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30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK
A3
Reaction coefficient of molecular fluorine at wall coated with amorphous
silicon: preliminary results
G. F. Leu1, A. Sublet1, P. Grünenfelder1, T.v.Braucke1, P. Modrzynski1
1
OC Oerlikon Solar, Hauptstrasse 1a, 9477, Trübbach Switzerland
Many industrial relevant PECVD processes require that the reactor is cleaned before every new
deposition. Fluorine containing plasma, as for example SF6, CF4, NF3 or even F2, is used for
cleaning after Silicon based deposition. An important question when modelling or optimizing the
cleaning process is the contribution of the molecular fluorine to the cleaning process.
Cleaning experiments using molecular fluorine (F2) in the absence of a plasma were performed in
a chamber previously deposited with amorphous silicon. It was observed that the main reaction
product was silicon Tetrafluoride (SiF4). The reaction coefficient of molecular fluorine at the
deposited wall was inferred by use of a simple diffusion model. The dynamic behaviour of this
coefficient was measured: it was found to be about 3.4E-3 at the in the first few seconds after
fluorine input and to reach about 3.1E-4 after approximately 100 seconds of fluorine treatment.
1. Introduction
Thin film Silicon deposition processes for the
photovoltaic industry use large area reactors to
deposit amorphous or microcrystalline silicon [1].
To ensure process reproducibility, a fluorine
containing plasma is produced in the reactor before
every new deposition. The fluorine reacts at the
walls with the deposited silicon; volatile silicon
Tetrafluoride (SiF4) is created and pumped out. The
two main cleaning-reactions can be roughly
described as follows:
2F2(g) + Si(s) Î SiF4(g)
(1)
4F(g) + Si(s) Î SiF4(g)
(2)
where (s) means solid state and (g) means gas
state
A coefficient for this kind of gas-wall reaction
can be defined as:
γ react
Γ
=1− r
Γi
(3)
where γ react is the reaction coefficient, Γr is the
flux of reflected particles, and Γi is the flux of
incident particles. If there is no wall reaction, then
the flux of reflected particles equals the flux of
incident particles and the reaction coefficient is zero.
If every incident particle reacts at the wall, then the
flux of reflected particles is zero and the reaction
coefficient equals one.
Both modelling and optimization of the PECVD
cleaning process require knowledge of the reaction
coefficients for reactions (1) and (2).
The aim of this work is to estimate the value of
the reaction coefficient for the reaction (1)
2. Experimental setup
The experimental device is extensively described
elsewhere [1]. A rough drawing and a brief
description of the reactor are provided below (see
Fig. 1). Since no plasma was used for these
measurements, the description of the electrical part
is omitted in this paper.
MFC F2
MFC Ar
Gas shower
Reactor
Pumping lines
Butterfly
valve
QMA
Pressure
gauge
FTIR
Vacuum
pump
Fig. 1 – Experimental device
The reactor is an isothermal rectangular
parallelepiped with the dimension 1.4 m x 1.2 m x
0.028 m. The gas flow is controlled with mass flow
controllers (MFC) and the gas mixture is prepared
outside the reactor. Molecular fluorine is generated
on-site from the electrolytic decomposition of
anhydrous HF. The generator, a Generation-F®
30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK
800SC from The Linde Group, is integrated with
purification, compression, and temporary process
buffer in a compact enclosure. It is able to produce
up to 800 standard liters per hour of high purity
fluorine.
The gas is let into the reactor via a gas shower to
ensure a homogeneous distribution. The gas is
pumped out through symmetric lateral ports. The
pressure is controlled with a system consisting of a
butterfly valve and a pressure gauge. A quadrupole
mass analyser (QMA) was mounted at the inlet of
the pump and a Fourier Transform Infrared
Spectrometer (FTIR) at the pump outlet.
Pump performance curves (Pressure = f(Flow)
without plasma were taken.
