurement system (2182A, Keithley). To prepare the test specimens, the Cu NP paste was printed onto high temperature PI film using a specially-made metal mask with a thickness of 30 µm and an area of 10×10 mm2. The printed films were also dried in the atmosphere at 60 ºC for 10 min and then annealed in an atmosphere of 5%H2+95%N2 for 5 min from 200 ºC to 320 ºC, and the assist pressure was 10 MPa. The thickness of the sintered films was approximately 10±2 µm.

Results and discussion

Characteristics of Cu nanoparticles

Fig. 2a displays the SEM and high-resolution transmission electron microscopy (HRTEM) images of Cu NPs prepared using a modified polyol method in which PVP acted as the capping agent and dispersing agent to hinder the surface oxidation and to ensure the dispersibility of Cu NPs. The average particle size is approximately 30 nm, and the spherical Cu NPs are coated with an amorphous layer with a thickness of approximately 1.5 nm. The coating layer is identified as PVP by the follow-up surface composition analysis. The addition of an appropriate amount of PVP could ensure the dispersibility of the synthesized Cu NPs and may provide a positive protection for Cu NPs against oxidation to a certain degree. As shown in Fig. 2b, the typical XRD pattern of the synthesized Cu NPs contains three main diffraction peak at 43.3°, 50.4°, and 74.08°, which represent the diffraction of planes (111), (200) and (220), respectively, of the face-centered cubic structure of bulk copper; no 7



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oxide peaks, such as those for CuO or Cu2O, are observed.

However, the formation of surface copper oxide is inevitable when Cu NPs are synthesized in an ambient atmosphere. The two primary origins for the oxidation of the Cu NPs are their reaction with the oxygen dissolved in the reaction solvent and their contact with oxygen in the air. The coating layer of PVP only hinders the further oxidation of copper nanoparticles. The copper oxide already present would lead to a high electrical resistivity and a weak bonding strength, and it should be removed completely to obtain a bonding structure with high conductivity as well as high bonding strength. Therefore, the formic acid treatment was performed to eliminate the uncontrolled surface copper oxide. Cu NPs without formic acid treatment were used as the control group. To facilitate the presentation, we denominate the f-Cu NPs and u-Cu NPs, which represent the formic acid-treated Cu NPs and untreated ones. The chemical reaction occurring during the formic acid treatment is presented in Eq. (1).

CuO + 2HCOOH → Cu(COOH) 2 + H 2 O

(1)

To study the oxidation removal effects of the formic acid treatment, XPS measurements were obtained to investigate the changes in the surface chemical composition of Cu NPs. As shown in Fig. 3a, the Cu 2p3/2 spectra of u-Cu NPs shows that the peak at 932.1 eV is assigned to copper, and the peaks of 934.3 eV, 941.6 eV, and 943.7 eV represent copper oxide, indicating the existence of surface oxidation. In the spectra of C1s of u-Cu NPs in Fig. 3b, the four peaks at 284.5 eV, 285.3 eV, 286.2 eV, and 287.9 eV originate from the coating layer of PVP. Thus, the surface amorphous layer consists of copper oxide and PVP, which cannot be detected by XRD. The surface oxidation may occur through the reactions between Cu NPs and O2 or H2O, which cannot be completely avoided. It is interesting that the peak of oxygen atom bonding with copper disappears after the Cu NPs are soaked with formic acid.

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Instead, a new peak of 935.1 eV which belongs to formate appears, as shown in the Cu 2p3/2 spectra of f-Cu NPs in Fig. 3c. This finding indicates that the surface copper oxide is removed totally via the chemical reaction. Copper formate is generated as a byproduct, which could be easily decomposed during low temperature heat treatment.33 In addition, based on the C 1s spectra of f-Cu NPs in Fig. 3d, the coating layer of PVP remains, even though the Cu NPs were soaked with a mixture of formic acid and absolute ethyl alcohol, and this layer would go on preventing the oxidation of the Cu NPs.

