Presentation on theme: "HfO2 thin films prepared by sol-gel method"— Presentation transcript:
1HfO2 thin films prepared by sol-gel method A.Barău1, M.Gartner1, M.Anastasescu1, V.S.Teodorescu2, M.G.Blanchin3, J.Tardy4 and M.Zaharescu11Institute of Physical Chemistry "Ilie Murgulescu" - Roumanian Academy202 Splaiul Independentei, Bucharest, ROUMANIA2National Institute of Material Physics, 105 bis Atomistilor Street,Bucharest-Măgurele, ROUMANIA3Universite Claude Bernard Lyon 1, 43 Boulevard du 11 Novembre 1918,69622 Villeurbane CEDEX-FRANCE4Ecole Centrale de Lyon , LEOM , 36 avenue Guy de Collongue,69134 Ecully, FRANCE_____________________________________________________________________________________________
2Objectives The preparation of HfO2 thin films by sol-gel method The establishing the correlation between the way of preparation and optical and structural properties of these materials.
3Why the HfO2 ? HfO2 properties: High thermal and chemical stability High thermodinamic stability in contact with siliconHigh refractive index (~ 2.00)Large band gap (5.86 eV)High dielectric constant (K ≈ 15-50)High density (9.86 g/cm2)Stable structure – SGP- (14) monoclinic : symmetry P121/c1(a = nm, b= nm, c= nm , b = 99,2)
4Why the HfO2 ? Possible applications: in micro and optoelectronics: - material for replacing SiO2 in metal/oxide/semiconductor (MOS) devices- optical coatings when high optical damage thresholds are needed- waveguide fabricationas material for nanofiltration membranes andfilms with high pencil hardness (over 9H) and hydophobicity
5Why the HfO2 ? Methods of film preparation (literature): Sputtering (Kang et al – 2000, Lee et al – 2000)Chemical vapor deposition - thermal (Balog et al – Lee et al – 2000)- plasma enhenced (Choi et al – 2002)- UV photo induced (Fang et al – 2004)Pulsed layer deposition (Esang et al – 2004)Atomic layer deposition (Zhang and Solanski – 2001, Ferari et al , Boher et al – 2004, Aarik et al – 2004)
6Why the HfO2 ? Methods of film preparation (literature): Sol-gel methods:- starting with HfCl4 in ethanol (Nishide et al – 2000,Shimada et al – 2002, Yu et al – 2003)- starting with HfCl4 in 1-methoxy-2 propanol (Blanc et al – 2000)- starting with HfCl4 in water, via hafnia hydroxide formation and peptization with formic/oxalic acid (Takahashi and Nishide – 2004, Nishide et al – 2005)- starting with HfOCl2 in ethanol (Gonçalves et al – 2004)- starting with Hf(OC2H5)4 and Acac (Villanueva-Ibanez et al – 2003)
7Experimental: Film preparation • The reagents: - hafnium ethoxide Hf(OC2H5)4 (Alfa Aesar) as HfO2 source,- acetyl acetone AcAc (Fluka) as stabilisator and- absolute alcohol p.a. (Merck) as solvent.Molar ratio: Hf(OC2H5)4/Acac = 1.• Solution preparation: mixing of the reagents in N2 atmosphere at 1000C for two hours.Synthesis were also performed starting with Hf-acetyl-acetonate or Hf-chloride, that allows working in ambinet atmosphere.
8Experimental: Film preparation • Film deposition: - substrates: silicon wafer;- deposition method: - dip-coating (5-8 cm/min withdraw speed),- spinning (5000 rpm)Before deposition the native SiO2 was removed in HF• Film densification:- 10 min at 100C and 30 min at 450o or 600oC with a heating rate of 1C/min.- For the multi-layered films, the same thermal treatment was applied, after each deposition
9Experimental: Films characterization Spectroellipsometric (SE) measurements in the nm spectral rangeTEM (Topcon 00B and a Jeol 200 CX electron microscopes working at 200kV)AFM (MultiMode SPM equipment - Instrument Veeco Metrology Group)RBS (4+He:E = 1.5 MeV)Preliminary electrical measurements were performed.
10Results obtained Spectroellipsometry Spectroellipsometric results on samples obtained by dip-coating, thermally treated at 450oCSamplesNumber of layersThermal treatmentd (A)HfO2 (%)Voids (%)ErrorF1HF1non42854.2045.80F1THF14256.0044.00F2THF222866.0133.90F3THF331973.0326.97the refractive indexes (n), the thickness of the samples (d) and the volume fractions of film components were obtained from the best fit of the SE experimental data with a multilayer and multicomponent Bruggemann-EMA model►The thickness of one layer deposition by spinning was 200 Å
11Results obtained Spectroellipsometry (a) (b) The thickness (a) and refractive indexes (n) of the samples with 1-3 layers (b) from spectroellipsometric results►by multilayer deposition the thickness of the films increases linearly►due to the densification by the repetitive thermal treatments the refractive index of the film increases
12Dip coated film – one layer dried Results obtainedAtomic Force MicroscopyVery low RMS roughness between 0.7 and 1.5 nmVery small surface roughness(~1 and ~1.5 nm)Dip coated film – one layer dried
13Spin coated films – one layer Results obtainedAtomic Force Microscopydrieddriedannealed450oCAnnealed450oCMaximum profile roughnessup to 10 nmLarge surface roughnessSpin coated films – one layer
14Results obtained Rutherford Backscattering spectrometry Dip coated filmsSpin coated films► No deformation of the Hf and Si peaks► Dissymmetry and Deformation of Hf and Si peaks:
15Results obtained Transmission Electron Microscopy Plan view TEM image and SAED pattern of the HfO2 film dried at 100oC and then annealed at 150oC (to be stable in the microscope)Amorphous structure with an non-uniform density in the nanometric scale
16Results obtained Transmission Electron Microscopy Plan view TEM image and SAED pattern of the HfO2 film annealed at 450oCThe structure is still amorphous with a beginning of crystallization
17Plan view HRTEM image of the HfO2 film annealed at 600oC Results obtainedTransmission Electron MicroscopyPlan view HRTEM image of the HfO2 film annealed at 600oCThe crystallization of the monoclinic HfO2 is observed.The crystallites are like a sponge.Pores with an average dimension of about 4.6 nm are observed.
18Results obtained Transmission Electron Microscopy Thermally treated at 450oC filmThermally treated at 6000CHigh resolution XTEM image of the cross section of the HfO2 films deposited by dip-coating
19Results obtained Electrical Properties I-V curves variation and mobility for the HfO2 sol-gel films thermally treated at 450oCLow operation voltageAlmost no hysteresisLow threshold voltageGood mobility
20Results obtained Electrical Properties The low operation voltage was assigned to the very thin dielectric filmImproved stability was correlated to the porous nature of HfO2 with air inclusionThe extremely low threshold voltage (VT ~ -0.4V) and high mobility arerelated to the very smooth surface of the film
21ConclusionsThe possibility to obtain HfO2 thin films by the sol-gel method was confirmedThe films have shown a dependence of the refractive indices and of the thickness on the number of depositions and the thermal treatments appliedThe structural evolution with the thermal treatment was establishedPreliminary electrical measurements were performed
22AcknowledgmentsThe work was realized as a collaboration (UMR No. 5586) of the Institute of Physical Chemistry of the Romanian Academy, Bucharest, Romania with the Laboratoire de Physique de la Matière Condensée et Nanostructures, Lyon, France, as a part of the existing cooperation agreement between the Romanian Academy and CNRS-France.The work was also supported by the Romanian Academy with Grant No. 41/2005.