Qin Wng ,Shuye Zhng ,Tiesong Lin＊ ,Pengzhe Zhng ,Peng He＊＊ ,Kyung-Wook Pik

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

b Department of Materials Science and Engineering,Korea Advanced Institute of Science and Technology,Daejeon,305-701,South Korea

c Joining and Welding Research Institute,Osaka University,Mihogaoka 11-1,Ibaraki,Osaka,567-0047,Japan

Keywords:Electronic packaging Ag paste Cu-Cu joints Mechanical property Joining morphology

ABSTRACT Today,a growing number of third-generation semiconductor-based power devices are used in products that can continuously operate at high temperatures for extended periods of time.Hence,traditional tin-lead and lead-free solders are no longer suitable for modern electronic packaging.A common method is to apply Ag paste for bare Cu-Cu joints under an inert or reductive atmosphere.In this study,the citrate-coated nanosized Ag paste was utilized to generate robust bare Cu-Cu joints under atmospheric conditions.The average size of citrate-coated Ag particles was approximately 4.76 nm after being cleaned by deionized water and acetone.The effects of process parameters,such as cleaning,joining temperature,holding time,and joining pressure,on the mechanical properties of the bare Cu-Cu joints were thoroughly investigated.Increasing washing and joining temperatures resulted in a shear strength increase of up to 28.2 MPa at a joining temperature of 260 °C after seven washes.In addition,a holding time of 30 min and a joining pressure of 1 MPa were selected as optimal process conditions for the application of citrate-coated nanosized Ag paste onto bare Cu-Cu joints.The newly developed Cu-Cu joints showed excellent thermal stability at 150 °C using the citrate-coated nanosized Ag paste.After long-term aging,the joints exhibited stability at 250 °C for 144 h,indicating a good high-temperature reliability for threedimensional integrated circuits (3D ICs) fabricated under atmospheric conditions.

1.Introduction

In recent years,the development of electronic information technology has become more and more rapid,thereby increasing the occupancy and need for corresponding technical derivatives in people’s daily life.The core of these technical derivatives is the internal integrated circuit(IC),whose function depends not only on the layout of internal wiring but also on the packaging and interconnection of internal semiconductor components [1-5].The quality of packaging and interconnection directly affects the mechanical,thermal,and electrical joining of the internal IC,which is closely related to the reliable operation of devices.

The emergence of third-generation semiconductor materials (represented by SiC and GaN) has raised the upper limit for the operating temperature of devices and puts forward higher requirements for device packaging and interconnection [6-8].According to the Restriction of Hazardous Substances(RoHS)directive,the use of lead-containing solder is prohibited as it is a hazard to the ecological environment and to human health [9].Currently,lead-free,soft solders,such as Sn-Ag,Sn-Zn,and Sn-Cu,are the most commonly used multistage packaging and interconnection materials in the electronics industry [10-13].Nevertheless,their electrical and thermal properties are insufficient and do not provide the high-performance standards required for high-temperature operations,failing to replace lead-containing solder in electronic products completely [14,15].Therefore,the electronics industry has an urgent need for new packaging and metallic interconnect materials to solve problems concerning device heat dissipation,reliability,and the presence of lead.

Fig.1.Typical synthesis process of Ag nanoparticles.

Fig.2.Secondary washing process of Ag nanoparticles.

Fig.3.Joining process of Cu-Cu joints using citrate-coated nanosized Ag paste.

Fig.4.Schematic diagram of the compression shear test.

Fig.5.Effect of washing times on the Cu-Cu joints morphologies using citrate-coated nanosized Ag paste:(a) not washing,washed (b) 3,(c) 7,and (d) 15 times.

Fig.6.Effect of washing times on the shear strength of the bonded Cu-Cu joints using citrate-coated nanosized Ag paste.

