JOURNAL OF APPLIED PHYSICS
VOLUME 85, NUMBER 1
1 JANUARY 1999
Heteronuclear and homonuclear surface abstraction reactions
of Cl, Br, and F
Gowri P. Kota, J. W. Coburn, and David B. Graves
Department of Chemical Engineering, University of California, Berkeley, California 94720
~Received 21 May 1998; accepted for publication 1 October 1998!
Surface reactions of atomic halogen atoms play important roles in various plasma etching processes,
commonly used in microlectronics manufacturing. However, relatively little is known about the
surface chemistry of these key reactive intermediates. Previous measurements of the recombination
coefficients of Cl, Br, and F on various surfaces in a molecular beam apparatus indicated that the
recombination reaction is pseudofirst order @G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci.
Technol. A 16, 270 ~1998!; 16, 2215 ~1998!#. One mechanism that would result in pseudofirst order
kinetics is a two-step process in which the first halogen atom adsorbs into a relatively strongly
bound chemisorbed state, and the second atom reacts with it either through a direct reaction, or after
being physisorbed onto the halogenated surface. In this article, we report experiments in which
surfaces are first exposed to a molecular beam of one type of halogen atom, then the surface is
exposed to a second type of halogen. During the second exposure, the heteronuclear reaction
product is monitored with a mass spectrometer. Finally, the surface is sputtered and the mass
spectrometer is used to detect any remaining presence of the original halogen atom. Analogous
experiments were also performed with isotopically enriched mixtures of chlorine. These
experiments unambiguously demonstrate that halogen atom surface recombination involves a two
step adsorption-abstraction mechanism. Under all conditions studied, the surface recombination
reactions proceeded at rates on the order of surface collision frequencies. The relative magnitudes
of the heteronuclear rates ~as a function of surface composition and halogen atom type! scaled in the
same way as the homonuclear recombination probabilities measured previously. In every case
examined, after the second halogen exposure, the surface retained a significant coverage of the
halogen that had been originally exposed to the surface. This leads to the conclusion that only a
fraction of the strongly bound surface sites are available for abstraction by free radical attack.
Absolute calibration of the incident and evolved species fluxes allowed an estimate to be made of
the reactive site densities for several surfaces. These ranged from 1012 to 1015 cm22 depending on
the surface. © 1999 American Institute of Physics. @S0021-8979~99!08701-0#
I. INTRODUCTION
greater than 0.05 for all the surfaces. An important result of
the recombination experiments is that the recombination coefficient is not a function of the incident halogen atom flux.
This implies that the recombination reaction is first order in
the incident halogen atom flux. One possible mechanism for
the recombination reaction is that the incident halogen atom,
either directly or through a weakly bound precursor state,
reacts with an adsorbed atom to form a molecule. The additional requirement is that the rate of desorption of the adatoms is small relative to the rate of reaction to form a molecule, thus making the coverage of the adatoms nearly
constant with respect to the incident flux. In other words,
since the desorption rate is small we can say that the adatom
is strongly bound relative to a weakly bound physisorbed
atom. There is some evidence in the literature that this
mechanism has been observed for atomic recombination at
surfaces. For example, Muller–Markgraf and Rossi have
studied Cl interactions with polycrystalline Ni surfaces.12
They reported that a kinetic simulation of their experiments
suggests a weakly bound precursor state for Cl, which is
bound by 3.9 kJ/mol, interacts with a strongly bound adatom
state. Linnett and Marsden have studied the kinetics of the
recombination of oxygen atoms at glass, potassium chloride,
Surface-catalyzed recombination reactions play an important role in determining the gas-phase concentration of
atomic and molecular neutral species as well as the composition of the ionic species in a plasma reactor.1–3 Since the
atomic radicals ~e.g., F or Cl! are usually more reactive than
the molecular species ~e.g., F2 or Cl2 !4–9 the recombination
probability of the atoms may influence the evolution of microfeatures being etched. We have previously reported the
recombination coefficients ~g! as a function of temperature
for Cl, Br, and F atoms on various surfaces.10,11 We also
developed a simple phenomenological model to fit the g data
as a function of temperature for all the surfaces. In general,
the recombination coefficient decreases as the temperature
increases. g Cl and g Br are similar, with values ranging from
near zero to near unity depending on the surface and temperature. However, g F is below 0.1 irrespective of the surface even at a temperature of 80 K. More specifically, g Cl
and g Br , at room temperature, are highest ~.0.6! for W and
stainless steel, are moderate ~0.2–0.4! for poly-Si, WSix ,
and anodized Al, and lowest ~,0.05! for c-Si, photoresist,
and quartz. On the other hand g F at room temperature is no
0021-8979/99/85(1)/74/13/$15.00
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© 1999 American Institute of Physics
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
lithium chloride, lead monoxide, and molybdenum trioxide
surfaces.13,14 They suggest that the oxygen atom recombination is first order and that the probable mechanism is oxygen
atoms from the gas phase recombining with strongly bound
oxygen atoms on the surface which are then immediately
replaced from the gas phase. Sinniah et al.15,16 have studied
kinetics of recombinative desorption of hydrogen from the
monohydride phase on the Si~100! surface. They also find
that the reaction is first order in the atomic hydrogen coverage and propose a mechanism where a localized adatom reacts with a delocalized adatom to form a molecule which
then desorbs. Ogryzlo17 also found that Cl atoms appear to
recombine with first order kinetics on quartz. Cadez et al.18
have studied the recombination of H atoms on metals and
suggest that a physisorbed atom could be involved in the
recombination reaction to form a H2 molecule. Smith19 has
studied recombination of H atoms and OH radicals at various
surfaces ~quartz, combustion glass, Al2O3 , ZnO.Cr2O3 , Pt,
etc.!. He finds that the recombination reaction is first order in
the atomic concentration and that the second-order reaction
at the surface and triple collisions in the gas phase to form a
molecule are negligible.
In this article, to study the recombination mechanism,
we have probed the existence of strongly bound adatoms that
react to form molecules. We have conducted experiments in
which the surface is first exposed to one type of halogen
atom flux followed by exposure to another type of halogen
atom flux, while simultaneously monitoring the product flux
from the surface using a modulated beam mass spectrometer.
These experiments are referred to as ‘‘heteronuclear’’ and
‘‘homonuclear’’ experiments depending on whether the two
types of halogen atom fluxes that the surface is exposed to in
sequence are different or are the same, respectively. The results of these experiments show that an incident halogen
atom abstracts a strongly bound adatom from the surface.
The transient mass spectrometer signal of the product resulting from the abstraction reaction is fit to a model which is an
extension of the steady state recombination model
described.10,11 In this study, we report results for c-Si, polySi, and W samples. At room temperature, for W, poly-Si and
c-Si, g Cl is 0.8, 0.2, and ,0.01, respectively, and g Br is 0.4,
0.2, and ,0.01, respectively.
II. EXPERIMENTAL APPARATUS
The experimental setup has been described in detail
previously.10,11 The apparatus consists of a differentially
pumped high vacuum chamber containing a rotatable carousel. The various samples mounted on the carousel include
poly-Si film deposited on a quartz crystal microbalances
~QCM!, crystalline Si (c-Si), and tungsten. The temperature
of the samples can be varied and is monitored by thermocouples attached to the sample or the carousel. A sputter ion
gun ~PHI model 04-191! is also mounted on the chamber.
Ar1 ions of energy 500 eV are used to sputter clean the
samples. Beams of atomic chlorine and bromine are generated from two external inductively coupled ~13.56 MHz!
plasma sources mounted on the chamber. Fluorine atoms are
produced with a 2.45 GHz microwave plasma source. Halo-
Kota, Coburn, and Graves
75
gen atoms ~along with undissociated halogen molecules! effuse into the vacuum chamber through an aperture at the end
of their respective plasma sources. The sample to be studied
is positioned in the center of the main chamber at the intersection of the atom beams and the ion beam. The base pressure in the chamber is 1028 Torr and the pressure during the
source operation is typically 1026 Torr. Species evolved or
reflected from the sample surface are detected with a modulated beam quadrupole mass spectrometer. The region containing the mass spectrometer is differentially pumped in
four stages with respect to the main chamber. The pressure in
this mass spectrometer chamber is in the 10210 Torr range.
