1. What is the limit of nanolaminate layer thickness in ALD
What is the limit of nanolaminate layer thickness in ALD? Oskari Elomaa,
The topic of my presentation:

2. But are there limitations for the layer thickness?
Nanolaminates by ALD:
tailored properties & controlled growth!
But are there limitations for the layer thickness?
Nanolaminates are a perfect material:
-tailored properties like hardness on permittivity
ALD is perfect technique
-monolayer growth
-comformal
But there are limit to the layer growth, which I will go through here.

3. Contents Nanolaminates: introduction and examples Nanolaminates by ALD
Layer thickness limitations
Property related
Thick layers
Thin layers
Process and modelling examples
Conclusions
References
Here are the contents:

4. Nanolaminates: introduction
Multilayer coatings
Repeating layers of different materials
One or more bilayers in a stack
Bilayer thickness from few to tens of nm
Growth methods
CVD
PVD
ALD
Sol-Gel
Cross-sectional TEM image of Al2O3–TiO2 film
nanolaminated by alternate ALD growth of 100-cycle Al2O3 and 350-cycle TiO2. [2]
 Nanolaminates in simple words:
Multilayer films/coatings:
-repating 2 or more different materials
-bilayers (EXAMPLE IN PICTURE)
-individuallayer thicknesses nanometer scale 2-50?
Growth methods:
-basically anything like CVD, PVD methods
-even sol-gel nanolaminates
-Magnetron Sputtering for hard coatings
-ALD for dielectrics
[1-5]

5. Nanolaminates: introduction
Tunable nanocomposites
Materials (single layer properties), composition
Thickness and number of bilayers
Iso-structural vs. non iso-structural bilayers
Crystal sructure (polycrystalline, amorphous)
Possibility to tailor the properties
High strenght and hardness
Corrosion/erosion resistance
Fracture toughness
High film quality (low roughness)
High/low thermal/electrical conduction
High/low optical refractive index
Nanolaminates are more, they can be tailored: properties you want!
Tunable thin films, tailoring:
-starting material properties, compositions of layers
-bilayer thickness, number of etc and copmpostion
-Isostructural and non-isostructural BI
same crystal structure (super lattice like TiN) or not
dislocations move or not
-crystalline, polycrystalline, amorphous
-interfacial density.
Properties:
High strenght and hardness, 100 Gpa with titanium nitrides
Corrosion/erosion resistance (chemically inert, durable)
Fracture toughness (dislocations dont move over layers)
High film quality (low roughness) with amorphous
High/low thermal/electrical conduction
High/low optical refractive index
[1-5]

6. Nanolaminates: examples
Hard coatings and high strenght materials
TiAlN/VN, TiAlN-CrN, AlN/Si3N4…
Machine tooling
Thin high-k dielectric layers
Al2O3/HfO2, Ta2O5/HfO2, Ta2O5/ZrO2, ZrO2/HfO2...
Gate dielectric candidates to replace SiO2, SiON
Capasitor dielectrics
Other tailored multilayer coatings
W/AI2O3…
Thermal barrier coatings
Optical filters, x-ray mirrors, gas sensors
Hard coatings and high strenght materials
TiAlN/VN, TiAlNYN-VN, TiAlN-CrN, AlN/Si3N4…
titaniumnitrides, aluminum nitrides and combinations
Machine tooling , cutting, drills
Thin high-k dielectric layers
Al2O3/HfO2, Ta2O5/HfO2, Ta2O5/ZrO2, ZrO2/HfO2...
Gate oxide candidates to replace Silicon oxide transistor
Dielectrics for capasitors
Tailored multilayer coatings
W/AI2O3…Tungsten/Aluminumoxide
Thermal barrier coatings
Optical filters, x-ray mirrors
Gas sensors
[1-15]

7. Nanolaminates by ALD Basic ALD process
Precursors changed after each individual layer to get bilayers
3 or more precursors
ALD advantages compared to PVD, CVD...
Accurate thickness control
Large-scale uniformity
Conformal layering
Sharp interfaces
Diverse sizes and shapes can be coated
ALD limitations
Speed (slow)
Precursors (none, toxic, expensive)
Basic ALD process
-3 or more precursors needed
-first one layer, then the other to bilayers
ALD advantages compared to PVD, CVD...
Accurate thickness control (up to 1 Å) of the individual nanolayers
Large-scale uniformity
Conformal layering
Sharp interfaces
Diverse sizes and shapes can be coated
ALD limitations
Speed (slow)
Precursors, some dont exist, some are expensive, some toxic
[1, 5-6]