Ag paste is a promising interconnect material for electronic packaging,especially die-attachment,that can overcome the shortcomings of the above solder.First,Ag has superior electrical and thermal conductivity (6.2×107S/m and 410 W/(m·K)) to meet the requirements of reliable electrical joining and even heat dissipation in high power density systems[16,17].Furthermore,the high melting point(962°C)of Ag itself makes it highly adaptable in high-temperature environments [18].Although many Ag pastes have been reported to demonstrate their potential applications,they generally require high joining temperature or pressure in forming joints [19-22].In addition,the use of Ag paste for die-attachment often requires the implementation of additional substrate metallization to ensure and secure a robust bond between the paste and substrate[23,24].A typical bonding method for bare Cu-Cu joints is the application of Ag paste,which is generally performed under an inert or reductive atmospheric environment [25,26].However,additional metallization or requirements for a specific atmosphere undoubtedly complicates the manufacturing process and increases costs.More importantly,it does not ensure the consistent,long-term reliability of Ag paste for high-temperature applications[27,28].

The purpose of this work was to develop a new type of Ag paste for Cu-Cu bonding processing under ambient air,particularly for dieattachment.Considering the practical application value,the emphasis was focused on the mechanical properties and thermal stability of the joints.Large quantities of small-sized Ag nanoparticles were synthesized at room temperature and used to prepare a paste for the bare Cu-Cu joints.The effects of the process parameters,specifically the effects of cleaning,joining temperature,holding time,and joining pressure,on the mechanical properties of the joints at room temperature were investigated in detail.The mechanical properties and fracture morphology of the joints after long-term working at 150°C and 250°C,respectively,were also investigated.

2.Materials and methods

2.1.Citrate-coated Ag nanoparticles

Citrate can absorb onto the surface of Ag,thus effectively preventing the agglomeration of newly generated Ag nanoparticles [29,30].Additionally,citrate is not a polymer and has a low decomposition temperature.Hence,sodium citrate (Na3C6H5O7) was selected as the surface coating agent for the reduction of Ag+of silver nitrate (AgNO3) by sodium borohydride (NaBH4),thereby mass-producing small-sized Ag nanoparticles.The typical synthesis process is displayed in Fig.1.

Fig.7.SEM images of shear fractures of Cu-Cu joints using citrate-coated nanosized Ag paste with different washing times:(a)unwashed,washed(b)3,(c)7,and(d)15 times.

Fig.8.Effect of joining temperature on the shear strength of Cu-Cu joints using citrate-coated nanosized Ag paste.

The AgNO3solution was first produced by adding 0.34 g of the AgNO3crystals to 200 mL of deionized water.Similarly,1.88 g of Na3C6H5O7·2H2O was added to 160 mL of deionized water to generate the Na3C6H5O7solution,and 0.011 g of NaBH4was added to 40 mL of deionized water to produce the NaBH4solution.The Na3C6H5O7solution was then rapidly poured into the AgNO3solution under magnetic stirring until the solution was transparent to obtain the desired oxidation solution.After that,the NaBH4solution was added dropwise to the oxidation solution at a rate of 20 drops/min under magnetic stirring at 400 rpm.The color of the reaction solution changed from colorless to light red,wine red,and finally to dark black,indicating the end of the reaction.To ensure the full reaction,the solution was continuously stirred for an additional 30 min.The resulting dark black solution,also known as nano-Ag sol,included the required small-sized Ag nanoparticles (average diameter:4.76 nm).

The small-sized Ag nanoparticles in the nano-Ag sol exhibited notably excellent dispersion.Traditional high-speed centrifugation cannot separate the Ag nanoparticles from the nano-Ag sol given the nature of sol,wherein the colloidal particles can precipitate due to the loss of surface charge or the destruction of the surface coating layer.Hence,a significant amount of acetone was poured into the nano-Ag sol to disrupt the original balance and promote the precipitation of the Ag nanoparticles.Acetone was first added to the nano-Ag sol at a volume ratio of 1:1(i.e.,400 mL).After 10 h of static storage,all Ag nanoparticles settled to the bottom of the cup,and the upper liquid became colorless and transparent.The sediment was removed,and 200 mL of acetone was added.After magnetic stirring for 10 min and ultrasonic vibration for 10 min,the solution was stored statically for 2 h.All Ag nanoparticles settled to the bottom of the cup once again.The sediment was removed and centrifuged at 3000 rpm for 10 min.After drying at 45°C,the resulting black powder (also called nano-Ag powder) was then produced,which contained the desired pure Ag nanoparticles.

