22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Remote plasma sources sustained in NF 3 mixtures
S. Huang1, J.R. Hamilton2,3, J. Tennyson2,3 and M.J. Kushner1
1
Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Avenue,
US-48109-2122 Ann Arbor, MI, U.S.A.
2
Quantemol Ltd., University College London, London WC1E 6BT, U.K.
3
Department of Physics and Astronomy, University College London, London WC1E 6BT, U.K.
Abstract: Remote plasma sources (RPS) are being developed for low damage materials
processing during semiconductor fabrication. Plasmas sustained in NF 3 are often used as a
source of F atoms. NF 3 containing gas mixtures (e.g., NF 3 /O 2 , NF 3 /H 2 ) provide the
additional opportunity to produce and control desirable F atom containing reactive species.
In this paper, results from computational investigations of RPS are discussed using global
and multidimensional models. A comprehensive reaction mechanism for plasmas sustained
in Ar/NF 3 /N 2 /H 2 /O 2 was developed using ab initio cross sections for NF x based on the
molecular R-matrix method. NF x rapidly dissociates due to dissociative attachment.
Addition of O 2 enables formation of NO, FO and FNO from the dissociation products.
Keywords: nitrogen trifluoride (NF 3 ), remote plasma source, capacitively coupled plasma
1. Introduction
Remote plasma sources (RPS) are used in
microelectronics fabrication to produce fluxes of radicals
for etching and surface passivation in the absence of
damage that may occur by charging and energetic ion
bombardment. RPS reactors use distance, grids or other
discriminating barriers to reduce or eliminate charged
particle fluxes from reaching the surface of the material
being treated [1]. Nitrogen trifluoride (NF 3 ) is frequently
used in RPS for the ease with which F atoms are produced
by dissociative attachment. F atoms are the main etchants
of silicon-containing materials such as SiO 2 , SiC and
Si 3 N 4 .
RPS sustained only in NF 3 typically limits the reactive
fluxes reaching the processing chamber to F, F 2 , NF x , N
and N 2 . RPS sustained in NF 3 gas mixtures, such as
NF 3 /O 2 , increases the variety of reactive species
produced. For example, the use of NF 3 /O 2 mixtures
increases the etch rate of Si 3 N 4 by production of NO
which aids in the removal of the N atom from the surface
[1]. The production of NO may, however, increase
roughening of the surface [2, 3]. The use of NF 3 /N 2
mixtures increases the etch rate of SiO 2 by aiding the
removal of the O atom, which in turn enhances the
formation of the SiF x etch product [4].
In this paper, we discuss results from a computational
investigation of RPS sustained in NF 3 containing gas
mixtures. Two modelling approaches were used – global
modelling to
investigate
fundamental
reaction
mechanisms and 2-dimensional modelling to address the
spatial dynamics of flow. Cross sections for NF x were
generated using ab initio computational techniques based
on the molecular R-matrix method.
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2. Description of the Models
The RPS was simulated using two techniques – a global
model using a plug flow approximation and a
2-dimensional model to address flow and electron kinetics
in a capacitively coupled plasma (CCP). The global
model, Global_Kin, is a 0-dimensional simulation for
plasma chemistry, plasma kinetics and surface chemistry
[5]. By also computing a heating enhanced flow speed,
the time integration in the global model can be used to
represent plug flow. Electron impact and transport
coefficients in the global model are provided by the
electron energy distribution, f(ε), based on solutions of the
stationary Boltzmann’s equation [6]. The global model
was used to investigate and develop the reaction
mechanism.
More system specific issues were
investigated using 2-dimensional modelling with the
HPEM [7]. In the HPEM, continuity, momentum and
energy equations for all species are solved coincident with
Poisson’s equation for the electric potential. Electron
transport is addressed using fluid equations for bulk
electrons and a kinetic Monte Carlo simulation for sheath
accelerated secondary electrons.
A reaction mechanism was developed for plasmas
sustained in mixtures containing Ar/NF 3 /N 2 /H 2 /O 2 . The
mechanism contains 66 species and 940 reactions. The
electron impact cross sections for NF 3 , NF 2 and NF will
be updated using the molecular R-matrix method [8]. The
R-matrix method divides the physical space for the
problem of interaction between electron and molecule
into two regions – an inner region containing the target
molecule and an outer region containing the incident
electron. The method solves the Schrödinger equation in
the inner region independent of the energy of impact
electron and then uses this solution to solve the
Schrödinger equation in the outer region, which is energy
1
dependent.
The cross-sections for electron impact
processes including elastic scattering, super-elastic
scattering from stable electronic states, dissociative
attachment, dissociative excitation and ionization are
calculated using the molecular geometries collected from
the NIST database [9]. All cross sections are calculated
accurately from threshold to 20 eV. Above 20 eV, the
cross section for dissociative attachment goes to 0, while
the cross sections for other processes are extrapolated to
higher energies using the Born correction (for excitation
and ionization) or assuming dominant dipole transition
(for other processes) and scaling with ln(E)/E, where E is
the electron energy.
A schematic of the CCP reactor is shown in Fig. 1.
With the plug flow approximation in the global model,
integration in time is exchanged with integration in space
by computing advective speed based on flow rate and
thermal expansion. Total power deposition by electrons
is specified for a tube 2.8 cm in diameter and 10 cm long.
The afterglow then extends for another 12.4 cm. In the
2-d model, the total power deposition by all species is
specified and the voltage applied to the electrodes at
10 MHz is adjusted to deliver this power. The fraction of
the power dissipated by electrons is computed for use in
the global model so that side-by-side comparisons can be
made.
resulting in a rebound in neutral densities due to
contraction of the gas. NF 2 and NF recombine to form
NF 3 (NF 2 + F 2 → NF 3 + F and NF 2 + NF 2 → NF 3 +
NF), which accounts for some increase in the density of
NF 3 . NO molecules are partially consumed to form NO 2 .