Thermal behaviors of Cu nanoparticles

It has been widely accepted that the presence of surface oxide would seriously affect the sintering properties of the Cu NPs, but the most direct effect is the sintering temperature. Here, we performed TG-DSC experiment to explore the effect of elimination of surface copper oxide on the thermal behavior of Cu NPs during the sintering process. To ensure the reduction effect of copper oxide by the mixture of formic acid and absolute ethyl alcohol, the soaking time was set to 10 min, 20 min and 30 min, as shown in Fig. 4. The weight loss before 120 ºC along with an endothermic peak is due to the evaporation of moisture, and the weight loss for all samples at approximately 300 °C could be a result of the volatilization and decomposition of short-chain PVP. For u-Cu NPs, a sharp exothermic peak between 260 ºC and 320 ºC is found, as shown in Fig. 4a. In general, there are three reasons that the exothermic peaks of metal particles can also be observed in the thermal analysis curve: the crystallization of amorphous metal particles, the recrystallization of strained metal particles by heating and the mutual diffusion of extremely unstable atoms on the surface of metal nanoparticles during the surface sintering process.34,35 Thus, in the case of metal nanoparticles sintering, the exothermic peak primarily resulted from the generation of sintering necks on a large scale, which indicates the proceeding of the sintering process. Therefore, the temperature range of the exothermic peak is usually 9



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supposed to be the sintering temperature.14,36 Accordingly, the sintering process of u-Cu NPs is expected to be conducted at the temperature of the exothermic peak (260 ºC) or higher.

For f-Cu NPs that are soaked for 10 min, there are two exothermic peaks, as shown in Fig. 4b. The first exothermic peak is a small peak lies 120 ºC ~ 160 ºC, indicating the formation of sintering necks. In addition, the temperature range of 120 ºC ~ 160 ºC is coincidentally the decomposition temperature range of copper formate, which suggests that the formation of sintering necks is simultaneous with the decomposition of copper formate. The chemical reaction occurring during the annealing process over the temperature range of 120 ºC ~ 160 ºC is exhibited in Eq. (2).

Cu(COOH)2 → Cu + CO 2 + H 2

(2)

The second one is an exothermic peak between 200 ºC and 320 ºC, which is broad compared with the other peaks, implying the occurrence of a relatively uniform heat flow. One possible reason for this uniformity is that the Cu NPs have experienced a relatively mild ripening process, which is a slow exothermic process. As a result, the annealing process of f-Cu NPs can be conducted over the temperature range of 120 ºC ~ 160 ºC, and the second exothermic peak suggests that a temperature higher than 200 ºC can aid the performance of the sintered body.

However, when the soaking time increased to 20 min and 30 min, as shown in Fig. 4c and Fig. 4d, there is a small weight gain when the annealing temperature raised up from 150 ºC to 250 ºC, which was primarily due to the oxidation of copper in the two situations. The weight gain shown in Fig. 4d was approximately 0.55%, which is nearly double the weight gain shown in Fig. 4c (0.25%), indicating a higher degree of oxidation. However, for the Cu nanoparticles soaked for 10 min, as shown in Fig. 4b, there is no sign of weight gain, which illustrates a good antioxidant capacity. A long soaking time with 10


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formic acid would facilitate the oxidation of Cu nanoparticles. As a result, the Cu NPs treated with formic acid for 20 min and 30 min are not suitable for use as a bonding material.