2.2.Cleaning effect

In the process of preparing Ag nanoparticles,citrate plays an important role with regards to the surface coating layer.To obtain a good coating effect of Ag nanoparticles,Na3C6H5O7·2H2O was added in excess.As a result,excess citrate settled down with the Ag nanoparticles.Too much citrate in the subsequent paste is unfavorable for improvement of the joint performance.The specific implementation for the secondary washing of the nano-Ag powder that was prepared in Section 2.1,is shown in Fig.2.First,the nano-Ag powder was re-dispersed in a mixed solvent of deionized water and acetone at a volume ratio of 2:1.After magnetic stirring for 10 min,ultrasonic vibration for 10 min,and standing for 13 h,the Ag nanoparticles were completely precipitated.Secondary washing was achieved by repeating the above steps.Finally,the precipitate was dried at 45°C to generate the nano-Ag powder (as described in Section 2.1).In this study,the effect of not washing versus washing for 3,7,and 15 times on the subsequent paste was investigated.

Fig.9.SEM images of shear fractures of Cu-Cu joints using citrate-coated nanosized Ag paste at different joining temperatures:(a)180 °C,(b)200 °C,(c)220 °C,(d)240 °C,(e) 260 °C,and (f) 280 °C.

2.3.Joining process

In the making process of paste,ethylene glycol(EG)was chosen as the organic additive.The nano-Ag powder,EG,and deionized water were uniformly mixed in a shaker at a mass ratio of 8:1:1 to form a paste,that is,the required Ag nanoparticle paste.The Ag nanoparticle paste was used to join two pieces of polished pure Cu with dimensions of 3×3×1 and 15×5×3 mm3.The joint sintering was carried out in a muffle furnace,as shown in Fig.3.The heating rate was set at 5°C/min,and the temperature drop indicated furnace cooling.In this study,the effects of joining temperature(180-280°C),holding time(0-60 min),and joining pressure(0-10 MPa)were especially investigated.

2.4.Characterizations and test method

Scanning electron microscopy(SEM,HELIOS Nanolab 600i,FEI,USA)and transmission electron microscopy (TEM,Tecnai G2 F30,FEI,USA)were used to characterize Ag nanoparticles,pastes,and joints.X-ray diffraction (XRD,D8A25,Bruker,Germany) was adopted to identify the phases of the joint fracture.The mechanical properties of the bonded Cu-Cu joints were tested using an electronic universal testing machine(5569,Instron,USA).The test method was compression shear,and the loading rate was 0.5 mm/min,as shown in Fig.4.Six joints were measured under each set of parameters,and the average shear strength was taken.

Fig.10.SEM images of joint fracture (on the Cu substrate side) at the joining temperature of 280 °C:(a) low and (b) high power morphologies.

Fig.11.Effect of holding time on the shear strength of the bonded Cu-Cu joints using citrate-coated nanosized Ag paste.

3.Results and discussion

3.1.Mechanical properties of Cu-Cu joints using citrate-coated nanosized Ag paste

3.1.1.Cleaning effect

The number of washings (in the secondary washing) greatly influenced the organic matter content in the citrate-coated nanosized Ag paste,thereby affecting the mechanical properties of the bonded Cu-Cu joints.Fig.5 shows the influence of washing times of Ag nanoparticles on the joint microstructure.The composition of points A,B,C,and D is displayed at the upper right of Fig.5(a)-(d).According to Fig.5(a),visible dark gray lumps are observed in the joint,especially the interface between the Cu substrate and the paste.The composition analysis of point A shows that it has 99.6%of the C element,indicating that the Ag nanoparticles without secondary washing do contain excessive organic residues.The joint microstructure significantly improved when the Ag nanoparticles were washed three times(Fig.5(b)).The dark gray lumps disappeared from the joint,and the composition analysis of point B showed that the C element content was only 9.8%.An increase in washing to seven times slightly decreased the C element content in the joint to 7.4% (Fig.5(c)) and presented a minimal change in the overall joint microstructure.A comparison of Fig.5(c) and (d) shows that the change in C element content is even weaker after washing more than seven times.The above results clarify that the increase of washing times does have a positive effect on the removal of organic residues,but the more washing times,the weaker the effect.In particular,there are obvious pores in Fig.5(d);this is because too many washing times would lead to excessive removal of the surface coating layer of Ag nanoparticles and weaken the low-temperature sintering ability of the corresponding Ag paste [31-33].The appearance of pores is exactly evidence of over-washing,suggesting that the number of washings should not be excessive.

