Point of Use (POU) Gas Purification for
Atomic Layer Deposition (ALD) Processes
Anthony Ricci
ALD Process Overview
A promising new method of depositing very
thin films has been developed for a variety of
applications, including gate dielectrics,
capacitor dielectrics, and diffusion barriers.1
The new technique, called atomic layer
deposition (ALD), solves many of the most
significant problems associated with traditional
thin-film deposition technologies, including
conformal film deposition over high-aspect
ratio structures, high-temperature processing,
large-area film uniformity, and accurate film
thickness control.
Traditional thin-film deposition technologies,
such as physical vapor deposition (PVD)
require ultra-high vacuum conditions and highdensity plasmas for accurate metal or metal
nitride deposition over high-aspect ratio
structures, and are often plagued by
conformality and plasma damage problems.
With chemical vapor deposition (CVD)
processes, accurate film thickness and
uniformity over large areas are difficult to
control.
ALD uses a precursor and a reactant, which are
introduced separately in a pulsed fashion to
bond chemically with the heated wafer surface
until self-limiting surface saturation is reached
(all available surface sites are occupied). The
precursor and reactant pulses are performed
with fast -actuating valves in order to control
the amount of precursors introduced into the
reactor chamber. An inert purge gas separates
the pulses, carrying away unreacted precursor
and reactant molecules, and surface reaction
byproduct molecules. ALD reactions will only
take place if the starting surface is prepared
to chemically react directly with each precursor
and reactant type. The sequence of surface
reactions (precursor + inert gas purge +
reactant + inert gas purge) is called the ALD
deposition cycle (Figure 1).2
Figure 1. Generic ALD Process Flow
Time
Flow, Pressure
Application Bulletin
ABG-105-0305
Cycle time
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Process Parameters:
• Reaction A
• A Purge
• Rx B (Plasma Opt)
• B Purge
Courtesy of Genus
Figure 2. Contaminants in ALD Processes
• heated walls are recommended to reduce
physisorption of undesirable molecules,
• diffusion of precursors limits control of pulse
separation, and
• purge gas velocity governs the macro flow.
Deposition of an ALD Al2O3 Layer
Ti (first layer)
ALD Chemical Framework
The precursors and reactants used in ALD are
introduced into the ALD reactor separately.
This is done by separating precursors and
reactants with an inert gas (N2 or Ar) purge
step.
Ti (second layer)
Dimethylamino
or Diethylamino
N
Top surface layer in <111> plane
of TiN
Adsorption of Ti(NR2)4 on Ti surface
TiN. Coverage is limited due to
steric hinderance between adsorbed
precursor molecules because of
interference of ligands on adjacent
precursor molecules
After exposure to NH3, the ligands
are removed leaving a partial
monolayer coverage of Ti and N.
Subsequent exposure will then
fill in the gaps.
Courtesy of Novellus
The surface-saturated film growth process in
ALD ensures excellent conformality and step
coverage, excellent within-wafer and waferto-wafer uniformity, and accurate film
thickness control. All of these aspects are
critical when depositing ultra-thin films on
high-aspect ratio structures.
ALD Tools
The ALD reactor can be either single or batch;
the following are key reactor characteristics2:
• self-limiting growth ensures precursor fluxes
do not need to be uniform over the wafer,
• rapid precursor switching and purging is
needed,
2
For example, in the deposition of tantalum
nitride films, first hydrogen gas is passed over
a SiN layer, forming SiN-H to promote tantalum
adhesion. Then the tantalum metal precursor
pulse reacts with the SiN-H layer to form SiNTaCl3 and HCl vapor. An inert purge gas step
is used to remove the HCl vapor from the
ALD reactor.
To continue from the previous example, the
nitride layer is deposited on the tantalum layer
by pulsing the reactant gas (NH3) to form SiNTa-N. Again, HCl is the volatile byproduct
molecule. An inert purge gas step removes
the reaction byproduct (HCl) and any
unreacted reactant (NH3) from the vacuum
chamber. A sub-nm conformal layer of TaN
has been deposited.
