ALD is a thin film deposition technique based on self-terminating surface reactions of gas precursors. It involves alternating exposure of a substrate to different precursors separated by purge steps, resulting in one atomic layer of film growth per cycle. ALD provides highly conformal and uniform coatings with atomic-level thickness control due to its self-limiting growth mechanism. It is widely used for depositing oxides, nitrides and some metals in applications such as semiconductors, coatings, MEMS and solar cells.

1. Mukhtar Hussain
Department of Physics &
Astronomy
King Saud University, Riyadh
awanchep@gamil.com

2.  What is ALD Process ?
 Basic Characteristics of ALD
 Principles of ALD Technique
 ALD Cycle for Al2O3 Deposition
 Requirements for Precursors
 Types of ALD Reactors
 Closed System Chambers ALD Reactor
 ALD Applications
 Advantages & Limitations
 Summary

3.  “ It’s a film deposition technique based on sequential use of self
terminating surface reactions”
 ALD is a CVD technique suitable for inorganic material layer as
oxides, nitrides and some metals.
 Perfect for deposition of very thin layers of the size of a
monolayer.

4.  Steps:
◦ Self-terminating reaction of the first reactant (Reactant A)
◦ Purge or evacuation to remove non-reacted reactant and by
products
◦ Self-terminating reaction of the second reactant (Reactant B)
◦ Purge
This is considered as one reaction cycle
 The surface must be in a controlled state, e.g. heated
 Parameters to be adjusted:
◦ Reactants (precursors)
◦ Substrate
◦ Temperature

5. Self-termination of adsorption provides atomic scale control of
the film thickness and ensures uniform coverage.
PrinciplesPrinciples ofof ALDALD TechniqueTechniquePrinciplesPrinciples ofof ALDALD TechniqueTechnique

6. In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group.
With silicon this forms: Si-O-H (s)
 After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is
pulsed into the reaction chamber.
Tri-methyl
aluminum
Al(CH3)3(g)
C
H
H
H
H
Al
O
Hydroxyl (OH)
from surface
adsorbed H2O
Methyl group
(CH3)
Substrate surface (e.g. Si)

7. Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2 (s) + CH4
 Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
producing methane as the reaction product
C
H
H
H
H
Al
O
Reaction of
TMA with OH
Methane reaction
product CH4
H
H
H
H
H C
C
Substrate surface (e.g. Si)

8. C
HH
Al
O
Excess TMA
Methane reaction
product CH4
H
H C
 Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
until the surface is passivized.
 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.
Substrate surface (e.g. Si)

9. C
HH
Al
O
H2O
H
H C
O
HH
 After the TMA and methane reaction product is pumped away,
water vapor (H2O) is pulsed into the reaction chamber.

10. 2 H2O (g) + :Si-O-Al(CH3)2 (s) : Si-O-Al(OH)2 (s) + 2 CH4
H
Al
O
O
 H2O reacts with the dangling methyl groups & form aluminum-oxygen (Al-O)
bridges and hydroxyl surface groups, waiting for a new TMA pulse.
 Again Methane is the reaction product.
O
Al Al
New hydroxyl group
Oxygen bridges
Methane reaction product
Methane reaction
product

11. H
Al
O
O
 The reaction product methane is pumped away.
 Excess H2O vapor does not react with the hydroxyl surface groups,
 That caused perfect passivation to one atomic layer.
O O
Al Al

12.  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.
O
H
Al Al Al
HH
OO
O O
O OO
Al Al Al
O O
O OO
Al Al Al
O O
O OO
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:

13.  Ligand Precursor
◦ To prepare the surface for next layer, and define the kind of
material to growth i.e. H2O for oxides, N2 or NH3 for nitrides, etc.
 Main Precursor (metallic precursor)
◦ Highly reactive (usually this means volatile precursors)
◦ Thermally stable
◦ Full-fill the requirement for self terminating reaction
◦ No self-decomposition
◦ No etching of the film or substrate material
◦ No dissolution into the film or substrate
◦ Sufficient purity

14. 14
Four main types of ALD reactorsFour main types of ALD reactors
 Closed system chambersClosed system chambers
 Open system chambersOpen system chambers
 Semi-closed system chambersSemi-closed system chambers
 Semi-open system chambersSemi-open system chambers

15. 15
 Closed System ChambersClosed System Chambers
 The reaction chamber walls are designed to effect theThe reaction chamber walls are designed to effect the
transport of the precursors.transport of the precursors.
• Open system chambersOpen system chambers
• Semi-closed system chambersSemi-closed system chambers
• Semi-open system chambersSemi-open system chambers
Schematic of
a closed ALD
system
Ref:Ref: "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06."Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06.
<<www.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdfwww.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdf>.>.

16. 16
The Verano 5500™
A 300-mm ALD system by
Aviza Technology, Inc [2].
Process Temperature [1]
[1] [1]
11
"Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06. <"Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06. <
www.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdfwww.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdf>>
22
”Atomic Layer Deposition," Aviza Technology. 26 April 06. <”Atomic Layer Deposition," Aviza Technology. 26 April 06. <
http://www.avizatechnology.com/products/verano.shtmlhttp://www.avizatechnology.com/products/verano.shtml>.>.

17.  Semi & Nanoelectronics
 Coatings on Polymers
 Protective Coatings
 Magnetic Heads
 Thin Film Electroluminescent Displays (TFELs)
 MEMS
 Nanostructures
 Chemical
 Solar Cell

18. 18
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

19.  Self-limiting growth process
 Precise film thickness control by the number of deposition cycles
 No need to control reactant flux homogeneity
 Excellent uniformity and conformity
 Large-area and batch capability
 Dense, uniform, homogeneous and pinhole-free films
 Atomic level composition control
 Good reproducibility and straightforward scale-up
 Surface exchange reactions by separate dosing of reactants

20.  Expensive equipment
 Low Effective Deposition Rate
 Critical adjustment of the flow:
too much flow => clogging of valves
too low flow => under-performance

21. Summary
 Its unique self-limiting growth mechanism which gives perfect
conformality and uniformity.
 Easy and accurate thickness control down to an atomic layer level.
 Closed System Chambers ALD Reactor is one of the mostly used one.
 ALD is a slow method
 Expensive equipment & Low Effective Deposition Rate
 ALD has many applications in the field of Nanoelectronics, Optical,
MEMS, Nanostructures & in Solar cell