Atomic Layer
Deposition (ALD)
overview
Oxford Instruments Plasma
TechnologyShohei Ito
Customer support Manager© Oxford Instruments 2021

Outline• What is ALD?
• Processes• System Features• Application examples• In situ diagnostics• Control of ion energy© Oxford Instruments 2021

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What is Atomic Layer Deposition (ALD)?
ALD is a sequential, self limiting surface reaction,
based on two or more reactants• Precise thickness control

Thermal ALD Al2O3
InitialsurfacePurge• Growth rate is independent of precursor flux once surfaces
are saturated• Conformal coating even in high aspect ratio structures• Low pin hole levels• No gas phase reactions generating particles• Very thin and dense films• Low deposition rates, typically 1Å/cycle• Wide variety of materials possible• limited by precursors© Oxford Instruments 2021

Al(CH3)3

H2O

Purge

Plasma ALD Al2O3
InitialsurfacePurge

O2 Plasma

Al(CH3)3

Purge
3ALD cycle of Al2O3 thermal

Initialsurface

Purge

Al(CH3)3

H2 O© Oxford Instruments 2021

Purge

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ALD cycle of Al2O3 plasma enhanced

InitialsurfacePurge

O2 Plasma© Oxford Instruments 2021

Al(CH3)3Purge5How to identify true ALD growth

Growth per cycle(GPC)

1. Saturation

2. Linear growth

ALD

Dose time

3. ALD temperature window

ALD

ALD

Number of cycles

Temperature• A plateau in GPC as dose times increase indicates self-limited, ALD.
The metal precursor and plasma dose times should follow this trend.
• A linear trend indicates layer-by-layer deposition. Exponential trend
indicates under-saturation or CVD.
• Low GPC at low T indicates slow deposition kinetics, i.e. undersaturation, poor nucleation. High GPC at high T indicates precursor
decomposition.
© Oxford Instruments 2021

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Growth per cycle vs temperature• Differing possibilities depending on reaction mechanism• ALD temperature window not always flat, since equilibrium densityof reactive surface sites (e.g., OH), depends on temperature.

Growthpercycle

Condensation

Decomposition

ALD window

Incomplete reaction

Desorption of loss of
surface species

Temperature© Oxford Instruments 2021

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Metrics for film deposition

Knoops et al., Handbook of Crystal Growth, 2nd edition, p. 1101–1134 (2015).

Strong points of ALD:
• Growth control at the subnanometer level• Uniformity of the films on large substrates• Conformality for complex surface features• Choice of deposition temperature down to low
substrate temperatures (using plasma)© Oxford Instruments 2021

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Thermal and Plasma
ALD benefits from Plasma•

Improved material properties• Film density and composition can be influenced• Lower impurities•

Increased choice of precursors and materials• Nitrides

Plasma essential for lower
oxygen content and lowerresistivities.

• Metals•

Reduced substrate temperatures•

Surface pre-treatment• Surface activation and surface cleaning•

Increased growth rate•

Shorter nucleation periods

Plasma not recommended for•

Aspect ratios above 50:1•

Substrates reactive with plasma© Oxford Instruments 2021

Plasma allows growth often at
100 °C or lower temperatures.

Often soft plasma
conditions can be used tominimize damage.

9Plasma ALD – reactor types

Direct plasma:

Substrate part of plasma creation zone
Strong ion bombardment
Plasma damage to sensitive devices© Oxford Instruments 2021

Remote plasma:

Substrate “downstream” of plasma creationzone
Mild ion bombardment at low pressure,virtually no ion energy at high pressure
Excellent compromise between direct plasma
ALD and radical enhanced

Radical Enhanced:

No plasma above substrate. Often bought
in source.
No ions, low radical flux
Poor film quality and uniformity

10FlexAL ALD system•

Cutting edge plasma ALD systems withthermal ALD as standard•

Mixed mode operation within a single recipe•

No hardware changes required to switch mode
between plasma and thermal ALD•

start with thermal Al2O3 on sensitive interface butcontinue with plasma ALD for best material properties•

for platinum start with plasma to minimise nucleation
delay and continue with thermalFlexAL© Oxford Instruments 2021

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Outline• What is ALD?
• Processes• System Features• Application examples• In situ diagnostics• Control of ion energy© Oxford Instruments 2021

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Plasma ALD processing advantages•

Reduced substrate temperatures
••

•

•

Nitrides

Al2O3AlN

Co3O4

Improved material properties

Ga2O3GaN

HfO2HfN•

Film density and composition can be influenced

In2O3

•

Lower impurity levels

Li2CO3

Increased choice of precursors and materials•

Nitrides including AlN, Si3N4, TiN•Metals

Surface pre-treatment••

Advantageous for temperature sensitive substrates
such as LiNbO3, LiTaO3, polymers, plastics, etc.

