Interfacial enhancement of Ag and Cu particles sintering using (111)-oriented nanotwinned Cu as substrate for die-attachment

Guo Rumeng ,Xiao Yubo ,Gao Yue ,Zhou Shiqi ,Liu Yang ,Liu Zhiquan

1.Shenzhen Institute of Advanced Electronic Materials,Shenzhen Institute of Advanced Technology,Chinese Academy of Sciences,Shenzhen 518055,China;2.Shenzhen College of Advanced Technology,University of Chinese Academy of Sciences,Shenzhen 518055,China;3.School of Material Science and Engineering,Harbin University of Science and Technology,Harbin 150040,China

Abstract The electroplated (111)-oriented nanotwinned-Cu (nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that both Cu and Ag sinter joints using (111)-oriented nt-Cu film exhibited a higher bonding strength than that using traditional random-oriented Cu film.Especially,the joints sintered with Cu paste on (111)-oriented nt-Cu film possessed a higher bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2 atmosphere,compared to that on random-oriented Cu film with a value of 31.3 MPa.The results show that as metal substrate layer,the (111)-oriented nt-Cu film can improve the connection performance of Ag and Cu sinter joints,which could further promote their application in dieattachment technology for the next-generation power semiconductors.

Key words nanotwinned copper,sinter joining,Ag and Cu paste,interface,bonding strength

0 Introduction

Electric energy is expected to become the main energy consumption around the world due to the foreseeable energy shortage and environment problem.The third-generation semiconductor materials such as SiC and GaN,which possess intrinsic advantages in high output power,high operating frequency,high thermal conductivity and high stability,ensure the high energy utilization efficiency of electric and are expected to replace the traditional Si-based power devices[1].Due to the rapid development of consumer electronics,automobile industry,military field and aerospace industry,the operating temperature of the power device would rise to over 250 °C[2].Traditional tin (Sn)-based solders are no longer suitable for the application in high-temperature operation[3].In decades,sinter joining with metal nanoparticles (NPs),has been considered as a promising substitute technology for traditional soldering owing to the fact that the metal NPs can realize low-temperature joining and high-temperature service[4].

Currently,most of the reports on sinterable paste are about the Ag paste and Cu paste.The low sintering temperature and antioxidant properties of silver nanoparticles allow the wide usage of Ag paste[5–6].On the other hand,the low cost of copper as well as the similar thermal and electrical properties ensures the great development potential of Cu paste[7].However,the relevant reports on the reliability of Ag and Cu sinter joining showed that most of the failures occur at the interface between the sinter layer and the metal substrate[8].Although some studies have reported that the Ag and Cu sinter joining under an inert or reduced gas atmosphere can prevent Cu oxidation[9–11],the bonding quality of the interface between Ag and Cu sinter layer and bare Cu is still unsatisfied.Therefore,forming a reliable connection interface between the sinter layer and the substrate is a prerequisite for realizing a high-strength and high-reliability interconnection structure based on sintering technology.

Compared to the random-oriented Cu film,nanotwinned Cu (nt-Cu) film is a high-performance material with high strength,high toughness,and a comparable conductivity[12].Besides,the nt-Cu with highly (111)-oriented microstructure has not only good performance in mechanical and electrical properties but also fast atomic diffusion because of the intrinsic diffusion ratio of (111) lattice[13–15].Thus,the(111)-oriented nt-Cu film is expected to solve the interface problem of sinter joining[16–18].

Herein,highly (111)-oriented nt-Cu film was applied as substrate for Ag and Cu particles sinter joining.The results showed the (111)-oriented nt-Cu film had good compatibility with Ag and Cu paste:(1) Higher bonding strength could be obtained than that of random-oriented Cu film.(2) Better interface between the metal sinter layer and the (111)-orientednt-Cu film could be formed.The above advantages of (111)-oriented nt-Cu film were proved by SEM,EDX and cross-section observation.These results show that the(111)-oriented nt-Cu film could improve the interfacial interconnection during Ag and Cu sintering,indicating its potential application as the metal layer for power devices in the next generation of power semiconductors.

