Mukhtar HussainDepartment of Physics &

AstronomyKing Saud University, Riyadh

awanchep@gamil.com

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

“ 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.

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

Self-termination of adsorption provides atomic scale control of the film thickness and ensures uniform coverage.

PrinciplesPrinciples of of ALDALD Technique TechniquePrinciplesPrinciples of of ALDALD Technique Technique

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-methylaluminumAl(CH3)3(g)

CH

HH

H

Al

O

Hydroxyl (OH)from surfaceadsorbed H2O

Methyl group(CH3)

Substrate surface (e.g. Si)

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 ofTMA with OH

Methane reactionproduct CH4

H

HH

HH C

C

Substrate surface (e.g. Si)

C

HH

Al

O

Excess TMAMethane reactionproduct CH4

HH 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)

C

HH

Al

O

H2O

HH C

OHH

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

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

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

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 OO OO

Al Al AlO O

O OO

Al Al AlO 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:

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

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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

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Closed System ChambersClosed System Chambers The reaction chamber walls are designed to effect the The 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 "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06. <April 06. <www.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdf>.>.

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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 "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April 06. <April 06. <www.icknowledge.com/misc_technology/Atomic%20Layer%20Deposition%20Briefing.pdf>>

22 ”Atomic Layer Deposition," Aviza Technology. 26 ”Atomic Layer Deposition," Aviza Technology. 26 April 06. <April 06. <http://www.avizatechnology.com/products/verano.shtml>.>.

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

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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

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

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

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