hfo 2 thin films prepared by sol-gel method a.barău 1, m.gartner 1, m.anastasescu 1, v.s.teodorescu...
TRANSCRIPT
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, 060021 Bucharest, ROUMANIA
2National Institute of Material Physics, 105 bis Atomistilor Street,
077125 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
_____________________________________________________________________________________________
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.
Objectives
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 , = 99,2)
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
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)
Why the HfO2 ?
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)
Why the HfO2 ?
Methods of film preparation (literature):
• 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.
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 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
Experimental:
Film preparation
• 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 (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 450oC
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
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 dried
annealed 450oC
Annealed 450oC
Results obtained
Atomic Force Microscopy
Spin coated films – one layer
Dip coated films Spin coated films
100 200 300 400 500 600 700 800 900 1000 1100 12000
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
Hf
CO
Si Dissymmetry of Hf peaks
Deformation of Si peaks
1 layer dried
1 layer annealed 600 °C
1 layer annealed 450 °C
Yie
ld
Channel
►► 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 HfO2 film dried at 100oC and then annealed at 150oC (to be stable in the microscope)
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
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.
Results obtained
Transmission Electron Microscopy
Thermally treated at 450oC film Thermally treated at 6000C
Results obtained
Transmission Electron Microscopy
High resolution XTEM image of the cross section of the HfO2 films
deposited by dip-coating
0,0 -0,5 -1,0 -1,5 -2,02
0
-2
-4
-6
-8
-10
-12
-14
-16
Vg= 0 V
Vg= -0.8 V
Vg= -1.6 V
Vg= -2.4 V
Vd (V)
I d (µ
A)
Red lines: ramp upPurple squares: ramp down
0,0 -0,5 -1,0 -1,5 -2,0 -2,5
1E-8
1E-7
1E-6
1E-5
Von
VD = -1.5 V
Lo
g I D
(µ
A)
VG (Volts)
0,0 -0,5 -1,0 -1,5 -2,0
0,00
0,02
0,04
0,06
0,08
0,10
0,12
0,14
µ for VD= -0.2V
mo
bili
ty µ
(cm
2 /V.s
)
VG (Volts)
Low operation voltage Almost no hysteresis Low threshold voltage Good mobility
Results obtained
Electrical Properties
I-V curves variation and mobility for the HfO2 sol-gel films thermally
treated at 450oC
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
Results obtained
Electrical Properties
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
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.