The reactor was then coated with 250 nm thick
amorphous silicon layer. Different F2 flows ranging
from 500 sccm to 5000 sccm were let into the
deposited reactor. Mass and FTIR spectra were
recorded. After every measurement, the reactor was
completely cleaned with standard cleaning plasma
and then coated again for the next measurement.
higher the SiF4 production, the lower the total
outflow. Is the SiF4 production equals zero,
then the outflow equals the F2 inlet flow. If the
whole F2 inlet flow is converted to SiF4, then
the outflow equals half of the inlet flow.
If the F2 and SiF4 outflows are known the
partial pressure of these components can be
easily determined:
nF 2 / SIF 4 =
FlowF 2 / SiF 4, out
p
•
(5)
k BT FlowF 2, out + FlowSiF 4, out
where nF 2 / SiF 4 is the number density (m-3) of the
given species, p is the total pressure in the reactor
(Pa), kB is the Boltzmann constant, T is the reactor
temperature (K) and the flows are expressed in
sccm.
3.2. Measurement of the reaction coefficient of F2
at the wall coated with amorphous silicon
The balance equation for F2 in reactor can be
written as:
3. Measurement principle
3.1. Measurement of the partial pressure of F2
and SiF4 in the reactor
The first step in determining the partial
pressure of F2 and SiF4 is the determination of
the respective outlet flows.
Considering that the reactor fulfils the
conditions for a "well mixed reactor" [2] we
assume that the conditions at the outlet are
identical with the conditions inside the reactor.
General pump theory, as well as experimental
observations lead to the conclusion that the
pump performance - Flow = f(pressure) - is almost
identical for many gases. As a consequence, the
measurement of the outlet pressure leads to the
calculation of the total gas outflow. The mass
conservation leads to the conclusion that the SiF4
outflow is half of the F2 consumption expressed in
flow units (if no fluorine atoms remain in the reactor
via adsorption). After some straightforward
calculations we get:
FlowSiF 4, out = FlowF 2,in − Flowtotal , out
FlowF 2, out = FlowF 2,in − 2 • FlowSiF 4, out
(4)
An interesting consequence of mass
conservation is that the total outlet flow and the
SiF4 production have opposite behaviours: the
dn dn
=
dt dt
where
−
flow in
dn
dn
−
dt wall chemistry dt
(6)
flow out
dn
is expressed in (m-3s-1)
dt
In steady state we get:
dn
dt
=
flow in
dn
dn
+
dt chemistry dt
(7)
flow out
The first term can be written as:
dn
dt
=
flow in
FlowF 2,in ( sccm) • 4.48 E17
V
(8)
where V is the volume of the reactor (m3)
Similarly, the last term of equation (7) can be
written as:
dn
dt
=
flow out
FlowF 2, out ( sccm) • 4.48 E17
(9)
V
For the second term of the equation (7), we
follow the analysis of Chantry [3].
30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK
dn
D
=n 2
dt wall chemistry
Λ
(10)
where D is the diffusion coefficient and
Λ2 = Λ20 + l0λ
(11)
is the effective squared fundamental mode
diffusion length of the reactor.
l0 is the ratio between volume and total surface of
the reactor.
λ is the "linear extrapolation distance" [3] and is
expressed as:
D • 2 2 − γ react
•
vth
γ react
The results of a typical combined FTIR-QMA
measurement versus time are depicted in Fig. 3.a.
and the corresponding pressure curve in Fig. 3.b.
The reactor was previously deposited with 250 nm
amorphous silicon layer, and the F2 flow was 5000
sccm. The curves were shifted, so that the gas
stabilization ends at t = 100 s.
(12)
Λ 0 is the fundamental mode diffusion length
under the following conditions:
a) total absorption at the wall surface
b) the mean free path much smaller than the
reactor dimensions.
