deposition techniques for thin films and sensing references: 1. shaestagir chowdhury, ph. d. thesis,...
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Deposition Techniques for Thin Films and Sensing
References:1. Shaestagir Chowdhury, Ph. D. Thesis, Dublin City University, 1998.
2. Deposition Techniques for Films and Coatings by R. F. Bunshah, Noyes Publication, 1982
3. H. Lee and S.A. Akbar, “Sensing behavior of TiO2 thin-film prepared by rf reactive sputtering,” Sensors Letter, in print (2008).
Historical Perspective of Material Synthesis
-200 -100 Now 100Time(year)
SEM STM
Bulk Fabrication
Catalysis, Polymers
Thin Film
Nano-materialsCarnot
Bohr Rutherford
CVD & PVD thin film growth techniques
General Application Comment
CVD
MOCVDoptical III-V semiconductors, some metallization processes
Highly flexible
Highly toxic, very expensive source material
PECVD Dielectric coatingLow operating temperature due to
plasma
Plasma damages substrate
PVD
Thermal Evaporation
Metallization
Poor step coverage
Contamination from heating element
Limitation on the target
SputteringMetallization and dielectric
material growthGood adhesion, low contamination
Sputtering Process
DC Sputtering
e-Cathode AnodeAr+
e-
e-
Cathode AnodeAr+
e-
Ar collides with the target surface
Metal atom and secondary electron release from the target
e-Cathode AnodeAr+
e-
e-
Cathode AnodeAre-
e-
Ar+
Secondary electron ionizes Ar to Ar+
ionized Ar+s hit target surface
e-Cathode AnodeAr+
e-
e-
Cathode AnodeAre-
e-
Ar+
more secondary electrons are generated
Plasma will self-sustain
e-Cathode AnodeAr+
e-
e-
Cathode AnodeAr+ Ar+ Ar+
e-e-
e-
RF Sputtering
Ar+
Ar+
Ar+
Ar+
Ar+ ions hit the target surface.
Non-conductive targetConductive backing plate
Ar+
Ar+
Ar+
Ar+
++
++
+
+++
This bombardment will last for approximately 10-7 sec.
++
++
+
+++
Electrons hit target surface.
e-
e-e-
e-
Ar+
Ar+
Ar+
Ar+
--
--
-
---
e-
e-e-
e-
Ar+
Ar+
Ar+
Ar+
This bombardment will last for approximately 10-9 sec.
--
--
-
---
e-
e-e-
e-
Ar+
Ar+
Ar+
Ar+
Self-bias
Reactive Sputtering (I)
Substrate
cathodes
Ar O2
Gas mixture (Ar + O2)
Easy compound fabrication
Oxide (oxygen) Nitride (nitrogen) Carbide (methane, acetylene, propane) Sulfide (H2S)
Reactive Sputtering (II)
SubstrateAr O2
Gas mixture (Ar + O2)
Sputtering Yield vs. Ion Energy
Sputtering Yield vs. Ion Energy
Bounce Back Sputtering Ion implantation
Energy < 5eV Intermediate energy Energy > 10KeV
Sputtering yield vs. chamber pressure
Sputtering yield decreases as pressure increases.
Sputtering yield vs. Incidence angle
Schematic diagram of main chamber and cathodesSchematic diagram showing the relationship between ion angle of incidence and sputtering yield
In order to achieve high sputtering yield, cathodes are inclined
Substrate
cathodes
Phase Transformation Control
Phase of TiO2 : Anatase, Rutile, Brookite
Microstructure Control
Substrate temperatureWorking Pressure(Ar Pressure)
Columnar grains
Porous structureconsisting of taperedcrystallites separatedby voids
Transition structureconsisting of packed fibrous grains
RecrystallizedGrain structure
Sputter System at CISM
Schematic diagram of the sputter system
Mechanical pump
Chamber TC
ManoMain Chamber
Gate Valve
Loadlock chamber
LL TC
Chamber CC
High Vacuum Valve
Turbo pump
Back TC
Turbo Backing Valve
Rough ValveLL Rough Valve
Observed sensitivity trends with respect to thickness of TiO2 thin film sensors
Comparison of the sensitivity of sputtered films toward 250 ppm of CO at 550 C.
