Study On The Formation Of Sidewall Intermetallic Compounds In Micron-Level Copper Pillar Bumps

The 3?dimensional integrated circuit(3D IC)technology is currently one of the most significant methods in semiconductor industry,as it satisfies the huge commercial interest in high interconnect density at finer bump pitch,and provides possibility to continue Moore's law in Si?based technology.In three different sized solder balls,copper pillar becomes the most promising one due to its great heat conduction and electric conduction.In recent research,a new type of compound formed in small-sized bumps was frequently observed:it was located on the sidewalls of copper pillars,often accompanied by the formation of voids.However,there is no systematic and comprehensive study on the formation mechanism of this sidewall compound.In this paper,three different sized copper pillar bumps(with diameter of 10?m,20?m,50?m)are utilized to study the growth of intermetallic compounds on the sidewall after reflow and aging treatments;the growth rate of sidewall intermetallic compounds is also calculated.The Arrhenius formula is applied to calculate the formation activation energy of sidewall intermetallic compounds in different sized bumps,and the electron backscatter diffraction(EBSD)technique is applied to analyze the texture of the sidewall intermetallic compound crystals.The main contents and conclusions are as follows:After aging treatment,the formation of sidewall intermetallic compound is observed in both non-reflowed bumps and reflowed bumps.The composition of the sidewall intermetallic compounds formed during aging treatment is Cu₃Sn.As the aging temperature increases,the growth rate of sidewall intermetallic compound becomes faster.In the non-reflowed bumps,the formation of sidewall intermetallic compounds requires an"incubation period",namely,the growth of sidewall intermetallic compounds would not be observed until being aged for a certain time.In the reflowed bumps,the rapid growth of sidewall IMC could be observed in the initial stage of aging treatment.The growth of sidewall IMC is often accompanied by the formation of voids,which are densely distributed at the boundary between the sidewal IMC and the copper pillars.After the same aging treatment,the size of the voids in small bumps is larger than that in large bumps.The volume of the sidewall IMC is counted after different aging periods,and is fitted to the aging time.The results show that the volume of the sidewall IMC is proportional to the square root of the aging time,which indicates a diffusion controlled mechanism.In different sized bumps,the growth rate and growth degree of sidewall IMC are significantly different.At 170°C,the growth rate of the sidewall IMC in 20?m bump is 23.66?m³h²,while the growth rate of the sidewall IMC in the 10?m bump is only 8.14?m³h².After aging for 64 hours,the volume ratio of the sidewall IMC to the copper pillar in the 20?m bump is 0.15;while in the 10?m bump,the ratio is 0.22.The formation activation energy of the sidewall IMC is calculated according to the Arrhenius formula,and the results are 42.2 k J?mole for 10?m bumps and 71.9 k J?mole for 20?m bumps.The calculation results are significantly lower than the activation energy of the traditional interfacial IMC.The EBSD results show that both the interfacial IMC and the sidewall IMC have obvious preferancial orientations.The texture of the interfacial IMC is<100>crystal orientation being parallel to axial direction of copper pillars,and the texture of sidewall IMC is<100>crystal orientation being parallel to radial direction of copper pillars.By calculating the atomic density of the low-index crystal planes in Cu₃Sn grains,we find that the atomic density on the(100)crystal plane is the lowest.Considering that the growth of the sidewall IMC is controlled by the diffusion mechanism and the activation energy of sidewall IMC is low,it is anticipated the while Cu and Sn atoms diffusing to form the sidewall IMC,the newly formed Cu₃Sn grains will spontaneously arrange themselves in<100>crystal orientation being parallel to the concentration gradient directions,to facilitate the further formation of IMC.Building on our results,we develope a model based on surface diffusion to explain the formation of sidewall IMC in micron-level copper pillar bumps.