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Overview
 Introduction
 Principle
 Precursor Requirements
 Process
 Comparison to CVD Process
 Applications
 Advantages and Disadvantages
 ALD Tool in NNfC - CeNSE
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Atomic layer deposition (ALD) is a chemical vapour deposition technique
based on the sequential self terminating gas-solid reactions to give thin films of
nanometer range.
•
ALD reactions use two chemical sources, A and B of as shown in Figure. 1, typically called precursors1.
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These precursors react with a surface one at a time in a sequential, self-limiting2 manner.
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By exposing the precursors to the growth surface repeatedly, a thin film is deposited as per required
thickness.
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Atomic layer growth can be obtained as
1 Å per cycle
and Atomic layer can be controlled upto ῀0.1 Å (10 pm3) per cycle
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ALD can be used to deposit several types of thin films, including various metal oxides (e.g. Al2O3, TiO2,
SnO2, ZnO, HfO2), metal nitrides (e.g. TiN, TaN, WN, NbN), metals (e.g. Ru, Ir, Pt), and metal sulfides (e.g.
ZnS).
Definition:
1 One of the compounds that participates in the chemical reaction that produces another compound
2 The amount of film material deposited in each reaction cycle is constant
3 1pm ( picometer) = 1x10-12
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• Atomic layer deposition principle was first published under the
name “Molecular Layering” (ML) in the early 1960s by Prof. S.I.
Kol’tsov from the Leningrad (Lensovet) Technological Institute
(LTI), Russia
• Introduced in 1974 by Dr. Tuomo Suntola and co-workers in
Finland to improve the quality of ZnS films used in
electroluminescent displays
• Recently, it turned out that ALD method also produces
outstanding dielectric layers and attracted semiconductor
industries for making High-K dielectric materials.
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Principle of ALD Technique: [Per Cycle]
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•
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First precursor gas (A Source) is introduced into the process chamber and produces a monolayer on
the wafer surface. Then a second precursor gas (B Source) is introduced into the chamber, which
reacts with the first precursor to produce a monolayer of film on the wafer surface.
Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen or argon)
after each precursor pulse to remove excess precursor from the process chamber and prevent
'parasitic' CVD deposition on the substrate.
Two fundamental mechanisms:
 Chemisorption saturation process
 Sequential surface chemical reaction process
Example: ALD cycle for Al2O3 deposition
A Source : H2O ; B Source: Tri methyl Aluminum (TMA)
Four stages of one ALD cycle:
1.
Exposure of substrate to precursor A
: Absorption
2.
Purge with N2 or Ar
: Removal of excess precursor A
3.
Exposure of substrate with precursor B
:R e a c t i o n
4.
Purge with N2 or Ar
: R e m o v a l of excess precursor
B and reaction products
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Figure. 1
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Good ALD precursors need to have the following behaviours:

Volatile
Vapour pressure ( > 0.1Torr at T < 200°C )
Liquid at volatilization temperature without decomposition

Reactive
Able to quickly react with substrate in a self-limiting fashion
(most precursors are air-sensitive)

Stable
Thermal decomposition in the reactor or on the substrate is not allowed

By products
Should not etch growing film and/or compete for surface sites

B Source
A Source
Available
The second precursor (B Source) must react with adsorbed monolayer (A Source) in order to form bonds
and prepare the surface for another dose of (A Source)
Some common precursors include:

Oxidants – for Oxides
Water (H2O), Ozone, Alcohols (ROH), Metal Alkoxides [M(OR)x]
 Reductants
Metals
Hydrogen gas (H2), Ammonia (NH3), Silanes (Si2H6)
Nitrides
Ammonia (NH3), Hydrazine (NH2-NH2)
Sulfides
Hydrogen Sulfide (H2S)
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Closed System Chamber( Most Common)
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Process Temperature
Step Coverage & Deposition Rate
Vs. Deposition Technique
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Per Cycle
One Cycle
Acceptable
temperature range for
deposition.
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Process Temperature
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Process: Step 1a
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Step 1b
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Step 1c
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Step 2a
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Step 2b
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Step 2c
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After 3 Cycles
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ALD
CVD
 Highly Reactive Precursors
Less Reactive Precursors
 Precursors react separately on the
Precursors react at the same time on
the substrate
 Precursors must not decompose at
Precursors can decompose at process
temperature
substrate
process temperature
 Uniformity ensured by the
saturation mechanism
 Thickness control by counting the
number of reaction cycles
 Surplus precursor dosing
acceptable
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Uniformity requires uniform flux of
reactant and temperature
Thickness control by precise process
control and monitoring
Precursor dosing important
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MBE: Molecular Beam Epitaxy ; CVD: Chemical Vapour Deposition ; PLD: Plasma Layer Deposition
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Doping in ALD..!!!!
Doping is carried out by substituting a pulse of precursor A with dopant precursor B
This allows exploration of a wide concentration window without having to prepare new
targets for each concentration (as in sputtered depositions)
Substrate
Dopant cycle B
Grated films with uniform properties possible
Annealing films to “activate” dopants is not required
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
High-K dielectrics for CMOS:
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•
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Reduces Leakage Current
Faster Switching Speed
Cooler Transistors
Candidates for High-K dielectrics
Film
Precursors
Al2O3
Al(CH)3, H2O or O3
HfO2
HfCl4 or TEMAH, H2O
ZrO2
ZrCl4, H2O
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


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Semiconductor memory (DRAM)
ZnO
Si
Cu interconnect barrier
Deposition in porous structures
Very good at coating non planar surfaces (On Si High Aspect Ratio Trenches)
Mixed Oxide Deposition: Layer by Layer
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Advantages of ALD:
 Uniformity
 Conformal Step Coverage
 High Density Film and No Pinholes
 Precise Thickness Control of Deposited Films
 Gentle Deposition Process for Sensitive Substrates
Disadvantages of ALD:
 Deposition Rate Slower than CVD
 Expensive Equipment
 Critical adjustment of the flow:
Too much flow => Clogging of valves
Too low flow => Under-performance
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Technical Specification:
Manufacturer: Beneq
Model: TFS 200
No. of Tool : 1
Year of Manufacture: 2009
Place of Manufacture: Vantaa, Finland
Tool Installed : 2010, CeNSE, IISc - Bangalore
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Substrate temperature range: 25 - 500 °C
Reaction chamber types and dimensions:
Single wafer: Ø 200 × 3 (mm)
Gas lines: up to 8
Liquid sources (+5 °C to ambient): up to 4
Hot source HS 300 (ambient to 300 °C): up to 4
Hot source HS 500 (ambient to 500 °C): up to 2
Plasma specifications (PEALD)
Power: 300 W
Type: Capacitive Coupled Plasma (CCP)
Fluidized bed particle coating option:
- particle size (min.) : 100 nm - 1 μm
- sample volume (max.) : 50 - 75 cm3
- temperature (max.): 450
°C
Control system: PLC control with PC user interface
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