For the rectangular parallelepiped we have
1 π2 π2 π2
=
+
+
Λ20 x02 y02 z02
SiF4 time behavior
12000
2.4E-09
FTIR
QMA
10000
1.6E-09
6000
1.2E-09
4000
8.0E-10
2000
4.0E-10
0
0
200
Pressure time behaviour
0.20
brt_PM1_pres
0.15
0.10
gas
stabilization
phase
0.05
An overview FTIR spectrum is shown in Fig. 2
Only SiF4 and HF can be identified at the pump
outlet.
overview FTIR spectrum
0.00
0
100
200
300
400
500
600
700
t (s)
Fig.3b – Typical pressure curve
0.30
0.25
I (arb)
SiF4
0.15
0.10
0.05
HF
2000
1500
1000
0.00
500
1/λ (cm-1)
Fig. 2 – Typical FTIR overview spectrum
I (ppm)
0.20
2500
0.0E+00
800
0.25
p (mbar)
3. Experimental results
3000
600
Fig.3a – Typical FTIR-QMA measurement
The equations (7), (8) and (9) lead to the
estimation of the net F2 loss through diffusion and
wall chemistry, the equations (10), (11), (12) and
(13) lead then to the calculation of the reaction
coefficient.
3500
400
t (s)
(13)
x0, y0, z0 are the dimensions of the reactor.
4000
2.0E-09
8000
I_amu85 (A)
D is the diffusion coefficient, vth the thermal
velocity and γ react is defined in equation (3).
I FTIR (ppm)
λ=
HF is always present both in the QMA as in the
FTIR spectra, but since hydrogen content of the
layer is generally lower than 10%, the HF was not
considered in the estimations presented in this work.
Two different regimes can be identified in Fig. 3:
non steady-state regime in the approximate first 100
s and a steady-state regime in the following 400 s.
The first 100 s after gas stabilization show a
rapidly changing state. Lower pressure at the pump
inlet indicates a lower total outflow and indirectly a
higher flow of SiF4 (since SiF4 flow is only half of
the F2 consumption expressed in the same volume
flow units). Both QMA and FTIR measurements
30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK
also show a higher SiF4 production compared with
the following 400 s.
Since none of the measured parameters indicates
a steady-state situation, the main assumptions behind
formula (4) are not fulfilled. That is why it was not
possible to determine the reaction coefficient for the
whole time interval. We can only get an indication
about the order of magnitude of this parameter by
considering the minimum pressure reached a few
seconds after letting F2 into the chamber. From this
analysis, a reaction coefficient of about 3.4E-3 is
obtained.
All of the curves reach the steady state after ca.
100 s from the gas stabilization. Using formulas (4)
– (13) for the region t = 200 s – t = 600 s, we obtain
a recombination coefficient of approx. 3.1E-4.
4. Discussions and conclusions
Various measurements show that the duration of
the unstable regime is independent of the deposition
thickness. Furthermore, the ignition of F2 plasma
after the stability interval, i.e. after the 500 s, leads
to a drastic increase of the SiF4 signal. The decrease
of the SiF4 signal and the corresponding increase of
the pressure during the unstable regime are not
related with the "end-of-cleaning" but rather to a
change in the surface properties.
It is too early to make suppositions regarding the
physics behind the dynamic behaviour of the
reaction coefficient. The behaviour of the system
during the unstable regime could suggest that the
deposited surface gets "partially covered and
inactivated" by the F2 molecules leading to a
decrease of the reaction coefficient and of the SiF4
production. Further surface analysis are necessary in
order to clarify this aspect.
The equations used in this work are not able to
model the unstable regime. Nonetheless, the
estimation of the reaction coefficient corresponding
to the very beginning of the process can still be used
as a first approximation.
The measurement of the reaction coefficient
during the stable regime can be performed within the
limits of these equations, as long as the well mixed
reactor approximation is still valid.
A dynamic behaviour of the reaction coefficient
of molecular fluorine at wall coated with amorphous
silicon was identified, and a variation of this
coefficient from ca. 3.4E-3 to 3.1E-4 was measured.
5. Acknowledgements
The research leading to these results has received
funding from the European Community's seventh
Framework Programme FP7/2007-2013 under grant
agreement no 249782.
4. References
[1] D. Chaudhary et al, 24th European
Photovoltaic Solar Energy Conference, 21-25
September 2009, Hamburg, Germany
[2] M. Meyyappan Computational Modelling in
Semiconductor Processing, Artech House 1994
[3] P.J. Chantry, J. Appl. Phys. 64 (1987), pp
1141-1148