1. Deposition of TCO films by Atomic Layer Deposition (ALD)
Gurram Sanjeev Kumar
Fraunhofer Institute for Surface Engineering and Thin Films (IST), Germany
TCM-2010, 3rd International Symposium on Transparent Conductive Materials
ORAMA Summer School ,15-17 Oct, 2010

2. Outline
ALD Fundamentals
ALD of TCOs
ALD in ORAMA Project
Summary

3. Atomic Layer Deposition Fundamentals Introduction
Atomic Layer Deposition follows
Sequential steps from [1] to [4]
Self limiting property
Reaction completion/cycle
No gas phase reactions
Beneq Oy
Ea – activation energy
Ed - desorption energy
T. Suntola, M.S. Report 4, 1989
Matti Putkonen and lauri Niinisto, 2005
Condensation
Decomposition
Desorption
Low reactivity
Low reactive sites

4. Atomic Layer Deposition Fundamentals Comparision with other techniques
Properties
PVD
CVD
ALD
Uniformity
~80 Å range
~10 Å range
Å range
Conformity
< 50%
< 70%
100%
Cleanliness
Particles
No particles
Vacuum
High
High /Med.
Medium
Temp. range
Low
Wide
Technology
~100 nm
~90 – 65nm
No limit
Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06.
VTT Finland

5. Atomic Layer Deposition Fundamentals History
Beneq Oy
Picosun Oy

6. Atomic Layer Deposition Fundamentals Thermal ALD
Thermal energy is the main source
ALD without radicals or plasma
Negative H (heat of reactions, sponteneous at any temperature)
(eg. ZnO, Al2O3, CdS ..)
Beneq Oy

7. Atomic Layer Deposition Fundamentals Plasma Enhanced ALD (PEALD)
PEALD is a useful extenstion of thermal ALD adding additional capabilties:
Low temperatures and higher deposition rates (eg. SiO2 )
Enhanced film quality and tunability
It has good film adhesion than thermal ALD (Creation of more active sites)
- Enhances reaction chemistry for specific films
Cycle time can be lowered ( easy switch on and off of plasma)
It helps in the deposition of single elements ( eg. Noble metals like Pt, Cu..)
Beneq Oy
Steven M George, Chem. Rev. 2010

8. Atomic Layer Deposition Fundamentals Advantages of Plasma over Thermal ALD
Processes requiring Plasma ALD
Solution with Plasma ALD
Example:
Problem : Desorption of precursor and no ALD processs
Solution : Use Plasma of second reactant and reduce deposition temperature

9. Atomic Layer Deposition Fundamentals Precursor Chemistry
The basic requirements for a good precursor for ALD processes are:
- Sufficient volatility at the deposition temperature
- No self-decomposition at the deposition temperature
- Preferably gases and liquids, solids as well (if no sintering problems)
- Precursors must adsorb or react with the surface sites
- Sufficiently reactivity towards the other precursor
- No etching of the substrate or the growing film
- Safe for handling and lower costs

10. Atomic Layer Deposition Fundamentals Precursor Chemistry
Inorganic
Metal Halides
Metallo organic
ß-diketone complexes and Alkoxides
Organo metallic
Adv:
Thermal stability
Reactivity
Disadv:
By products
Vapour pressure
Molecule size
Cost and availability
H2O or O3 are commonly used as one of the reactant for Oxide materials

11. Atomic Layer Deposition Fundamentals Applications
Anti-reflection and optical filters : Al2O3, ZnS, SnO2, Ta2O5
Electroluminescent devices : SrS:Cu, ZnS:Mn, ZnS:Tb, SrS:Ce
Optical applications : SnO2, ZnO, MgF2
Sensors : SnO2, Ta2O5
Wear and corrosion inhibiting layers : Al2O3, ZrO2
High-k dielectrics :Al2O3, HfO2, ZrO2, Ta2O5, La2O3
Conductive gate electrodes : Ir, Pt, Ru, TiN,
Metal interconnects and liners : Cu, WN, TaN, WNC, Ru, Ir
Catalytic materials : Pt, Ir, Co, TiO2, V2O5
Nanostructures : DRAM, MEMS
Biomedical coatings: TiN, ZrN, CrN, TiAlN, AlTiN
Metals :Ru, Pd, Ir, Pt, Rh, Co, Cu, Fe, Ni
Piezoelectric layers :ZnO, AlN, ZnS
Transparent Electrical Conductors : ZnO:Al, ITO)
UV blocking layers :ZnO, TiO2
OLED passivation : Al2O3
Photonic crystals : ZnO, ZnS:Mn, TiO2, Ta2N5,
Special structures requiring high conformity
Ali Javey et al 3D solar cells

12. ALD of Transparent Conductive Oxides TCOs Deposition and Types
There are different TCOs :
N-type TCOs P-type TCOs
ZnO based for eg. ZnO, ZnO:Al CuO based for eg. CuO, CuAlOx
ZnSn2O4 , ITO, SnO (delafossite structures), ZnO based amorphous Spinel structures, ZnO:N
The main important parameters for deposition of the TCO layers by ALD
- Precursors
- Growth temperature
- Type of ALD used (Thermal or Plasma)

