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