Surface etching mechanism of silicon nitride in fluorine and nitric oxide
containing plasmas
B. E. E. Kastenmeier
IBM Semiconductor Research and Development Center, 2070 Route 52, Hopewell Junction,
New York 12533
P. J. Matsuo
Department of Physics, The University at Albany, State University of New York, 1400 Washington Avenue,
Albany, New York 12222
G. S. Oehrlein
Department of Materials and Nuclear Engineering and Institute for Plasma Research,
University of Maryland, College Park, Maryland 20742-2115
R. E. Ellefson and L. C. Frees
Leybold Inficon Inc., East Syracuse, New York 13057-9714
共Received 27 August 1999; accepted 2 October 2000兲
The etch rate of silicon nitride (Si3N4 ) in the afterglow of fluorine-containing plasmas is strongly
enhanced when both nitrogen and oxygen are added to the remote discharge. This effect is attributed
to the formation of nitric oxide 共NO兲, which we identify as a highly reactive precursor for the
etching of Si3N4 . The Si3N4 etch rate, surface oxidation, and the depletion of the surface of N atoms
show a linear dependence on the NO density. In order to determine the products of the NO reaction
at the Si3N4 surface, mass spectrometry was performed in immediate proximity to the surface with
a specially designed movable sampling orifice. Both SiF4 and N2 are identified as primary etch
products, but a smaller amount of N2O was also detected. Based on our results, we suggest that NO
enhances the removal of N from the Si3N4 surface by the formation of gaseous N2 , and leaving
behind an O atom, while the overall surface oxidation remains very low, and the reactive layers are
very thin. This modified surface reacts more readily with F atoms than the Si3N4 surface. © 2001
American Vacuum Society. 关DOI: 10.1116/1.1329118兴
I. INTRODUCTION
In remote plasma chemical dry etching 共CDE兲, the synergistic effect of direct ion bombardment on the removal rate
of materials is not available. Instead, all etching reactions are
purely chemical. The absence of ion bombardment on the
surfaces allows for high etch rate ratios of different materials
and processes free of ion- or charge-induced damages. However, in many cases removal rates are suppressed due to the
absence of sufficient activation energy. Small changes in the
feed-gas composition of the remote discharge can significantly alter the rate of removal of certain materials due to the
formation of reactive intermediates. The new species can
open up additional pathways for etching reactions. In this
paper, we describe the effect of nitric oxide 共NO兲 addition to
a F and O containing gas phase on the etching of silicon
nitride (Si3N4 ). The changes in this system upon injection of
NO are significant enough to allow experimental characterization of the new reaction channels.
We first describe the experimental setup. Subsequently
the results allowing the characterization of the Si3N4 etch
mechanism by NO and F are presented. Etch rates of Si3N4
and their relation to the NO partial pressure are shown. The
effects of NO on the chemical composition of the reactive
layer are demonstrated by x-ray photoelectron spectroscopy
measurements. The main products of the chemical reaction
of NO with Si3N4 are determined by mass spectrometry, al25
J. Vac. Sci. Technol. A 19„1…, JanÕFeb 2001
lowing the determination of the mechanism by which NO
enhances the etching of Si3N4 .
II. EXPERIMENTAL SETUP
The remote plasma chemical dry etching 共CDE兲 system
used in this investigation is shown in Fig. 1 共see Refs. 1–3
for a more detailed description兲. A 2.45 GHz microwave
discharge is ignited in a sapphire applicator. The species effluent from the plasma travel through tubing of variable
length and lining material into the reaction chamber. For
some experiments, nitric oxide 共NO兲 is injected directly into
the reaction chamber downstream from the discharge. The
sample is placed on the center of an electrostatic chuck. Etch
rates are measured in situ by ellipsometry. Two orifice designs were employed for the mass spectrometry measurements reported here. For gas phase sampling, an orifice with
a diameter of 70 ␮m is placed at the chamber wall. Additionally, for sampling the gas phase in immediate proximity
to the sample, a cone-shaped 70 ␮m sampling orifice is
welded onto a 0.25-in.-o.d. stainless steel tube. The cone
shape allows for uninhibited flow of species to and from the
surface. The mass spectrometer is mounted on top of the
reaction chamber by means of a bellows and a drive gear.
This allows the mass spectrometer and orifice setup to move
vertically within a distance of 5 cm from the sample surface.
