1. 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).

2. Historical Perspective of Material Synthesis
-200
-100
Now
100
Time
(year)
SEM
STM
Bulk Fabrication
Catalysis, Polymers
Thin Film
Nano-materials
Carnot
Bohr Rutherford

6. CVD & PVD thin film growth techniques
General Application
Comment
CVD
MOCVD
optical III-V semiconductors, some metallization processes
Highly flexible
Highly toxic, very expensive source material
PECVD
Dielectric coating
Low operating temperature due to plasma
Plasma damages substrate
PVD
Thermal Evaporation
Metallization
Poor step coverage
Contamination from heating element
Limitation on the target
Sputtering
Metallization and dielectric material growth
Good adhesion, low contamination

7. Sputtering Process

8. DC Sputtering e- Cathode Anode Ar+ Ar collides with the target surface
Metal atom and secondary electron release from the target e-
Cathode
Anode
Ar+ Ar
more secondary electrons are generated
Plasma will self-sustain e-
Cathode
Anode
Ar+ Ar
Secondary electron ionizes Ar to Ar+
ionized Ar+s hit target surface

9. Ar+ ions hit the target surface.
RF Sputtering
Ar+
Ar+ ions hit the target surface.
Non-conductive target
Conductive backing plate - e-
Ar+
This bombardment will last for approximately 10-9 sec. - e-
Ar+
Self-bias +
Electrons hit target surface. e-
Ar+
Ar+ +
This bombardment will last for approximately 10-7 sec.

10. Reactive Sputtering (I)
Easy compound fabrication
cathodes
Gas mixture
(Ar + O2)
Substrate
Oxide (oxygen)
Nitride (nitrogen)
Carbide (methane, acetylene, propane)
Sulfide (H2S) Ar O2

11. Reactive Sputtering (II)
Gas mixture
(Ar + O2) Ar O2
Substrate

12. Sputtering Yield vs. Ion Energy

13. Sputtering Yield vs. Ion Energy
Bounce Back
Sputtering
Ion implantation
Energy < 5eV
Intermediate energy
Energy > 10KeV

14. Sputtering yield vs. chamber pressure
Sputtering yield decreases as pressure increases.

15. Sputtering yield vs. Incidence angle
cathodes
Substrate
Schematic diagram of main chamber and cathodes
Schematic diagram showing the relationship between ion angle of incidence and sputtering yield
In order to achieve high sputtering yield, cathodes are inclined

16. Phase Transformation Control
Phase of TiO2 : Anatase, Rutile, Brookite

17. Microstructure Control
Transition structure
consisting of packed
fibrous grains
Columnar grains
Porous structure
consisting of tapered
crystallites separated
by voids
Recrystallized
Grain structure
Substrate temperature
Working Pressure
(Ar Pressure)

18. Sputter System at CISM

19. Schematic diagram of the sputter system
Mechanical pump
Chamber TC
Mano
Main Chamber
Gate Valve
Loadlock chamber
LL TC
Chamber CC
High Vacuum Valve
Turbo pump
Back TC
Turbo Backing Valve
Rough Valve
LL Rough Valve

20. Observed sensitivity trends with respect to thickness of TiO2 thin film sensors
This thickness is comparable to the depletion length.
Comparison of the sensitivity of sputtered films toward 250 ppm of CO at 550 C.
Dutta et al., “Reactively sputtered titania films as high temperature carbon monoxide sensors,” Sensors and Actuators B, 106 (2005) 20

21. 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) ppm of H2; (b) 400 ppm of H21.
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) 21

22. Theoretical calculation of the depth of electron depletion layer
The Debye length as a function of carrier concentration can be represented as
Ogawa1:
McAleer2:
The Debye length can be used to determine the possible maximum width
of the depleted region.
In this presentation, the width of the depleted region is the same as the
Debye length.
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.
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) 22

23. Calculation of the Debye lenght
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.
The Debye length of TiO2 using Ogawa equation
1U. Diebold, Surf.Sci.Rep. 48, 53 (2003)
2Y. Choe, Sensors and Actuators B. 77, 200 (2001) 23

24. 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) 24

25. The sensitivity changes as the thickness varies
T < L
The resistivity decreases as the thickness
of the film increases due to increases in
grain size.
The sensitivity increases as the thickness
of the film increases for T < L1.
1A. Ashour, Surf.Rev.Lett. 13, 87 (2006) 25

26. 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). 26

27. The sensitivity changes as the thickness varies
T > L
The sensitivity decreases as the thickness of
the film increases.
As the resistivity of the bulk increases, the
rate of the sensitivity decrease is reduced. 27

28. 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. 28

29. Reactive sputtering method for TiO2 film deposition
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. 29

30. Reactive sputtering schematic for TiO2 film deposition
cathodes
Ti metal target
Gas mixture
(Ar + O2)
Substrate Ar O2
Reactive gas is added to Ar gas for reactive sputtering; He, N2, O2 can be used for reactive gas 30

31. TiO2 film at room temperature deposition
Amorphous films are obtained. 31

32. 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 32

33. XRD results for TiO2 films
Amorphous TiO2
Anatase TiO2
Rutile TiO2
XRD data of TiO2 films after 2 hours, then annealed at 800 or 1000 C for 2 hours 33

34. XPS result of heat treated TiO2 films
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. 34

35. Cross-sectional image of a TiO2 thin filmAu
0.5 m
TiO2
Polished alumina
1 m
Cross-sectional image of a TiO2 film after 8 hours, then annealed at 1000 C for 2 hours 35

36. Properties of TiO2 thin film after depositions
The films seem to be continuous and dense from SEM observations.
The film is amorphous as-deposited, but transforms to crystalline phases of depending on the annealing temperature.
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.
The deposition rate is approximately 1 nm / min. 36

37. Sensing behaviors of TiO2 thin film (300 nm thickness) after 1000 C annealing for 2 hours under various CO
1000 ppm
750 ppm
500 ppm
250 ppm 37

38. 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.

39. Conclusions As-deposited film is amorphous.
The film shows two different phases, anantase and rutile, depending on annealing temperature.
Thin film sensors shows maximum sensitivity at a thickness comparable to the Debye length.
The theoretical model predicts the experimental trend. 39