1. Atomic Layer Deposition (ALD)
Maryam Ebrahimi
University of Waterloo
January 17th, 2006
Chem 750/7530

2. Outline ALD Theory and Process Precursor Requirements
Deposition Advantages
Comparison to CVD Process
Applications

3. 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

4. ALD Example Cycle for Al2O3 Deposition
Tri-methyl
aluminum
Al(CH3)3(g) C H Al O
Hydroxyl (OH)
from surface
adsorbed H2O
Methyl group
(CH3)
Substrate surface (e.g. Si)
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

5. ALD Cycle for Al2O3 C H Al O
Reaction of
TMA with OH
Methane reaction
product CH4
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

6. 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.

7. ALD Cycle for Al2O3
After the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.

8. 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

9. 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.

10. Two reaction steps in each cycle:
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 (s)  :Al-O-Al(CH3)2 (s) + CH4
2 H2O (g) + :O-Al(CH3)2 (s)  :Al-O-Al(OH)2 (s) + 2 CH4

11. 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

12. 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

13. Comparison of ALD and CVD
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

14. ALD Applications summary
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)
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 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)

15. References
Cambridge NanoTech Inc., Cambridge, MA USA Atomic%20Layer%20Deposition%20tutorial%20Cambridge%20NanoTech%20Inc.pdf
spr2003_final_project_results/ALD-finalpres-465-spr2003.pdf
ICKNOWLEDGE LLC, Georgetown, MA 01833, Atomic%20Layer%20Deposition%20Briefing.pdf
B.S.Lim, A. Rahtu and R.G. Gordon, Nature Materials, 2 (2003)