Oxides

Surface activation and surface cleansing, e.g. GaN

Shorter nucleation periods, e.g. plasma ALD ofmetals© Oxford Instruments 2021

Metals

MoO3

Pt

Nb2O5

RuNiO

Fluorides
AlF3

SiO2

NbN

Si3N4

SnO2
Ta2O5

TaN

Sulphides

TiO2TiN

MoS2

WO3

WN

WS2ZnOTiS2/TiS3

ZrO213

Plasma surface pre-treatments• Optimal pre-treatment for surface can be integrated in recipe (plasmapressure, power, gas flow).
• Improved nucleation for certain surfaces (short O2 plasma on a-Si)• Functionalizing graphene, CNTs (O2 or H2 plasma)• Improving adhesion (H2 plasma for Pt on TiO2)• In-situ cleaning (TMA pulses to remove oxide, O2 plasma for carbon)
ALD process

Pretreatment
plasma,precursor,gas© Oxford Instruments 2021

With biasing chemical sputter cleanof surfaces (e.g. Ar/H2 to remove
native oxide and carbon)• Before deposition of dielectrics
or metal contact• Adhesion improvement14

Ar sputter etch rates of Al2O3, HfO2, SiO2 and Si3N4

Etch rates (Å/min) under Ar
plasma exposure weredetermined using thedifference in film thickness,before and after etch asmeasured by ellipsometry.

SiO2 removal byin-situ bias
plasma?

ALD Al2O3
as sputtermask?

Increasing bias

© Oxford Instruments 2021

Plasma cleaning by SF6 for TiN• Plasma cleaning using SF6
plasma without bias

Etch rates of TiN coated on Si test pieces
placed at various locations in the system.
Effective cleaning for surfaces >100 °C(temperature at which TiFx becomes volatile).

• Data available for TiN deposition(should also work for Si, W, Mo,and Ta compounds)• Expected frequency of cleaningand downtime• Clean every >450 nm• <10 min plasma clean for TiN© Oxford Instruments 2021

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Low damage for depositing on polymers
Rate of the resist removal during 50
plasma exposures of 5s each• To limit etching of a polymer plasma
conditions can be set to reduce etching.

• High pressure plasma allows growth onpolymers without etching them.
• ALD deposit will also function asprotective layer, so generally etching isminimal.

© Oxford Instruments 2021

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We enable material property control by substrate biasing

A wide range of properties can be tuned by substrate biasing in ALD• First to introduce substrate biasing in ALD• >10 years of plasma ALD experience© Oxford Instruments 2021

Faraz et al., ACS Appl. Mater. Interfaces 10, 13158 (2018)18Deposition of Nb2O5• Plasma and thermal ALD using
TBTDEN and O2 plasma or H2O.
Wide temperature range for plasma andthermal ALD with higher GPC for plasma
process

Material
Precursor
Co-reactant
Temperature range
Growth per cycle
Refractive Index
Uniformity

Nb2O5
TBTDENO2 plasma, H2O thermal
150ºC – 350 °C
0.56 Å/cycle (plasma)0.38 Å/cycle (thermal) @ 200°C
2.35 – 2.46
± 3-4% over 200mm

Nb2O5 on c-Si
giving high surface
passivation afteranneal

Basuvalingam et al., J. Vac. Sci. Technol. A 36, 041503 (2018)
Macco et al., Sol. Energy Mater. Sol. Cells 184, 98 (2018)© Oxford Instruments 2021

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ALD process schemes: multi-component materials
Besides the regular AB process scheme, there are also ABC multistep and ABAB supercycle
schemes, which are easily implemented in recipe based software.

e.g. Al2O3, TiO2,Pt

e.g. Pt at low
temperaturee.g. ZnO:Al,HfSiOx, SrTiO3
Knoops et al., Handbook of Crystal Growth, 2nd edition, p. 1101–1134 (2015).

© Oxford Instruments 2021

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Conclusions• Extremely flexible thermal and plasma ALD system• Wide range of precursors and films• Low temperature capability for temperature sensitive deposition• Pretreatments prior to deposition to improve interface and nucleation• Turbopumping for superior nitride films• Substrate bias capability for film densification and stress control• Clusterable with other modules for transfer without vacuum break© Oxford Instruments 2021

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