Reserch On Failure Behaviors And Mechanisms Of Electrical Contact Films In Simulated Space Environments

Electrical contact materials and devices with nanometer or micrometer scale are used as an electrical interconnect or contact electrode in many applications for microelectronics, protection coatings and microelectromechanical systems. The electrical contact films are mainly composed of metal films, such as gold, silver, copper, nickel, etc, which have good electrical property and chemical inertness. Bilayer Au-Cu electrical contact film, as a basic material system, has comprehensive applications in the electrical system of space and aviation. This electrical contact films and devices work in the low earth orbit （LEO） environments and suffer from the space environments attack. Meanwhile, the size of contact devices decreases with the increasing severity of service conditions, for example, increasing current densities and therefore higher local temperatures. Thus the thin film and multilayered system are subject to the risk of undergoing severe interdiffusion phenomena, which will result in the materials’failure and affect reliability of the devices even the whole system. Therefore, analysis and evaluation of failure behaviors and mechanisms of the electrical contact in service environments, have great significance in improving and developing electrical contact film system.The complicated multi-factor space environments in which the electrical contact films were applied were selected as objective environments in this thesis. The evolution rule of failure behaviors of Au/Cu bilayer films in simulated environmental condition and simulated direct current condition was characterized and analyzed by using Auger electron spectroscopy （AES）, X-ray photoelectron spectroscopy （XPS）, high resolution transmission electron microscopy （HRTEM） and atomic force microscope （AFM）. Surrounding the relationship between the structural evolution and properties evolution, the microstructural evolution and formation process of structural defects in the simulated space environments were studied systematically. Based on the interfacial thermodynamics, the failure model and mechanisms of Au/Cu thin films in single factor and coupling factor environments were investigated. The main contents and conclusions are as follows:（1） The space environments simulators were designed and constructed. One is a small environments simulator that could partly simulate the low earth orbit environment. The device is applied to failure treatment of film sample in the simulated space environments which included thermal factor, ultraviolet （UV） radiation, atmosphere factor, direct current （DC） and related coupling factors. And the other is an in-situ simulator for environments failure of electron component. The device is used to surface temperature measurement and in-situ analysis for film sample in the simulated space environments which mainly included thermal factor, DC and related coupling factors.（2） The evolution processes of surface and interface structure in Au/Cu films upon working temperature and coupled with other typical factors in the simulated space environments were investigated. The defects created on the film surface by the raising temperature provided more passageways for the diffusion of Cu atoms to the surface layer, and resulted in the changes in the surface structure and chemical components. Based on the interfacial thermodynamics, the intermetallic compounds （IMCs）<AuCu> phase firstly initiated formed in the Au grain boundaries. The stress gradient produced by the formation of <AuCu> compound at the Au layer, which would lead to the diffusion of Cu atoms to the free surface through the Au grain boundaries.（3） The sample temperature of Au/Cu film was changed from room temperature to 44℃ under UV radiation in a vacuum within 120 minutes, and then remained stable with treatment time increased. Meanwhile, the calculation results show that the concentration gradient of Cu atoms at Au/Cu interface fell to 2.24 in 360 minutes from 3.45 at the beginning. The increase of defects in the grain boundaries of the Au layer was induced by UV radiation, because Cu element had a smaller work function relative to Au element and it was more likely to migrate to the surface layer through the grain boundaries of the Au layer.（4） The defects formation and nanoscale interfacial evolution at the Au/Cu and Cu/Si interface under DC in a vacuum UV environments were investigated. The increase of defects at the heterointerface and in the surface layer was induced by the coupling effect, which could provide more channels for the removal of atoms. The directed migration of atomic clusters in the films was caused by the effect of DC, which also aggravated the defects’expansion and led to the formation of Au-Cu intermetallic compounds. In addition, the voids formed at the interface between the Au/Cu films and the Si substrates were found to be mainly related to the generation of IMC<Au2Cu3> phase.（5） The structural evolution of Au-Cu IMCs and formation process of structural defects under DC in a vacuum were studied systematically. The directed migration and diffusion of Cu atoms to the surface layer in Au/Cu films drived by electron wind force, which resulted in the increase of interface energies upon the formation of compound phase from the corresponding solid solution phase. The microstructural evolution rate of the Au-Cu IMCs was improved by the intensifying effect of electron wind force, which could ultimately lead to the nucleation of voids. During the late stage of the effect of electron wind force, the voids growth and the thickening of Cu₂O line were connected with the directed migration of interstitial oxygen atoms in the Cu layer（6） The formation and growth processes of the structural defects at the film-substrate interfaces under DC in a vacuum were investigated. The generate of amorphous Cu₂O lines and voids in the Cu layer were caused by the electron wind force under DC, and it would be expanded into the Cu/Si substrate. When the defects migrated to the film-substrate interfaces, the Cu-Si-O bond would be found in the amorphous structure of SiO2 layer. Following further driving of the electron wind force, the Cu₂O oxides and voids would ultimately exist alongside the film-substrate interfaces as a stable state.From the point of view of material defects, the failure behaviors and mechanisms of electrical contact films in specific environments were discussed in this thesis. And the conclusions and methods have significant academic value for research of failure analysis and reliability of electronic devices.