1 Experimental

1.1 Materials

Polyvinyl pyrrolidone K23,(PVP,Mw=50 000),LAscorbic acid and sodium hypophosphite (NaH2PO2) were purchased from Aladdin.Ethylene glycol (EG),silver nitrate (AgNO3),ethanol and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co.,Ltd.Copper sulfate pentahydrate (CuSO4·5H2O,98%),isopropanolamine (MIPA) and triethylene glycol (TEG) were supplied by Sigma-Aldrich.Deionized (DI) water was made in the laboratory.All the raw materials were used as received without purification.

1.2 Preparation of Ag nanoparticles (Ag-NPs)

Initially,L-Ascorbic acid was dissolved in deionized water as the reducing agent.Then PVP was added to the solution as capping molecule.After that,the pH of the solution was adjusted by NaOH.The AgNO3was then injected into the solution.After 40 mins of the chemical reaction,the prepared Ag nanoparticles (NPs) were subsided by centrifugal separation.Then the washing process with deionized water and absolute ethyl alcohol were carried out for four times to remove excess PVP and other impurities.

1.3 Preparation of Cu nanoparticles (Cu-NPs)

Initially,CuSO4·5H2O was completely dissolved in a mixed solution of MIPA and TEG under magnetic stirring at 60 °C.Then add the complex to 40 ml of ethylene glycol solution containing NaH2PO2,reacting for 10 mins at 60 °C to form Cu-NPs.Cu-NPs were centrifuged and washed by ethyl alcohol for four times to remove excess PVP and other impurities.

1.4 Electrodeposition of Cu film substrate

The highly (111)-oriented nanotwinned and random-oriented Cu films were fabricated via DC electroplating.The random-oriented Cu plating solution is commercial electroplating solution.The (111)-oriented plating solution containing basic electrolytes (CuSO4·5H2O and H2SO4) and additives.Cu substrates (12 mm × 12 mm × 1.5 mm) and dummy Cu chips (4 mm × 4 mm × 1.5 mm) were electroplated with Cu films at a thickness of about 80 μm.After electroplating,the substrate electroplated with Cu film was rinsed by deionized water and dried for further process.

1.5 Fabrication of Ag and Cu sinter joints

Ag NPs were mixed with EG at a mass ratio of 8.5:1.5 to fabricate the Ag paste.Cu NPs were mixed with MIPA at a mass ratio of 8.5:1.5 to fabricate the Cu paste.

Prior to the bonding process,the electroplated Cu substrates and Cu chips were polished and cleaned by an HCl solution (10 wt.%) for 30 s,then rinsed with DI water.Then,Ag paste and Cu paste were printed on the substrate through a stainless-steel mask with the thickness of 0.1 mm,and the Cu dummy chip was placed on the paste to produce a sandwich structure.The prepared samples were sintered at 300 °C for 60 mins in the N2atmosphere with a pressure of 0.4 MPa,and the heating rate was controlled to about 15 °C/min.The schematic of the sinter joints is illustrated in Fig.1.

Fig.1 Schematic illustration of Ag and Cu sinter joint during bonding experiment

1.6 Characterization and shear test

The morphology observation was performed by a scanning electron microscope (Nano SEM 450,FEI) equipped with electron backscattered diffraction (EBSD) and energy dispersive X-ray Spectroscopy (EDX).The cross-section of nt-Cu film was examined with a FEI Nova 200 dual-beam focused ion beam (FIB).The grain orientation was observed by EBSD.The bonding strength of sinter joints were evaluated by a bonding tester (DAGE 4 000) with a shear speed of 100 μm/s,and the shear height is 100 μm.Before observation,the cross-section samples were fabricated via an ion-milling polishing process (Gatan 697).