III. EXPERIMENTAL PROCEDURE AND RESULTS
A. Heteronuclear experiments
The sample is first sputter cleaned and then exposed to
one type of halogen atom flux ~e.g., Br atoms!. After the
sample is exposed to about 1000 L, the halogen atom flux is
shut off. Then the system is allowed to pump down without
any halogen atoms/molecules incident on the surface for
about 10 min. Since the physisorbed atoms are weakly
bound, only strongly bound adatoms and no physisorbed atoms are present on the sample surface at the end of the
pump-down time. This surface with strongly bound halogen
atoms ~Br! is then exposed to a flux of another type of halogen molecules ~e.g., Cl2 ! and then atoms ~e.g., Cl atoms!.
During the first part of the second exposure, only halogen
molecules (Cl2 ) are allowed to impact the sample surface by
flowing gas through the atom source with no plasma. Then
the plasma is turned ‘‘on’’ in the atom source, thus exposing
the sample to atomic halogen atoms ~Cl!. During this second
exposure to halogen molecules and atoms, the modulated
mass spectrometer signal of the product formed by the abstraction of the strongly bound adatoms by the incident
molecules/atoms ~ClBr! is monitored. For all of the heteronuclear experiments, no abstraction product mass spectrometer ~MS! signal is detected when the sample is exposed to
molecules. On the other hand, for most of the heteronuclear
experiments when the sample is exposed to halogen atoms an
abstraction product is detected with the mass spectrometer.
Upon exposure to halogen atoms if a product MS peak is
detected then the incident atom is able to abstract a strongly
bound adatom from the surface. If no product signal is detected then the incident atom is not able to abstract a strongly
bound adatom from the surface. The greater the ability of the
incident atom to abstract a strongly bound adatom, the larger
is the MS peak height or the area under the MS signal versus
time curve. The peak height of the product MS signal as
soon as the plasma is turned on will be proportional to the g
of the sample and to the incident flux. The time constant for
decay of the MS signal versus time curve will be a function
of g, incident flux and the number density of the strongly
bound adatoms. A detailed derivation of the ClBr product
flux is discussed in Sec. IV A. An obvious question at this
point is whether all the strongly bound adatoms resulting
from the first exposure are abstracted from the surface by the
incident halogen atoms during the second exposure. In order
to test this, when the product MS peak intensity has decayed
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76
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
FIG. 1. ~a! Modulated mass spectrometer ClBr1 signal at m/e5114 during
exposure of poly-Si sample ~that has been pre-exposed to Br atoms! to Cl2
and Cl. Before the plasma is turned on in the Cl-atom source, the sample is
exposed to only Cl2 and after the plasma is turned on in the Cl-atom source,
the sample is exposed to Cl atoms and some undissociated Cl2 . ~b! Normalized G ClBr as a function of time calculated using Eq. ~10! or ~25! with a time
constant t510 s.
to zero, we shut off the halogen atom flux and initiate bombardment of the sample with 500 eV Ar1 ions. During the
ion bombardment, the modulated MS signal of the halogen
that the surface was first exposed to ~Br!, is monitored. The
area under the MS signal versus time curve is a qualitative
measure of the number of halogen atoms on the surface. The
results of these heteronuclear and subsequent ion-induced
desorption experiments are discussed below.
1. Reaction between Br and Cl
a. Incident Cl abstraction of adsorbed Br. The poly-Si
sample is first exposed to a flux of Br atoms, followed by
pump down and is then exposed to a flux of chlorine ~Cl2
FIG. 2. ~a! Modulated mass spectrometer ClBr1 signal at m/e5114 during
exposure of crystalline-Si sample ~that has been pre-exposed to Br atoms! to
Cl2 and Cl. ~b! Normalized G ClBr as a function of time calculated using Eq.
~10! or ~25! with a time constant t51 s.
Kota, Coburn, and Graves
FIG. 3. ~a! Modulated mass spectrometer ClBr1 signal at m/e5114 during
exposure of W sample ~that has been pre-exposed to Br atoms! to Cl2 and
Cl. ~b! Normalized G ClBr as a function of time calculated using Eq. ~10! or
~25! with a time constant t515 s.
and then Cl!. We have chosen to record ClBr MS signal
intensity at m/e5114 because it is the largest signal. The
114
ClBr MS signal results from recombination of 35Cl and
79
Br ~where 35 and 79 are the atomic masses!. 35Cl and 37Cl
exist in the natural abundance ratio of 0.755:0.245 and 79Br
and 81Br exist in the natural abundance ratio of 0.505:0.495.
The 114ClBr MS signal intensity during the poly-Si exposure
to chlorine is shown in Fig. 1~a!. When the brominated
sample is exposed to molecular Cl2 , no ClBr peak is detected with the mass spectrometer. As soon as the discharge
is turned on in the Cl-atom source @i.e., t;60 s in Fig. 1~a!#,
the brominated sample is exposed to atomic Cl and a large
ClBr peak is detected with the mass spectrometer. This
clearly indicates that the incident Cl is able to abstract
strongly bound Br from the sample. Similar ClBr MS signal
intensity curves are shown for c-Si and W in Figs. 2~a! and
3~a!. It should be noted that the areas under the MS signal
versus time curve follow the order W.poly-Si.c-Si.
After the ClBr MS signal has decayed to zero intensity,
the Cl-atom flux is shut off and the sample is exposed to a
flux of 500 eV Ar1 ions. Figures 4–6 show the ion-induced
desorption of the remaining strongly bound Br atoms ~from
the first exposure! from the poly-Si, c-Si, and W surfaces,
respectively. This indicates that only a fraction of the
FIG. 4. Modulated mass spectrometer Br1 signal at m/e579 during exposure of poly-Si sample ~that has been previously exposed to Br atoms and
then Cl atoms! to 500 eV Ar1 ions. At time535 s, the ion beam is turned
on.
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
FIG. 5. Modulated mass spectrometer Br1 signal at m/e579 during exposure of crystalline-Si sample ~that has been previously exposed to Br atoms
and then Cl atoms! to 500 eV Ar1 ions. At time525 s, the ion beam is
turned on.
strongly bound Br atoms have been abstracted by the incident Cl atoms from the surface. Therefore, the strongly
bound or chemisorbed Br atoms resulting from the first exposure to Br flux can be divided into ‘‘reactive’’ chemisorbed atoms and ‘‘unreactive’’ chemisorbed atoms. The reactive chemisorbed atoms are those that can be abstracted by
the halogen atoms and the unreactive chemisorbed atoms are
those that cannot be abstracted. From Figs. 4 and 5 it is
interesting to note that the areas under the sputtered Br MS
signals are about the same for both c-Si and poly-Si even
though the ClBr MS signal is much greater for poly-Si than
for c-Si. However, the area under the Br MS signal for W
~Fig. 6! is greater than those for poly-Si and c-Si. It is also
interesting to note the shape of the Br MS signal in Fig. 6 for
W sample, which is distinctly different from the Br MS signals in Figs. 4 and 5 for poly-Si and c-Si, respectively. Figure 7 shows the ion-induced desorption of Br when the
poly-Si sample has been previously exposed to only Br and
no Cl. Comparing the areas under the Br MS signal curves in
Figs. 4 and 7, we note that more Br is desorbed when the
poly-Si sample is previously exposed to only Br than when
the poly-Si sample is previously exposed to Br followed by
Cl. On the other hand, the ion-induced desorption Br MS
signal for a c-Si sample, when the c-Si sample is previously
exposed to only Br ~not shown!, is about the same as that
when the c-Si sample is previously exposed to Br and then
Cl ~Fig. 5!. Therefore, we can conclude that for a c-Si
sample, a relatively small amount of chemisorbed Br is abstracted by the incident Cl.
