Study On Thethermal Stability Of Nanostructured Metallic Multilayers

Cu-based Nanostructured multilayers have been widely used in the fields of microelectronics and micromachine due to their extraordinary electrical conductivity and great electromigration resistance.Many microelectronic devices serve under adverse working environment;one of the most typical problems is high temperature.Nanostructured metallic multilayers are far from the thermodynamic equilibrium state so that the change of temperature are likely to cause the change of microstructure.The phenomenon such as grain growth,interface instability and layered structure damage that will lead to degradation or failure of material and reduce the service life of the multilayers.Therefore,the poor thermal stability of nanostructured metallic multilayers limit their application in the field of advantage performance devices.The stability of microstructure and properties of nanostructured metallic multilayers following elevated temperature exposure is very significant if their unusual properties exploited in advanced engineering applications.In the dissertation,a series of Cu/X nanistructured multilayers with special structure are designed to meet the needs in the field of microelectronics.The fcc/fcc system of Cu/Ag multilayers,the fcc/hcp system of Cu/Ru multilayers and fcc/bcc system of Cu/Mo(V)multilayers multilayers were prepared by dc magnetron sputtering on Si substrates.The length scale effect on the thermal stability of thier microstructure,mechanical properties and electric properties were investigated.The relationship between microstructure and stability,especially for the detailed structure of the heterogeneous interfaces were discussed,and the intrinsic mechanism of thermal stability in multilayers was explored.The research offers some scientific references not only for practical application of nanostructured Cu/X multilayers,but also for the future design and manufacture novel thermal stable nanostructured-functional films.The main conclusions are as follows:(1)The thermal stability of Cu/Ag multilayers exhibits strong length scale dependence.The annealing induced grain growth and heterogeneous interface evolution of Cu/Ag multilayers with individual layer thickness(h)varying from 2.5 to 50 nm were investigated by transmission electron microscopy.For samples with h<20 nm,the heterogeneous interfaces completely disappear when the annealing temperature exceeds 200?.However,the temperature for stable layered structure can reach 300? as the h>20 nm,where the interfaces remain remarkably intact.In-situ TEM observation confirmed that grooves formed in both layers simultaneity due to the epitaxial interface structures,which is different from the conventional microstructural evolution.There is a competition between the effect of grain boundary diffusion and heterogeneous interface resistance on grain growth.Eventually,the equilibrium groove angles are achieved to pin the grain boundaries and resist grain growth.The formation of the low angle columnar grain and coherent twin boundaries increase the thermal stability due to the low interface energy.Moreover,the formation of annealing twins in multilayer also significantly improve the microstructural stability.(2)The growth of Cu grains slowed with Cu layer thickness decreasing in Cu/Ru multilayers,illustrating an unusual size dependence,that is,"smaller is more stable." The thermal stability of nanoscaled Cu/Ru multilayers with different modulation ratio(a fixed Ru layer thickness of 3 nm and varied Cu layer thickness(h)from 5 nm to 200 nm)were investigated by TEM analyses and nanoindentation tests.All of the Cu/Ru multilayers retained the layered structures up to 400? annealing.Especially the average grain size was maintained as small as 25 nm for the Cu/Ru multilayers with h=10 nm and the hardness only decreased by 6.5%after 400? annealing,which exhibits an excellent stability.The enhanced thermal stability of Cu/Ru multilayers originates from the textured columnar grains and semicoherent heterogeneous interfaces,which have a low interface energy.The amount of textured columnar grains and semicoherent heterogeneous interfaces increase with the Cu layer thickness decreasing is responsible for the enhanced thermal stability of Cu/Ru multilayers.The Cu/Ru multilayers with h=100 nm achived high strength and high electrical conductivity with enhanced thermal stability.Grain growth make a greatly reduction of the amount of high angle grain boundaries and hence reduce the electron scattering at the grain boundaries,which contributes to the conductivity.Through in situ heating TEM,detwinning-mediated grain growth was observed in the Cu/Ru multilayers and a schematic of the microstructural evolution of Cu/Ru multilayers were presented.(3)The length scale effects on the interfaces is the key factor that influence the thermal stability of of Cu/Mo(V)multiyaers.The thermal stability of nanoscaled Cu/Mo(V)multilayers with different modulation ratio(a fixed Mo(V)layer thickness of 3 nm and varied Cu layer thickness from 5 nm to 100 nm)were investigated by TEM analyses and nanoindentation tests.The room temperature hardness of Cu/Mo multilayers increases with the decreasing of h and then decreased at the scale of 10 nm,which is because the cryatal strtuctures are transformed in the order from polycrystalline to texture and then to superlattice.While the hardness of Cu/V multilayers increases with the decreasing h without softing phenomenon,and the coherency stess strengthening contributes to the high hardness at small scales.After annealing treatment,all of the Cu/Mo(V)multilayers retained the layered structures and with no obvious grain growth up to 400?,illustrating Cu-Mo and Cu-V interfaces contribute to increasing thermal stability.The hardness of Cu/Mo multilayers decreased with annealing temperature rising due to the crystallization of amorphous phases during the annealing process.The hardness of CuV multilayers with h=5 nm increased to 5.34 GPa result from the intermixing induced by annealing.The differences in thermal stability between Cu/Mo and Cu/V multilayers origins from the different interface structures at small scales.