| With the rapid development of electronic and information technology in recent years,the world has gradually entered a new era dominated by the Internet of Things and intelligent manufacturing.The integration degree of electronics is increasing while its geometry size is decreasing.Therefore,the characteristic size of the materials used in electronic devices at present is also decreasing to the micron or even the nanometer scale.The micro/nano-scaled thin metal films(such as Au,Cu alloy and Al)are important electrode materials and conductive interconnect materials in integrated circuits and flexible electronic devices due to their excellent conductive properties,and undertake the function of transmitting electrical signals among the discrete functional units of electronic devices.These thin metal film structures are often subjected to high temperatures or mechanical deformation in everyday use.For example,in large-scale integrated circuit,the metal film at micro/nanoscale is inevitably subjected to high current density and periodic thermal mismatch strain.In flexible electronic devices,it would also bear large and complex mechanical deformation such as cyclic bending and tensile straining.Under cyclic loading,the material will produce fatigue damage and form fatigue cracks,finally leading to the function degradation or even failure of the device.However,the fatigue damage behavior of these materials is different from that of the bulk materials due to the constraint effect of the geometric scale and the microstructure scale.At the same time,the change of material scale from micron to nanometer will also cause the change of its damage mechanism,resulting in a significant size effect.Clarifying the fatigue damage behavior and deformation mechanism of the micro/nanoscale materials is one of the key issues in the current material science field,and it also provides important theoretical guidance for the design and fabrication of electronic devices.In this work,flexible polyimide(PI)substrate constrained Au films with different thicknesses were prepared by magnetron sputtering.Furthermore,rigid Si substrate constrained Au lines with different thicknesses were prepared by electron beam evaporation and lithography.The film thickness is at the micron to the nanometer scale and the line widths are at the micron scale.The effects of the characteristic size of the films,applied loading conditions and the ultra-thin interlayer on the fatigue damage behavior of micro/nano Au thin films and their physical mechanisms were systematically studied by uniaxial tensile,dynamic bending fatigue and alternating current induced thermal fatigue experiments,combined with in-depth microstructure characterization.The main research results of this work are as follows:1.Nanocrystalline(NC)Au films with different thicknesses(40,90,170,930 nm)were subjected to the dynamic bending fatigue loading.The effects of material size,applied strain,and number of loading cycles on the fatigue damage behavior were detailedly investigated.When the film thicknessh?90 nm,the fatigue damage of Au films was mainly manifested as fatigue extrusions and short intergranular cracks.Furthermore,according to different applied strain amplitude and the number of loading cycles,the strain-cycle coordinate system was used to construct the damage transition maps of thin films for the first time.The damage behavior could be divided into three regimes:?-cracking along slip bands;?-a mixture of cracking along slip bands and grain boundaries;?-cracking along grain boundaries.When the film thickness decreases to 40 nm,the fatigue damage behavior of Au thin film only showed intergranular cracks(Regime ?).We proposed the parameters of cumulative irreversible strain by dislocation slip ?f,DScum or grain boundary sliding ?f,GBS,cum,to quantitatively characterize the critical conditions for describing the transition of the two kinds of damage behaviors and it is in good agreement with the damage transition maps.2.Combined with the quantitative characterization of fatigue extrusion height,the cross-section observation of fatigue damage and the characterization of vacancy defects by slow positron beam annihilation technique,it was found that the generation and diffusion of supersaturated vacancies in the fatigue process played an important role in the formation of micro/nano scale thin film fatigue extrusion.It was found that the generation and diffusion of oversaturated vacancies during the fatigue process might affect the fatigue life of the thin films.With the decrease of film thickness,due to the decrease of dislocation interaction ability and the intensification of vacancy diffusion,the delay of the accumulation of vacancies in the material and the saturation time inhibited the fatigue extrusion nucleation,and improved the damage tolerance of the material,thus prolonging the fatigue life of the material.Based on the vacncy model and the experimental results,we have successfully predicted the transision of the fatigue damage behavior from the bulk scale to the small scale.The vacancy assisted extrusion growth mechanism proposed in this paper shows that it would be reasonable to control the vacancy behavior by regulating the interface for the design of small-scale metals with good fatigue properties.3.The effects of the film-substrate interface on the fatigue damage behaviors of films with different thicknesses were investigated by adding an ultra-thin Ti interlayer(?5 nm)between the film and substrate.The results showed that the ultra-high cycle fatigue performance of 1 ?m thick Au films could be significantly improved by adding Ti interlayer,while the fatigue performance of 200 nm and 10 nm film was not improved.for?1?m thick films,the good fatigue damage resistance caused by the addition of Ti interlayer was mainly attributed to the effective suppression of intrusion-like voids at Au/Ti interface and further suppressed surface extrusion growth,thus reducing the tendency of cyclic strain localization and fatigue crack initiation.This finding strongly supports the vacancy-assisted extrusion growth mechanism proposed in this paper and provides a potential strategy for the design of flexible devices with extremely long fatigue life.4.The ultra-high cycle(108 cycles)fatigue damage behavior and related mechanisms of NC Au and Au/Ti films have been studied.Except for the fatigue extrusions and intergranular cracks that were commonly observed in the thin films,hillocks as a new kind of damage behavior formed in all films,which may be accompanied by the formation of holes.The hillock and hole size showed a strong film thickness dependence,and the density of the hillock and hole had a strong dependence on the film thickness and applied strain.The thicker film had a larger hillock size but a lower density.In addition,the size and density of the hillocks formed in Au/Ti films were slightly lower than those in Au films.The analysis showed that the surface diffusion,grain boundary diffusion,interface diffusion and the applied strain amplitude gradient promoted the formation of hillocks,in which the surface diffusion and grain boundary diffusion were the main factors affecting the hillocks formation.5.Thermal cyclic loading of NCAu lines with different thicknesses(50,100,200 nm)and line width(5,10,15 ?m)was carried out by applying an alternating current to study their thermal fatigue behaviors.It is found that the thermal fatigue life shows a strong line-thickness effect,that is,the thinner the line thickness is,the higher the thermal strain as well as the temperature oscillation can be borne.However,no significant line-width effect was found.In addition,the damage behavior of thermal fatigue was obviously dependent on the line thickness and the range of applied strain.Based on the experimental observation and theoretical analysis,the thermal fatigue mechanism diagram controlled by applied strain and line thickness was proposed.Atomic diffusion and dislocation slip were the two important mechanisms in thermal fatigue,and atomic diffusion is the main factor controlling fatigue failure. |
PDF Full Text Request