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

THANK YOU FOR LISTENING!

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

•  

• 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

•  

• 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

•  

• 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

•  

• 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, 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