This setup allows one to sample mass spectrometric data in
0734-2101Õ2001Õ19„1…Õ25Õ6Õ$18.00
©2001 American Vacuum Society
25
26
Kastenmeier et al.: Surface etching mechanism
FIG. 1. Schematic of the chemical downstream etcher used in this investigation. The gases are fed into the sapphire applicator, where a microwave
discharge is ignited. The species effluent from the plasma travel through
tubing of variable length and lining material to the reaction chamber. The
sample is placed on the center of an electrostatic chuck. A quadrupole mass
spectrometer is mounted on the chamber on top of the sample, and monochromatic ellipsometry is used to determine etch rates. NO is fed directly
into the chamber downstream from the discharge.
two different modes: 共1兲 At constant sample-orifice distance,
a multilayer stack is etched. The intensities of etch products
are expected to show changes at transitions between materials. 共2兲 During the etching of one material, the sample-orifice
distance is varied, thus scanning the concentration profile of
products.
Experiments were performed with Si3N4 /SiO2 /c-Si
multilayer samples. The mass spectrometry experiments designed to sample the etch products are repeated with a
sample covered with a Teflon™-like fluorocarbon film for
comparison. This film was grown using CHF3 in an inductively coupled high-density plasma reactor.4 Teflon-like films
are not attacked by the chemistries at the wafer temperatures
employed here, therefore the amount of etch products desorbing from that surface is small compared to a Si3N4 surface. All samples were placed on a silicon wafer covered
with the same inert fluorocarbon film in order to minimize
the generation of etch products from surfaces other than that
of the sample.
26
FIG. 2. Normalized Si3N4 etch rates and NO partial pressures for two distinctively different plasma chemistries. The data shown in the top panel are
taken in the afterglow of a CF4 /O2 discharge, with either 0 or 20 sccm of N2
added to the discharge. The bottom panel shows the same set of data, measured in the afterglow of a NF3 /O2 discharge. For both cases, the Si3N4 etch
rates and the NO partial pressure show the same trend as a function of the
O2 /CF4 ratio. Data are taken from Refs. 1 and 2.
bottom panel shows the normalized etch rates of Si3N4 in the
afterglow of a NF3 /O2 discharge and the NO partial pressure
in the reaction chamber. Again, the etch rates and the NO
partial pressure follow the same trend. The close correlation
between Si3N4 etch rates and NO partial pressure allowed
this group to identify NO as a chemically active species in
Si3N4 etching,7 which was later verified by Blain et al.8
The chemical composition of the reactive layer during
Si3N4 etching is changed by the presence of NO. This is
shown in Fig. 3, in which photoemission spectra from
Si(2p), O(1s), and N(1s) electrons are plotted for experi-
III. RESULTS
The starting point of this investigation are Si3N4 etch rates
in the afterglow of various plasma chemistries. These etch
rates are shown for CF4 /O2 and NF3 /O2 discharges in Fig. 2,
normalized to their respective maximum values 共data taken
from Refs. 1 and 2兲. The CF4 /O2 etch rates 共top panel兲 show
a significant enhancement by a factor of 7 or more when 20
sccm of N2 are added upstream to the discharge as compared
to the case when only CF4 and O2 are used. The etch rates of
Si and SiO2 are not significantly changed by the upstream
addition of N2 . 5,6 Also included in the top panel is the normalized partial pressure of nitric oxide 共NO兲 in the reaction
chamber for the case of 20 sccm of N2 added upstream, as
determined by mass spectrometry. Both the NO partial pressure and the Si3N4 etch rates follow the same trend. The
J. Vac. Sci. Technol. A, Vol. 19, No. 1, JanÕFeb 2001
FIG. 3. Photoemission spectra of Si(2p), O(1s), and N(1s) electrons. The
spectra were taken from Si3N4 samples processed with CF4 /O2 400/20 sccm
and NO addition of 0 sccm 共top row兲 and 10 sccm 共bottom row兲. The
addition of NO enhances the surface oxidation and aides the removal of N
atoms from the surface.
27
Kastenmeier et al.: Surface etching mechanism
27
Two reactions of the NO molecule on the Si3N4 surface
have been proposed to explain the observed etch rate
enhancement.1,8 The first produces N2 :
Nsurface⫹NOgas→N2⫹Osurface .
共1兲
The O atom left on the surface by this reaction must desorb as the etching progresses, most likely as O2 . The net
reaction then becomes 2Nsurface⫹2NOgas⫹→2N2gas⫹O2gas .
