Interfacial Reaction And Design Of Reaction Barrier Layer Of Ga-21.5In-10Sn/Cu Liquid-solid Interface

Flexible and stretchable electronics own high extensibility,excellent bending and can meet the deformation requirements of equipment.Its broad application prospect makes it undertake an important mission in the reform of electronic industry.Combined with the excellent conductivity of metal and fluidity of liquid,room temperature Ga-based liquid alloys can meet the design requirements of flexible electronic devices for special functions such as the properties of mechanics and electricity.Therefore,room temperature Ga-based liquid alloys are selected as one of the preferred materials for designing the stretchable and wearable flexible electronics.During the service process,the interfacial instability of Ga-based liquid alloys interconnection electrode structure makes the reliability issues of flexible electronic devices increasingly prominent.In this work,the effects of different alloy compositions,temperature,temperature gradient,cooling rate,long-term room temperature aging,current density and Joule heat on these interfacial reactions at Ga-21.5In-10Sn/Cu interface were systematically studied.Based on the interfacial reaction kinetics,thermodynamics and atomic cluster nucleation theory,the microstructure evolution behavior of intermetallic compound（IMC）at the liquid-solid interface under heating and current stressing conditions and the competitive reaction mechanism between different metal atoms are discussed.According to its interfacial reaction characteristics and failure modes of Ga-21.5In-10Sn/Cu conductors under different conditions（including heating and the thermoelectric coupling service process）,different methods for constructing interfacial reaction barrier layers which can solve the instability problems of Ga-21.5In-10Sn/Cu interface structure were proposed.The feasibility of carbon materials,pure metals with high thermal stability and electrodeposited coatings as interfacial reaction barrier layer was systematically investigated,respectively.The main results are summarized as follows:（1）For the interfacial reaction at Ga-21.5In-10Sn/Cu interfaces,the existing competitive mechanism between different atomic pairs（Cu-Ga,Cu-In and Cu-Sn）and the interfacial reaction priority were revealed.The study on the liquid-solid interfacial reaction between four types Ga-based alloys（Ga,Ga-24.5In,Ga-30Sn,Ga-21.5In-10Sn）and Cu showed that although the alloy composition and content both had a significant effect on the chemical short-range orders in the liquid alloy,the alloy composition difference had no obvious influence on the liquid-solid interfacial reaction of Ga-based liquid alloy/Cu system where Ga atoms existed.The reaction products at Ga-based alloys/Cu liquid-solid interface are tetragonal Cu Ga₂which own the typical bricks-like morphology.The chemical reaction priority of Cu-Ga atom pairs during the interfacial reaction at Ga-based liquid alloys/Cu interface was confirmed by our experimental results and thermodynamic calculation results.It was found that the chemical reaction between Cu and Ga was the dominant factor causing the failure of Ga-based liquid alloys electronic devices in the practical service.（2）Changing the temperature can control the preferred orientation of intermetallic compounds Cu Ga₂crystals.When the diffusion direction of dissolved Cu atoms was roughly aligned with a axis or b axis,due to the open structure along a axis or b axis,the preferred orientation plane of{001}which was perpendicular to the substrate plane facilitated faster interstitial diffusion of Cu than along c axis direction.The effect of temperature on the crystallographic orientation of interfacial reaction products showed that when the interfacial reaction temperature was 140℃,the weak preferred orientation plane of Cu Ga₂was{001}which was perpendicular to the tested plane of Cu substrate and the preferred growth direction was<001>.When the interfacial reaction temperature was 220℃,the preferred orientation crystal plane of Cu Ga₂was{001}which was parallel to the tested plane of Cu substrate and the preferred growth direction was<001>.When the interfacial reaction temperature was310℃,the weak preferred orientation plane of Cu Ga₂was{001}which was parallel to the tested plane of Cu substrate and the preferred growth direction was<001>.Besides,the temperature gradient had no significant influence on the morphologies of as-formed intermetallic compound grains at Ga-21.5In-10Sn/Cu interface.（3）The obvious electric-induced dissolution behavior at cathode Cu surface during the current stressing service process was revealed.The study on the interfacial reaction behavior of Cu/Ga-21.5In-10Sn/Cu conductor under the current stressing service conditions suggested that the electric-induced dissolution behavior of cathode Cu surface and the interface reaction at the cathode and anode interface were greatly affected by current density.With the extension of current stressing time and the increase of current density,tetragonal Cu Ga₂will be formed on the Cu surface of both cathode and anode.The dissolved Cu atoms migrated from the cathode to the anode through Ga-21.5In-10Sn liquid alloy and then caused the interfacial reaction of the anode,which was confirmed by the experimental design under the thermoelectric coupling.According to the decoupling research concept,when the same force of electronic wind and electric field was ensured,it is proved that the single variable Joule heat had an important contribution to the electric-induced dissolution behavior at cathode interface.In addition,inspired by the concept of decoupling research,it is proved that the electric-induced dissolution behavior at cathode Ga-21.5In-10Sn/Cu liquid-solid interface was also affected by single variable current stressing.（4）W and Mo layer can significantly inhibit the interfacial reaction at Ga-21.5In-10Sn/Cu liquid-solid interface.In order to inhibit the liquid-solid interfacial reaction of Ga-21.5In-10Sn/Cu,it was necessary to confirm that whether the carbon materials and pure metal materials with good electrical properties can be employed as the interfacial reaction barrier at Ga-21.5In-10Sn/Cu interface or not,and the research results are as follows:the graphene film prepared by chemical vapor deposition technology could be used as the reaction barrier layer at Ga-21.5In-10Sn/Cu interface whose limiting maximum temperature is 350℃.Besides,the integrity and thickness of graphene films are the important factors to ensure the interfacial reaction barrier effect and electrical properties.Within the processing temperature and service temperature range of electronic devices（less than 230℃）,W and Mo showed excellent inhibition effects on interfacial reaction at Ga-21.5In-10Sn/Cu interfaces.At higher temperature（500℃）,W as the interfacial reaction barrier layer could still exhibit excellent chemical inertia at Ga-21.5In-10Sn/Cu interface,which is of great significance in the design of interfacial reaction barrier layer.（5）Ni-W alloy coating can be employed as the barrier coatings to inhibit the interfacial reaction at Ga-21.5In-10Sn/Cu interfaces even under the experimental tests with the high current density.Ni-W alloy coating was designed and successfully prepared using electro-deposition method.The effects of electrodeposition conditions on coating quality（composition,uniformity and thickness）were investigated.The inhibitory effect of Ni-W alloy coating on Ga-21.5In-10Sn/Cu interface reaction under thermal and thermoelectric coupling conditions was verified.The safe service temperature of Ni-W coating and Ni-W-graphene coating as a reaction barrier material for Ga-21.5In-10Sn/Cu liquid-solid interface was 360℃and 260℃,respectively.When the ambient temperature was 410℃,because the integrity of Ni-W and Ni-W-graphene coating was destroyed in a short time,both Ni-W and Ni-W-graphene coatings would lose its expected function of interfacial reaction barrier.It was worth noting that the intermetallic compounds at the liquid-solid interface on both anode and cathode Cu surfaces protected by Ni-W composited coatings were not found,even when the service temperature was 190℃and the current density was 550 A/cm².Therefore,the existence of Ni-W diffusion barrier layer could significantly inhibit the interfacial reaction of Ga-21.5In-10Sn/Cu liquid-solid interface.