8. Limits of thickness 1: nanolaminate
Critical thickness
Nanolaminate property (mechanical, elecrical etc.) related
Optimum bilayer thickness for spesific property
AlN/TiN hardness maximum when AlN 2 nm
TiO2 amorphous between 2,5 and 9 nm
Critical thickness, something that just has to be fulfilled:
Nanolaminate property (mechanical, elecrical etc.) related
Optimum bilayer thickness for spesific property
AlN/TiN hardness maximum when AlN 2 nm
TiO2 amorphous between 2,5 and 9 nm
[8, 16-17]

9. Limits of thickness 2: thick ALD
Defects and imperfections multiply (in crystalline)
Surface roughness increases
Unwanted-wanted depending on application
Faceting increases
layers in the stack not parallel with the substrate
Stresses, cracks
Some of the problems can be avoided by amorphous layer
crystalline/amorphous bilayer nanolaminate
see example 1
When thickness increases, problems:
-Defects and imperfections multiply in crystalline films
-Surface roughness increases
-Unwanted in most applications such as optical applications (refractive index)
Wanted in gas sensors (large area)
-Faceting increases, the case of crystalline layers, the development of faceting will ultimately limit the ability to keep the layers in the stack parallel with the substrate
Stresses, cracks
These can be avoided by using a thin amorphous layer
-Crystalline/amorphous bilayer nanolaminate
-see example 1
[1, 5, 8]

10. ALD process: example 1 ZnO2/Al2O3 nanolaminates Process
ZnO polycrystalline electric conductor
Al2O3 amorphous insulator
Process
Diethyl zinc (DEZ), trimethyl aluminum (TMA) and H2O
1-128 bilayers
Deposited at 177 C
ZnO2/Al2O3 nanolaminates
-ZnO conducting, polycrystalline film
-rough surface topography, used in gas sensors
-Al2O3 insulating,
-amorphous film with high conformality
Process
Diethyl zinc (DEZ), trimethyl aluminum (TMA) and H2O
Silicon substrate
1-128 bilayers
Deposited at 177 C on silicon substrate
[1]

11. ALD process: example 1 Results Speed of growth similar to normal ALD
Surface roughness: from 6 to 0,2
Minimum Al2O3 needed: one monolayer
Results
Speed of growth similar to normal ALD
(2,01 for ZinkOXIDE and 1,29 Å/cycle for ALUOXIDE)
Surface roughness:
for ZnO up to 6
from 1.45 nm at 1 bilayer to 0.2 nm at 128 bilayers
Minimum Al2O3 needed: one monolayer
[1]

12. Limits of thickness 3: thin ALD
Basically monolayer by monolayer but:
the surface is only gradually converted from into actual film
adsorption, limited number of reactive sites,
density of reactive sites on substrate and film
Nucleation phenomena
islands
especially in polycrystalline films
can affect the growth (unperfect films)
W on Al2O3 limits the minimum thickness of a continuous W nanolayer to ~25Å
When thickness decreases: problems
Basically monolayer by monolayer but
reactant molecule will adsorb onto the substrate,
and after some time, will react to form the film
The outermost surface is gradually converted from pre-deposited substrates into an actual film
limited number of reactive sites
density of reactive sites on the underlying surface and films surface itself can be different
Nucleation phenomena
nucleation especially in case of polycrystalline films, islands etc.
can critically affect the growth of thin nanolaminates
W on Al2O3 limits the minimum thickness of a continuous W nanolayer to ~25Å
[5-6,17-18]

13. ALD modelling : example 2
Hypothesis:
Reactant adsorption depends on properties of the surface and absorbant
The surface changes during the initial deposition from substrate to film
Difference between initial stage and the stabilized stage
Film thickness is not linearly dependent of cycles during the first few
The film thickness becomes linearly dependent After the initial cycles
Hypothesis:
The probability of reactant adsorption depends on:
The properties of the surface and absorbant
The surface changes during the initial deposition
Substrate to film
Thus differences exist between
The initial stage and the stabilized stage of ALD
The film thickness is not linearly dependent
Of deposition cycles during the first few deposition cycles
The film thickness becomes linearly dependent
After the initial cycles
[17-18]

14. ALD modelling : example 2
Results
On the left, the results of the proposed modelling,
Transient region
-starting growths, not linear
Converged region
-linear
On the right model tested in:
TiN-ALD using TDMAT and NH3 as the reactant gases.
[17-18]

15. Limits of thickness 3: process
Chamber atmosphere
Optimum temperature/pressure window for each layer
No unwanted growth, thickness control
Precursors
Temperature, dependence of the substrate
self-decomposition and residues need to be avoided
Substrates
Oxide or stripped affects growth mode
Amorphous or crystalline (surface roughness limitations)
Speed = time = money
ALD is slow to be economical (but batch processing)
Typically few nm/min
Process parameters are limiting factors for thickness control, compostion crystall structureetc.
Chamber atmosphere
Optimum temperature/pressure window for each layer (crystal structure)
No unwanted growth, thickness control
Precursors
Temperature, dependence of the substrate
self-decomposition and residues need to be avoided
Substrates
Oxide or stripped affects growth mode
Amorphous or crystalline (surface roughness limitations)
Speed = time = money
ALD is slow to be economical (but batch processing)
Typically few nm/min
[1, 5, 13-16]

