1. HfO2 thin films prepared by sol-gel method
A.Barău1, 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, Bucharest, ROUMANIA
2National Institute of Material Physics, 105 bis Atomistilor Street,
Bucharest-Măgurele, 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
_____________________________________________________________________________________________

2. 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.

3. 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 = nm, b= nm, c= nm , b = 99,2)

4. 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

5. Why 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)

6. 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)

7. 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.

8. 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

9. Experimental: Films characterization
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.

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

11. 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

12. Dip coated film – one layer dried
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

13. Spin coated films – one layer
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

14. 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:

15. Results 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

16. Results obtained Transmission Electron Microscopy
Plan view TEM image and SAED pattern of the HfO2 film annealed at 450oC
The structure is still amorphous with a beginning of crystallization

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

18. 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 films deposited by dip-coating

19. 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

20. 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

21. 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

22. 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.