Fig.6 shows the effect of washing times on the shear strength of the bonded Cu-Cu joints.The joint strength presented an initial increase and subsequent decrease in its trend as the number of washings increased.The macroscopic fracture analysis(insets of Fig.6)shows that its surface included Cu substrate and the Ag nanoparticle paste.A proportion of the Ag nanoparticle paste in the fracture increased with increased washing,indicating that the crack propagation path of the joint passes through the bonding interface and the Ag nanoparticles to some extent.The joint strength was highest at 23 MPa when the Ag nanoparticles were washed with deionized water and acetone until seven times.

Fig.7(a)-(d) present the individual shear fractures.First,Fig.7(a)suggests that the joint belongs to the non-ductile fracture.The presence of many Ag sintered necks indicates that the joint structure was not dense,as it presented a large number of nanopores,thereby resulting in a relatively low joint strength with the unwashed Ag nanoparticles.Second,the presence of large area dimples in Fig.7(b) and the obvious plastic deformation in Fig.7(c) indicate that both the joints exhibit ductile fracture.Therefore,the joint strength can exceed 20 MPa after the used Ag nanoparticles are washed three or seven times in advance.When the used Ag nanoparticles were washed 15 times,however,the plastic deformation capacity of the joint in Fig.7(d) weakened,and may be related to the large pores in this joint(Fig.5(d)).

3.1.2.Joining temperature effect

Fig.12.SEM images of shear fractures of the bonded Cu-Cu joints using citrate-coated nanosized Ag paste with different holding time:(a)0 min,(b)10 min,(c)30 min,and (d) 60 min.

Fig.13.Effect of joining pressure on the shear strength of the bonded Cu-Cu joints using citrate-coated nanosized Ag paste.

The joining temperature is an important process parameter.Fig.8 shows the effect of joining temperature on the shear strength of the bonded Cu-Cu joints.The joint strength increased sharply with the joining temperature before 220°C but did not change much after 220°C.A maximum joint strength of 28.2 MPa was recorded at a joining temperature of 260°C.Nonetheless,the joint strength slightly decreased when the joining temperature reached 280°C.

This phenomenon may be a result of the joint fractures at different joining temperatures (Fig.9(a)-(f)).The fracture of the joint was powdery at the joining temperature of 180°C(Fig.9(a)),indicating that the sintering of Ag nanoparticle was insufficient and the corresponding joint strength was extremely low.At a joining temperature of 200°C,the sintering degree of the Ag nanoparticles obviously improved,but no plastic deformation was found,as shown in Fig.9(b).At a joining temperature of 220°C,the distinct dimples and plastic deformation was observed after joint breakage(Fig.9(c)).Therefore,the sharp increase in the joint strength as well as the joining temperature after 220°C can be attributed to the increased sintering of the Ag nanoparticles and the appearance of dimples and plastic deformation.A comparison of Fig.9(c)-(f)shows that a joining temperature of up to 220°C resulted in the ductile fracture in all of the joints,such that the joint strength did not change significantly.The plastic deformation gradually became obvious from 220°C to 260°C and weakened from 260°C to 280°C,such that the maximum joint strength was observed at 260°C.