To further verify this assumption, the three types of Cu NPs treated with formic acid were annealed at 200 ºC for 5 min under an atmosphere of 5% H2 + 95% N2, and the SEM images of the sintered microstructure are shown in Fig. 5. When sintered at 200 ºC for 5 min, for Cu NPs soaked with formic acid for 10 min in Fig. 5a, the neighboring nanoparticles fuse together to form continuous sintering venations through visible sintering necks, and these venations interweave with each other, extending throughout, forming a cohesive mesh like a spider's web. However, for Cu NPs soaked with formic acid for 20 min and 30 min, as shown in Fig. 5b and Fig. 5c, the sintering methods are different from that used for the Cu NPs soaked for 10 min. Although visible sintering necks are also dispersed, continuous sintering venations did not form. Sintering venations only exist within the scope of a small and isolated range, as shown in the red circle in Fig. 5b and Fig. 5c. Moreover, isolated nanoparticles that were not involved in the sintering process can still be found. A thick, sintered microstructure still can be found in Fig. 5b, while in Fig. 5c, only a small, scattered sintering microstructure can be seen, which coincides with the DSC-TG results given in Fig. 4c and Fig. 4d. Thus, the Cu NPs soaked for 30 min experience a higher degree of oxidation than the Cu NPs soaked for 20 min during the annealing process, resulting in a sintering barrier for Cu NPs.

The weak antioxidant capacity of Cu NPs soaked with formic acid for 20 min and 30 min leads to discontinuous sintering venations and isolated nanoparticles. The slight oxidation hinders the sintering process of Cu NPs and changes the way to be sintered. As illustrated in the schematic diagram in Fig. 6, for Cu NPs treated with formic acid for 10 min, nearly all of the adjacent nanoparticles merge into one during the heat treatment, which is called a long-range sintering process. In contrast, only limited areas 11



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of Cu NPs treated with formic acid for more than10 min fuse together due to oxidation. These limited areas are separated from each other like islands, and this process is called the short-range sintering process in which sintering only occurs in the limited areas. The Cu NPs treated with formic acid for 10 min are long-range sintered and are more suitable for use as a bonding material considering the mechanical support and electrical connection requirements.

Bondability of Cu nanoparticles

The above results show that the Cu NPs treated with formic acid for 10 min are more suitable for use as a bonding material because they form sintering necks at a low temperature and they show good resistance to oxidation. The sintering method of Cu NPs was controlled by the formic acid treatment in a favorable direction towards a long-range sintering process. Therefore, we used the f-Cu NPs (treated with formic acid for 10 min) for bonding experiments, and the u-Cu NPs were used as a control group.

Fig. 7 shows the shear strengths of Cu-Cu joints using nanoparticle paste at different temperatures for 5 min with a certain pressure of 10 MPa. The shear strengths exhibited by the Cu-Cu joints with f-Cu NPs are much higher than those with u-Cu NPs at all tested temperatures. It is interesting that when using f-Cu NPs as the bonding material, the shear strength of the obtained Cu-Cu joints is approximately 13.6 MPa at 160 ºC, while the shear strength for u-Cu NPs at this low sintering temperature is much lower (nearly 0 MPa). This result indicates that the f-Cu NPs are sintered much easier than the u-Cu NPs. When the processing temperature rises to 320 ºC, the shear strength exhibited by the Cu-Cu joints with f-Cu NPs is 51.7 MPa, which, to the best of our knowledge, is the highest shear strength using Cu NPs as a bonding material.8,15,37-39

By contrast, the shear strength presented by the Cu-Cu joints with u-Cu NPs is 31.2 MPa, which is 12


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much lower than that of the Cu-Cu joints with f-Cu NPs. Even when the sintering temperature is 260 ºC, a temperature close to the traditional soldering temperature, a shear strength of 43.4 MPa is obtained by f-Cu NPs, which, to the best of our knowledge, is also the highest shear strength using Cu NPs as a bonding material at such a low temperature.8, 15, 39 Note that the removal of surface oxidation by the formic acid treatment greatly promotes the sintering process of Cu NPs, leading to a high shear strength of the Cu-Cu joints.

