Transcript Atomic Layer Deposition (ALD)

Atomic Layer Deposition
(ALD)
Maryam Ebrahimi
University of Waterloo
January 17th, 2006
Chem 750/7530
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Outline
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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
Tri-methyl
aluminum
Al(CH3)3(g)
Methyl group
(CH3)
Al
C
H
H
H
Hydroxyl (OH)
from surface
adsorbed H2O
H
O
Substrate surface (e.g. Si)
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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
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into the reaction chamber
ALD Cycle for Al2O3
Methane reaction
product CH4
H
Reaction of
TMA with OH
H
H
C
H
H
H
H
C
C
H
H
Al
O
Substrate surface (e.g. Si)
• Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl
groups, producing methane as the reaction products
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.
Two reaction steps in each cycle:
Al (CH3)3 (g) + :Al-O-H
2 H2O (g) + :O-Al(CH3)2
(s)
(s)

:Al-O-Al(CH3)2

(s)
:Al-O-Al(OH)2
+ CH4
(s)
+ 2 CH4
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ALD Precursor Requirements
 Must be volatile and thermally stable
 Preferably 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
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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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ALD Applications summary
Piezoelectric layers (ZnO, AlN, ZnS)
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,
Transparent Electrical Conductors (ZnO:Al, ITO)
UV blocking layers (ZnO, TiO2)
OLED passivation (Al2O3)
Solid Lubricant layers (WS2, )
Conductive gate electrodes (Ir, Pt, Ru, TiN, )
Photonic crystals (ZnO, ZnS:Mn, TiO2, Ta2N5, )
coatings inside porous alumina, inverted opals
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
Anti-reflection and optical filters (Al2O3, ZnS, SnO2,
Ta2O5)
Fabry-Perot, Rugate, Flip-Flop optical filters
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
nanostructures
and 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)
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)
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References
• Cambridge NanoTech Inc., Cambridge, MA 02139 USA
www.cambridgenanotech.com/.../
Atomic%20Layer%20Deposition%20tutorial%20Cambrid
ge%20NanoTech%20Inc.pdf
• www.mne.umd.edu/.../465_spring_2003/465_
spr2003_final_project_results/ALD-finalpres-465spr2003.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
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