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.