Presentation on theme: "HfO 2 thin films prepared by sol-gel method A.Barău 1, M.Gartner 1, M.Anastasescu 1, V.S.Teodorescu 2, M.G.Blanchin 3, J.Tardy 4 and M.Zaharescu 1 1 Institute."— Presentation transcript:
HfO 2 thin films prepared by sol-gel method A.Barău 1, M.Gartner 1, M.Anastasescu 1, V.S.Teodorescu 2, M.G.Blanchin 3, J.Tardy 4 and M.Zaharescu 1 1 Institute of Physical Chemistry "Ilie Murgulescu" - Roumanian Academy 202 Splaiul Independentei, Bucharest, ROUMANIA 2 National Institute of Material Physics, 105 bis Atomistilor Street, Bucharest-Măgurele, ROUMANIA 3 Universite Claude Bernard Lyon 1, 43 Boulevard du 11 Novembre 1918, Villeurbane CEDEX-FRANCE 4 Ecole Centrale de Lyon, LEOM, 36 avenue Guy de Collongue, Ecully, FRANCE _____________________________________________________________________________________________
The preparation of HfO 2 thin films by sol-gel method The establishing the correlation between the way of preparation and optical and structural properties of these materials. Objectives
HfO 2 properties: High thermal and chemical stability High thermodinamic stability in contact with silicon High refractive index (~ 2.00) Large band gap (5.86 eV) High dielectric constant (K ) High density (9.86 g/cm2) Stable structure – SGP- (14) monoclinic : symmetry P121/c1 (a = nm, b= nm, c= nm, = 99,2) Why the HfO 2 ?
Possible applications: in micro and optoelectronics: - material for replacing SiO 2 in metal/oxide/semiconductor (MOS) devices - optical coatings when high optical damage thresholds are needed - waveguide fabrication as material for nanofiltration membranes and films with high pencil hardness (over 9H) and hydophobicity Why the HfO 2 ?
Methods of film preparation (literature): Sputtering (Kang et al – 2000, Lee et al – 2000) Chemical vapor deposition - thermal (Balog et al – 1979 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) Why the HfO 2 ?
Sol-gel methods: - starting with HfCl 4 in ethanol (Nishide et al – 2000, Shimada et al – 2002, Yu et al – 2003) - starting with HfCl 4 in 1-methoxy-2 propanol (Blanc et al – 2000) - starting with HfCl 4 in water, via hafnia hydroxide formation and peptization with formic/oxalic acid (Takahashi and Nishide – 2004, Nishide et al – 2005) - starting with HfOCl 2 in ethanol (Gonçalves et al – 2004) - starting with Hf(OC 2 H 5 ) 4 and Acac (Villanueva-Ibanez et al – 2003) Why the HfO 2 ? Methods of film preparation (literature):
The reagents: - hafnium ethoxide Hf(OC 2 H 5 ) 4 (Alfa Aesar) as HfO 2 source, - acetyl acetone AcAc (Fluka) as stabilisator and - absolute alcohol p.a. (Merck) as solvent. - Molar ratio: Hf(OC 2 H 5 ) 4 /Acac = 1. Solution preparation: mixing of the reagents in N 2 atmosphere at C for two hours. Synthesis were also performed starting with Hf-acetyl-acetonate or Hf- chloride, that allows working in ambinet atmosphere. Experimental: Film preparation
Film deposition: - substrates: silicon wafer; - deposition method: - dip-coating (5-8 cm/min withdraw speed), - spinning (5000 rpm) Before deposition the native SiO 2 was removed in HF Film densification: - 10 min at 100 C and 30 min at 450 o or 600 o C with a heating rate of 1 C/min. - For the multi-layered films, the same thermal treatment was applied, after each deposition Experimental: Film preparation
Spectroellipsometric (SE) measurements in the nm spectral range TEM (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. Experimental: Films characterization
Spectroellipsometric results on samples obtained by dip-coating, thermally treated at 450 o C SamplesNumber of layers Thermal treatment d (A)HfO 2 (%)Voids (%)Error F1HF1non F1THF F2THF F3THF the 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 Results obtained Spectroellipsometry
(a) (b) The thickness (a) and refractive indexes (n) of the samples with 1-3 layers (b) from spectroellipsometric results Results obtained Spectroellipsometry 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
Results obtained Atomic Force Microscopy Very low RMS roughness between 0.7 and 1.5 nm Very small surface roughness (~1 and ~1.5 nm) Dip coated film – one layer dried
Large surface roughness Maximum profile roughness up to 10 nm dried annealed 450 o C Annealed 450 o C Results obtained Atomic Force Microscopy Spin coated films – one layer
Dip coated filmsSpin coated films Dissymmetry and Deformation of Hf and Si peaks: Dissymmetry and Deformation of Hf and Si peaks: No deformation of the Hf and Si peaks Results obtained Rutherford Backscattering spectrometry
Amorphous structure with an non-uniform density in the nanometric scale Plan view TEM image and SAED pattern of the HfO 2 film dried at 100 o C and then annealed at 150 o C (to be stable in the microscope) Results obtained Transmission Electron Microscopy
Plan view TEM image and SAED pattern of the HfO 2 film annealed at 450 o C The structure is still amorphous with a beginning of crystallization Results obtained Transmission Electron Microscopy
Plan view HRTEM image of the HfO 2 film annealed at 600 o C The crystallization of the monoclinic HfO 2 is observed. The crystallites are like a sponge. Pores with an average dimension of about 4.6 nm are observed. Results obtained Transmission Electron Microscopy
Thermally treated at 450 o C filmThermally treated at C Results obtained Transmission Electron Microscopy High resolution XTEM image of the cross section of the HfO 2 films deposited by dip-coating
Low operation voltage Almost no hysteresis Low threshold voltage Good mobility Results obtained Electrical Properties I-V curves variation and mobility for the HfO 2 sol-gel films thermally treated at 450 o C
The low operation voltage was assigned to the very thin dielectric film Improved stability was correlated to the porous nature of HfO 2 with air inclusion The extremely low threshold voltage (VT ~ -0.4V) and high mobility are related to the very smooth surface of the film Results obtained Electrical Properties
The possibility to obtain HfO 2 thin films by the sol-gel method was confirmed The films have shown a dependence of the refractive indices and of the thickness on the number of depositions and the thermal treatments applied The structural evolution with the thermal treatment was established Preliminary electrical measurements were performed Conclusions
Acknowledgments The 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.