FIG. 6. Modulated mass spectrometer Br1 signal at m/e579 during exposure of W sample ~that has been previously exposed to Br atoms and then Cl
atoms! to 500 eV Ar1 ions. At time525 s, the ion beam is turned on.
Kota, Coburn, and Graves
77
FIG. 7. Modulated mass spectrometer Br1 signal at m/e579 during exposure of poly-Si sample ~that has been previously exposed to Br atoms only!
to 500 eV Ar1 ions. At time560 s, the ion beam is turned on. Note that the
area under the curve is greater than that in Fig. 4, thus more Br is present on
the poly-Si surface before exposure to Cl relative to that after exposure to
Cl.
b. Incident Br abstraction of adsorbed Cl. In these experiments, the sample is first exposed to the Cl-atom flux,
followed by pump down and then exposed to bromine ~Br2
and then Br!. The ClBr MS signal intensity at m/e5114
during the poly-Si sample exposure to bromine is shown in
Fig. 8. When the chlorinated sample is exposed to molecular
Br2 , no ClBr peak is detected with the mass spectrometer.
As soon as the discharge is turned on in the Br-atom source
and the chlorinated sample is exposed to atomic Br, a ClBr
peak is detected with the mass spectrometer. This clearly
indicates that an incident Br is able to abstract a strongly
bound Cl from the sample. When the ClBr MS signal has
decayed to zero intensity, the flux of Br atoms is shut off.
The sample is then exposed to 500 eV Ar1 ions. During the
ion-induced desorption a Cl peak ~not shown! is detected
with the mass spectrometer thus indicating that not all the
chemisorbed Cl are abstracted by Br. For the case of c-Si
sample, no ClBr MS peak is detected. Similar to the results
of the Cl abstraction of Br ~Sec. III A 1 a above! for the case
of W sample, the area under the ClBr MS curve is greater
than that for the poly-Si sample. It should also be noted that
for the case of poly-Si the magnitude of the ClBr MS peak in
Fig. 8 is smaller than that in Fig. 1~a!. However, the time
required for the decay of the ClBr MS signal appears to be
longer in Fig. 8 than in Fig. 1~a!.
FIG. 8. Modulated mass spectrometer ClBr1 signal at m/e5114 during
exposure of poly-Si sample ~that has been pre-exposed to Cl atoms! to Br2
and Br. When the plasma is turned on in the Br-atom source, the sample is
exposed to Br atoms.
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78
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Kota, Coburn, and Graves
FIG. 9. Modulated mass spectrometer ClF1 signal at m/e554 during exposure of poly-Si sample ~that has been pre-exposed to Cl atoms! to F2 and
F. When the plasma is turned on in the F-atom source, the sample is exposed
to F atoms.
2. Reaction between F and Cl
a. Incident F abstraction of adsorbed Cl. Similar to the
Cl and Br heteronuclear exposure experiments, a sputtercleaned sample is first exposed to Cl atoms and then exposed
to fluorine ~F2 and then F!. Figure 9 shows the ClF MS
signal intensity at m/e554 during poly-Si exposure to fluorine. With only F2 incident on the sample no ClF is evolved
but when the discharge is turned on in the F-atom source,
ClF is detected with the mass spectrometer. The ClF MS
signal intensity takes a relatively long time to decay to zero
intensity, unlike the ClBr MS signal intensity curves in Figs.
1~a! and 8. This may be due to the fact that F atoms etch Si
spontaneously and therefore expose the underlying reactive
chemisorbed Cl which can then react with the incident F
atoms to form ClF. We also detect SiFx Cly etch products
using the mass spectrometer indicating that the chlorinated
layer of Si is being etched by the incident F atoms.
b. Incident Cl abstraction of adsorbed F. The sputtercleaned sample is first exposed to F atoms followed by exposure to chlorine. Figure 10 shows the ClF MS signal intensity during poly-Si exposure to chlorine ~Cl2 and Cl!. The
magnitude and the decay time of the ClF MS signal intensity
are smaller compared to the F abstraction of adsorbed Cl.
This may be due to the fact that the spontaneous etch rate of
Cl atoms is lower than that of F atoms.7,20 In addition the
energetics of Cl atoms interacting with adsorbed F are different compared to F atoms interacting with adsorbed Cl.
During the exposure of the sample to 500 eV Ar1 ions, no
clear F MS peak was detected with the mass spectrometer.
However, this could be in part due to the large noise in the
MS signal at m/e519 ~F! relative to that at m/e535 ~Cl! or
79 ~Br!.
B. Cl uptake experiments
As described in Sec. III A above, it is proposed that the
chemisorbed atoms can be classified into reactive chemisorbed atoms and unreactive chemisorbed atoms. The MS
signal curves along with the quartz crystal microbalance
~QCM! are used to estimate the total chemisorbed Cl when a
clean surface is exposed to a flux of Cl atoms. We use this
estimate of total chemisorbed Cl to compare with the number
FIG. 10. Modulated mass spectrometer ClF1 signal at m/e554 during exposure of poly-Si sample ~that has been pre-exposed to F atoms! to Cl2 and
Cl. When the plasma is turned on in the Cl-atom source, the sample is
exposed to Cl atoms.
density of the reactive chemisorbed Cl in Sec. V. In this
Section, we will describe how we use both the QCM and the
70
Cl2 MS signal curves to obtain the total chemisorbed Cl
number densities for poly-Si and W samples.
First, the poly-Si ~QCM! is sputter cleaned and exposed
to atomic Cl while simultaneously monitoring the frequency
change of the QCM and the 70Cl2 MS signal intensity. The
total frequency change of the poly-Si QCM is then used to
calculate the total chlorine uptake. This is about 1.6
31015 Cl atoms/cm2. Layadi et al.21 used angle-resolved
x-ray photoelectron spectroscopy ~XPS! to estimate the chlorinated layer thickness and composition during Cl2 plasma
etching of Si ~100!. They report a Cl content of 1.8
31015 Cl atoms/cm2 at 40 eV ion energy to 3.531015 Cl
atoms/cm2 at 280 eV ion energy, with a 6–10-Å-thick chlorinated silicon layer. Figure 11 shows the Cl2 mass spectrometer signal when a sputter-cleaned poly-Si surface is first
exposed to a flux of Cl atoms. The initial depletion of the Cl2
signal indicates that the incident Cl is being adsorbed on the
poly-Si surface, and therefore, until the surface is saturated
with chemisorbed Cl, less Cl2 is formed and evolved from
the surface. Then the Cl2 MS signal increases to a steady
state value which is lower than the Cl2 MS signal intensity
when the plasma is ‘‘off.’’ The steady state Cl2 MS signal
intensity is lower in the plasma on condition than in the
plasma off condition because of the dissociation of Cl2 when
the plasma is turned on. In order to confirm that the additional decrease in the Cl2 intensity when the plasma is first
turned on is not due to some transient in turning the plasma
on in the atom source, we turn the plasma off and then on
again. As seen in Fig. 11, when the plasma is turned on for
the second time the Cl2 MS signal decreases immediately to
a steady state value with no additional decrease in the Cl2
MS signal. We can relate the area of the additional decrease
in Cl2 MS signal ~with respect to the steady state Cl2 MS
signal! in Fig. 11 to the uptake of Cl ~in Cl atoms/cm2! on
the poly-Si surface measured using QCM. A similar Cl2 MS
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Kota, Coburn, and Graves
79
C. Homonuclear „isotopic Cl… experiments
FIG. 11. Modulated mass spectrometer Cl1
2 signal at m/e570 during exposure of a sputter-cleaned poly-Si sample to Cl2 and Cl. At time575 s, the
plasma is turned on in the Cl-atom source, thus exposing the sample to Cl
atoms ~in natural abundance!. The plasma is turned off at time5140 s and
on at time5180 s.
signal intensity curve ~Fig. 12! is measured when a sputtercleaned W sample is first exposed to Cl atoms. The additional decrease in the Cl2 MS signal from the steady state
value ~Fig. 12! is larger than that for the poly-Si sample ~Fig.