2 Results and discussion

The Ag and Cu particles and electroplated Cu films are shown in Fig.2.SEM images of Ag particles and Cu particles produced by the reaction are shown in Fig.2 a and 2b respectively.It can be observed that the synthesized particles had a nearly spherical shape.The particles had good dispersibility and no significant agglomeration can be observed.The diameter of the Ag nanoparticles was in the range of 50?250 nm,while the diameter of the Cu particles was between 30 nm and 300 nm.

Fig.2 SEM images of Ag NPs (a) and Cu NPs (b).Cross-sectional FIB images of electroplated random-oriented Cu film (c)and (111)-oriented nt-Cu film (d).Surface SEM images of electroplated (111)-oriented nt-Cu (e) and its plan-view EBSD OIM image (f),the inserted image in (f) is the inverse pole figure of Cu

Microstructures of as-deposited Cu films were characterized by FIB and EBSD.Fig.2 c and Fig.2 d present cross-sectional FIB images of electroplated random-oriented Cu film and (111)-oriented nt-Cu film,respectively.Compared to the randomly arranged grains in random-oriented Cu film (Fig.2 c),the densely packed columnar grains with nanotwinned structures was observed in (111)-oriented nt-Cu film (Fig.2 d).The surface of (111)-orientednt-Cu film was checked by SEM (Fig.2 e) and EBSD(Fig.2 f).The plan-view EBSD OIM image of Fig.2 f showed the high orientation of (111) plane,which occupied 98% of the whole surface.

2.1 Mechanical properties of sinter joints with(111)-oriented and random-oriented Cu films

Fig.3 shows the shear strength of sinter joints by Ag paste and Cu paste with (111)-oriented and random-oriented Cu films as chips and substrates.Visible strength difference can be observed.When the (111)-oriented Cu film was applied,the shear strength of sinter joint had a significant increase.The sinter joints obtained by Ag paste exhibited higher bonding strength of 22.7 MPa using the (111)-oriented Cu films,while the shear strength was only 16 MPa when the random-oriented Cu films were used.

Fig.3 The shear strength of Ag and Cu sinter joints on random-oriented Cu and (111)-oriented nt-Cu film

On the other hand,the shear strength of Cu sinter joints increased from 31.3 MPa to 53.7 MPa,when the chips and substrate shifted from random-oriented Cu films to (111)-oriented nt-Cu films.The difference of bonding strength between Ag paste and Cu paste might attribute to the intrinsic lattice incompatibility of Ag and Cu.However,the enhancement effect of (111)-oriented Cu film on the mechanical properties of sinter joints was clearly confirmed.

2.2 Fracture surface and cross-section morphology of Ag sinter joints

Fig.4 showed the fracture surface images of Ag sinter joints after shear test.It could be observed that both substrates were not covered by Ag paste entirely,indicating that the fracture occurred at the interface between sinter layer and Cu film.In order to quantify the composition of the fracture surface,EDX was applied to the fracture surface of specimens using different substrate.It can be obviously observed that there was a significant difference in the amount of Ag paste remaining on different electroplated films.The fracture surface on the (111)-oriented nt-Cu film contained 48.9 wt% Ag,which was higher than the content of Ag on the random-oriented Cu film surface (20.9 wt%).The high residual amount of Ag on the fracture surface indicated that more interconnection between sinter layer and Cu film could be formed.The result shows that the (111)-oriented nt-Cu film can promote the combination of the paste and the substrate,which was also confirmed by the result of shear strength test.

Fig.4 The fracture surface images of Ag sinter joints after shear test (a) EDX analysis on the fracture surface after shear test of Ag sinter joints with random-oriented Cu film (b) (111)-oriented nt-Cu film

Fig.5 shows the cross-section observation of Ag sinter joints using different substrate.Although the formation of sintering necks could be observed in both Cu films,an obvious crack occurred on the interface between Ag paste and random-oriented Cu film,as shown in Fig.5a,suggesting a low bond strength.In contrast,there were basically no cracks on the (111)-oriented nt-Cu film (Fig.5 b).The interface in Fig.5 b was sintered more densely than that in Fig.5 a.However,strong interconnection was not formed at the interface between (111)-oriented nt-Cu film and Ag sinter layer.Therefore,the shear strength of this joint was also not high.But due to the cracks at the interface of random-oriented Cu film,the shear strength of sinter joints with the (111)-oriented nt-Cu film was slightly higher than that of sinter joints with random-oriented Cu film.

