The Synthesis Of Cu Nanoparticles And Their Low Temperature Joining Behavior During Sintering

Because of the size effect of metal nanoparticles(NPs),the melting point will decrease with the decrease of particle size,and thus Cu NPs can be sintered at temperatures much lower than the melting point of the bulk copper.At the same time,after sintered into bulk copper,the sintered body of Cu NPs can work steadily for long time at high temperatures,which well meets the need of �low temperature molding,high temperature service�.In recent years,many researchers have attempted to use Cu NPs as the main material for conductive inks and bonding material for high power semiconductor devices.However,due to the easily oxidation of Cu NPs,the current research has certain insufficient,such as high sintering temperature,long sintering,high sintering pressure,corrosive sintering environment and poor performanc,and so on.These drawbacks greatly lower the application value of Cu NPs in the field of electronic packaging.In order to solve these problems caused by the oxidation of Cu NPs,this paper presents a method to remove the surface oxide of Cu NPs by using formic acid to realize the low temperature sintering of Cu NPs.Besides,the effects of formic acid treatment on the sintering behavior of Cu NPs are studied.By optimizing the surface composition of Cu NPs,the sintering temperature of Cu NPs is lowered and the conductivity of sintered Cu NP body is improved.At the same time,the sintering behavior of Cu NPs under pressure is also investigated.Furthermore,high strength Cu-Cu joints for high power semiconductor devices are obtained.In this paper,the surface oxide of Cu NPs is removed by formic acid treatment.The surface composition of Cu NPs shows that the surface oxide is completely removed by formic acid treatment,and the newly formed formate can easily decompose at low temperature.The thermal analysis of Cu NPs shows that the sintering temperature of Cu NPs decreases from 260 oC to 160 oC after the treatment of formic acid.Meanwhile,the exothermic peak which occurs in the thermal curve is confirmed as sintering peak.Furthermore,the time of formic acid treatment has a significant effect on the sintering process of Cu NPs: the long time formic treated Cu NPs are short-range sintered and the short time formic treated Cu NPs are long-range sintered.In this paper,a low temperature sintered formic acid treated Cu NP conductive ink with high conductivity is developed.The resistivity of the Cu NP film sintered at 260 oC for 60 min is 6.12 ??�cm,while that of the original Cu NP film is 18.59 ??�cm,which is almost three times more than that of the formic acid treated Cu NP conductive ink.The result shows that the better conductivity of the Cu NPs treated with formic acid is due to the low-void and continuous sintered venation,and the absence of oxide in the sintered film is also a reason for the excellent conductivity.In addition,the diffusion mechanism of intergranular sintering interconnection is discussed through the in situ TEM observation of the sintering of Cu NPs.In this paper,the pressure-assisted sintering behavior of Cu NPs is explored;besides,its low-temperature sintering interconnecting process and the joint performance are also studied.The results show that the performance of formic acid treated Cu NP paste is better than that of the original Cu NP paste under 10 MPa pressure assisted sintering process.The resistivity of the formic acid treated Cu NP film is 5.65 ??�cm after sintered for 5 min at 260 oC while the resistivity of the sintered original Cu NP film is 28.70 ??�cm.In addition,the shear strength of the formic acid treated Cu NP joint is 43.41 MPa,while the shear strength of the original Cu NP paste joint is only 23.92 MPa.The formic acid treatment of Cu NPs has important meaning for the practical industrial application.Moreover,high-density coherent twins are found in sintered Cu NP body,which is one of the main reasons for high shear strength and high conductivity of Cu-Cu joints.And the results of TEM show that the combination part between Cu NPs and Cu pad is metallurgical joints.Based on the microstructure of the joint,and combined with the �ball-plate model� of the metal NPs and the Cu pad,it is concluded that the obtuse angle contact between the sintered Cu NPs and Cu pad is beneficial to improve the mechanical properties of the Cu-Cu joint.