11!, indicating that more Cl is adsorbed on the W than on the
poly-Si sample. The ratio of the areas of additional decrease
in Figs. 12 and 11 is about 3, resulting in a Cl uptake of
about 531015 cm22 for W when exposed to Cl atoms. It
should be noted that the Cl-uptake ratio corresponds to the
total chemisorbed Cl and does not necessarily correspond to
the reactive chemisorbed atom densities.
FIG. 12. Modulated mass spectrometer Cl1
2 signal at m/e570 during exposure of a sputter-cleaned W sample to Cl2 and Cl. At time535 s, the plasma
is turned on in the Cl-atom source, thus exposing the sample to Cl atoms ~in
natural abundance!. The plasma is turned off at time5110 s and on at
time5160 s.
In Sec. III A the types of halogen atoms used in the first
and second exposure are different. Therefore, as will be seen
in Sec. IV, this complicates modeling the abstraction kinetics. For example, we assume that the recombination coefficient for the formation of the ClBr abstraction product is the
average of the recombination coefficients of Cl and Br measured under steady state conditions. In order to simplify the
analysis we have conducted experiments where the sample is
exposed to the same type of halogen atoms during both the
first and the second exposures. In conducting this experiment
we have used different concentrations of the two Cl isotopes
~35Cl and 37Cl! in the first and the second exposure sequences. In the homonuclear experiments, the sputtercleaned sample is first exposed to a flux of isotopically enriched chlorine. The isotopically enriched chlorine contains
95% 35Cl and 5% 37Cl. Then the flux of isotopic Cl is shut
off and the chamber is allowed to pump down without any
flow of gases on to the sample surface for about 10 min.
Similar to the heteronuclear experiments, at this point, the
sample should have only strongly bound Cl and no physisorbed Cl. Next, this surface is exposed to a flux of chlorine
in natural abundance. Naturally occurring chlorine contains
75% 35Cl and 25% 37Cl. During this second exposure, we
record the modulated beam mass spectrometer 72Cl2 signal.
The analysis of the Cl2 signal in this experiment is not as
straightforward as the analysis of ClBr MS signal intensity
discussed previously. This is because of ~1! the presence of
both 35Cl and 37Cl in the isotopically enriched chlorine used
in the first exposure and the naturally occurring chlorine used
in the second exposure and ~2! the Cl2 MS signal changes
~decreases! when the plasma is turned on as a result of Cl2
dissociation in the atom source. The Cl2 MS signal recorded
during the homonuclear experiment looks very similar to the
Cl2 MS spectra recorded for Cl uptake ~Figs. 11 and 12!. The
difference is that in the Cl-uptake experiment the sample is
sputter cleaned and in the homonuclear experiment the
sample is sputter cleaned and exposed to isotopically enriched Cl atoms, prior to exposure to Cl atoms ~in natural
abundance!. As shown in Fig. 13, when the plasma is turned
on in the atom source and the W sample is exposed to atomic
Cl, there is a depletion in the Cl2 MS signal intensity at
m/e572. When the plasma is off in the Cl-atom source, only
Cl2 impacts the sample surface and when the plasma is
turned on in the Cl-atom source, both Cl and some undissociated Cl2 impact the sample surface. Therefore, with the
plasma on in the Cl-atom source, the steady state 72Cl2 MS
signal intensity decreases relative to that when the plasma is
off. As shown in Fig. 13, in addition to this decrease in the
72
Cl2 MS signal intensity when the plasma is turned on, we
observe an additional decrease when the sample ~preexposed to isotopically enriched 35Cl! is initially exposed to
naturally occurring Cl. This additional decrease in 72Cl2 MS
signal intensity ~with respect to the steady state signal! is
attributed to the abstraction of the adsorbed isotopically enriched Cl by the incident naturally occurring Cl. That is,
since there is a deficiency of 37Cl in the adsorbed layer, less
72
Cl2 is produced. In order to confirm that the additional
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80
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Kota, Coburn, and Graves
IV. KINETIC MODEL FOR HALOGEN ATOM
ABSTRACTION REACTIONS
A. Model for heteronuclear abstraction reactions
FIG. 13. Modulated mass spectrometer Cl1
2 signal at m/e572 during exposure of W sample ~that has been pre-exposed to isotopically enriched Cl
atoms! to Cl2 and Cl. At time550 s, the plasma is turned on in the Cl atom
source, thus exposing the sample to Cl atoms ~in natural abundance!. The
plasma is turned off at time5110 s and on at time5180 s exposing the
sample to Cl2 and Cl, respectively. ~b! Normalized G 72Cl2 as a function of
time calculated using Eq. ~41! with a time constant t i 55 s.
depletion in the 72Cl2 signal is not a transient due to the
plasma being turned on in the Cl-atom source, we turn the
plasma off in the source after the 72Cl2 MS signal appears to
have reached a steady state value. As shown in Fig. 13, following the further exposure of the sample to naturally occurring Cl2 , the plasma is turned on again. The 72Cl2 MS signal
intensity decreases to a steady state value due to dissociation
of Cl2 in the plasma source but there is no additional depletion observed as in the case of initial exposure to naturally
occurring Cl. This clearly indicates that the initial enhanced
depletion in the 72Cl2 MS signal intensity is because of the
abstraction of adsorbed isotopically enriched Cl by the naturally occurring incident Cl and not because of a transient in
turning the plasma on in the atom source.
We do not report the 70Cl2 and 74Cl2 MS signals for W
or the 70Cl2 , 72Cl2 , and 74Cl2 MS signals for poly-Si and
c-Si because the additional decrease or increase ~with respect to the steady state MS signal! resulting from the abstraction reaction is indistinguishable from the noise and
other transients in the MS data. More specifically, in the case
of 70Cl2 MS signal since the transient result is a decrease in
the MS signal with time ~as shown in Sec. IV!, it is camouflaged by the decrease of the Cl2 MS signal due to turning
the plasma on in the source. In the case of 74Cl2 MS signal,
the transient is similar to that of 72Cl2 signal but the signalto-noise ratio for the 74Cl2 MS signal is small, thus making it
difficult to clearly detect the transient resulting from the abstraction reaction. A detailed analysis of the transients in the
Cl2 MS signal intensities during the exposure of the sample
~pre-exposed to isotopically enriched chlorine! to naturally
occurring Cl is described in Sec. IV B.
It has been reported previously that g Br and g Cl are independent of the incident Br flux and Cl flux,
respectively.10,11 This suggests that the recombination reaction is pseudofirst order in the incident halogen atom flux. It
is probable that the incident halogen atom reacts either directly @Eley–Rideal mechanism ~ER!# or through a weakly
bound precursor @Langmuir–Hinshelwood mechanism ~LH!#
with a reactive chemisorbed atom to form a molecule. In
addition, the coverage of the reactive chemisorbed atoms
should be independent of the incident halogen atom flux.
Though we refer to the reaction between a physisorbed atom
and a reactive chemisorbed atom as LH mechanism, it is
possible that the adatoms may not be fully accommodated to
the surface. If an incident atom were to react directly with a
reactive chemisorbed atom, the reaction is considered to be
ER type. The heteronuclear abstraction reactions can be
modeled using either ER or LH type of reaction mechanism.