To understand why the plastic flow of the joint at a joining temperature of 280°C significantly weakened,a micro-morphology analysis was performed on another side,specifically on the Cu substrate side of the joint fracture,as shown in Fig.10.The low-power morphology in Fig.10(a) suggests that,unlike the Cu surface before joining,the Cu on the fracture surface was granular,and the size of the Cu particles was approximately 500 nm.Partial magnification was performed to obtain Fig.10(b).Composition analysis of points E and F in Fig.10(b)is shown in Table 1.The content of the Ag element in point E is 100%,indicating that it is the residual Ag nanoparticle paste in this side fracture.Although the content of the Cu element in point F was as high as 82.9%,12.5%of the O element was still present.This suggests that excessively high joining temperature leads to oxidation at the interface,which results in joint breakage along the interface before the Ag nanoparticles undergo plastic deformation.

Table 1 Composition analysis of the joint fracture(on the Cu substrate side)at the joining temperature of 280 °C.

Table 2 Composition analysis of the joint fracture (on the Ag nanoparticle paste side)under different aging conditions.

It is worth noting that the surface of the Cu substrate after joint fracture was granular,and the size was on the nanometric scale.To explain this phenomenon,the thermal decomposition of citrate in the paste was analyzed.The results indicate that the thermal decomposition of citrate produced an intermediate product:formic acid(HCOOH).The decomposition reaction of HCOOH occurred above 160°C.The specific reaction formula is as follows:

Fig.14.SEM images of shear fractures of the bonded Cu-Cu joints using citrate-coated nanosized Ag paste at different joining pressures:(a)0 MPa,(b)1 MPa,(c)5 MPa,and (d) 10 MPa.

It is clear that HCOOH decomposed into hydrogen (H2) and carbon dioxide (CO2) when heated to 160°C.Generally,the surface of the Cu substrate was obviously oxidized to form cuprous oxide(CuO)or cuprous oxide (Cu2O) during heating.In this study,H2,as a thermal decomposition product of HCOOH,has strong reducibility and can reduce the generated CuO or Cu2O to Cu.The Cu produced by the chemical reaction was generally granular.Therefore,nanoscale Cu particles were observed on the side of the fracture near the Cu substrate.Further analysis of the joint fractures on the Cu substrate side at other joining temperatures revealed that Cu nanoparticles were produced to varying degrees at 220°C and above.

3.1.3.Holding time effect

Fig.11 shows the effect of holding time on the shear strength of the bonded Cu-Cu joints using the citrate-coated nanosized Ag paste.The joint strength continuously increased with the extension of holding time.The macroscopic morphology of the fracture surface (insets of Fig.11)also shows that with the increase of the holding time,the proportion of the citrate-coated nanosized Ag paste gradually increased.This is understandable as the increase in holding time contributes to the thermal decomposition of organic matter in the Ag nanoparticle paste.

Fig.12(a)-(d) presents the joint fractures with different holding times.Indeed,with increasing holding time,the sintering of Ag nanoparticles gradually became more sufficient,and the plastic deformation became increasingly obvious.Therefore,the joint strength was positively correlated with the holding time.

3.1.4.Joining pressure effect

Fig.13 displays the shear strength of the bonded Cu-Cu joints using the citrate-coated nanosized Ag paste,which varied with the joining pressure.As the joining pressure increased,the joint strength continuously improved.From the macroscopic morphology of the fracture surface(insets of Fig.13),the increase of the joining pressure resulted in a closer bonding interface,and the proportion of the Ag nanoparticle paste increased.When no pressure was applied,the sintered structure of the Ag nanoparticle paste was distributed at the edge due to the obvious coffee ring effect.When the pressure reached 10 MPa,the maximum strength of the joint was measured to be 37.9 MPa.The joining pressure on the joint greatly affected the density.Specifically,the density of the joint significantly increased with increasing joining pressure,such that mechanical properties of the joint can be significantly improved with a more compact joint.

The fractures of the joints at different joining pressures are shown in Fig.14(a)-(d).The absence of joining pressure resulted in a powdery joint fracture (Fig.14(a)).Obviously,the Ag nanoparticles were not sintered together.As the joining pressure increased,the Ag nanoparticles gradually sintered and densified,resulting in a more obvious plastic deformation in the joint fracture.This can be clearly seen by comparing Fig.14(b)-(d).