Fig. 1. Schematic of the CCP reactor. The top electrode
is powered and the bottom electrode is grounded as are
other boundaries.
The bounding dielectrics to the
electrodes are alumina. Gases are injected from the left
and are pumped from the right.
3. Scaling of RPS Sustained in NF 3 Gas Mixtures
The base case in our investigation is an NF 3 /O 2 = 70/30
gas mixture at 400 mTorr and a flow rate of 500 sccm.
The total power is 200 W of which 60 W is dissipated by
electrons, the input power to the global model. Spatial
variation of the densities and temperatures from the plug
flow model are shown in Fig. 2. NF 3 and O 2 rapidly
dissociate in the plasma zone largely due to dissociative
attachment, and F atoms are the dominant product. The
fractional dissociation of NF 3 is 32% at the end of the
plasma zone. The maximum F atom density reaches
7.5 × 1014 cm-3. NO is primarily formed by the reactions
N 2 + O 2 → NO + NO and N 2 + O → N + NO which are
aided by an increase in gas temperature to about 1100 K.
The increase in gas temperature is due to charge exchange
and dissociative excitation and attachment which create
high-energy neutrals by the Franck-Condon effect. The
density of NO increases as more N 2 molecules are
formed. In the downstream zone, the gas temperature
decreases due to thermal conduction to the walls,
2
Fig. 2. Densities of (a) neutrals, and (b) charged species
obtained from the global model. The conditions are
NF 3 /O 2 = 70/30, 200 W (60 W into electrons), 400
mTorr, 500 sccm.
The plasma is highly electronegative with a ratio of
negative ions to electrons of about 30. The electron
density increases during the flow to a maximum of
3.6 × 1010 cm-3. This increase is due to the increase in the
proportion of atomic species which have a lower rate of
specific power deposition. To maintain the desired total
power deposition, the electron density must increase.
Formation of negative ions are due to dissociative
attachment (e + NF x → NF x-1 + F, x = 1 – 3 and
In plasma zone, the electron
e + O 2 → O + O-).
temperature is about 5-6 eV. This high value is necessary
to offset the high rate of loss due to attachment.
Downstream the plasma rapidly transitions to an ion-ion
plasma mainly composed of F-, NO+ and NF 2 +. This is
due to the large rate coefficient for attachment to NF x by
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thermal, low energy electrons. As the electrons cool in
the afterglow, the rate coefficient for attachment
increases. Charge exchange predominantly favors the
formation of NO+ and NF 2 + ions as the ionization
potentials of these two species are the lowest among all
positive ions, so charge neutrality is maintained by
[F-] ≈ [NO+] + [NF 2 +].
Time averaged densities of all positive ions, NF 3 , F and
NO, and gas temperature from the 2d model are shown in
Fig. 3. NF 3 and O 2 dissociate and undergo rarefaction as
they flow between the electrodes and are heated. The
electric field is enhanced at the edge of the electrodes at
the intersection with the alumina insulators, a triple point.
This field enhancement locally heats electrons which
increases the local rate of ionization.
This local
enhancement is reflected by the local maximum in ion
density at the edge of the electrodes. F- ions are
dominantly produced by dissociative attachment of NF 3
which is a thermal process. The production of F atoms is
therefore less sensitive to the electric field enhancement.
At the end of the plasma zone, the fractional dissociation
of NF 3 reaches about 24%, which is commensurate with
but lower than the fractional dissociation predicted by the
global model. This disparity is in part due to ionization in
the 2d model having a large component caused by high
energy secondary electrons.
predicted by the global model. The density of F is nearly
constant in the downstream zone as opposed to
rebounding with the cooling gas as with the global model.
This indicates more volumetric losses for F atoms, or the
spatially dependent gas temperature. Axial diffusion, a
processing not accounted for in the global model, likely
also plays a role. The density of NO increases as the gas
temperature increases due to the rate coefficients for its
formation having an activation energy. Rate coefficients
for most reactions responsible for the depletion of NO
(e.g., formation of NO 2 ) do not have such activation
energies. Once NO flows into a cooler region, the overall
rate coefficients for formation of NO decreases more
rapidly than the overall rate coefficient for depletion of
NO. There is then a gradual decrease in the NO density.
4. Concluding remarks
Global and multi-dimensional modeling has been used
to investigate RPS sustained in NF 3 /O 2 mixtures. NF x
(x = 1 - 3) rapidly dissociates in the RPS primarily by
dissociative attachment by thermal electrons. Gas heating
aids in the formation of NO, generated by the dissociation
products of NF 3 and O 2 . The trends predicted by the
global and 2d models generally agree with some
exceptions, such as the axial distribution of F atoms.
These differences are likely attributable to the spatial
distribution of gas temperature and axial diffusion, neither
of which are accounted for in the global model.
5. Acknowledgements
This work is supported by the Samsung Electronics Co.
Ltd., the Semiconductor Research Corp., the DOE Office
of Fusion Energy Science (DE-SC0001319) and the
National Science Foundation (CHE-1124724).
Fig. 3. Time averaged densities of (a) all positive ions,
(b) NF 3 , (c) F and (d) NO, and (e) gas temperature in a
capacitively
coupled
discharge
sustained
in
The conditions are total power =
NF 3 /O 2 = 70/30.
200 W, 10 MHz, 400 mTorr, 500 sccm.
In the downstream zone, the density of NF 3 increases
from 2 × 1015 to 3 × 1015 cm-3 as the gas cools due to
thermal conduction to the wall, a similar trend as
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