Therefore, we develop the abstraction reaction model using
both the ER and LH mechanisms. It should be noted that we
have previously used LH kinetics for modeling g X as a function of temperature.10,11 Helmer and Graves discuss a recombination model that includes both LH and ER mechanisms.22
1. Eley – Rideal reaction kinetics
For the case of ER reaction mechanism,
X ~ g ! 1X ~ * ! →X 2 ,
~1!
where X(g) is the incident halogen atom and X( * ) is the
reactive chemisorbed atom. The site balance for reactive
chemisorbed atoms at steady state is,
*,
s v G X,inc~ 12 u X* ! 5s r G X,incu X* 1k *
d s *u X
~2!
where s v is the sticking or trapping probability of an incident
atom into a vacant reactive chemisorption site, G X,inc is the
halogen atom flux ~cm22 s21!, s* is the density of the reactive chemisorption sites ~cm22!, u X* is the coverage of the
reactive chemisorbed atoms, s r is the reaction probability for
an incident atom with a reactive chemisorbed atom, and k *
d
is the rate constant for desorption of reactive chemisorbed
atoms ~s21!. As reported previously,10,11 since the recombination reaction is pseudofirst order in the incident halogen
flux, it follows that u X* should be independent of G X,inc . For
the model described in Eq. ~2!, this will be true if k *
d !s r .
Therefore,
u X* 5
sv
.
s v 1s r
~3!
Recombination coefficient ( g X ) defined as the fraction
of the atomic flux incident on the surface that recombine to
form molecules can be expressed as,
g X5
2s r G X,incu X*
G X,inc
5
2s r s v
.
s r 1s v
~4!
It should be noted that g X is independent of G X,inc .
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
According to the ER model described above, in the case
of the transient heteronuclear experiment described in Sec.
III A 1, the recombination reaction occurs between an incident 35Cl @ 35Cl(g) # and a reactive chemisorbed 79Br
@ 79Br( * ) # to form a ClBr114 molecule.
Cl~ g ! 1 79 Br~*!→114ClBr.
~5!
35
We make a simplifying assumption that the reactive sites are
the same for both Cl and Br. The ClBr mass spectrometer
signal intensity versus time curve during the sample exposure to Cl atoms can be modeled by first writing a balance
for reactive chemisorbed Br sites where the brominated
sample is exposed to Cl atoms at time t50.
s*
*
d u 79
dt
Br
* 2k d* s * u 79
* .
52s r G Cl,incu 79
Br
~6!
Br
In the above @Eq. ~6!#, as stated earlier, the desorption term is
small relative to the reaction term. We assume here that s v
and s r are the same for both Cl and Br. Therefore, the total
* 1u Br
* ) is concoverage of reactive chemisorbed atoms ( u Cl
stant and equal to s v /(s v 1s r ) even though the individual
* and u Br
*,
coverages of reactive chemisorbed Cl and Br ~u Cl
respectively! are varying with time ~when the ClBr MS signal is nonzero!. The coverage of 79Br( * ) after the first exposure to Br atoms but before the second exposure to 35Cl ~also
the initial condition, i.e., at t50! is,
* ~ t50 ! 50.505~ u Cl
* 1u Br
* !,
u 79
~7!
Br
where 0.505 is the natural abundance fraction of 79Br. Integrating Eq. ~6! with respect to time and substituting the initial condition in Eq. ~7! we get,
S
* 50.505~u Cl
* 1u Br
* )exp 2
u 79
Br
D
g ClBr GCl,inc t
.
* 1u Br
*)
2 s * ( u Cl
~8!
The flux of 114 ClBr~G114ClBr) evolved from the sample surface can be expressed as,
* .
G 114ClBr5s r G 35Cl,incu79
~9!
Br
*
In the above @Eq. ~9!# we substitute for u 79
S D
Br
S D
using Eq. ~8!.
0.505
t
G 114ClBr5
gClBrG35Cl,inc exp 2
,
2
t ER
~10!
Kota, Coburn, and Graves
81
*,
k v s s u X s * ~ 12 u X* ! 5k r* s s u X s * u X* 1k *
d s *u X
~13!
where k v is the rate constant for adsorption of a physisorbed
atom into a vacant reactive chemisorbed site ~cm2 s21!, s s is
the density of the physisorption sites ~cm22!, u X is the coverage of physisorbed atoms, and k r* is the rate constant for
reaction of a physisorbed atom and a reactive chemisorbed
atom ~cm2 s21! as represented in Eq. ~12!. Once again, u X*
should be independent of G X,inc , k d* !k r* s s u X and Eq. ~13!
reduces to,
k v s s u X s * ~ 12 u X* ! 5k r* s s u X s * u X*
or,
u X* 5
* 1u Br
*!
2s*~u Cl
gClBrGCl,inc
.
~11!
The g ClBr in the above @Eq. ~11!# is the recombination coefficient for the formation of ClBr. Before interpreting the experimental results in Figs. 1~a!–3~a! we will derive the expression for G ClBr using LH reaction kinetics.
2. Langmuir – Hinshelwood reaction kinetics
For the case of LH reaction mechanism,
X ~ p ! 1X ~ * ! →X 2 ,
~12!
where X(p) is the physisorbed atom. The following equation
is the balance for reactive chemisorbed atoms at steady state.
kv
k v 1k r*
~15!
.
Similar to the balance for reactive chemisorbed atoms
@Eq. ~13!#, we can also write a balance for the physisorbed
atoms. We assume that the incident atom is trapped into a
weakly bound physisorbed state. The physisorbed atom is
then assumed to diffuse on the surface and either desorb
before recombining or recombine to form X2 and then desorb. Based on this model, at steady state,
z G X,inc~ 12 u X ! 5k r* s s u X s * u X* 1k d s s u X
1k v s s u X s * ~ 12 u X* ! ,
~16!
where z is the sticking or trapping probability of the incident
atom into a physisorbed state, G X,inc is the incident halogen
atom flux ~cm22 s21!, and k d is the desorption rate constant
~s21! for physisorbed atoms. Since the frequency factor for
desorption of a physisorbed atom is of the order
;1012 – 1013 s21 and the energy of physisorption is of the
order of tenths of an eV, u X !1. Therefore, in Eq. ~15!, (1
2 u X );1. Substituting Eq. ~14! in Eq. ~16! and rearranging,
u X5
z G X,inc
.
k d s s 12k r* s s s * u X*
~17!
Recombination coefficient ( g X ) for LH mechanism can be
expressed as
g X5
where
t ER[
~14!
2k r* s s u X s * u X*
G X,inc
5
S
z
11
kd
2k r* s * u X*
D
.
~18!
Once again, it should be noted from the above Eq. ~18! that
g X is independent of G X,inc .
According to the LH model, for the case of heteronuclear reaction, the recombination reaction occurs between
a physisorbed 35Cl @ 35Cl(p) # and a reactive chemisorbed
79
Br @ 79Br( * ) # to form a ClBr114 molecule.
Cl~ p ! 1 79Br~*!→114ClBr.
35
~19!
Once again, we assume that the reactive sites are the same
for both Cl and Br. The ClBr mass spectrometer signal intensity versus time curve during the sample exposure to Cl
atoms can be modeled by first writing a balance for reactive
chemisorbed Br sites where the brominated sample is exposed to Cl atoms at time t50.
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82
s*
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
*
d u 79
Kota, Coburn, and Graves
Br
* 1k v s s u79 s*~12u Br
* 2u Cl
*)
52k r* s s u Cls*u 79
dt
Br
Br
* .
2k d* s * u 79
~20!
Br
The time required for u Cl to reach steady state is small rela* is varying @;1–100 s from
tive to the time over which u 79
Br
Figs. 1~a!–3~a!#. Therefore, u Cl can be treated as a constant
and its steady state value can be used in Eq. ~20!. Also,
u 79 Br 50 since there are no Br atoms in the incident beam
and therefore, no physisorbed Br. In addition, the loss of
reactive chemisorbed atoms due to desorption is relatively
small because k *
d !k r* s s u X . Equation ~20! reduces to,
*
d u 79
Br
dt
* .
