Study On The Structure,Dieletric And Ferroelectric Properties Of Yttrium Doped Hafnium Oxide Films

In recent years,as one of the most typical high-k dielectrics,hafnium oxide based thin films have been widely studied in microelectronics industry.Recently,doped-Hf02 thin films prepared by Atomic Layer Deposition(ALD)technique are discovered to exhibit pronounced ferroelectricity.Due to its superior compatibility to silicon-CMOS integration technology,this novel ferroelectric material might lead to a significant breakthrough in development of integrated ferroelectric semiconductor devices such as non-volatile ferroelectric memories.Research shows that the dielectric properties of HfO2 based films are mainly dependent on phase structure.In order to control the electric properties of films,the phase stabilization mechanism of HfO2 should be investigated.At present,the research of ascertaining the doping content,film thickness and deposition temperature is mainly relying on experiments,which will lengthen research development cycle time and increase cost.Furthermore,processing method for high-quality HfO2 based ferroelectric films is usually ALD methods.Becaue of the high cost and technical difficulty for scale-up production the ALD is not competitive as compated with sputtering.So the use of sputtering processes for the fabrication of the high-quality HfO2 based ferroelectric films are worth to be investigated systematically.According to these ideas,a series of HfO2 based thin films are prepared by twin-target mid-frequency magnetron sputtering method.The effects of reaction gas ratio,deposition pressure and substrate temperature on the structure,surface morphology and dielectric performance are systematically investigated.The phase transition mechanism of the HfO2 based thin films is discussed in depth.A new method of using Hf buffer layer to improve electrical properties of HfO2 film is presented.Main conclusions are listed as follows:(1)HfO2 films with very low leakage current density and smooth and dense surface morphology have been deposited on Si(100)substrates by facing-target mid-frequency magnetron sputtering under oxygen atmosphere.The thickness of films could be controlled from a few to hundreds of nanometers.The influence of process parameters on microstructure and properties is investigated in detail.Results indicate that the facing-target mid-frequency magnetron sputtering system provides effective approach to eliminate target poisoning phenomenon.The concentration level of oxygen vacancy reduced and the films tend to be smoother,the root-mean-square roughness reduces from 0.876 nm to 0.333 nm with the increase of O/Ar gas flow ratio.The dominant conduction mechanism of the as-deposited HfO2 film is Ohmic type conduction at low electric field and trap-assisted tunneling at high electric field,respectively.The leakage current density decreases from 1.0 × 10-3 to 1.2 × 10-6 A/cm2 as the O/Ar gas flow ratio increases from 1/4 to 1/1.The decreasing deposition pressure causes the reduction of deposition rate and densification of film structure.(2)The Y-doped HfO2 thin films have been prepared by reaction magnetron sputtering deposition with Y content ranging from 0 mol%to 9.2 mol%.Y is incorporated into the HfO2 layers by simultaneously sputtering from Y and Hf metal targets.The effects of Y content on the composition,valence state,crystallographic structure,and dielectric properties of HfO2 film are investigated.Results indicate that the sample is completely oxidized while the oxygen deficiency emerges after the stripping depths have increased and the interfacial layer is mainly Hf silicate.A pure cubic phase is stabilized for Y content about 7.4 mol%-9.2 mol%.We introduce surface energy to correct the classical thermodynamic conditions of phase transition and establish thickness-composition and thickness-temperature phase diagram of Y doped HfO2 based thin film.Theoretical results agree well with the experimental results,confirming that this theory is correct.The permittivity and leakage current density are found to be strongly dependent on the yttrium content.The best combination of low leakage current of 1×10-7 A/cm2 at 1 V and a highest permittivity value of 29 is achieved when the doping ratio of Y increases to 9.2 mol%.(3)10 nm-thick ferroelectric(FE)Hf02 films with 1.5 mol%yttrium-doping are fabricated by mid-frequency reactive magnetron co-sputtering deposition on bare Si and underlying metal Hf.The effects of interfacial layer on structural and electrical properties of Y:HfO2 films have been systemically studied.Results indicate that we effectively altered the interface layer thickness form 2.5 nm to 1.3 nm by deposit Hf buffer layer,and the films mainly consist of non-centrosymmetric orthorhombic phase which is responsible for ferroelectricity in HfO2 film.It is demonstrated by experimental analysis and theoretical calculation that the interfacial layer has crucial effect on the electrical characteristics of MIS capacitor.As the thickness of layer increases,the effective electric field applied to the ferroelectric Y:HfO2 film is decreased and the hysteresis behavior can be overshadowed in P-V measurement.The crystal structure as well as electrical properties of sputtering deposited Y-doped Hf02 thin films are investigated for different deposition temperature.It can be observed that an enhanced suppression of ferroelectric phase(orthorhombic phase)fraction in favor of the paraelectric phase(monoclinic phase)with increasing deposition temperature.With deposition temperature increases to 200? the film shows excellent ferroelectricity with a large remnant polarization(Pr?20?C/cm2)and low leakage current density(about 10-6 A/cm2 at 3 MV/cm).