22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Reactive plasmas in multi-ICP system: spatial characterization by threedimensional simulation
J. Brcka1
1
Tokyo Electron US Holdings Inc., Technology Development Center, Austin, TX, U.S.A.
Abstract: A multi-ICP system can be used to increase plasma uniformity, which makes it
possible to increase the processing area and provide additional variables for controlling the
plasma. The 3D model of an asymmetric multi-coil ICP using COMSOL multiphysics
software is explored to characterize reactive plasma parameters of different gases (Ar, H 2 ,
CH 4 , CO) and their mixtures.
Keywords: inductively coupled plasma, multi-coil, 3D simulation, COMSOL
1. Introduction
Beginning from the first generations of the tools
designated for plasma processing of the planar silicon in
ICs fabrication, there was always development trend to
match tool capabilities with increased wafer size. Scaling
plasma sources to larger substrate size was always
complicated due to many technical issues that were
needed to be resolved. The most important aspect was to
provide uniform performance of reactive plasma source
over the full size silicon wafer. Current uniformity
requirements are very stringent; typically nonuniformity
should be below 3 % across the wafer. A multi-ICP
source is suggested to enhance the plasma uniformity for
large-area, low pressure plasma processing [1]. A parallel
configuration of the ICP sources will decrease the total
impedance of the system and enable a higher electron
density. Additionally, it offers additional variables for
controlling the plasma.
Scaling plasma sources is also challenged by the plasma
chemistry. Recently, Okada [2] reported preparation of
nano-structured carbon materials (such as nanocrystalline
diamond, carbon nanotubes, and carbon nanocapsules)
from a CH 4 /CO/H 2 mixture in an ICP plasma and
speculated on the role of CO in nanocarbon generation.
For industrial applications this process would need to
be scaled to large area semiconductor manufacturing
technology with a minimized design and development
cost. A plasma discharge model with reasonable accuracy
is needed to support the prediction of the scaled up
plasma tool and processing procedure. The model can
further be used to design the optimal configuration and
operational parameters of the final plasma tool design to
be used for semiconductor manufacturing.
Models of ICP plasmas were described elsewhere [3-7].
The numerical simulation techniques commonly used for
simulating low-temperature plasma discharge mainly
include fluid dynamic, kinetic and hybrid models. These
models are significantly different in principles, strengths,
applications and limitations. The fluid models are used
widely for simulation of plasma tools because of its
efficient computational cost. This option makes special
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significance when the technical solution brings additional
factors into consideration such are asymmetry,
dimensional scale, transient performance, etc. Content
and approach in this work is challenging technological,
computational, dimensional scaling and plasma chemistry
aspects under one framework.
2. Model
Plasma simulation is becoming an essential technology
used to develop new semiconductor manufacturing
equipment and develop improved process control
schemes. In ICP systems the inductive antenna is coupled
to the excited plasma inside the low pressure gas reactor.
Using multiple antennae the mutual inductance between
them and to the plasma can influence plasma distribution
inside the chamber and impact the overall performance of
the source. Assembling the source from individual
sources changes the symmetry of the system. Operation
of the multi-source configuration is sensitive either to
return RF currents in the hardware or inside the plasma.
To perform our feasibility investigation we formulated
a 3D plasma model using COMSOL Multiphysics [8]
suite which is time-efficient for computation for most of
the two-dimensional models and flexible in setting the
geometry of the actual chamber.
As mentioned
previously, the proposed plasma tool configuration
involves a certain degree of asymmetry due to the multicoil ICP source configuration (Fig. 1). Each coil is
represented by a spiral tube (∅3 mm copper tube) with
five turns (R max =85 mm, R min =50 mm and 7 mm radial
pitch). Formally, for modelling purpose the power from
the RF generator (13.56 MHz) is delivered to a small
section of each coil to establish identical conditions in
power distribution between the coils. The actual power
distribution requires additional equipment which is not
described within the scope of this work. The coils are
separated from the plasma by a dielectric window.
Electrostatic coupling between the coils and plasma is
inevitable and may cause an electrical asymmetry in the
plasma. We accounted for this within the geometrical
model with a domain under the dielectric window to be
1
assigned features to serve as a grounded shield with
embedded slots.
In this work we used a cross section set obtained from
IST-LISBON data at LXCat open-access database [10-13]
and limited the reaction scheme (Fig. 3) to primary
electron collisions with hydrogen molecules.
Fig. 2. Primary electron collisions with argon atoms.
a)
Fig. 3. Primary electron reactions assumed in simple
plasma model of hydrogen.
b)
Fig. 1. Chamber cross-section (a) and top view on multiICP source with four spirals.
