Study On The Mechanical Properties Of Through Silicon Via Copper

Through silicon via(TSV)technology is the core technology for achieving three-dimensional(3D)electronic packaging.The 3D TSV packaging makes up of through via for vertical interconnection and redistribution layer(RDL)for plane interconnection.TSV has excellent performances,such as,small package size,short interconnection distance and low signal delay,which makes it possible for 3D electronic packaging technology to develop continuously to miniaturization and high performance.The key technical issues,such as the optimizing designs of micro-nano chips and the reliability predictions of packaging systems in 3D electronic packaging,also attract increasing attentions.Thereinto,the mechanical properties including Young’s modulus,yield strength and breaking strength of micro-nano scale materials are especially important for enhancing the function and reliability of the package body.However,the mechanical properties of materials at micro-nano scale are significantly different from those at the macro-scale.These properties are not only sensitive to scale effects,but also closely related to the manufacturing process.If the design,manufacture and application of micro-nano chips and package system is still guided by traditional mechanical properties,the analyzing results are often the opposite.Therefore,obtaining the reliable basic mechanical properties of the TSV interconnect material are one of the foundations for chip design,interconnect structure design,manufacturing process optimization,and failure analysis.At present,there are few studies on the characterization of TSV copper interconnect materials.Therefore,it is of great significance to study and characterize the mechanical properties of micro-scale TSV copper interconnect materials,and to grasp the basic mechanical properties of TSV copper interconnect materials and the corresponding influence induced by the preparation process of TSV.For the above requirements,a uniaxial micro-stretching method is adopted to test the mechanical properties of TSV copper cylinder and uniaxial micro-stretching method is used in RDL copper interconnects deposited from methanesulfonic acid bath.A model for the test sample is established,which is suitable for both micro-compression and uniaxial micro-stretching.The yield strength of the TSV copper cylinder is achieved by the micro-compression experiment and the Young’s modulus can be extracted by the nanoindentation method.The Young’s modulus,yield strength and fracture strength of the RDL copper interconnects are measured with the uniaxial micro-stretching.The effects of sample size,current density and heat treatment temperature on the mechanical properties are analyzed.The main research topics of this paper are described as follows:Firstly,a micro-compression experimental method is established for in-situ TSV copper cylinder.1)The initial conditions for the micro-compression experiment were obtained by simulation analysis.In this way,the test results distortion induced by the aspect ratio(λ)of the TSV copper cylinder and the great tilt angle of the pressure head,are reduced.The simulation results show that the buckling does not occur when the aspect ratio of specimen is 150μm:50μm(3:1)with friction on pressure head with less than 5-degree tilt angle.The buckling probability increases when the aspect ratio is greater than 3:1,and a visual system is introduced to eliminate buckling interference.2)The micro-compression test platform constructs to test the TSV copper cylinder under different process conditions.With the increase of current density,the grain size of TSV copper column decreases,the grain orientation and the ratio of CSL grain boundaries change,and the content of C element increases.Thus,the influence of current density(1-9 mA/cm~2)is nonlinear.When the current density is3mA/cm~2,the yield strength reaches maximum,175.1MPa.The heat treatment will decrease the yield strength from 175.1MPa@25?C to 148.3MPa at@400?C due to increasing the grain size and decreasing the grain boundary density.The yield strength of the TSV copper cylinder increases gradually with the aspect ratio increases.A maximum of 208.0 MPa is reached atλ=6:1(120μm:20μm).At the same time,the strain-jump phenomenon of single crystal appears in the stress-strain curve.Secondly,the Young’s modulus and hardness of the TSV copper cylinder were obtained by nanoindentation testing technology.1)The experimental results show that the variation trend of Young’s modulus of TSV copper cylinder under different current densities is basically the same as that of yield strength.Young’s modulus of the DC-deposited TSV copper cylinder is approximately the same(101.2~128.9GPa)at different regions(top,middle and bottom).The Young’s modulus is closely related to grain orientation and grain boundary density.Different grain orientations have their respective contributions to Young’s modulus and the increase of grain boundary density will increase the value of Young’s modulus.The value of Young’s modulus increases when grain orientation changing from(100)to(111)and grain boundary density increasing.The average hardness values are2.4GPa@1mA/cm~2,2.6GPa@3mA/cm~2,2.7GPa@6mA/cm~2 and 2.4GPa@9mA/cm~2.2)In the variable current density electrodeposition sample,Young’s modulus in low current region and high current region are 133.9 and 141.6GPa,respectively.The hardness value of the low current density region is 2.7GPa,which is higher than the high current density region(2.2GPa)due to the large number of grain boundariesΣ3 in the low current density region.The heat treatment eliminates the microstructural difference in the high and low current density regions.The mechanical properties of the two regions are roughly the same,and the young’s modulus is about 109.9GPa,and the hardness is about 1.7GPaFinally,the yield strength and Young’s modulus of RDL copper interconnection line were obtained by dynamic mechanical analysis(DMA)uniaxial micro-tensile test.The shape,length,radius of curvature and thickness of the micro-tensile specimens were optimized using finite element simulation software.The yield strength of the RDL Cu interconnection line reaches the highest value(347.3MPa)at 10mA/cm~2,when the current density increased from 5mA/cm~2 to 15mA/cm~2,the.The increase of current density leads to that the main crystal surface of the sample changes from(111)to(220),and the change of the main crystal surface and integral ratio of the sample results in the decrease of Young’s modulus from 110.2MPa to 96.6MPa.The increase of current density will lead to the appearance of pinhole defect in the sample,which will lead to the decrease of mechanical properties.Heat treatment reduces the residual stress and storage energy in the material,and also reduces the mechanical properties of the material.After 400?C heat treatment,the yield strength of RDL Cu interconnection line decreases from 347.3MPa@25?C to 228.4MPa,and the main crystal surface of the RDL Cu-MSA interconnection changes from(220)to(311),and Young’s modulus fluctuates between 95.7GPa and 77.8GPa.