What is the limit of nanolaminate layer thickness in ALD? What is the limit of nanolaminate layer...

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

•  

• 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)

•  

• 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

•  

• 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

•  

• 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