The goal was to investigate the plasma characteristics in
a chamber scaled towards large area (450 mm and above)
silicon wafer processing and coupled to CH 4 /CO/H 2
chemistry. Baseline plasma characterization of the multiICP system was initially performed in pure argon gas and
further investigations were done under increased
complexity of the reaction scheme, either in CH 4 , CO, H 2
or their mixtures. We used limited cross sections for the
reaction scheme (Fig. 2) of electron-argon collisions that
were obtained from MORGAN data at LXCat openaccess database [9].
Electron collisions drive the entire processing plasma
chemistry and are among the most important and critical
processes that we need consider. Hydrogen (H 2 ) is
considered a carrier gas (typically, 90 % of the mixture).
2
Methane (CH 4 ) has been the subject of investigation for
many years and its cross sections were easily the most
studied ones when comparing to other gases heavily used
in semiconductor manufacturing, such are CF 4 or SiH 4 .
Mantzaris et al. [14] developed a self-consistent, 1D
simulator for the physics and chemistry of radio
frequency (RF) plasmas. The model for CH 4 chemistry
considers four species, CH 4 CH 3 CH 2 , and H. The
authors determined that CH 4 plasmas are electropositive
with negative ion densities one order of magnitude less
than those of electrons. The high-energy tail of the
electron energy distribution function (EEDF) in CH 4 , lies
below both the Druyvensteyn and Maxwell distributions.
In our model we used a limited reaction scheme (Fig. 4)
for CH 4 cross sections and recommendations from work
by Morgan [15] and references therein. Species and rates
for neutral gas reactions in CH 4 and H 2 (see details in
work by Petrov and Giuliani [16]) either unimolecular or
bimolecular were not included in the model at this stage
and will be considered later together when coupling the
plasma model to a heat transfer model. The following
species due to primary collisions with electrons were
considered for CH 4 molecules: the ions CH 4 +, CH 3 +,
CH 3 , CH 4 * vibrational excitations into σ v (2,4) and
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σ v (1,3), and total electronic excitations into CH 4 **
leading to dissociation [15].
Fig. 4. Electron reactions in plasma model for methane.
Fig. 5. Electron reactions used in plasma model of CO
gas.
Carbon monoxide (CO) cross section data downloaded
from the website jila.colorado.edu contained compilation
of Phelps data, and according to the author’s note is very
similar to the original cross sections by Land [17]. We
accounted for individual cross sections for published
vibrational excitations but a single excited molecule CO*
was considered as resultant product. Similarly, the
electronic excitation state CO** represents 5 different
excitation levels. Considered plasma species in the case
of CO molecules, were CO, CO+, CO* (all vibrational
excitations) and CO** (all electronic excitations).
Suggested reaction scheme for CO plasma is shown in
Fig. 5.
Oxygen (O) cross sections [18,19] were
considered as the second order collisions in plasma of
H 2 +CH 4 +CO mixture.
Molecular gases tend to be readily dissociated by
electron collisions (Maxwell EEDF was used in the
model). Rate coefficients for dissociative recombination
(e.g., AB++e-→A+B) of molecular ions CO+, O 2 +, CH 4 +,
CH 3 +, CH 2 + and CH+ were obtained from Mitchell [20],
where gas temperature was substituted by electron
temperature. Volumetric electron loss process (collisional
radiative recombination) was approximated by net
recombination rate [21].
Consideration of the species and cross sections
mentioned does not mean we included all possible
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collision processes that will occur in the plasma. The
proposed scheme is a formal approach to match the actual
reaction set but with a limited set of participating species,
thus our model is rather limited on chemistry aspects.
3. Multi-ICP source operation and simulation results
Operation of the multi-ICP source can be performed in
several modes. The system can run with all coils in
parallel connection to the RF generator or controlled
individually by a controller. In this work a spatial plasma
distribution was investigated with respect to the phase
applied at each coil. Three configurations were proposed
for simulation (Fig. 6). The baseline case assumed an
identical phase on each coil. The antiphase case consisted
of π phase difference between the coils. The last
configuration refers to π phase difference between two
sets of coils.
Fig. 6. Simulation cases in spatial characterization of the
reactive plasma.
Work on computation is ongoing, robust cases are
converging within several days up to week. Discussion of
ongoing simulations and obtained results extends the
limited space within the abstract. The results will be
reported during conference event.
4. Conclusions
Computational framework in 3D geometry allowed
successful prediction of the spatial distributions of plasma
and reactants in large area multi-ICP system. Global
uniformity of the plasma at diverse process conditions can
be controlled by the RF phase on individual coils. MultiICP configuration provides dynamic control of plasma
composition.
5. References
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[8] www.comsol.com
[9] MORGAN database, www.lxcat.laplace.univ -tlse.fr
[10] IST-LISBON database, www.lxcat.laplace.univ tlse.fr
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[15] W. L. Morgan, Plasma Chemistry and Plasma
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