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Atomic Layer Deposition (ALD)

Maryam EbrahimiUniversity of Waterloo

January 17th, 2006Chem 750/7530

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Outline

• ALD Theory and Process

• Precursor Requirements

• Deposition Advantages

• Comparison to CVD Process

• Applications

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What is ALD?

• ALD (Atomic Layer Deposition) • Deposition method by which precursor

gases or vapors are alternately pulsed on to the substrate surface.

• Precursor gases introduced on to the substrate surface will chemisorb or surface reaction takes place at the surface

• Surface reactions on ALD are complementarity and self-limiting

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ALD Example Cycle for Al2O3 Deposition

• In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group.• With silicon this forms : Si-O-H • After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is pulsed into

the reaction chamber

Tri-methylaluminumAl(CH3)3(g)

CH

HH

H

Al

O

Hydroxyl (OH)from surfaceadsorbed H2O

Methyl group(CH3)

Substrate surface (e.g. Si)

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ALD Cycle for Al2O3

• Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, producing methane as the reaction products

C

H

H

H

H

Al

O

Reaction ofTMA with OH

Methane reactionproduct CH4

H

HH

HH C

C

Substrate surface (e.g. Si)

Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2 (s) + CH4

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ALD Cycle for Al2O3

• Trimethyl Aluminum (TMA) reacts with the adsorbed • hydroxyl groups, until the surface is passivated. TMA does not react with itself,

terminating the reaction to one layer. This causes the perfect uniformity of ALD. The excess TMA is pumped away with the methane reaction product.

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ALD Cycle for Al2O3

• After the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.

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ALD Cycle for Al2O3

• H2O reacts with the dangling methyl groups on the new surface forming Aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse. Again methane is the reaction product.

2 H2O (g) + :Si-O-Al(CH3)2 (s) :Si-O-Al(OH)2 (s) + 2 CH4

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ALD Cycle for Al2O3

• The reaction product methane is pumped away. Excess H2O vapor does not react with the hydroxyl surface group, again causing perfect passivation to one atomic layer.

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ALD Cycle for Al2O3

One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle. Each cycle including pulsing and pumping takes e.g. 3 sec.

Al (CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2 (s) + CH4

2 H2O (g) + :O-Al(CH3)2 (s) :Al-O-Al(OH)2 (s) + 2 CH4

Two reaction steps in each cycle:

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ALD Precursor RequirementsMust be volatile and thermally stablePreferably liquids and gases Should Chemisorb onto the surface or rapidly

react with surface and react aggressively with each other

-Short saturation time, good deposition rate, no gas phase reactions Should not self-decompose - Affect thickness, uniformity Should not etch, dissolute into film or substrate

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Deposition Advantages Alternating reactant exposure creates unique

properties of deposited coatings: Thickness is determined simply by number of deposition cycles

Precursors are saturatively chemisorbed → stochiometric films with large area uniformity and 3D conformality

Intrinsic deposition uniformity

Low temperature deposition possible

Gentle deposition process for sensitive substrate

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Comparison of ALD and CVD

ALD• Highly reactive precursors• Precursors react separately on

the substrate• Precursors must not

decompose at process temperature

• Uniformity ensured by the saturation mechanism

• Thickness control by counting the number of reaction cycles

• Surplus precursor dosing acceptable

CVD• Less reactive precursors• Precursors react at the same

time on the substrate• Precursors can decompose at

process temperature

• Uniformity requires uniform flux of reactant and temperature

• Thickness control by precise process control and monitoring

• Precursor dosing important

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High-k dielectrics (Al2O3, HfO2, ZrO2, Ta2O5, La2O3,)

for transistor gates and DRAM capacitors in Si, GaAs, Heterostructures, compound semiconductors, Mesfets, III-V Semiconductor materials, organic transistors, graphene, graphite, nanotubes, nanowires, molecular electronics,

Conductive gate electrodes (Ir, Pt, Ru, TiN, )

Metal interconnects and liners (Cu, WN, TaN, WNC, Ru, Ir)

Metallic diffusion barrier layers for copper interconnects and semiconductor vias for transistor gate and memory cell applications, DRAM capacitors, Passivation layers

Catalytic materials (Pt, Ir, Co, TiO2, V2O5)Coatings inside filters, membranes, catalysts (thin

economical Pt for automobile catalytic converters), fuel cells ion exchange coatings

Nanostructures (all materials)Conformal deposition around and inside

nanostructuresand MEMS

Biomedical coatings: (TiN, ZrN, CrN, TiAlN, AlTiN)Biocompatible materials for in-vivo medical devices

and instruments

ALD 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)

Solid Lubricant layers (WS2, )

Photonic crystals (ZnO, ZnS:Mn, TiO2, Ta2N5, )coatings inside porous alumina, inverted opals

Anti-reflection and optical filters (Al2O3, ZnS, SnO2, Ta2O5)Fabry-Perot, Rugate, Flip-Flop optical filters

Electroluminescent devices (SrS:Cu, ZnS:Mn, ZnS:Tb, SrS:Ce)

Processing layers (Al2O3, ZrO2, Etch barriers, ion diffusion barriers, fill layers for magnetic read heads

Optical applications (AlTiO, SnO2, ZnO)Photonics, Nanophotonics, Solar cells, integrated optics, optical coatings, lasers, variable dielectric constant nanolaminates

Sensors (SnO2, Ta2O5, )Gas sensors, pH sensors,

Wear and corrosion inhibiting layers (Al2O3, ZrO2)

ALD Applications summary

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References• Cambridge NanoTech Inc., Cambridge, MA 02139 USA

www.cambridgenanotech.com/.../ Atomic%20Layer%20Deposition%20tutorial%20Cambridge%20NanoTech%20Inc.pdf

• www.mne.umd.edu/.../465_spring_2003/465_ spr2003_final_project_results/ALD-finalpres-465-spr2003.pdf

• ICKNOWLEDGE LLC, Georgetown, MA 01833, www.icknowledge.com/misc_technology/ Atomic%20Layer%20Deposition%20Briefing.pdf

• B.S.Lim, A. Rahtu and R.G. Gordon, Nature Materials, 2 (2003) 749-754