Research On Low Temperature And Low Pressure Cu-Cu Sintering Bonding Technology For Die-Attachment

Electric energy has become the world's main energy consumption.The strong demand for electricity has stimulated continuous technological development in power generation,storage,distribution and management.At present,the emergence of third-generation solid-state power semiconductor devices is expected to greatly increase the utilization rate of electric energy and reduce transmission losses.And it can control more power in the same area,which is helpful to the miniaturization of the device.However,the heat generation problem caused by high integration restricts the application of traditional tin(Sn)-based solder and cannot withstand the harsh working environment that may be in the future.Therefore,high-reliability die attach materials urgently need to be developed.The Cu particle sintered connection material has quickly attracted the attention of scientific researchers due to its low price,excellent electrical/thermal conductivity,and excellent high-temperature reliability,and has become a promising next-generation die attach material.The Cu sintering connection technology is different from the fusion welding mechanism of Sn solder.The particle size effect promotes the diffusion of atoms on the particle surface,and then the welding is carried out at a lower temperature.The particle size effect in the block formed after welding disappears,and the remelting temperature restored to the melting point of bulk Cu,ensuring its high-temperature reliability.However,the biggest problem in the sintering process of Cu particles is the easy oxidation of the Cu particles.During the sintering process,the Cu oxide layer covers the Cu particles,which greatly reduces the sintering performance of the Cu particles.Therefore Cu-Cu bonding was conducted at relative high temperature and high pressure in reductive atmosphere in order to eliminate the harmful effect of surface oxide,which limited its application in industry.Herein,a novel low-temperature low-pressure sinterable Cu nanoparticle(Cu-NPs)paste was developed using Cu Metal Organic Decomposition(MOD)ink as solvent and surface modifier.The Cu MOD ink in paste introduced a surface modification on Cu nanoparticles which not only eliminated the surface oxidation of Cu-NPs,but also modified Cu-NPs by Cu atoms obtained from thermal decomposition.As a result,robust Cu sinter joints for die-attachment could be realized at a low pressure of 0.4 MPa in non-reductive nitrogen atmosphere.Using the paste with 20 wt.%Cuf addition for sintering,a relative high shear strength of 17.1 MPa could be achieved at 300°C,which was about 3.8 times higher than that sintered without MOD ink(4.5 MPa).When bonding temperature rose to 350°C,even higher shear strength over 20 MPa was obtained.The bonding strength was comparable to the traditional Pb-5Sn solder,which promised the industrial potential for the presented bonding method.At the same time,in order to solve the problems of low sintering strength and high porosity of commercial Cu particles under low temperature and low pressure sintering conditions,a new type of bimodal micro-nano Cu particle paste with great potential was prepared by a liquid phase chemical reduction method.A high-strength Cu joint with a shear strength of 60.4 MPa could be obtained at 300°C.At the same time,the porosity of the sintered body could be reduced to 11.25%,and the resistivity could be reduced to 11.21??·cm.Compared with the Cu sintered joint of pure Nano-Cu paste at 300°C,the shear strength of 1:1 Cu paste was increased by 27.26 MPa,the porosity was reduced to 33.7%of the pure Nano-Cu paste,and the resistivity was reduced to 16.7%of the pure Nano-Cu paste.The paste was applied to DBC(direct bond copper)devices to analyze the influence of different plated chips on sintering.Finally,in order to solve the problem of low interface connection rate between the sintered layer and the substrate under the condition of low temperature and low pressure sintering.We used electroplating methods to electroplate<111>oriented nano-twinned Cu(nt-Cu)films with high diffusion coefficients and randomly oriented commercial Cu films on Cu substrates,using two different grain orientation Cu as the substrate.After sintering,it was found that<111>-oriented nt-Cu thin film was used as the base material of Cu sintered joints,and the bonding interface between the sintered layer and the base material could be greatly improved.The joint composed of Cu paste and<111>-oriented nt-Cu film had an extremely high shear strength of 53.7MPa under sintering conditions at 300°C,0.4 MPa and N₂ atmosphere.Compared with the joint composed of randomly oriented Cu film,the shear strength was increased by73.2%,and the interface connection rate was increased by 13.8%.