The Influence Of Alloying Elements On The Cu/(Al、Cu、Au) Interfacial Properties By First-Principles Calculations

With the development of integrated circuit chips to miniaturization and multi-function,higher requirements are put forward for the bonding wire,the key material of electronic packaging.Due to its high electrical and thermal conductivity,as well as high cost-effectiveness,copper bonding wires have become the preferred material to replace gold bonding wires and have broad application prospects in the field of electronic packaging.However,after bonding copper wire to substrate pad（generally Al,Cu,Au）,there are still some problems,such as poor stability of the bonding interface,easy to fall off and lead to bonding failure.Therefore,the reliability of the bonding interface has become the main factor restricting the wide application of copper wire.At present,the main direction of research and development of high quality copper bonding wire is by adding trace alloy elements to pure copper wire to improve its mechanical properties,bonding reliability and oxidation resistance,etc.Due to the limitation of experimental conditions,it is difficult to study the interface information such as the characteristics of chemical bonds,electronic structure,and interface energy of the bonding interface on the atomic scale.With the rapid development of computational materials science,first principle calculation has been widely used in the prediction and evaluation of material properties.Using theory to guide experiments can shorten the research cycle and save research costs.Therefore,in this thesis the first-principles calculation method is adopted to study the alloying elements X（X=Ag,Al,Be,Fe,In,Mg,Ni,P,Pd,Si,Sn,Zn）doped in Cu on the three kinds of Cu/（Al,Cu,Au）interface properties,and calculate the work of separation,interface energy and electronic structure of the interface.The influence of alloying elements on the interface stability is discussed from the perspective of interface energetics,and the micro mechanism of alloying elements on the interface is discussed according to the electronic structure,which provides theoretical reference for the development of new copper bonding wire.The main findings are as follows:1.The surface energy of Cu,Al and Au low-index crystal planes was studied.The results show that the surface energyσof the three metals has the following relationship:σ_（111）<σ_（100）<σ_（110）.The close-packed surface（111）has the lowest surface energy and the most stable structure.Therefore,the interface model composed of（111）surface is selected for study in this thesis.The surface energy of Cu,Al and Au is:σ_Cu（111）=1.328 J/m²,σ_Al（111）=0.781J/m²,σ_Au（111）=0.669 J/m²,respectively.When the number of atomic layers is 5,the surface energy has converged.Therefore,5 atomic layers are adopted for Cu（111）,Al（111）and Au（111）to construct the interface model.The UBER method was used to determine the optimal interfacial distance of the three groups of Cu/Al,Cu/Cu,and Cu/Au interfaces,which were 2.5（?）,2.4（?）,and 2.6（?）,respectively.2.The influence of alloying elements on the Cu/Al interfacial properties was studied.The separation work and interface energy of the Cu₂₀/Al₂₀interface are 1.423J/m²,0.686J/m²,respectively,indicating that the interface bonding is relatively stable.After doping with Al,Be,Fe,Pd and Zn,the separation work of Cu₁₉（X）/Al₂₀interface increases,the interface energy decreases,and the interface stability increases.Particularly,the level of stability is significantly improved by Pd,the Cu₁₉（Pd）/Al₂₀interface has the highest work of separation of 1.57 J/m²and the lowest interface energy of 0.539 J/m².However,after doping with Ag,In,Mg,Ni,P,Si and Sn elements,the separation work of Cu₁₉（X）/Al₂₀interface decreases and the interface energy increases,indicating that the interface stability decreases.The electronic structure analysis of the interface shows that new bonding peaks are formed after the doping of alloying elements,and more charge accumulates at the interface,which enhances the strength of Cu-Al covalent bond at the interface,which is the main factor to improve the stability of the Cu/Al interface.The covalency of the Cu-Al bond is weakened,resulting in a decrease in interface stability.For example,the Cu（1）-Al（1）average bond population of the Cu₂₀/Al₂₀interface is 0.187,and it is reduced to 0.174 after Si doping.3.The effect of alloying elements on the Cu/Cu interfacial properties was studied.The separation work and interface energy of the Cu₂₀/Cu₂₀interface are 1.756J/m²and0.899J/m²,respectively.Except for the Ag and Zn elements,the Cu₁₉（X）/Cu₂₀interface separation work increases and the interface energy decreases after doping,which is beneficial to the improvement of interface stability.The order of the alloying elements increase the Cu₁₉（X）/Cu₂₀interface separation work is as follows:Be>Sn>Si>P>Al>In>Ni>Mg>Fe>Pd.The stability of Cu₁₉（Be）/Cu₂₀interface,Cu₁₉（Sn）/Cu₂₀interface is higher,the separation energy is 1.877J/m²,1.851J/m²,interface energy is 0.779J/m²,0.805J/m²,respectively.The electronic structure analysis of the interface shows that the doping of alloying elements Be,Sn,Si,P,Al,In,Ni,Mg,Fe and Pd increases the charge accumulation between Cu atoms at the interface,and the population value of Cu-Cu bond at the interface increases,so the stability of the interface is improved.After doping with Ag and Zn elements,the bond length of Cu-Cu at the interface increases,which weakens the bonding between Cu atoms,so the stability of Cu₁₉（Ag）/Cu₂₀and Cu₁₉（Zn）/Cu₂₀interfaces are reduced.4.The effect of alloying elements on the Cu/Au interfacial properties was studied.The separation work and interface energy of the Cu₂₀/Au₂₀interface are-0.723J/m²,2.757J/m²,respectively.Compared with the Cu₂₀/Al₂₀and Cu₂₀/Cu₂₀interfaces,the Cu₂₀/Au₂₀interface has the smallest separation work and the largest interface energy.Therefore,the stability of Cu₂₀/Au₂₀interface is the worst from the perspective of energy.When the alloy element X is doped,the separation work of the Cu₁₉（X）/Au₂₀interface increases and the interface energy decreases,indicating that the 12 alloying elements doped are all conducive to the improvement of the stability of the Cu/Au interface.The order of the alloying elements increase the Cu₁₉（X）/Au₂₀interface separation work is as follows:Al>Fe>In>Zn>Be>Sn>Si>P>Ni>Pd>Ag>Mg.Among them,the stability of Cu₁₉（Al）/Au₂₀interface and Cu₁₉（Fe）/Au₂₀interface are higher,the separation work is0.706J/m²,0.645J/m²respectively,and the interface energy is 1.328J/m²and 0.645J/m²,respectively.However,after doping with Mg element,the separation work of Cu₁₉（Mg）/Au₂₀interface is-0.440J/m²,the interface energy is 2.474J/m².The separation work is still negative,indicating that the interface bonding is still unstable.From the perspective of the electronic structure of the interface,the doping of alloying elements enhances the chemical bonding between Cu and Au atoms,and the increasing law of the average bond population value is basically consistent with the increasing law of the the separation work of the interface.