Fig. 8 shows the microstructures of the sintering interfaces (upper and lower parts) of the Cu-Cu joints. Throughout the annealing process, the f-Cu NPs experience a sufficient long-range sintering process, while the u-Cu NPs undergo a short-range sintering process. When the sintering temperature is 160 ºC, the f-Cu NPs are sintered more densely than are the u-Cu NPs. This small sintering degree results in higher shear strength. For u-Cu NPs, there are visible cracks between the sintered Cu NPs and the Cu pad for both the upper part and the lower part. Because the reduction of copper oxide by hydrogen could not occur at this low temperature and because the surface oxide hinders the formation of sintering necks, the diffusion channels between Cu NPs and the Cu pad are blocked; thus, the value of shear strength is approximately 0 MPa. For f-Cu NPs annealed at 200 ºC, the sintering characteristics were obvious, and the particle size of Cu NPs increases drastically. Moreover, a small quantity of Cu NPs that contacted the bulk copper integrated with the Cu pad, greatly increasing the shear strength. For u-Cu NPs sintered at 200 ºC, the previous cracks disappear; however, the NPs are still not sintered sufficiently at this low temperature, and a border between the sintered Cu NPs and the Cu pad is distinct. When the sintering temperature is 260 ºC, the microstructures of f-Cu NPs are coarsened, indicating a higher sintering level. In addition, vast quantities of Cu NPs are fused with the Cu pad, and the bridge between the sintered Cu NP layer and this Cu pad fused with the Cu NPs greatly 13



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enhances the mechanical strength of the Cu-Cu joints. These observations agree well with the shear strengths shown in Fig. 7. Nevertheless, although the u-Cu NPs started to fuse with the Cu pad, the particle size is much smaller than that of f-Cu NPs, leading to a fine, fragile structure. In addition, there are many microvoids in the sintering layer that degrade the integrity of the joints and the shear strength of the u-Cu NP joints.

When the sintering temperature increases to 320 ºC, the microstructures of f-Cu NPs are further coarsened, especially at the junction between the sintering layer and the Cu pad. Many thick, sintered venations fuse with the Cu pad and extend to the interior of the sintered layer; thus, the mechanical property of the Cu-Cu joints is greatly improved via the long-range sintering process. There are also many thick, sintered venations that fuse with the Cu pad in the sintered u-Cu NPs; however, they are sintered less densely than the f-Cu NPs. In addition, there are more microvoids both at the interface of the Cu pad and in the interior of the sintered Cu layer due to the poor densification of sintered u-Cu NPs as a result of the short-range sintering process. Although there are also many microvoids in the sintered layer of f-Cu NPs, the thick sintered venations interact with one another and include the Cu pad, thus building a solid structure in long-range sintering manner. Therefore, the shear strength of Cu-Cu joints with u-Cu NPs is much smaller than that of Cu-Cu joints with f-Cu NPs.

Fig. 9 exhibits the SEM micrographs of the typical fracture surfaces of Cu-Cu joints formed at different temperatures. The fracture surfaces of Cu-Cu joints with f-Cu NPs formed at 160 ºC exhibit the apparent particle growth as well as obvious sintering necks, which contribute to the mechanical properties of the Cu-Cu joints. However, there is no significant evolution in the morphology of u-Cu NPs, which still exist as isolated nanoparticles with low shear strength. When the sintering temperature increases to 200 ºC, a fracture trace can be found in the fracture surfaces of sintered f-Cu NPs, while 14


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sintered u-Cu NPs only exhibit particle growth and the formation of sintering necks, without any fracture trace. The formation of a fracture trace indicates that the high-strength joints began to form, but they are still not sufficient for reliable interconnections.

