Research On Cu Based Nanosolders For Low Temperature Cu-cu Bonding

As one of the most important part in semiconductor industry,the development of integrated circuits in recent years has been regarded as a technology development strategy in our country.In order to overcome the bottleneck encountered by Moore’s Law in the rapid development of the semiconductor industry,the 3D-IC(three-dimensional integrated circuit)technology was emerged as a promising solution and put forward higher requirements for the bump density,energy consumption and packaging performance in the packaging process.However,the Sn or Sn-based lead-free solders used in the bonding process at this stage has been unable to meet the more stringent packaging requirements,and gradually appeared some problems,such as the overflow of Sn during the bonding process,the Sn whisker growth during the server process and the Kirkendall voids formation.These phenomena will lead the short circuit,open circuit,and the degradation of electrical performance in integrated circuits.Therefore,it is an urgent need to find a high-performance interconnection material that can replace Sn material for the development of microelectronics packaging technology.Because of the excellent electrical conductivity,thermal conductivity and resistance to electromigration,Cu and Ag are always considered as ideal interconnect materials.However,the high melting point of Cu and Ag limits their application in microelectronic packaging,because they can not meet the process requirements.With the rapid development of nanotechnology,researchers had achieved a lot of breakthroughs in the research field of low-temperature sintering of crystalline materials.Therefore,reducing the the Cu-Cu bonding temperature by using the Cu and Ag materials with nanotechnology is a viable research idea.Since Ag is a resource shortage material,it is not suitable for sustainable development.So,this study mainly focuses on Cu-Cu bonding with Cu-based nanosolders.The specific research contents are as follows.Firstly,based on the size effect of nanocrystalline materials,we proposed a method of reducing the Cu-Cu bonding temperature by sinterable Cu nanosolders,which were prepared by Cu nanoparticles.In this paper,Cu nanoparticles with an average size of about 100 nm were synthesized and Cu nanosolders were also prepared.The sintering and bonding experiments were carried out at 250 oC to 400 oC.The lowest electrical resistivity of 12.9 μΩ·cm was achieved at 400 oC,while the highest shear strength of 31.88 MPa was reached after bonding at the temperature of 400 oC and the pressure of 40 MPa.The sintering mechanism of Cu nanosolders and the bonding mechanism of Cu-Cu bonding process have also been proposed in this paper.These mechanisms provide a theoretical basis for the research of low temperature bonding.By using the Cu nanosolders,the Cu-Cu bonding temperature was effectively reduced to 400 oC.Although it can not fully meet the microelectronic packaging requirements,but it confirms the feasibility of Cu-Cu bonding by using sinterable Cu nanosolders,which laid the foundation for the following study.Secondly,based on the size effect and the sintering mechanism,a method of further reducing the Cu-Cu bonding temperature by smaller Cu nanoparticles were proposed.By improving the synthesis process,Cu nanoparticles were obtained with an average size of 60 nm.By using these nanoparticles,the high sintering performance of Cu nanosolders was achieved at 300 oC,and the resistivity was as low as 12.0 μΩ·cm.A high-quality Cu-Cu bonding interface was also achieved at 300 oC,with a much lower bonding pressure of 1.08 MPa.The shear of Cu-Cu joint can reach a high value of 31.88 MPa.Then,the aging test had been taken for the bonded samples by heating at 150 oC for 200 h.After testing,the bonding interface and the mechanical properties of bonded samples did not show obvious changes,which demonstrate the compactness of the bonding joint.We have also achieved the high-quality wafer level Cu-Cu bonding by using this type of Cu nanosolders.After bonding,the wafer substrates showed high degree of integrity,which indicate the Cu nanoparticles of 60 nm have a better prospect in microelectronic packaging.Thirdly,according to the fact that Ag nanoparticles have better sintering performance than Cu nanoparticles with similar size,we proposed a method of preparing the Cu base Cu-Ag composite nanosolders by adding Ag nanoparticles of 70 nm into Cu nanoparticles.At the temperature of 250 oC,the Cu2-Ag composite nanosolders(the Cu-Ag atomic ratio of 2:1)exhibited high sintering properties of low temperature.The resistivity of sintered Cu2-Ag film had reached 19.9 μΩ·cm,almost close to the resistivity of 12.0 μΩ·cm,which was brought by pure Ag nanosolders.The diffusion layer formation of Cu-Ag interface was observed by TEM,the sintering mechanism of Cu-Ag composite nanosolders was analyzed,and the key effect of Ag nanoparticles in enhancing the sintering performance was explained.With the bonding temperature of 250 oC,a compact bonding interface with a hige shear strength of 25.41 MPa was achieved by Cu2-Ag nanosolders,which was similar as that by using Ag nanosolders.Therefore,by using the Cu2-Ag nanosolders,the sintering and bonding performance were enhanced,and the cost-efficiency was guaranteed.Fourthly,to further research the low temperature Cu-Cu bonding method,we have proposed a novel type of Cu nanoaggregates with large amounts of ultra-small Cu nanoparticles on the surface.Based on these nanoaggregates,we have done the sintering and bonding researches.Excellent sintering and bonding properties can be achieved within 250 oC.By using Cu nanoaggregates,the lowest resistivity can reach 4.37 μΩ·cm,which is only 2.5 times than that of Cu bulk.The shear strength of the Cu-Cu joint can also achieve 25.36 MPa.This study has made great progress in the Cu-Cu bonding by using pure Cu nanosolders,which has a momentous research significance.In this work,a series of studies on the Cu-Cu bonding by using Cu-based nanosolders were carried out with the size effect as the theoretical main line.The research ideas and research results have the guiding significance for the development of microelectronic packaging.