The direct formation of O2 from two adsorbed NO molecules
without the O atoms adsorbing on the surface is second order
in both the NO density and the surface site density and is
therefore much less likely to occur. The second equation
produces N2O:
Nsurface⫹NOgas→N2Ogas .
FIG. 4. Normalized Si3N4 etch rate and oxidation during NF3 /O2 etching as
a function of normalized NO density. For these experiments, the flow of
NF3 was kept constant, and O2 was added to the discharge 共see Fig. 2兲.
Thus, the NO density, determined in the reaction chamber with a quadrupole
mass spectrometer, varied in a nonobvious fashion 共Ref. 2兲. The surface
oxidation is determined using x-ray photoelectron spectroscopy. Both etch
rate and oxidation depend on the NO density in the same fashion.
ments where samples were processed with a CF4 /O2 remote
discharge. The effects of NO were investigated by adding a
small amount 共10 sccm兲 of NO gas directly into the reaction
chamber downstream. After processing, the samples were
transferred in vacuum to the surface analysis chamber. The
spectra were taken with an emission angle of 15° with respect to the surface to enhance surface sensitivity. The top
row shows spectra taken from a sample that was processed
with the remote CF4 /O2 discharge only. In the bottom row,
spectra from a sample processed with the remote discharge
and 10 sccm of NO are shown. The Si(2p) peak, shown in
the left column, shows a shoulder at high binding energies
when 10 sccm of NO is injected downstream 共bottom panel兲.
This corresponds to an increased number of Si atoms bonded
to O atoms during etching as compared to the case where no
NO is used 共top panel兲. Correspondingly, the O(1s) signal,
shown in the center column, is significantly higher for the
case in which NO is used. In the right column of Fig. 3, the
N(1s) peak is shown. The peak intensity is reduced significantly for the case in which NO is added downstream.
The correlation between the NO partial pressure in the
reaction chamber, the Si3N4 etch rate, and the surface oxidation is summarized in Fig. 4. The data are acquired under
NF3 /O2 processing conditions. The etch rate and NO partial
pressure data are the same as in the bottom panel of Fig. 2,
normalized to their respective maximum values. Samples for
x-ray photoelectron spectroscopy measurements were prepared under the same experimental conditions and then
transferred to the surface analysis chamber. The
O(1s)/Si(2p) photoemission intensity ratio, normalized to
its maximum value, is used as a measure for the surface
oxidation. Within experimental error, both the Si3N4 etch
rate and the degree of surface oxidation as expressed by the
O/Si ratio are proportional to the partial pressure of NO in
the reaction chamber.
JVST A - Vacuum, Surfaces, and Films
共2兲
Although the above-mentioned results and those shown in
Fig. 4 favor reaction 共1兲, contributions from Eq. 共2兲 cannot
be ruled out based on these data. In fact, in a recent
publication,9 Blain monitors the N2O partial pressure during
Si3N4 etching in the afterglow of a NF3 /Ar discharge. His
data show a decline of the N2O partial pressure after the
Si3N4 is removed. The N2 produced by reaction 共1兲 on the
Si3N4 surface, however, cannot be distinguished from the
background N2 produced by the NF3 /Ar discharge employed
in his work.
In order to clarify the relative importance of reactions 共1兲
and 共2兲, mass spectrometry experiments were carried out
with the orifice placed in proximity to the sample surface.
The partial pressure of etch products is higher close to the
sample surface as compared to a recessed wall position. The
experiments with this orifice setup were conducted in two
modes. In the first mode, the sample–orifice distance was
kept constant, and a Si3N4 /SiO2 stack was etched. Changes
of the signal intensities can be attributed to the amount of
species produced during the etching of either Si3N4 or SiO2 .
In the second mode, the sample–orifice distance was increased in steps. The intensity of product species is expected
to decrease with increasing distance in this case.
For the experiments reported here, sulfurhexafluoride
(SF6 ) with a small amount of O2 was injected into the sapphire microwave applicator. This gas mixture was used, because it does not yield discharge products with mass m/e
⫽28. Other gases like CF4 /O2 and NF3 /O2 produce significant amounts of CO and N2 , respectively. The possible contribution of Si as a cracking product of SiF4 to the mass peak
at m/e⫽28 could be ruled out based on specifically designed
experiments in which the SiF4 partial pressure showed significant changes, but the mass peak at m/e⫽28 remained
constant at background levels. For these experiments, polycrystalline Si on SiO2 was etched in SF6/02 discharges. The
intensity of the sif3⫹ peak declined significantly as the
poly-Si was removed and SiO2 etching started at a much
lower rate. The intensity of the peak at m/e⫽28, however,
remained unchanged. By using SF6 , it is thus possible to
detect N2 as an etch product against a relatively small background at 28 u. For the SF6 /O2 system, the downstream injection of NO has the same etch rate enhancing effect as for
CF4 /O2 gases. This is demonstrated in Fig. 5, where Si3N4
28
Kastenmeier et al.: Surface etching mechanism
28
FIG. 5. Etch rates of Si3N4 as a function of downstream NO addition. A
similar behavior is observed for SF6 /O2 and CF4 /O2 as discharge gases.