Fig.5 Cross-section observation of Ag sinter joints on random-oriented Cu film (a) and (111)-oriented nt-Cu film (b)

2.3 Fracture surface and cross-section morphology of Cu sinter joints

The SEM observation on fracture surface of the sinter joints using Cu paste with different electroplated film is showed in Fig.6.For the joints on electroplated randomoriented Cu film,both substrate and Cu paste can be seen in the image of fracture surface (Fig.6 a),suggesting that the broken occurred at the interface between sinter layer and electroplated Cu film.When the substrate was electroplated with (111)-orientednt-Cu film,the fracture surface of joints was completely covered by the Cu sinter layer and no bare substrate was visible (Fig.6 b),indicating the fracture occurred within the sinter layer.Due to the difference in the fracture mechanism,the shear strength was significantly different as presented in Fig.3.

Fig.6 The SEM observation on fracture surface of Cu joints on random-oriented Cu film (a) and (111)-orientednt-Cu film (b) The inserted image illustrates the broken type

The cross-section observations of sintered joins using different Cu films are presented in Fig.7.Throughout the same sintering process,both sinter layers in Fig.7 a and Fig.7 b exhibited a sufficient sintering process.The microstructures of Cu NPs are fully coarsened.However,there are some differences at the interface.In order to quantify the differences,the length of the interface-bonding region was measured and showed in Table.1.The results revealed that the interface connection ratio between the Cu sinter layer and the (111)-oriented nt-Cu substrate layer was about 85.4%,which is higher than that of the Cu sinter joint on random-oriented Cu film (only about 71.6 %).Due to the low interface connection ratio between sintered Cu and randomoriented Cu film,the fracture occurred at the interface as shown in Fig.6 a,causing an inferior bonding strength(Fig.3).Therefore,when using substrate with the (111)-oriented nt-Cu film,the shear strength of Cu sintered joint was higher than that using random-oriented Cu film.

Fig.7 Cross-section observation of Cu sintered joints on random-oriented Cu film (a) and (111)-oriented nt-Cu film(b) after sintering

Table 1 Interfacial connection ratio at the interface between electroplated Cu films and sinter layer

Consequently,the (111)-oriented nt-Cu film exhibited an interface enhancement effect for Ag and Cu sinter joining,which greatly improved the bonding performance of Ag and Cu sinter joints.Generally,the bonding performance is largely determined by the state of interlayer,while the roughness and grains size could greatly influence the interface[19–20].In the present work,the enhancement effect could be largely attributed to the high diffusion ratio of (111) lattice[21].It is reported that the (111)-plane has the highest diffusion ratio,which is 3 to 4 orders of magnitude faster than that on other surfaces[22–23].Thus,the (111) surface atoms could easily diffuse into Ag and Cu sinter layer,and form the stable interface.This work demonstrates the interface enhancement effect of (111)-oriented nt-Cu film during Ag and Cu sinter joining,which would promote the application as substrate of sintered paste for next-generation power semiconductors.

3 Conclusions

The highly (111)-oriented nt-Cu film was utilized as substrate for Ag and Cu sintering,and the interfacial connection could be largely improved.It was observed that both Cu and Ag sinter joints exhibited higher bonding strength on (111)-oriented nt-Cu film than that on traditional random-oriented Cu substrate.Especially,the joints sintered with Cu paste on the (111)-oriented nt-Cu film possessed extremely high bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2atmosphere.It is concluded that the (111) surface of nt-Cu film has an interfacial enhancement effect during Ag and Cu sinter joining,which has potential application as the metal layer for die-attachment of next-generation power semiconductor devices.