This thickness is comparable to the depletion length.
Dutta et al., “Reactively sputtered titania films as high temperature carbon monoxide sensors,” Sensors and Actuators B, 106 (2005) 810-815
Observed sensitivity trends with respect to thickness of SnO2 thin film sensors
Measured gas-sensitivity as a function of film thickness, for various H2 concentration: (a) 1000 ppm of H2; (b) 400 ppm of H2
1.1Yong-sham Choe, “New gas sensing mechanism for SnO2 thin-film gas sensors fabricated by using dual ion beam sputtering,” Sensors and Actuators B, 77 (2001) 200-208
Theoretical calculation of the depth of electron depletion layer
The Debye length as a function of carrier concentration can be represented as
Ogawa1:
McAleer2:
1. H. Ogawa, M. Nishikawa, A. Abe, Hall measurement studies and an electrical conduction model of thin oxide ultrafine particle films, J. Appl. Phys. 53 (1982) 4448.
2. J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, Tin dioxide gas sensors part 1. Aspects of the surface chemistry revealed by electrical variations, J. Chem. Soc.,Faraday Trans. 1 (83) 1323-1346.
The Debye length can be used to determine the possible maximum widthof the depleted region.
In this presentation, the width of the depleted region is the same as the Debye length.
Calculation of the Debye lenght
The Debye length of TiO2 using Ogawa equation The value of the dielectric constant of TiO2 ranges from 86 to 170 depending on the orientation of the optical axis 1.
The dielectric constant of TiO2 is taken to be 128, an average value for a polycrystal.
The charge carrier concentration ranges from 1015 / cm3 to 1018 / cm3.
The Debye length is ~ 300 nm for a typical charge carrier concentration of 1017/ cm3 at 550 C.
For SnO2, the Debye length is ~ 100 nm for the charge carrier concentration of 1018 / cm3.
1U. Diebold, Surf.Sci.Rep. 48, 53 (2003)2Y. Choe, Sensors and Actuators B. 77, 200 (2001)
Sensitivity of the film as function of thickness (T<L)
When films having the thickness (T) less than the depletion length is exposed to air, it is completely depleted. After exposing to gas such as H2 or CO, the depleted region shows a different resistance state because of electron donation.
Sensitivity derivation for T < L
Ra,d = a,d / T
Rg,d = g, d / T
S = Rg,d/Ra,d = g,d / a,d
Where g,d , a,d are the resistivity of the air depleted region and the gas depleted region, respectively. For the model, the grain size in the film is assumed to increase with the thickness of the film1.
1 H. Chen, Y. Lu and W. Hwang, Thin Solid Films. 514, 316 (2006)
The sensitivity changes as the thickness varies
T < L
The resistivity decreases as the thickness of the film increases due to increases ingrain size.
The sensitivity increases as the thickness of the film increases for T < L1.
1A. Ashour, Surf.Rev.Lett. 13, 87 (2006)
Sensitivity of the film as function of thickness (T>L)
When films having thickness (T) greater than the depletion length is exposed to air, it has two different resistance parts: (a) depleted part and the (b) bulk part. After exposing to gas such as H2 or CO, the depleted region shows a different resistance state because of electron donation. The bulk part will remain constant.
where Rbulk is the resistance of the bulk part. bulk is the resistivity of the bulk part. Rair, Rgas are defined as the resistance of the film under air and gas, respectively. S is the sensitivity (Rgas / Rair).
The sensitivity changes as the thickness varies
T > LThe sensitivity decreases as the thickness of the film increases.