One typical cycle deposits roughly 1 Å of film
in a few seconds. The final film thickness may
be precisely determined by controlling the
number of deposition cycles (Figure 2).3
Contaminants in ALD Processes
The purity of the purge, carrier, and reactant
gases, and the control of outgassed materials
that were physisorbed to the reactor walls,
are key parameters because ALD is a relatively
slow and low-temperature process, compared
to other deposition technologies.3 As a result,
molecular contamination has a greater negative
effect on film quality and throughput. Also, in
order to reduce operating temperatures (lowtemperature ALD), longer purge gas steps are
needed to achieve highly uniform films.4 Highpurity inert gases such as nitrogen or argon
can contain combined water and oxygen
contamination levels as much as 2 ppm.5 These
inert purge gases are used to remove the
volatile byproduct and any unreacted precursor
and reactant molecules. Molecular contamination
in carrier gases such as hydrogen could
disintegrate precursor species and be deposited
onto the reactor walls.
POU Purification for ALD Processing
The placement of purifier assemblies in the
purge, carrier, and ammonia reactant gas lines
directly prior to reactor entry (i.e., at the pointof-use) will provide the best defense against
contamination from entering the reactor. Pall
AresKleen™ purification medium is capable of
removing gaseous contamination such as water,
oxygen, carbon monoxide, and carbon dioxide
to sub-ppb levels from inert purge and carrier
gases, such as nitrogen, argon, and hydrogen.
Pall Gaskleen® purifier assemblies are available
in Top Mount and in-line configurations.
(Refer to Pall Gas Filtration Purifiers Datasheets.)
POU filtration is effective at removing
particulate contamination ≥ 0.4 microns at
99.9% removal efficiency in the low-vapor
pressure precursors by using the in-line Pall
Gasket-Sert™ PSP filter.
(Refer to Pall Gas Filtration Metal Datasheet.)
Glossary
Aspect ratio – the ratio of height-to-width of a given
step, trench, or contact hole.
Chemisorption – to chemically (irreversibly) bind a
substance on the surface of another substance.
Conformality – a measure of how well a deposited
layer conforms to the layer beneath it.
References
1. Markku Leskela, “ALE research 6/99 – 9/00,”
University of Helsinki, Laboratory of Inorganic
Chemistry.
2. Tom Seidel, “ALD Reactor Architectures,” NSFSRC Network Forum, (4/24/2003).
3. Aaron Hand, “Industry Begins to Embrace ALD,”
Semiconductor International, (5/1/2003).
4. M.J. Biercuk, D.J. Monsma, C.M. Marcus, J.S.
Becker, and R.G. Gordon, Applied Physics Letters
83, 12 (2003).
5. Nitrogen, Matheson Purity 99.9995%,
www.mathesontrigas.com.
Gas Filtration Purifiers Data Sheet A79a
Mini-Gaskleen™ Purifier
Gas Filtration Purifiers Data Sheet A88
Gaskleen® II Purifier
Gas Filtration Purifiers Data Sheet A87a
Gaskleen® ST Purifier
Gas Filtration Purifiers Data Sheet A81a
Maxi-Gaskleen™ Purifier
Gas Filtration Purifiers Data Sheet A86a
Gaskleen® 11⁄8” C-seal Purifier
Gas Filtration Metal Data Sheet A94
Gasket-Sert™ PSP Filter
Deposition rate – the rate at which a given film is
deposited. It is usually measured in Angstroms/minute.
Physisorption – to physically (reversibly) bind a
substance on the surface of another substance.
Step coverage – the degree to which a film can evenly
cover the sides and bottom of vertical features, such
as trenches and contact openings.
Uniformity – a measure of how far a given parameter,
such as deposition rate, varies from the average value
across one wafer, from wafer to wafer in a batch, or
from wafer batch to batch.
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