Effects Of Alloying Elements On The Ag/(Al?Cu?Au) Interfacial Properties:A First-Principles Study

In recent years,as the price of gold rises and electronic devices tend to be miniaturized and multifunctional,there is an urgent need to move towards low-cost and high-density packaging.At present,Ag exhibits excellent electrical and thermal conductivity as well as good mechanical properties,and has developed into another basic material replacing Au wire in addition to Cu wire.But,the strength of pure Ag wire is relatively low,and there are some disadvantages such as easy to breakage and vulcanization under high-speed bonding condition.The effective way to improve the above problems is through multiple alloying.However,the Ag bonding wire has heterogeneous interface problems with Al,Cu,and Au pads use in practical.The effect of alloy elements on the interface properties has been studied by experimental analysis.However,due to the limitation of experimental testing technology and the complexity of interface observation,the understanding of the influence of alloy elements on the interface properties in microcosmic is still a problem to be solved.As the first principle is widely used in interface research,therefore,the first-principles calculations method based on density functional theory is adopted in this thesis.Through the calculation of the properties of interface distortion,the work of separation,interfacial energy and electronic structure,the influence of 11 different alloy elements?Au,Cu,Pd,Be,Mg,Al,Ca,Ni,Ti,Sn,In?doping on the Ag alloy on the properties of the Ag/?Al,Cu,Au?interface is analyzed.The mechanism of alloying elements is discussed at the micro-scale,which provided theoretical support for the selection of alloying elements for Ag bonding wires.The main research contents and conclusions of this thesis are as follows:1.The surface energy of Ag,Al,Cu and Au crystal with low-index is calculated.The results show that the surface energy of all metals satisfy the following requirements:??111?<??100?<??110?,which indicates that the surface of close-packed surface structure is the most stable,and the surface energy tends to converge when the number of atomic layers of?111?surface reaches 5 layers,in which?_Ag?111?=0.786 J/m²,?_Al?111?=0.840 J/m²,?_Cu?111?=1.206 J/m²,?_Au?111?=0.669 J/m².Among the nine groups of Ag/Al interface models by constructed,the Ag?111?/Al?111?interface has the largest work of separation W_sep=1.378 J/m² and the lowest interfacial energy?_int=0.248 J/m²,indicating that it is composed of densely packed surface has the best stability.In addition,the doping tendency of the doped alloying elements in the Ag/Al interface is analyzed.Au,Cu,Pd,Ni,and Ti tend to replace the Ag atom at the interface position 1.Be,Mg Tends to replace near the interface at site 3.Al,Ca,Sn,In tends to replace position 5 in which far from the interface.2.The influence of alloying elements on the properties of Ag/Al interface was studied.After the addition of alloy atoms,the volume of the Ag/Al interface is distorted to varying degrees.Due to Ca and Ag atoms have large differences in properties such as electronegativity,atom radius,and number of valence electrons,the degree of distortion formed at the interface is greater,while Cu atoms and Ag atoms are similar in nature and have lower distortion.In addition,the work of separation and interfacial energy of the Ag₂₀/Al₂₀0 interface are 1.378 and 0.248 J/m2,respectively,indicating good stability,but the alloy element doping reduces the stability of the Ag/Al interface,the decrease degree meeting to Pd<Ni<Ti<Cu<Be<Al<Au<Sn<Mg<In<Ca.The calculation of the electronic structure of the Ag/Al interface shows that the decrease in the strength of the Ag-Al covalent bond caused by the doping of alloying elements is the main factor leading to the decrease in interface stability.The bond population value before doped was 0.200,and it decreased to 0.193 after Ca doping.3.The effects of alloying elements on the properties of Ag/Cu interface were studied.The order of the alloy element doping increases the volume of the Ag/Cu interface is Ca>Sn>Mg>Au>Ti>Al>Pd>Ni>In>Be>Cu.When not doped,the work of separation and interfacial energy of the Ag₂₀/Cu₂₀0 interface are-0.563 and 2.555 J/m²,respectively.A negative separation work indicates poor interface bonding ability and stability;as the alloying element is doped,the W_sepep increases and decrease of the interfacial energy indicates that the doping is conducive to improving the stability of the Ag/Cu interface.The maximum W_sepep of Cu doping system is 0.364J/m²;the strengthening order satisfies Cu>Pd>Be>Al>Ti>Ni>In>Sn>Au>Ca>Mg.The electronic structure shows that the weaker bonding of the un-doped Ag/Cu interface is due to the weak chemical bonding between Ag and Cu atoms at the interface,while the alloying element doping provides more electrons to participate in bonding,making the Ag-Cu covalent is enhanced.The bond population values of the Ag/Cu interface indicate that the order of increasing interface stability caused by increasing covalent bond strength is:Cu>Pd>Be>Ti>Ni>In>Al>Sn>Ca>Au>Mg.Among them,the Cu?1?-Ag?1?,Cu?2?-Ag?2?average bond populations of the Ag₂₀Cu/Cu₂₀interface system are the largest,are 0.090 and 0.094.It is generally believed that the doping of Cu can greatly increase the stability and bonding ability of the Ag/Cu interface.In addition,the Pd element can also play a better reinforcing effect.4.The effects of alloying elements on the properties of Ag/Au interface were studied.5.The change law of the relative volume of the Ag/Au interface by alloying elements is:Ni>Be>Cu>Pd>Ca>Al>In>Ti>Sn>Mg>Au.The interface energetics calculation results show that the work separation and interfacial energy of the Ag₂₀/Au₂₀0 interface are 1.242and 0.213 J/m²,respectively,which is less stable than the Ag/Al interface but higher than the Ag/Cu interface are un-doped.Except for Au doping,doping is beneficial to improve the stability of the Ag/Au interface.The order of enhancement is Ti>Ni>Pd>Cu>Al>Be>Sn>In>Mg>Ca?>Au?.When undoped,Ag and Au have obvious PDOS bonding peaks overlapping,indicating that the two have strong chemical bonding and good interface stability.The main reason for the change in the stability of the interface by the doping is the increase in the strength of the covalent bond of Ag-Au,with the exception of Au doping,which strengthens the relative effect between Ag and Au.Its strengthening effect is:Ti>Ni>Pd>Cu>Be>Sn>Al>In>Mg>Ca?>Au?.Among them,Ti,Ni,Pd,Cu elements can better play the effect of further improving the stability of the Ag/Au interface.