Note that the fracture surfaces by the f-Cu NPs at the sintering temperature of 260 ºC exhibit many elongated dimples with sharp tips, suggesting that the strong Cu-Cu joints of Cu-Cu NPs-Cu are obtained at this low temperature, which coincides with the results shown in Fig. 7, presenting a high shear strength of 43.4 MPa. In addition, the elongated dimples illustrate that the bonding material of Cu nanoparticles has ductile characteristics.40 A similar structure has also been found by Yan et al15 where Cu NPs also were used as bonding material. And the difference is that the size of presented dimples is large than that of Yan’s study, that is the reason why the obtained Cu-Cu joints show a higher shear strength. Under the same experimental conditions as the former one, a few shallow dimples in the fracture surfaces of Cu-Cu joints with u-Cu NPs are observed. This result shows that the bonding strength of Cu-Cu joints with u-Cu NPs is not sufficiently high to compare with the bonding strength of the joints with f-Cu NPs. When the sintering temperature increases to 320 ºC, the elongated dimples exhibited by the sintered f-Cu NPs are substantially broadened, which is primarily due to the coarsening of the sintering microstructure. Those broadened dimples indicate an improvement in bonding strength and reveal that robust joints were successfully fabricated. For the Cu-Cu joints with u-Cu NPs, the shallow dimples are also broadened, but they are much smaller than those of the Cu-Cu joints with f-Cu NPs, resulting in a lower bonding strength. To this point, the elimination of surface copper oxide with formic acid treatment greatly promotes the sintering process of Cu NPs. The sintering of the f-Cu NPs proceeds to a higher degree than the sintering of the u-Cu NPs.

It is widely accepted that the oxidation of Cu NPs impairs their bonding properties. Kobayashi et 15



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al8 confirmed that although sintered at 400 ºC, the shear strength of Cu-Cu joints using Cu NPs was only 28.6 MPa. The most substantial difference between f-Cu NPs and u-Cu NPs is the presence of surface oxidation, and the residual copper oxide is the primary reason that the shear strength of Cu-Cu joints with u-Cu NPs is much lower than that of the Cu-Cu joints with f-Cu NPs in all situations. Once treated with a mixture of formic acid and absolute ethyl alcohol, through which the sintering method of Cu NPs was modified from a short-range sintering process to a long-range sintering process, the surface copper oxide of a high melting point is replaced by copper formate. The surface copper formate can be removed using a low-temperature heat treatment, resulting in the exposure of Cu NPs with high surface energy. This process becomes the driving force of the low-temperature sintering. Therefore, the Cu-Cu joints with f-Cu NPs possess higher shear strength than that of u-Cu NPs.

A low electrical resistivity is the precondition for the application of Cu NPs as a bonding material in various electronic devices. The electrical resistivity variations of the conductive copper films sintered under a pressure of 10 MPa are shown in Fig. 10. The samples were sintered at 200 ºC, 260 ºC and 320 ºC for 5 min under the same conditions as those used in the bonding experiments. For f-Cu NPs, when the sintering temperature rises up from 200 ºC to 260 ºC, the resistivity decreases from 40.5 µΩ·cm to 5.65 µΩ·cm because the Cu NPs are sintered more densely, as shown in Fig. 8 and Fig. 9. In addition, the coarsening of the continuous sintered venations promotes the electron transport, which also reduces the resistivity. When the sintering temperature increases from 260 ºC to 320 ºC, there is only a small decrease from 5.65 µΩ·cm to 3.16 µΩ·cm in resistivity, indicating that the sintered structure tends to be stable. Although the lowest resistivity is still slightly higher than that of the bulk copper, it is much lower than that reported in the vast majority of studies.15,38,41 The sintering time in this case is only 5 min, while this process usually requires one hour or longer; this fact cannot be 16


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neglected. However, for u-Cu NPs sintered in a short-range sintering process, the resistivities are much higher than those of f-Cu NPs sintered in a long-range sintering process in all situations (The resistivity was 150.21 µΩ·cm, 28.71 µΩ·cm and 9.41µΩ·cm when the sintered at 200 ºC, 260 ºC and 320 ºC respectively).