52k r* s s u Clu79
~21!
Br
We can rewrite Eq. ~17! for u 35Cl as follows:
u 35Cl5
z G35Cl,inc
* 1u Br
*!
k d s s 12k r* s s s * ~ u Cl
~22!
.
k v and k r* are assumed to be the same for both Cl and Br.
Therefore, the total coverage of reactive chemisorbed atoms
* 1u Br
* ) is constant and equal to k v /(k v 1k r* ). Integrat( u Cl
ing Eq. ~21! with respect to time and using the initial condition for reactive chemisorbed Br @Eq. ~7!#, and substituting
for u 35Cl using Eq. ~22!, we get,
S
* 50.505~u Cl
* 1u Br
* )exp 2
u 79
Br
D
g ClBrGCl,inc t
.
* 1 u Br
*!
2 s * ~ u Cl
~23!
The flux of 114ClBr (G 114ClBr ) evolved from the sample surface can be expressed as,
* .
G 114ClBr 5k r* s s u 35Cl s*u 79
~24!
Br
* using
In the above @Eq. ~24!# we substitute for u 35Cl and u 79
Br
Eqs. ~22! and ~23!, respectively.
S D
G 114ClBr 5
S D
t
0.505
gClBrG35Cl,inc exp 2 ,
2
t
~25!
where
t LH[
* 1u Br
*!
2s *~u Cl
gClBr GCl,inc
.
~26!
It should be noted that irrespective of the reaction
mechanism used ~ER or LH!, t ER5tLH([ t ) and the expressions for G ClBr @Eqs. ~10! and ~25!# are the same. Since the
values of g Cl and g Br are about the same at room temperature
we assume that g ClBr is equal to the average of g Cl and g Br .
Therefore, g ClBr ~poly-Si!;0.2, g ClBr (c-Si);0.01, and g ClBr
~W! ;0.6. The incident Cl-atom flux G Cl,inc is about 5
31014 cm22 s21 . The measured ClBr MS signal is proportional to G ClBr , but since the cross section for ionization in
the mass spectrometer is not known we are not able to compare the magnitude of the measured ClBr MS peak to the
calculated G ClBr using Eq. ~25!. However, the time constant t
for the decay of 114ClBr MS signal intensities shown in Figs.
1~a!–3~a! can be used in Eq. ~26! to calculate s*. Figures
1~b!–3~b! show the model curves corresponding to t ~polySi!510 s, t (c-Si!51 s, and t~W!515 s, respectively. The
G 114ClBr values at t50 in Figs. 1~b!–3~b! have been scaled to
equal the magnitude of the measured 114ClBr peaks when the
samples are initially exposed to Cl, atoms @Figs. 1~a!–3~a!#.
Therefore, using Eq. ~26!, the number of reactive chemi* 1u Br
* ) # on poly-Si, c-Si, and W sursorbed atoms @ s * ( u Cl
faces are 531014, 2.531012, and 231015 cm22 , respectively. The estimate of 2.531012 cm22 for c-Si is an upper
limit because during some of the experimental runs no ClBr
peak was detected, i.e. t ,1 s. In order to calculate s*, we
* 1u Br
* ) which as
first have to determine the value of ( u Cl
stated earlier is assumed to be a constant @Eqs. ~3! and ~15!#.
* 1u Br
* )50.5.
If we assume that s r ~or k r* )5s v ~or k v !, ( u Cl
Therefore, s* (poly-Si!5131015 cm22 , s* (c-Si!5531012
cm22 , and s* (W)5431015 cm22 .
From Eq. ~25!, at time t50, the 114ClBr MS signal is
directly proportional to g ClBr and the incident flux of Cl atoms. Integrating Eq. ~25! with respect to time from t50 to
t5`, we get,
E
`
0
* 1uBr
* !,
G 114 ClBr dt5 ~ 0.505!~ 0.755! s * ~ u Cl
~27!
where 0.755 is the natural abundance fraction of 35 Cl. Therefore, the area under the ClBr MS signal curve ~Figs. 1–3! is
directly proportional to the number density of the reactive
chemisorbed atoms.
We discuss the model interpretation of incident Br abstracting reactive chemisorbed Cl to form ClBr ~Fig. 8! in
Sec. V. We have not attempted to extend this model to the Cl
and F heteronuclear experiments shown in Figs. 9 and 10.
This is due to the fact that g Cl and g F values are quite different ~especially for poly-Si and W! and therefore using
either of these values for g ClF would not be justified. Another
complicating factor is that F atoms spontaneously etch Si
which may result in more than one time constant for the
decay of the CIF MS signal in Fig. 9.
B. Model for homonuclear „or isotopic Cl… abstraction
reactions
Similar to the ClBr transient model for the heteronuclear
experiments we can model the C2 transient for the homonuclear experiment. Once again, we could use either LH or
ER type reaction mechanism. Since the expression for abstraction product flux will be the same for both the cases
@Eqs. ~10! and ~25!#, we will consider only LH reaction
mechanism in this section. There are two isotopes of Cl: 35Cl
and 37Cl, which lead to the formation of the following Cl2 :
70
Cl2 , 72Cl2 , and 74Cl2 . Therefore, when a surface that has
been pre-exposed to isotopically enriched Cl is exposed to
naturally occurring Cl, we have the following possible reactions:
Cl~ p ! 1 35Cl~*!→70Cl2 ,
~28!
Cl~ p ! 1 37Cl~*!→72Cl2 ,
~29!
Cl~ p ! 1 Cl~*!→ Cl2 ,
~30!
Cl~ p ! 1 37Cl~*!→74Cl2 ,
~31!
35
35
37
37
35
72
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Kota, Coburn, and Graves
where 35Cl(p) and 37Cl(p) are the physisorbed atoms and
35
Cl( * ) and 37Cl( * ) are the reactive chemisorbed atoms. A
balance for the coverage of reactive chemisorbed 35Cl can be
written where at time t50, the surface is exposed to naturally occurring Cl.
s*
*
d u 35
Cl
dt
* 1k v s s u 35Cl s*~12u Cl
* ),
52k r* s s u Cls *u 35
Cl
~32!
where u Cl is the sum of physisorption coverages of Cl and
37
Cl, u 35Cl is the physisorption coverage of 35Cl only ~similar
nomenclature is used for u *!, and k v is the rate at which the
vacant reactive chemisorbed sites are filled by the physisorbed atoms ~cm2 s21!. In the above @Eq. ~32!# we have not
included the term accounting for the loss of 35Cl( * ) by desorption because k d !k r* s s u Cl by assumption. It should be
noted that in Eq. ~32! the total coverage of the reactive
* 1u37
* ), is constant even though
* 5u35
chemisorbed Cl ( u Cl
Cl
Cl
the individual coverages of the reactive chemisorbed 35Cl
* ! are varying during the initial expo* and u 37
and 37Cl ~u 35
Cl
Cl
sure to naturally occurring Cl. The coverage of 35Cl( * ) after
the first exposure to isotopically enriched Cl atoms but before the second exposure to naturally occurring Cl atoms
~also the initial condition, i.e., t50!,
35
* ~ t50 ! 50.95
u 35
Cl
S
kv
k v 1k r*
D
~33!
,
where 0.95 is the abundance of 35Cl in isotopically enriched
Cl incident flux. Equation ~17! can be rewritten for u 35Cl and
u 37Cl as follows:
u 35Cl 5
u 37Cl 5
z G35Cl,inc
*
k d s s 12k r* s s s * u Cl
,
~34!
.