Note that the absolute etch rate values differ significantly 共see the text兲.
etch rates are shown for both gas mixtures as a function of
downstream NO addition. The etch rates are normalized to
their respective maximum values. The normalized etch rates
show identical trends as a function of NO addition. The absolute etch rates for SF6 /O2 are greater by a factor of 5 than
those for CF4 /O2 , however. Typical etch rate values are 20
nm/min for CF4 /O2 and 100 nm/min for SF6 /O2 at 10 sccm
of NO. For all experiments reported here, the flow of SF6
FIG. 6. Mass spectrometric signal intensities during the etching of a
Si3N4 /SiO2 stack 共left column兲 and a Teflon-like fluorocarbon film 共right
column兲. The numbers in the panels indicate the magnitude of the ion current detected in the mass spectrometer.
J. Vac. Sci. Technol. A, Vol. 19, No. 1, JanÕFeb 2001
⫹
⫹
FIG. 7. Mass spectrometric intensities for SiF⫹
3 , N2 , and N2O during the
etching of Si3N4 and SiO2 in the afterglow of a SF6 /O2 discharge and NO
gas injected directly into the reaction chamber. At point A, NO gas is injected downstream, increasing the Si3N4 etch rate and the intensities of the
product species in the mass spectrometer. At points B – F, the sample–
orifice distance was increased in steps from 0.03 to 4 cm. The decrease in
the mass spectrometer intensities indicate that these species are produced by
the etching of the Si3N4 . At point G, the Si3N4 film is completely removed,
and SiO2 etching starts. At point H, the sample–orifice distance is again
reduced to 0.03 cm, causing the signal intensity of etch products to increase.
and O2 was kept constant at 300 and 50 sccm, respectively.
The microwave power was 700 W and the pressure 500
mTorr.
Figure 6 shows the mass spectrometric signal intensities
of etch products during SF6 /O2 etching. The orifice was
placed at a fixed position 0.3 mm above the sample surface,
corresponding to approximately two mean free path lengths
at the operating pressure of 500 mTorr. In the left column the
intensities as recorded during the etching of a Si3N4 /SiO2
stack are plotted. All curves of that column are obtained
during the same run, and the material that is being etched at
a certain time is indicated by the arrows at the top of the
column. Initially, the Si3N4 is being etched and only SF6 and
O2 is injected. At the time labeled ‘‘a,’’ 20 sccm of NO is
injected directly into the reaction chamber. This increases the
Si3N4 etch rate of by a factor of 6 共see Fig. 5兲 and thus the
signal intensity of reaction products increases. At point ‘‘b,’’
the Si3N4 is removed completely and etching of the underlying SiO2 starts. No other parameters are changed. At this
point, the signal intensities of Si3N4 etch products are expected to decline. At point ‘‘c,’’ the discharge is switched off
and the flow of gases is stopped. The right column shows the
intensities of the same peaks while a fluorocarbon film is
etched. The ordinates have the same scale as on the left-hand
side for comparison. At point ‘‘d,’’ NO is added downstream
of the SF6 /O2 discharge, and the flow of NO is stopped at
‘‘e.’’
In the top row, the intensity of the SiF⫹
3 peak at 85 u is
29
Kastenmeier et al.: Surface etching mechanism
29
TABLE I. The various heats of formation occurring in the two processes. The values are taken from Ref 10.
Reaction
Nsurface⫹NOgas→N2⫹Osurface
Nsurface⫹NOgas→N2Ogas
Bond energy 共eV兲
Formation of N–N in N2 : ⫺9.79
Formation of Osurface –Si: ⫺8.27
Breaking of N–O: 6.53
Breaking of Nsurface –Si: 4.54
Formation of N–N in N2O: ⫺4.50a
Formation of Osurface –Si: ⫺8.27
Weakening of N–O: 4.80b
Breaking of Nsurface –Si: 4.54
Sum
⫺6.99
⫺3.43
a
Values taken from Ref. 11.
