Research On Preparation And Properties Of Tantalum Oxide High-K Films

Upgrade of global IC manufacture technology is quickening pace from the 21st century. Transistor's size will pro rata reduce with the advancement of integration degree.As a result, transistor's channel length will go to mini and the gate thickness become thin.SiO₂,as gate dielectric material,had very approach the threshold thickness which can hold its energy band integrity.So researchers must find out new materials to replace SiO₂ as the gate dielectric to break down the barriers and keep Moore's Law rolling forward.High-K material,A material that can replace silicon dioxide as a gate dielectric perfectly.Ta₂O₅ was deemed to one replacement of SiO₂,because it has appropriate dielectric constant,lower leakage current density,high break out voltage,and resolve compatibility issues with the traditional IC technology.In this paper,tantalum oxide films were deposited on p(100)Si by dc magnetron reactive sputtering of Ta target in an Ar/O₂ mixture atmosphere.The influence of Ar/O₂ on the grown rate,crystal structure,chemistry composition and optical,dielectric and electrical properties of tantalum oxide films has been studied as mostly content.Results of experiment as following:Grown rate was influenced by Ar/O₂ gas ratio.It can be seen that more Ar in Ar and O₂ mixture ambience is favorable to the film crystallization and films growth from XRD of films. XRD shown that annealing temperature has a great affect to tantalum oxide film's microstructure.Films as sputtered and annealed at lower temperature(400℃)are all amorphous.Annealing temperature increased to 900℃,then XRD of films shownβ-Ta₂O₅ (001)and(200)peak obviously.So,when Ar/O₂ gas ratio increased,Crystal property also became better.And after annealed,β-Ta₂O₅(001)and(200)peak intensity increased,quality percentage of Ta₂O₅ in films increased too.The ratio of the oxygen to tantalum content in the films was studied by XPS analysis. The atomic ratio of O/Ta are calculated to be 2.71,2.70,2.60 and 2.50 for films deposited at Ar/O₂ flow ratio of 1:1,2:1,3:1,4:1,respectively.So for the films deposited at Ar/O₂=4:1, the atomic ratio of O/Ta accorded with the stoichiometric ratio of 2.5 for Ta₂O₅ perfectly.The results indicate that the proper Ar in Ar and O₂ mixture ambience is favorable to format Ta₂O₅ with stoichiometric ratio.Spectroscopic ellipsometry results showed that remarkably good fits are obtained in the whole wavelength range.The refractive index n and extinction coefficient k values as a function of photon wavelength were first determined and reported.The refractive index n value in our sample is about 2.11 and 2.06 in wavelength range 500-1000nm,which is very close to the value of bulk Ta₂O₅ of 2.2.The variation of k value is not significant at long wavelength range.The optical transmittance of the films higher than 80%in the visible region of the spectrum is obtained for most of samples.According to formula E=hc/λ,the optical band gap of the films are calculated to be 4.44eV,4.47eV,4.68eV,4.81eV,4.97eV for films deposited at Ar/O₂ flow ratio of 2:1,1:1,1:2,1:3,1:4,respectively.Dielectric spectrum shown that dielectric properties of the tantalum oxide films depended strongly on the frequency.The dielectric constant and loss tangent are quickly decreasing with the increasing of frequency at lower frequency,capacitance and loss tangent will not change with frequency at higher frequencies.Leakage current in ultra-thin insulating films affect strongly the measurement of the capacitance.It has been shown that in the case of metal-Ta₂O₅-metal structures the frequency dependence of the capacitance can be satisfactorily represented by an equivalent circuit model with parallel resistance only,or by an equivalent circuit model with serial resistance Rs only to the capacitor formed with the dielectric film between the electrodes.But dielectric constant increased with accretion of Ar/O₂ gas ratio.The max dielectric constant is 54.6 in all sputtered films.I-V test shown that Ar/O₂ gas ratio have great influence to leakage current mechanism of sputtered films.When Ar is more,tantalum oxide film has large breakdown voltage and low leakage current. Defining the voltage corresponding to leakage current density of 1×10^-5A/cm² as breakdown voltage V_B,then the max breakdown field strength E_B is 0.054MV/cm at Ar:O₂=4:1.