WSE

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HfO2 thin films prepared by sol-gel method
A.Barau1, M.Gartner1, M.Anastasescu1,
V.S.Teodorescu2, M.G.Blanchin3, J.Tardy4 and
M.Zaharescu1   1Institute of Physical Chemistry
"Ilie Murgulescu" - Roumanian Academy 202 Splaiul
Independentei, 060021 Bucharest,
ROUMANIA 2National Institute of Material Physics,
105 bis Atomistilor Street,
077125
Bucharest-Magurele, ROUMANIA 3Universite Claude
Bernard Lyon 1, 43 Boulevard du 11 Novembre
1918, 69622 Villeurbane CEDEX-FRANCE 4Ecole
Centrale de Lyon , LEOM , 36 avenue Guy de
Collongue, 69134 Ecully, FRANCE
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Objectives

• 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.

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Why the HfO2 ?

• HfO2 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 15-50)
• High density (9.86 g/cm2)
• Stable structure SGP- (14) monoclinic
  symmetry P121/c1
• (a 0.51156nm, b 0.51722 nm, c 0.52948 nm
  , b 99,2)

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Why 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 fabrication
• as material for nanofiltration membranes and
• films with high pencil hardness (over 9H) and
  hydophobicity

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Why the HfO2 ?

• 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 - 2004, Boher et al
  2004, Aarik et al 2004)

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Why 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)

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Experimental 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.

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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 SiO2 was
  removed in HF
• Film densification
• - 10 min at 100?C and 30 min at 450o or 600oC
  with a heating rate of 1?C/min.
• - For the multi-layered films, the same
  thermal treatment was applied, after each
  deposition

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Experimental Films characterization

• Spectroellipsometric (SE) measurements in the
  300-700 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 (4HeE 1.5 MeV)
• Preliminary electrical measurements were
  performed.

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Spectroellipsometric results on samples obtained
by dip-coating, thermally treated at 450oC
Results obtained Spectroellipsometry
Samples Number of layers Thermal treatment d (A) HfO2 () Voids () Error
F1HF 1 non 428 54.20 45.80 0.0000803
F1THF 1 1 142 56.00 44.00 0.0001520
F2THF 2 2 228 66.01 33.90 0.0000585
F3THF 3 3 319 73.03 26.97 0.0000826

• 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

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Results 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
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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
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Results obtained Atomic Force Microscopy

dried
dried
annealed 450oC
Annealed 450oC

• Maximum profile roughness
• up to 10 nm

• Large surface roughness

• Spin coated films one layer

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Results obtained Rutherford Backscattering
spectrometry
Dip coated films
Spin coated films
? No deformation of the Hf and Si peaks
? Dissymmetry and Deformation of Hf and Si peaks
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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)
Results obtained Transmission Electron Microscopy

• Amorphous structure with an non-uniform density
  in the nanometric scale

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Plan view TEM image and SAED pattern of the HfO2
film annealed at 450oC
Results obtained Transmission Electron Microscopy

• The structure is still amorphous with a
  beginning of crystallization

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Plan view HRTEM image of the HfO2 film annealed
at 600oC
Results obtained Transmission Electron Microscopy

• The 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.

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Results obtained Transmission Electron Microscopy
Thermally treated at 450oC film
Thermally treated at 6000C
High resolution XTEM image of the cross section
of the HfO2 filmsdeposited by dip-coating
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Results obtained Electrical Properties
I-V curves variation and mobility for the HfO2
sol-gel films thermally treated at 450oC

• Low operation voltage
• Almost no hysteresis
• Low threshold voltage
• Good mobility

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Results obtained Electrical Properties

• The low operation voltage was assigned to the
  very thin dielectric film
• Improved stability was correlated to the porous
  nature of HfO2 with air inclusion
• The extremely low threshold voltage (VT -0.4V)
  and high mobility are
• related to the very smooth surface of the film

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Conclusions

• The possibility to obtain HfO2 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

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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.
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