In addition, there is no deny the fact that thermal conductivity is one of the most important indicators to evaluate the bondability of Cu nanoparticles because it can directly determine the heat dissipation of the power semiconductor devices.42 The thermal conductivities of sintered Cu NP layer were calculated by the Wiedemann-Franz law (for more detail, see supplementary Table. S1). Overall, the thermal conductivities of the sintered f-Cu NP layer were higher than that of the sintered u-Cu NP layer. When sintered at 260 ºC and 320 ºC, the thermal conductivities of the sintered u-Cu NP layer was -1

25.77 W·m-1·K and 78.61 W·m-1·K-1 respectively, while for the sintered f-Cu NP layer, the thermal conductivities increased to 130.9 W·m-1·K-1 and 233.71 W·m-1·K-1. And the maximum value is about 58.87% of the theoretical value of bulk copper, which is comparable to that of the research conducted by Li et al14. It indicates a good heat emission ability of the Cu-Cu joints. Thus, the removal of surface oxidation not only enhances the mechanical properties of Cu-Cu joints but also improves their electrical and thermal performance.

Accordingly, the Cu-Cu joints with f-Cu NPs at a low temperature of 260 ºC possess a high shear strength (43.4 MPa) as well as good electrical conductivity (5.65 µΩ·cm), thus meeting the demands of next-generation power devices. Therefore, we selected the Cu-Cu joints bonded with f-Cu NPs at 260 ºC to investigate the joining mechanism at the interface in the formation process of Cu-Cu joints. The TEM observations of the joining interface between the sintered Cu NP layer and the Cu pad are shown in Fig. 11a, revealing that the sintered Cu NP layer is composed of poly-crystals with a size of hundreds 17



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of nanometers. Although there are many small voids, the densely sintered Cu NP layer forms, and no gaps can be found at the joining interface between the sintered Cu NP layer and the Cu pad. Fig. 11b shows the lattice image taken from area A in Fig. 11a, clearly exhibiting that metallurgical bonding between the sintered Cu NP layer and the Cu pad can be achieved at a low sintering temperature of 260 ºC, which ensures the high shear strength of Cu-Cu joints fabricated with Cu NPs. Note that high density coherent twins are found in the interior of sintered Cu NPs, as shown in Fig. 11c. It has been demonstrated that the introduction of high-density coherent twins can dramatically increase the strength and electrical properties of polycrystalline materials.43-46 And the high-density coherent twins can be obtained under special conditions during crystal growth. Therefore, it is reasonable to deduce that the high-density twins are also one of the primary reasons for the high shear strength and low electrical resistivity of the bonding structure.

For the sintered Cu NP layer with nanosized grains, the dislocation motion could be blocked by the coherent twin boundaries, and these boundaries are much more stable against migration because of their low excess energy compared with the conventional large grain boundaries. Therefore, the formation of high-density coherent twins enhances the strength of the sintered body. In addition, because crystalline defects serve as the scattering centers of the electron transporting in metals and because the scattering effect of the coherent twin boundary is much lower than that of a conventional high angle grain boundary,43 the problem in which intrinsic defects generally exhibit a significant increase in electronic resistivity because of the grain boundary scattering effect would also be overcome by high density twins to some extent.

Therefore, we believe that the formic acid-treated Cu NPs, which are long-range sintered, are more suitable for use as a low-temperature bonding material due to their outstanding bonding strength 18


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and excellent electrical performance. We believe our work presents an encouraging step toward the development of the next-generation power devices.