13. ALD of Transparent Conductive Oxides ZnO based TCO
Precursor choice
The important precursors for Zinc Oxide deposition are
Zinc alkyl compounds
DEZ
(Diethyl Zinc)
DMZ
(Dimethyl Zinc) +
H2O/O3
C2H5-Zn-C2H5
CH3-Zn-CH3
C2H5-Zn-C2H5 + H2O ZnO + 2C2H6
C2H5-Zn-C2H5 + O ZnO + COx +H2O
CH3-Zn-CH3 + H2O ZnO + 2CH4
CH3-Zn-CH3 + O ZnO + COx +H2O

14. ZnO based TCO: Precursor effect with Thermal ALD
Diethyl Zinc + H2O process
Dimethyl Zinc + H2O process
100 – 150 °C
104 – 200 °C
Higher growth rate with DMZ because of Steric effect (molecular size)
Decrease of growth rate at high temp.:- due to dehydroxylation
Orientation in the ALD window is <100> direction and above 200°C is <200>
Grzegory Luka et al, Poland, Baltic ALD 2010 & GerALD2 in Hamburg, [1]

15. ZnO based TCO: Precursor effect with Thermal ALD
Diethyl Zinc + H2O process
Dimethyl Zinc + H2O process
Zn intestitials
Oxygen vacancies

16. ZnO based TCO: Precursor effect with Plasma Enhanced ALD
Diethyl Zinc + O2 process
Dimethyl Zinc + O2 process
85 – 125 °C
(100 – 150, thermal ALD)
75 – 150 °C [*]
(104 – 200, thermal ALD)
The growth rate of both precursors are higher than Thermal ALD
The <002> orientation is obtained 50°C below 200°C (unlike Thermal ALD)
Resistivities are relatively lower to the thermal ALD : Hydrogen incorporation
*Sang-Hee Ko Park et al, Elec. Solid-State Lett., 2006, Vol. 9

17. ZnO based TCO: Precursor effect with Plasma Enhanced ALD
Better is DMZ than DEZ for PEALD
DEZ completely decomposes at low temperature (RT) in high vacuum.
DMZ is stable with formation of monomethylzinc with oxygen and without oxygen flow (this avoid CVD unlike DEZ)
A strong peak of <002> is obtained at 120°C with PEALD of DMZ and above 150°C for DEZ.
Quadrupole Mass Spectroscopy
Pieter C. Rowlette et al, Chem. Vap. Deposition, 2009, 15

18. ALD of Transparent Conductive Oxides ZnO based TCO: ALD of ZnO with O3
Properties:
ZnO with Ozone occurs at high temp.
Hydroxyl desorption at high temp.
ZnO ( O3 process) – SAW applications
ZnO (H2O process) – TCO applications
Increase in resistivity - aggresive O3 - Si
diffusion in to the films O3
H2O
C2H5-Zn-C2H5 + H2O ZnO + 2C2H6
C2H5-Zn-C2H5 + O ZnO + COx +H2O
Seong Keun Kim et al, Thin Solid Films 478, 2005

19. ALD of Transparent Conductive Oxides ZnO doped TCOs :ZnO:Al
For a film thickness of 200 nm
Grzegory Luka et al, Poland, Baltic ALD 2010 & GerALD2 in Hamburg

20. ALD of Transparent Conductive Oxides ZnO based P-TCOs : ZnO: N
Initial reaction mechanism by DFT
(Lin Dong et al, Thin Solid Films 517, 2009)
Doping of Zinc Oxide with Nitrogen
Chongmu Lee et al, Materials Letters 61, 2007

21. ALD of Transparent Conductive Oxides Copper Oxide based P - TCOs
CuO deposition from Cu(acac)2 and O3
With Solid state reactions: Delafossite, Spinel structures can be produced
CuO
Al2 O3 (TMA + H2O)
CuAlOx
Annealing at
High temp.
N times
Growth rate
0.038 nm/cycle
ALD window
140 – 230 °C
Mari Alnes , Norway, Baltic ALD 2010 & GerALD2 in Hamburg

22. ALD in ORAMA Project
To develop low defect density electronics with ALD
Deposition of Amorphous n-TCOs like Zn-Sn-O and Zn-Ga-Sn-O with ALD
Deposition of N-type ZnO and ZnMgO (counterpart to crystalline based p-n junction) having mobilities with high homogeneity and uniformity, using ALD technique.
ZnMgO to be deposited by ALD as new material for Ag seed layer
Novel ALD deposition of Poly-Crystalline (P type Active Semiconductor Oxides) e.g. Delafossite structures.
Single crystalline films to be grown on crystalline substrate

23. Thank you Summary A brief introduction to ALD process
Precursors effect on TCOs deposition
Different effects of Thermal and Plasma ALD on TCOs
ORAMA project goals with ALD
Thank you