As the resistivity of the bulk increases, the rate of the sensitivity decrease is reduced.
The sensitivity plot based on the model
The sensitivity decreases after crossing the Debye length.
Below the Debye length, the sensitivity increases as the thickness increases.
The proposed model explains the experimental trend.
• Reactive sputtering is a process in which a fraction of at least one of the coating species enters the deposition system in the gas phase.
• Advantage:
- compounds can be formed using relatively easy-to-fabricate metallic targets.
- insulating compounds can be deposited using DC power supplies.
Reactive sputtering method for TiO2 film deposition
Substrate
cathodes
Ar O2
Gas mixture (Ar + O2)
Reactive gas is added to Ar gas for reactive sputtering; He, N2, O2 can be used for reactive gas
Reactive sputtering schematic for TiO2 film deposition
Ti metal target
TiO2 film at room temperature deposition
Amorphous films are obtained.
Sputtering conditions for TiO2 film deposition in CISM
Distance between Ti metal target and alumina substrate 13 cm
RF Power 300 W
Deposition rate from a profilometer 1 nm / min
Ar gas flow rate / partial pressure 36.5 sccm (4.6 mTorr)
O2 gas flow rate / partial pressure 3 sccm (0.3 mTorr)
Total sputtering operating pressure 5 mTorr
Substrate heating temperature RT
XRD results for TiO2 films
Amorphous TiO2
Rutile TiO2
Anatase TiO2
XRD data of TiO2 films after 2 hours, then annealed at 800 or 1000 C for 2 hours
XPS result of heat treated TiO2 films
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 films
Rutile
Anatase
Ti 2p3/2
Ti 2p1/2
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 films
Rutile
Anatase
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 filmsFigure 3. XPS spectra of the Ti 2p region for the surface of the TiO2 films.
Rutile
Anatase
Ti 2p3/2
Ti 2p1/2
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 films
Rutile
Anatase
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 filmsFigure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 films
Rutile
Anatase
Ti 2p3/2
Ti 2p1/2
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 filmsFigure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 films
Rutile
Anatase
Intensity (Arbitrary unit)
Figure. 3 XPS spectra of the Ti2p region for the surface of the TiO2 filmsFigure 3. XPS spectra of the Ti 2p region for the surface of the TiO2 films.
Rutile
Anatase
Ti 2p3/2
Ti 2p1/2
XPS results show that these films are TiO2 . The films were prepared for 2 hours deposition and annealed at 800 or 1000 C for 2 hours, respectively.
TiO2
Au
Polished alumina
0.5 m
1 m
Cross-sectional image of a TiO2 thin film
Cross-sectional image of a TiO2 film after 8 hours, then annealed at 1000 C for 2 hours
Properties of TiO2 thin film after depositions
1. The films seem to be continuous and dense from SEM observations.
2. The film is amorphous as-deposited, but transforms to crystalline phases of depending on the annealing temperature.
3. The film annealed at 1000 C for 2 hours is identified as the rutile phase. The anatase phase of the film is obtained when the film is annealed at 800 C for 2 hours.
4. The deposition rate is approximately 1 nm / min.
Sensing behaviors of TiO2 thin film (300 nm thickness) after 1000 C annealing for 2 hours under various CO
250 ppm500 ppm
750 ppm1000 ppm
Sensing trend of TiO2 thin film after 1000 C annealing for 2 hours under 250 PPM CO with respect to the thickness of the films
The thickness of the film is controlled by the deposition time. 90, 300, and 500 nm thickness of the films are used to observe the sensing properties.
The sensitivity has a maximum value at the thickness comparable to the Debye length at two sensing operating temperatures.
Conclusions
1. As-deposited film is amorphous.
2. The film shows two different phases, anantase and rutile, depending on annealing temperature.
3. Thin film sensors shows maximum sensitivity at a thickness comparable to the Debye length.
4. The theoretical model predicts the experimental trend.
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