Conclusions

High strength and highly conductive Cu-Cu joints are obtained via the long-range sintering of formic acid-treated Cu NPs at temperatures as low as 260 ºC. A modified polyol method is used to prepare Cu NPs, and a mixture of formic acid and absolute ethyl alcohol is utilized to remove the surface copper oxide. The bonding properties are greatly improved by the formic acid treatment. Compared with Cu-Cu joints with u-Cu NPs, the shear strengths of Cu-Cu joints with f-Cu NPs increase from 23.9 MPa to 43.4 MPa at 260 ºC and from 31.3 MPa to 51.7 MPa at 320 ºC, which are values that are even higher than those of the standard Pb-free solders. Shallow dimples were found in the fracture structures of u-Cu NPs and f-Cu NPs when annealed at 260 °C and 320°C, but the shallow dimples relative to f-Cu NPs were usually larger than that of u-Cu NPs. In addition, the sintered venations of f-Cu NPs were thicker than those of u-Cu NPs. Moreover, the electrical resistivity of sintered f-Cu NP layer was 5.65 µΩ·cm at 260 ºC and 3.16 µΩ·cm at 320 ºC, while the electrical resistivity of u-Cu NP layer was 28.71 µΩ·cm and 9.41µΩ·cm with a sintering temperature of 260 ºC and 320 ºC, respectively. The enhancement in the shear strength and electrical conductivity is attributed to a higher level sintered body after the elimination of surface oxidation by the formic acid treatment. And based on the microstructure analysis, metallurgical bonding is realized between the Cu pad and the sintered Cu NP layer. High density twins are found in the interior of the Cu NP layer sintered at 260 ºC, and they contribute to the low resistivity and high strength of Cu-Cu joints. This joining process shows the great potential of this technology for use in the next-generation power devices.

19



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ASSOCIATED CONTENT Supporting Information Available: Structure schematic of DBC substrate, Structure schematic of SiC die, Schematic diagram of shear test of Cu-Cu joints and the detailed calculating course of the thermal conductivity of sintered Cu NP layer. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

1,2

*E-mail: [email protected];

Notes

¶

Equal contribution: Jingdong Liu1,2¶, Hongtao Chen2¶

The authors declare no competing financial interest.

Acknowledgments

This research was financial supported by the Shenzhen Science and Technology Plan Project under Grant JCYJ20160318095308401, JCYJ20150529152949390, Grant JCYJ20140417172417159 and the Guangzhou Science and Technology Plan Project under Grant 201509030004. References (1)

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Figure captions

Fig. 1 Schematic representation of the formation of highly conductive and high-strength Cu-Cu joint

Fig. 2 a) SEM and HRTEM images and b) XRD pattern of synthesized Cu NPs

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Fig. 3 X-ray photoemission spectra of Cu NPs: a) Cu 2p3/2 and b) C 1s of u-Cu NPs; c) Cu 2p3/2 and d) C 1s of f-Cu NPs

Fig. 4 TG-DSC curves of the a) u-Cu NPs and f-Cu NPs soaked for b) 10 min, c) 20 min, and d) 30 min.

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Fig. 5 SEM images of Cu NPs sintered at 200 ºC for 5 min under an atmosphere of 5% H2 + 95% N2; the soaking times with formic acid are a) 10 min, b) 20 min and c) 30 min, respectively.

Fig. 6 Schematic diagram representing the sintering methods of Cu NPs treated with formic acid for different times

Fig. 7 Shear strengths of Cu-Cu joints using nanoparticle paste containing f-Cu NPs and u-Cu NPs sintered at 160 ºC, 200 ºC, 260ºC and 320 ºC for 5 min.

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Fig. 8 SEM micrographs of the interfaces (upper and lower parts) of Cu-Cu joints. The sintering temperature varied from 160 ºC to 320 ºC: a) Cu-Cu joints with f-Cu NPs and b) Cu-Cu joints with u-Cu NPs

Fig. 9 SEM images of the fracture surfaces of Cu-Cu joints in which the sintering temperature varied

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from 160 ºC to 320 ºC; high magnification images and low magnification images of a) Cu-Cu joints with f-Cu NPs and b) Cu-Cu joints with u-Cu NPs

Fig. 10 Resistivities of the low-temperature sintered Cu films by the Cu NPs

Fig. 11 TEM observations of Cu-Cu joints sintered with f-Cu NPs at 260 ºC. a) TEM image of the Cu pad/sintered Cu NP layer interface; b) HRTEM image of area A showing the interface between the Cu pad and the sintered Cu NP layer; c) TEM image of twins formed in the sintered Cu NP layer. Inset: HRTEM image of the twins

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Table of Contents/Abstract Graphic

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