3.2.High-temperature reliability of Cu-Cu joints using citrate-coated nanosized Ag paste

Fig.15.Microstructure of the bare Cu-Cu joints prepared by the optimal process under different aging conditions:(a)150 °C for 72 h,(b)150 °C for 144 h,(c)250 °C for 72 h,and (d) 250 °C for 144 h.

Fig.16.Shear strength of the joints prepared by the optimal process under different aging conditions.Inset:fracture surfaces of joints under different aging conditions.

The shear strength of the joint at room temperature is an important index to evaluate the reliability of the joint.When the citrate-coated nanosized Ag paste was applied to package the high-power device,however,the joints that use the paste must be able to work for a long time under high-temperature conditions.Therefore,it is necessary to study the thermal stability of the joint.Process simplification implemented the following optimized conditions for the bare Cu-Cu joints:a low-pressure demand,chip failure risk,washing three times,a joining temperature of 260°C,a holding time of 30 min,and a joining pressure of 1 MPa.The bare Cu-Cu joints prepared by the optimal process were tested for aging at 150°C and 250°C.Fig.15 shows the microstructure of these joints under different aging conditions.It is evident that the Ag nanoparticle pastes were completely dense-sintered.Moreover,the Ag nanoparticle paste was tightly bonded to the Cu substrate after aging at 150°C for 72 h and 144 h,as shown in Fig.15(a) and (b).However,the interface bonding of the joint after aging at 250°C was different.After aging at 250°C for 72 h,a small number of micro-cracks were observed at the interface between the Ag nanoparticle and the Cu substrate(Fig.15(c)).After the aging time was extended to 144 h,large cracks formed at the interface to separate the Ag nanoparticle paste from the Cu substrate(Fig.15(d)).

Fig.17.TEM image of the unaged joint.

Fig.18.Schematic diagram of the robust interface bonding mechanism:(a)oxidation of Cu substrate surface,(b,c)oxide reduction on the surface of Cu substrate,and(d) interdiffusion of Ag and Cu elements to form a strong interface layer.

Fig.19.Microstructure(a)near the interface including the large crack and(b)near the tightly bonded interface,of the bare Cu-Cu joint after aging at 250 °C for 144 h.(c) XRD analysis of the corresponding joint fracture.

To quantitatively analyze the thermal stability,the joints under different aging conditions were subjected to shear tests,and the results are displayed in Fig.16.As compared to the unaged joint,the shear strength of the joint after aging at 150°C for 72 h did not significantly decrease and instead increased slightly.This indicates that the aging temperature of 150°C was helpful in promoting joint sintering.The aging time at 150°C was further extended to 144 h,and the joint strength decreased slightly,still up to 26.1 MPa.As indicated above,the increase in aging time minimally affected the mechanical properties of the joint.In other words,the bare Cu-Cu joint using the Ag nanoparticle paste has excellent thermal stability when aged at 150°C.Furthermore,the shear strength of the joints aged at 250°C was compared.The results indicated that the joint strength was significantly reduced to 14.8 MPa after aging for 72 h,and even as low as 4.9 MPa after aging for 144 h.Evidently,the thermal stability of the joint at 250°C was not as good as that at 150°Cpossibly due to the joint fracture surface,as shown in the insets of Fig.16.Unlike the aging at 150°C,the fracture surface of the joint aged at 250°C exposed a large area of the Cu substrate.Moreover,the exposed Cu substrate area increased with the extension of aging time at 250°C.The above indicates that there was a significant decrease in joint strength after aging at 250°C,which may be attributed to the deterioration of the interface between the Ag nanoparticle paste and the Cu substrate.