~35!
z G37Cl,inc
*
k d s s 12k r* s s s * u Cl
It should be noted that as shown in Sec. IV A, it is valid to
use the steady state expressions for u Cl @Eqs. ~34! and ~35!#
even though the reactive chemisorbed-atom coverage is
varying with time. Integrating Eq. ~32! with respect to time
and using the initial condition @Eq. ~33!#, and substituting for
u 35Cl @Eq. ~34!#, we get,
S
F
* 5uCl
* 0.75510.195 exp 2
u 35
Cl
g ClGCl,inc t
*
2 s * u Cl
DG
~36!
.
Similarly writing a balance for the reactive chemisorbed
Cl37, we get,
S
F
* 5u Cl
* 0.24520.195 exp 2
u 37
Cl
g ClGCl,inc t
*
2 s * u Cl
DG
.
~37!
72
74
The fluxes of Cl70
2 , Cl2 , and Cl2 evolved from the sample
surface can be expressed as,
* ,
G 70Cl2 5k r* s s u 35Cl s*u35
~38!
Cl
* 1k r* s s u 37Cl s*u35
* ,
G 72Cl2 5k r* s s u 35Cl s*u37
Cl
Cl
~39!
* .
G 74Cl2 5k r* s s u 37Cl s*u37
83
~40!
Cl
In the above @Eqs. ~38!–~40!# we substitute for u 35 Cl , u 37Cl ,
* , and u 37Cl using Eqs. ~34!–~37!, respectively.
u 35
Cl
F
S DG
S DG
S DG
1
t
G 70Cl25 gClG35Cl,inc 0.75510.195 exp 2
2
ti
F
1
t
G 72Cl25 gClGCl,inc 0.3720.0995 exp 2
2
ti
F
1
t
G 74Cl25 gClG37Cl,inc 0.24520.195 exp 2
2
ti
,
~41!
~42!
,
,
~43!
where
t i[
*
2 s * u Cl
gClGCl,inc
.
~44!
It is interesting to note from the above, Eqs. ~41!–~43!, that
the flux of 70Cl2 decreases whereas the fluxes of 72Cl2 and
74
Cl2 increase with time. The 72Cl2 MS signal versus time for
W shown in Fig. 13~a! can be modeled using Eq. ~42!. Figure 13~b! shows the model curve for 72Cl2 using t i 55 s and
a normalized peak height at t50. For W, g Cl;0.8 from previous experimental measurements.10 G Cl,inc during the second
exposure to Cl ~naturally occurring! is about 5
31014 cm22 . Therefore, using Eq. ~44!, the number density
of reactive chemisorbed atoms present on the W surface
* !5131015 cm22 . In order to calculate s* we first
( s * u Cl
* . Similar to the heteronuclear model,
have to determine u Cl
*
we assume that k r* ~or s r ! 5k v ~or s v ! and therefore, u Cl
50.5 and s * (W!5231015 cm22 .
V. INTERPRETATION OF EXPERIMENTAL DATA
USING THE MODEL
From the kinetic models for heteronuclear and homonuclear reactions: ~1! the area under the abstraction product
MS signal versus time curve is directly proportional to the
number density of the reactive chemisorbed atoms, ~2! the
time constant for the decay of the product MS signal is directly proportional to the number density of the reactive
chemisorbed atoms and inversely proportional to the recombination coefficient and the incident atom flux, and ~3! the
magnitude of the product MS signal when a pre-exposed
sample is first exposed to a flux of atoms ~i.e., at t50! is
directly proportional to the recombination coefficient and the
incident atom flux. Therefore, comparing the areas under the
ClBr MS versus time curves for the poly-Si, c-Si, and W
samples in Figs. 1–3, respectively, we note that since the
areas under the MS signal curves for W.poly-Si.c-Si, the
number density of the reactive chemisorbed atoms for
W.poly-Si.c-Si. Also, from previous experimental measurements we know that g Cl and g Br for W.poly-Si.c-Si.
Therefore, comparing Figs. 1–3 we note the magnitude of
the ClBr MS signal when the brominated sample is exposed
to Cl atoms for W.poly-Si.c-Si. From the model fits to
the ClBr MS signal versus time curves ~Figs. 1–3!, we have
calculated the reactive chemisorbed atom number densities
of 231015, 531014, and 2.531012 cm22 for W, poly-Si,
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84
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
and c-Si, respectively. From the model fit to the 72Cl2 MS
signal versus time curve ~Fig. 13!, we estimate the number
density of the reactive chemisorbed atoms of 131015 cm22
for W. The agreement between the reactive chemisorbed
number densities for W calculated using the heteronuclear
experiment ~ClBr! and the homonuclear experiment ( 72Cl2 )
is especially good considering some of the assumptions
made in the heteronuclear model. The assumptions made in
the heteronuclear model were that g ClBr is equal to the average of g Cl and g Br , and that the reactive chemisorption sites
and the values of k v and k r* are the same for both Cl and Br.
It is interesting to note the difference in the number densities
of the reactive chemisorbed atoms for poly-Si and c-Si. This
may be explained by the possibility that the reactive chemisorbed sites are present along the grain boundaries, steps, or
dislocations that are in greater number for poly-Si than for
c-Si. In addition, the reactive chemisorption sites may not be
strictly localized to the surface and may also exist in the
subsurface region.
The magnitude of the ClBr MS signal when the sample
pre-exposed to Br is initially exposed to Cl ~i.e., t50 in the
model!, is proportional to g ClBr ~with a constant G Cl,inc!. Using the magnitude of ClBr MS signal for poly-Si ~;7 arb.
units! as the reference, the magnitude of the ClBr MS signal
for c-Si and W at t50 should be ;0.5 and ;20, respectively. These values estimated using the model are within a
factor of two of the experimentally obtained values which
are ;1 for c-Si ~Fig. 2! and ;13 for W ~Fig. 3!.
The evidence for the existence of unreactive chemisorbed atoms is that only a fraction of the total chemisorbed
atoms are abstracted from the surface by the incident halogen
atoms. The MS signals of the ion-induced desorption of the
unreacted chemisorbed halogen are shown in Figs. 4–6. The
area under the Br MS signal versus time curve is proportional to the number of unreacted chemisorbed atoms. Comparing Figs. 4 and 5, we note that the areas under the Br MS
signal curves are about the same for both poly-Si and c-Si,
indicating that the number of unreacted chemisorbed atoms
are about the same for poly-Si and c-Si. This is an interesting result considering the fact that the reactive chemisorbed
atom number density is much larger for poly-Si than for
c-Si. From Fig. 6, the area under the Br MS signal curve for
W is larger than those for poly-Si and c-Si. Therefore, the
number of unreactive chemisorbed atoms for W.poly-Si
;c-Si.
It should be noted that the relative magnitude of the ClBr
MS peak ~when a pre-exposed poly-Si sample is initially
exposed to atoms, i.e., t50! is greater for the incident Cl
abstracting Br from the poly-Si surface @Fig. 1~a!# than for
the incident Br abstracting Cl from the poly-Si surface ~Fig.
8!. Since the magnitude of the ClBr peak is proportional to
both g ClBr and the flux of the incident atoms, the difference
in the magnitudes of the ClBr peaks may be due to the fact
that the energetics of Cl atoms interacting with reactive
chemisorbed Br are different compared to Br atoms reacting
with reactive chemisorbed Cl, therefore resulting in different
g ClBr values and thus influencing the amount of ClBr formed
in the two cases. As stated earlier, the area under the ClBr
curve is proportional to the number density of the reactive
Kota, Coburn, and Graves
chemisorbed atoms. Therefore, the area under the ClBr MS
curve in Fig. 1~a! corresponds to the number density of reactive chemisorbed Br whereas the area under the ClBr MS
curve in Fig. 8 corresponds to the number density of reactive
chemisorbed Cl. Though the magnitude of the ClBr signal is
greater in Fig. 1~a! than in Fig. 8, the time constant for the
decay of the ClBr MS signal appears to be larger in Fig. 8
than that in Fig. 1~a!. The area under the ClBr MS curve in
Fig. 1~a! is approximately 1.5 times that in Fig. 8. Once
again, this is not surprising considering that the energetics of
Cl atoms interacting with reactive chemisorbed Br are probably different compared to Br atoms reacting with reactive
chemisorbed Cl. Similarly, we note that the magnitude of the
ClF MS signal and the area under the ClF MS curve when
the chlorinated poly-Si sample is exposed to F atoms ~Fig. 9!
are greater than those when the fluorinated poly-Si sample is
exposed to Cl atoms ~Fig. 10!. We can attribute this difference to the fact that the g ClF values may be different for the
two cases. A complicating factor in the analysis of the ClF
MS curves ~especially Fig. 9! is that the etching reaction
probability of the F atoms is significant relative to the abstraction reaction probability.
