Study On The Preparation、Structure And Properties Of Ca-based Dielectric Thin Films

Ca-based dielectric materials can be widely used as resonator,oscillator,and key electronic components for wireless communication and military systems due to moderate dielectric constant（?）and very low dielectric loss（tan?）.In this dissertation,the fabrication of Ca(Mg_1/3Ta_2/3)O₃（CMT）,Ca(Mg_1/3Nb_2/3)O₃（CMN）and CaTiO₃（CT）precursor by Pechini method were investigted,and CMT,CT,CMT/CT and CMN/CT dielectric thin films were prepared by spin coating.Conditions optimation and dielectric properties of dielectric thin films on Pt/Ti/SiO₂/Si substrates were investigated.The effect of heat-treatment temperature,heat-treatment time and annealing temperature on microstructure and dielectric properties of CMT thin films were researched.The characterization of the mechanical properties of the CMT films/substrate composite systems annealed at different temperatures were conducted by nanoindentation measurements.The intrinsic mechanical properties of CMT and CT thin films were obtained by 10%rule and extracted by ZP and HC models.Also,the residual-stress were also estimated from the true mechanical properties of thin films.With delicate controlling of CMT/CT and CMN/CT thin films,the relations among material compositions,structure and properties was investigated.The main contents in this paper are as follows:Ca-based dielectric thin films precursors were investigated,and the pH value is very important for the preparation of thin films.By adjusting pH,citric acid（CA）combines with metal ions well.The Ca-based thin films precursor solutions are finally formed through the combination of citrate complexes with NH₄⁺acting as crosslinks.As a result of the formation of a stable polymer network,the homogeneity of metal ions in the network is improved.With the loss of free water,the sol transforms into the gel by heating.The thermal stability of the polymeric gel is also improved in the pre-sintering process,which avoids the early precipitation of partial metal ions from the gel.The growth model of films transformed from layered into island.Through the optimization of preparation conditions,it discovered that all the CMT films annealed from 650^oC to 800^oC show a single perovskite phase and the crystallization increases with increasing the annealed temperature but a secondary phase is observed in the film annealed at 900 ^o C.Further characterizations of the CMT dielectric properties reveal that the dielectric constants were increased for the samples annealed at higher temperature.The CMT thin films annealed at 800^oC exhibited the optimal dielectric properties.The optimum fabrication conditions for CMT and CT thin films are 180^oC/120s-380^oC/120s-500^oC/330s-700^oC/3600s and120^oC/120s--700^oC/270s--700^oC/1800s,respectively.It was found that the elastic modulus and hardness of CT with different thickness and CMT annealed at different temperatures film/substrate composite systems exhibited a strong size effect.The elastic modulus and hardness of composite systems increases with decreasing the normalized depth（h/t）in the small penetration depth at initial stage.For the CMT film/substrate composite systems,with the increase of annealing temperature for CMT films,the mechanical properties were improved.The recovery resistance R_s first decreases and then increases,elastic recovery energy first increases and then decreases with increasing annealing temperature.The CMT composite systems annealed at 700 ^oC exhibits minimum recovery resistance and maximum elastic recovery about 0.21.With increasing the thickness,the mechanical properties of CT thin films was improved.It was noticed that the intrinsic hardness increases from 15.6 GPa to 21.73 GPa,the intrinsic elastic modulus increases from 200 GPa to 228 GPa and residual-stress increases from 1.67GPa to 2.32 GPa when the CT thickness increases from 65 nm to 340 nm.By comparing the hardness of CT and CMT thin films with substrate,CT film/substrate composite system is hard film/soft substrate,and CMT film/substrate composite system is soft film/hard substrate system,respectively.The two systems are accord with reverse Hall-petch relationship in Hall-petch theory.By constructing the CMT/CT and CMN/CT heterogeneous thin films,the effects of stacking layer sequence,thickness,combination ratio and interface numbers on the structures and dielectric properties of the thin films were investigated.XRD results showed that the hetero-layered CMT/CT films with CT film as the first layer have single perovskite phase,but a secondary phase was discovered with CMT film as the first layer.With the increasing thickness and heterogeneous interface numbers,the dielectric properties of heterogeneous thin films can be improved.The dielectric properties can be effectively tuned by varying the combination ratio.The dielectric properties of heterogeneous thin films have better dielectric properties than that of single component.Through the series circuit model,a relationship among films thickness,period numbers and dielectric properties is also established.A coefficient related to residual stress was proposed and the relationship of hardness and dielectric constant can be derived based on the relationship among residual stress,hardness and dielectric properties,indicating the significant effect of residual stress direction on polarization and dielectric properties.The greater the interface numbers and the smaller the single-layer thickness,then the greater the percentage of residual stress with same thickness.It promotes the strain-induced polarization and henceforth enhancing the dielectric constant of heterogeneous CMT/CT and CMN/CT films.With increasing the thickness of Case-based films,the hardness of composite system gradually increases and dielectric properties are enhanced due to the stress joint action.The dielectric properties can also be effectively tuned by varying the single-layer thickness,total thickness and hence controlling the combination ratios of the thin films.