Before revealing the cause of interface deterioration,it is important to understand the formation of a strongly bonded interface.Fig.17 shows the TEM image of the unaged joint near the interface between the Ag nanoparticle paste and the Cu substrate.The area between the two white dotted curves is the interface layer,which was tightly bonded with the Ag nanoparticle paste and the Cu substrate.The diffraction pattern at point G(inset of Fig.17)shows the presence of Ag and Cu in the interface layer,suggesting the formation of a strongly bonded interface through the mutual diffusion of the Ag and Cu elements.These results contradicted previous research.When the bare Cu substrate was joined in ambient air,its surface was easily oxidized,resulting in the serious deterioration of the interface.Hence,the Cu substrate was often plated with Ag or Cu before bonding.In this study,the diffraction pattern at point G did have rings for CuO,Cu2O,and Ag2O,although there is only one diffraction ring for every oxide.It is guessed that there may be CuO,Cu2O,and Ag2O in the interface.However,the composition analysis of the interface shows that the content of the O element was almost 0.Even if these oxides do exist,therefore,their content should be extremely small and negligible.

These observations were produced by the reducing gas,H2,which was produced by the thermal decomposition of citrate,of which the detailed explanation is provided in Section 3.1.3.The bonding mechanism of the robust interface can be briefly described as follows.Generally,the Cu substrate was sintered in air,and its surface was easily oxidized to CuO and Cu2O (Fig.18(a)).However,the citrate coated on the surface of Ag nanoparticles was able to reduce the Cu oxides to Cu due to its own reduction effect and its generation of H2during thermal decomposition (Fig.18(b)) [34].As a result,Cu substrate surface cleansing was achieved (Fig.18(c)).The surface of the reduced Cu exhibited high cleanliness and was observed in the form of nanoparticles,which aided in its diffusion.Finally,at a certain joining temperature,the Cu and Ag elements diffused with each other to form a strong interface layer(Fig.18(d)).

On this basis,the causes of interface deterioration were further revealed,specifically for the joint aged at 250°C for 144 h,as shown in Fig.19.Fig.19(a)shows the microstructure near the interface including the large crack,indicating that the aging process separated the Ag nanoparticle paste from the Cu substrate.Fig.19(b) displays the microstructure near the tightly bonded interface,as well as the distribution of the Ag,Cu,and O elements.Evidently,the tight bonding was mainly due to the interdiffusion of the Ag element in the Ag nanoparticle paste and the Cu element in the Cu substrate.Nonetheless,the change of the O element is worth noting.The O element curve(marked by the blue curve) exhibited a peak at the interface,which implies O element enrichment at the interface.XRD analysis of the joint fracture(Fig.19(c))demonstrates that the formed oxides are mainly CuO,Cu2O,and Ag2O.The composition analysis of the joint fracture (on the Ag nanoparticle paste side) under different aging conditions was characterized,and the results are summarized in Table 2.The O element contents in the fracture of the joints after aging at 250°C for 72 h and 144 h were over 12%,which was much higher than that of the joints after aging at 150°C.Therefore,it can be concluded that the aging temperature of 250°C resulted in obvious oxidation at the interface,thus significantly reducing the joint strength.

4.Conclusions

Highly reliable Cu-Cu joints were successfully performed using citrate-coated nanosized Ag paste in ambient air.A detailed study of the mechanical properties of the joints at room temperature showed that the washing times of the Ag nanoparticles have an important effect on the joint strength.With the increase of washing times,the joint strength tended to first increase and then decrease.Similarly,the joint strength first increased and then decreased as the joining temperature rose.Nevertheless,the extension of the holding time and the increase of the joining pressure both contributed to the improvement of the joint strength from the beginning to the end.Based on the practical application considerations,three washing times,a joining temperature of 260°C,a holding time of 30 min,and a joining pressure of 1 MPa were selected as the optimal process conditions for the bare Cu-Cu joints.The thermal stability of the joints made by the optimal process at 150°C and 250°C was investigated.The results showed that the joint has excellent thermal stability at 150°C.After long-term aging at 250°C,however,its strength was significantly reduced.This is because the aging temperature of 250°C caused significant oxidation of the interface between the Ag nanoparticle paste and the Cu substrate,resulting in a significantly lowered joint strength.The present work aids in the understanding of the bare Cu-Cu joining for three-dimensional (3D) ICs fabricated under atmospheric conditions.

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical limitations.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant number:51805115 and 51975150) and the China Postdoctoral Science Foundation (Project number:2019M651280).