Difference between the bond energies of the N–O bond in NO 共6.53 eV兲 and the N–O bond in N2O 共1.73 eV兲.
b
plotted. The intensity follows the increase of the Si3N4 etch
rate at point ‘‘a,’’ and then declines at point ‘‘b,’’ since SiO2
is removed at a rate of only approximately 10 nm/min at
these conditions. For the Teflon-like sample, the SiF⫹
3 peak
remains at much lower levels. The background signal is
probably due to etching of unprotected areas of the Si carrier
wafer and the quartz windows.
The next two rows show the intensities of the N⫹
2 and
N2O⫹ products. The N2 intensity increases significantly and
abruptly at point ‘‘a.’’ When the Si3N4 is completely removed at ‘‘b,’’ the signal intensity decreases. The changes of
the N2O⫹ signal are much weaker by comparison, but there
is a small decrease upon completion of the Si3N4 etching,
indicative of the formation of N2O. The changes during etching of the fluorocarbon film are more subtle for both species.
Finally, in the bottom row the intensity of the NF⫹
2 peak
at 52 u is plotted. This ion is a cracking product of NF3 in the
ionization region of the mass spectrometer. The peak intensity shows the typical behavior of a Si3N4 etch product,
while no changes are observed when no Si3N4 is present 共see
the right column兲. The overall intensity of this peak, however, is smaller than that of other etch products by approximately two orders of magnitude.
In Fig. 7, peak intensity profiles of etch products are
shown for an experiment in which the sample–orifice distance was increased in steps while the Si3N4 surface was
etched in the afterglow of a SF6 /O2 discharge. The concentration profile as a function of distance from the sample surface allows one to distinguish the role of a given species in
the etch mechanism. An etchant species is expected to increase in intensity as the sampling orifice is removed from
the surface, which acts as a drain for that particular species.
A product species is expected to decrease in intensity. At a
location very close to the sample, the flux of product species
is spread out over an area equal to the sample area. At a
recessed orifice position, the same flux of species is spread
out over a larger area, resulting in a lower partial pressure.
At point A, 20 sccm of NO is injected directly into the
reaction chamber, leading to an increased Si3N4 etch rate.
Correspondingly, the SiF⫹
3 signal increases by a factor of 8.
Subsequently, the orifice is moved in steps from its starting
position at 0.03 to 4 cm above the sample surface, indicated
⫹
by letters B–F. The decline of the SiF⫹
3 and N2 signal indicates that SiF4 and N2 are produced at the Si3N4 surface. The
N2O⫹ signal in the bottom panel of Fig. 7 shows only a
slight increase upon NO addition to the reaction chamber,
JVST A - Vacuum, Surfaces, and Films
and a smaller relative change as the sample–orifice distance
is varied.
IV. CONCLUSIONS
It can be concluded from these data that N2 , N2O, and
NF3 are reaction products of the NO reaction on Si3N4 surfaces in the presence of atomic F. The signal intensities were
calibrated to partial pressure values by using pure N2 and
N2O gases. At point ‘‘b’’ in Fig. 6, the N2 signal declines by
15%, corresponding to a partial pressure decline of 60
⫻10⫺6 Torr. The N2O signal declines by 5% or 13
⫻10⫺6 Torr. The data indicate that reaction 共1兲 is the predominant pathway for N removal from the surface.
From an energetic point of view, the N2 producing reaction 共1兲 is preferred over reaction 共2兲. In Table I, the bond
energies relevant for the two reaction pathways are listed,
together with the total energy released by the reactions. Both
are exothermic, but the energy released by the N2 producing
reaction is almost twice as high as that of the N2O producing
reaction. The difference stems, for the biggest part, from the
formation of the extremely stable N2 molecule.
We have demonstrated how the introduction of a new
chemical reaction pathway can increase the overall reaction
rate. The chemically active species was obtained in a nonobvious fashion, i.e., not through electron impact of the parent
gases, but through chemical reactions in the gas phase 共for
CF4 /O2 /N2 and NF3 /O2 processes兲 or through direct downstream injection. This approach to increase the reaction rate
can be of importance in other systems in which the amount
of activation energy supplied to the surface is limited.
ACKNOWLEDGMENTS
This work was supported by Sandia National Laboratories, Leybold Inficon, and the New York State Science and
Technology Foundation. We also thank Matthew Blain and
John Langan for stimulating discussions.
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