16. Conclusions Nanolaminates by ALD: But layer thickness is limited by:
Tailored properties (mechanical, electrical etc.)
Controlled growth, conformality
From thin gate oxides to thick tool coatings
But layer thickness is limited by:
Critical thickness
Defects and imperfections
Surface roughness and faceting
Stresses (thermal, thickness related)
Non-linear growth in the beginning
Process parameters

17. THANK YOU FOR LISTENING! QUESTIONS?

18. References
1: J.W. Elam, Z.A. Sechrist, S.M. George, ZnO/Al2O3 nanolaminates fabricated by atomic layer deposition: growth and surface roughness measurements, Thin Solid Films 414 (2002) 43–55 
2: Yong Shin Kim, Sun Jin Yun, Nanolaminated Al2O3–TiO2 thin films grown by atomic layer deposition, Journal of Crystal Growth 274 (2005) 585–593
3: Philip C. Yashar,William D. Sproul, Nanometer scale multilayered hard coatings, Vacuum 55 (1999) 179}190
4: Lijuan Zhong, Fang Chen, Stephen A. Campbell, and Wayne L. Gladfelter ,Nanolaminates of Zirconia and Silica Using Atomic Layer Deposition, Chem. Mater. 2004, 16,
5: Markku Leskelä, Industrial Applications of Atomic Layer Deposition (ALD), 10th MIICS Conference
Mikkeli, March 18, 2010
6: J. M. Jensen, A. B. Oelkers, R. Toivola, and D. C. Johnson, X-ray Reflectivity Characterization of ZnO/Al2O3Multilayers Prepared by Atomic Layer Deposition, Chem. Mater. 2002, 14,
7: R. M. Costescu, D. G. Cahill, F. H. Fabreguette, Z. A. Sechrist, S. M. George, Ultra-Low Thermal Conductivity in W/Al2O3 Nanolaminates, Science 303, 989 (2004)
8:, D.R.G. Mitchell*, D.J. Attard, K.S. Finnie, G. Triani, C.J. Barbe´, C. Depagne, J.R. Bartlett TEM and ellipsometry studies of nanolaminate oxide films prepared using atomic layer deposition, Applied Surface Science 243 (2005) 265–277
9: T.M. Mayer, T.W. Scharf, S.V. Prasad, N.R. Moody, R.S. Goeke, M.T. Dugger, R.K.
Grubbs, S. M. George, R.A. Wind, J.M. Jungk, W.W. Gerberich, Atomic Layer Deposition of Highly Conformal Tribological Coatings, SANDIA REPORT 2005
10: Dr. Troy Barbee, Optical applications of nano-laminates, Technology Days in the Government
Mirror Development and Related Technologies

19. References
 11: Diana Riihelä, Mikko Ritala , Raija Matero, Markku Leskelä, Electronics, Optics and Opto-electronics, Introducing atomic layer epitaxy for the deposition of optical thin films, Thin Solid Films 289 (1996)
12: H. Zhang and R. Solankia, B. Roberds, G. Bai, and I. Banerjee, High permittivity thin film nanolaminates, JOURNAL OF APPLIED PHYSICS
13: Lijuan Zhong, Weston L. Daniel, Zhihong Zhang, Stephen A. Campbell, and Wayne L. Gladfelter
, Atomic Layer Deposition, Characterization, and Dielectric Properties of HfO2/SiO2 Nanolaminates and Comparisons with Their Homogeneous Mixtures. Chem. Vap. Deposition 2006, 12, 143–150
14: H. Zhang and R. Solankiz, Atomic Layer Deposition of High Dielectric Constant
Nanolaminates, Journal of The Electrochemical Society,
15: Kijung Yong and Joonhee Jeong , Applications of Atomic Layer Chemical Vapor Deposition for the Processing of Nanolaminate Structures, Korean d. Chem. Eng., 19(3), (2002)
16: Steven M. George, Fabrication of Nanolaminates with Ultrathin Nanolayers Using Atomic Layer
Deposition: Nucleation & Growth Issues, AFOSR Grant No. FA , Final Report 2009
17: Jung-Wook Lim, Hyung-Sang Park, and Sang-Won Kang, Kinetic Modeling of Film Growth Rate in Atomic Layer Deposition, Journal of The Electrochemical Society, C403-C
18: Jung-Wook Lim, Hyung-Sang Park, and Sang-Won Kang, Analysis of a transient region during the initial stage of atomic layer deposition JOURNAL OF APPLIED PHYSICS 88 (11) 2000