Study On Preparation And Dielectric Properties Of MOS Devices With Novel Rare Earth-based High K Films

With the continuous development of integrated circuits,the size of field effect transistors(FETs),the most basic unit,is shrinking.In this process,the biggest challenge is that the thickness of the gate dielectric layer is getting thinner and thinner,causing unacceptable leakage current and energy loss.Therefore,high k materials have been introduced to replace the traditional gate dielectric material SiO2.At present,the high k material used in industrialization is HfO2,but its dielectric constant is relatively low(?20)and crystallization temperature is low(<500?).It can only meet the needs of integrated circuit development in the next few years.Therefore,for the long-term development of integrated circuits,it is necessary to develop new high k gate dielectric materials.Among many high k material candidates,rare earth oxides stand out due to their high dielectric constant,large band gaps and excellent thermal stability.However,the dielectric constant of pure binary rare earth oxides is comparable to that of HfO2.Therefore,reasonable structural design is key to improving dielectric properties of rare earth oxides.In addition,dielectric constant and band gaps are often inversely proportional,that is,it is difficult to achieve uniformity of high dielectric constant and large band gap at one material,which is a difficulty in designing the gate dielectric material.In our work,a series of rare earth oxide thin films were prepared by magnetron sputtering.The dielectric constant and band gaps were adjusted by doping or lamination to meet the requirements of the gate dielectric materials.The main research includes:(1)The effects of sputtering atmosphere(argon-to-oxygen ratio),metal electrode type and target purity on the microstructure and dielectric properties of binary rare earth oxide thin films were studied in detail.The results show that the optimal Ar:O2 is(30?35):(5?10)sccm and the optimal metal electrode is Pt.In addition,the higher the target purity(>3N),the better the dielectric properties of rare earth oxide thin films,which provides data support for the subsequent preparation of rare earth-based high k films.(2)Two kinds of ternary rare earth oxides GdYOx and GdAlOx were prepared by magnetron co-sputtering.The comprehensive properties of these two rare earth oxides were found to be superior to pure Gd2O3,showing that the dielectric constants of both are more than 20,the band gaps are greater than 5 eV and the leakage currents meet the requirements of CMOS devices.Besides,by adjusting the annealing temperature,the thermal stability of two ternary rare earth oxides was studied in detail.The optimum annealing temperature range is 500?600?.Besides,the mechanism of annealing temperature effect is analyzed.(3)A series of Ta and Ti doped Gd2O3 films and a series of Fe and Ni doped La2O3 films were prepared.The microstructure,band gaps and dielectric properties of these metal-doped rare earth oxide films were systematically studied.The results show that the dielectric properties of Gd2O3 doped with trace(?1%)Ta or(?3%)Ti are significantly improved.The dielectric constant of Ta-doped Gd2O3 can reach 21.2 and the band gap is increased to 5.45 eV.The dielectric constant of Ti-doped Gd2O3 can reach 23.9 and the leakage current density is as low as 9.6×10-4 A/cm2.However,incorporation of trace Fe(0.94%)can seriously damage the dielectric properties of La2O3 films.It is a harmful impurity and should be removed in the process of rare earth metal purification.The proper incorporation of Ni(?10.04%)is beneficial to improve the dielectric properties of La2O3 film(k=22.08).Appropriate Ni doping changes the growth mode of the film,reduces the roughness of the film and has a suitable band gap(5.7 eV)and band offset structure.(4)After a reasonable lamination of an oxide with a large dielectric constant and another oxide with a large band gap,a breakthrough in dielectric properties was achieved.As for La2O3-ZrO2 bilayer film,our result indicated that Pt/La2O3-ZrO2/Si MOS capacitors have better performance than Pt/ZrO2-La2O3/Si MOS capacitors,featuring for larger dielectric constant of 12.37,lower leakage current density of 3.5×10-4 A/cm2.Schottky emission is identified as the main transport mechanism for leakage current.And the lower leakage current density for ZrO2/Si stack can be attributed to the larger band gap(6.4 eV)and larger band gap offsets(VB=2.85 eV,CB=2.43 eV)of ZrO2 films,which can offer a higher Schottky barrier height between Si substrate than La2O3 films.As for TiO2-Y2O3 bilayer film,by introducing Y2O3 as a passivation layer and compounding with TiO2 film,a novel composite structure was developed,which successfully achieved the advantage of TiO2 exerting its high dielectric constant and avoiding the short plate with small band gap.The stacking sequence of Y2O3 and TiO2 films and the thickness ratio of the sublayers were optimized,and the dielectric constant of TiO2-La2O3 bilayer film was as high as 28.24,which is more than 1.4 times that of the commercial high k material-HfO2.And the method was broadened to other rare earth oxide films,which has certain universality and provided ideas for the development and design of novel gate dielectrics.