What is the limit of nanolaminate layer What is the limit of nanolaminate layer thickness in ALD?thickness in ALD?
Oskari Elomaa, 20.4.2010
Nanolaminates by ALD:
tailored properties & controlled growth!
But are there limitations for the layer thickness?
Contents
• Nanolaminates: introduction and examples
• Nanolaminates by ALD
• Layer thickness limitations– Property related
– Thick layers
– Thin layers
• Process and modelling examples
• Conclusions
• References
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-GelCross-sectional TEM image of Al2O3–TiO2 filmnanolaminated by alternate ALD growth of 100-cycle Al2O3 and 350-cycle TiO2. [2]
[1-5]
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
[1-5]
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
[1-15]
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)
[1, 5-6]
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
[8, 16-17]
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
[1, 5, 8]
ALD process: example 1
ZnO2/Al2O3 nanolaminates– ZnO polycrystalline electric conductor
– Al2O3 amorphous insulator
Process– Diethyl zinc (DEZ), trimethyl aluminum (TMA) and H2O
– 1-128 bilayers
– Deposited at 177 C
[1]
ALD process: example 1
Results– Speed of growth similar to normal ALD
– Surface roughness: from 6 to 0,2
– Minimum Al2O3 needed: one monolayer
[1]
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
[5-6,17-18]
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
[17-18]
ALD modelling : example 2
Results
[17-18]
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
[1, 5, 13-16]
Conclusions
Nanolaminates by ALD:– 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
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QUESTIONS?
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, 1098-1103
•
• 5: Markku Leskelä, Industrial Applications of Atomic Layer Deposition (ALD), 10th MIICS Conference
• Mikkeli, March 18, 2010
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• 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, 2276-2282
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• 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
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) 250-255
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• 12: H. Zhang and R. Solankia, B. Roberds, G. Bai, and I. Banerjee, High permittivity thin film nanolaminates, JOURNAL OF APPLIED PHYSICS 87 4 2000
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• 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
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• 14: H. Zhang and R. Solankiz, Atomic Layer Deposition of High Dielectric Constant
• Nanolaminates, Journal of The Electrochemical Society, 148 4 2001
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• 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), 451-462 (2002)
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• 16: Steven M. George, Fabrication of Nanolaminates with Ultrathin Nanolayers Using Atomic Layer
• Deposition: Nucleation & Growth Issues, AFOSR Grant No. FA9550-06-1-0075, Final Report 2009
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• 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, 148 6 C403-C408 2001
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• 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