From the Cl-uptake experiments ~Sec. III B, Figs. 11 and
12! we estimated that the total number densities of chemisorbed atoms ~equal to the sum of the reactive and the unreactive chemisorbed atom number densities! are about 1.6
31015 and 531015 cm22 for poly-Si and W, respectively.
Comparing the estimated number density of the reactive
chemisorbed atoms and the calculated total number density
of chemisorbed atoms for both W and poly-Si, we note that
the reactive atom number density is less than the total number density of chemisorbed atoms as expected.
VI. DISCUSSION
The main results of the homonuclear and the heteronuclear abstraction experiments can be summarized as follows:
~1! The incident atom either directly or through a physisorbed precursor state reacts with a strongly bound adatom to form a molecule. Therefore, the recombination
reaction follows a pseudofirst order kinetics. The recombination reaction appears to be first order in the incident
flux.
~2! Only a fraction of the strongly bound adatoms appear to
be reactive, i.e., undergo recombination reaction ~Figs.
4–7!. Therefore, we have classified the strongly bound
adatoms that react as ‘‘reactive’’ chemisorbed atoms and
those that do not react as ‘‘unreactive’’ chemisorbed atoms.
~3! The abstraction reactions, for both heteronuclear and
homonuclear cases, are fast ~Figs. 1–3, 8–10, 13!.
~4! If the reaction follows LH kinetics then we can consider
that the physisorbed atoms play an important role in the
plasma-surface chemistry.
~5! The estimated reactive chemisorbed atom densities for
c-Si, poly-Si, and W range from 1012 to 1015 cm22 . As
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
required, these estimates are lower than the measured
total chemisorbed atom densities ~Figs. 11 and 12!.
Both the Eley–Rideal and Langmuir–Hinshelwood reaction mechanisms have been studied in great detail for various
systems by many researchers. Koleske and Gates23 have
studied atomic H abstraction of chemisorbed H on Si. They
found that the abstraction reactions are first order and have a
low activation energy ~;0.5–1 kcal/mol!. They report that
these observations are consistent with an Eley–Rideal abstraction mechanism. Cheng et al.24 have studied atomic H
abstraction of halogen from Si~100! surface. They also found
that the activation energy is low and the reaction is first order
in the incident atom flux which can be explained by an Eley–
Rideal reaction mechanism. Jumper et al.25 have measured
and modeled F-atom recombination on a Ni surface. Their
model assumes a reaction mechanism where a gas-phase F
atom reacts directly with a adsorbed F atom on the surface to
form a F2 molecule. They have referred to this reaction
mechanism as Langmuir–Rideal mechanism. Sinniah
et al.15,16 have studied the recombinative desorption of hydrogen from the monohydride phase on the Si ~100! surface.
They found that the reaction is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol
over the temperature range of 685–790 K. They propose a
mechanism where the rate limiting step is the promotion of a
localized adatom to a delocalized adatom state which then
reacts with a localized atom to form a molecule. Similarly,
for the halogen atom recombination, the physisorbed halogen
atom can be considered as the delocalized adatom state and
the reactive chemisorbed atom can be considered as the localized adatom state. Rettner and Auerbach have studied the
reaction of H with Cl/Au~111!.26,27 They found that the reaction between H and Cl occurs by both ER and LH mechanisms. The HCl product has a bimodal energy distribution:
the ER reaction product leaves the surface with high kinetic
energy in a narrow angular distribution and the LH reaction
product leaves the surface with near-thermal energy in an
angular distribution that is close to that of a cosine function.
In the case of halogen atom abstracting halogen atom from
various surfaces the reaction could be either ER or LH type.
Even without the knowledge of the exact reaction mechanism, as discussed in Sec. IV, the estimated reactive chemisorbed atom densities are the same whether the reaction is
ER or LH type.
The heteronuclear and the homonuclear experiments
have been used to investigate the existence of the reactive
sites and their role in the recombination reactions at surfaces.
However, we have not been able to conclude as to whether
the abstraction reaction is LH or ER type. The expressions
for g using ER and LH reaction mechanisms are given in
Eqs. ~4! and ~18!, respectively. If the reaction mechanism is
ER type and if s r and s v are assumed to be independent of
surface temperature, then g is also independent of surface
temperature. But previous studies10,11 have shown that g decreases as temperature increases. Therefore, we have chosen
to model the g dependence on temperature assuming that the
reaction mechanism is LH type. In Eq. ~18!, the rate constants for desorption and for reaction ~k d and k r ! are assumed
Kota, Coburn, and Graves
85
to have Arrhenius forms and therefore, g is a function of
temperature. Using Eq. ~3!, if s v and s r are assumed to be
independent of temperature, then the reactive chemisorbed
atom density ( s * u *
x ) is also independent of temperature.
Additional experiments of measurements of abstraction
product signal at different surface temperatures should enable us to determine if this assumption is true. According to
Eq. ~27!, if the u *
x is constant then the total abstraction product flux ~i.e., area under the curve! should be independent of
the surface temperature. In addition, the time constant for
decay of the abstraction product signal @Eq. ~26!# should increase and the abstraction product signal at t50 @Eq. ~25!#
should decrease as surface temperature is increased. This is
because g decreases as temperature increases.
VII. SUMMARY
Halogen atoms interacting with surfaces exposed to low
pressure plasmas appear to behave in the following manner.
Initially, halogen atoms will fill all available surface dangling bond sites, and will diffuse into subsurface sites as
well, depending on the surface material. The bonds are covalent and relatively strong, leading to very little spontaneous thermal desorption at room temperature. Subsequent
halogen atoms incident on the surface will abstract some
fraction of the strongly bound surface halogens ~referred to
as reactive chemisorbed atoms!, resulting in the formation
and desorption of halogen diatomic molecules. Under all
conditions studied, the surface recombination reactions proceeded at rates on the order of surface collision frequencies.
The relative magnitudes of the heteronuclear recombination
rates ~as a function of surface composition and halogen atom
type! scaled in the same way as the homonuclear recombination probabilities measured previously.10,11 In every case
examined, after the second halogen exposure, the surface retained a significant coverage of the halogen that had been
originally exposed to the surface. This leads to the conclusion that only a fraction of the strongly bound surface sites
are available for abstraction by free radical attack. After surface sites have been reopened due to abstraction, subsequent
halogen atoms will fill these reactive chemisorption sites. We
have estimated that the ~room temperature! reactive site densities for c-Si, poly-Si, and W are 2.531012, 531014, and
2.531015 cm22 , respectively. It is not possible to determine
with the experiments we have conducted whether the incident halogen atom reacts directly with an adsorbed halogen
atom ~‘‘Eley–Rideal’’ type reaction!, or whether the incident
halogen atom adsorbs into a physisorbed state, followed by
surface diffusion and abstraction reaction ~‘‘Langmuir–
Hinshelwood’’ type reaction!.
ACKNOWLEDGMENTS
The authors would like to acknowledge the donation of
the experimental apparatus by IBM Almaden Research Division. The authors would also like to thank the financial support provided by Lam Research Corporation and the California State MICRO program. The authors are very appreciative
of helpful discussions with Bryan A. Helmer.
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86
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
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