Role of N2 addition on CF4 /O2 remote plasma chemical dry etching
of polycrystalline silicon
P. J. Matsuo,a) B. E. E. Kastenmeier, J. J. Beulens, and G. S. Oehrleinb)
Department of Physics, University at Albany, State University of New York, Albany, New York 12222
~Received 22 March 1996; accepted 17 January 1997!
The remote plasma chemical dry etching of polycrystalline silicon was investigated using various
CF4 /O2 /N2 gas compositions. The effects of O2 and N2 addition on the etch rate and surface
chemistry were established. Admixing O2 to CF4 increases the gas phase fluorine density and
increases the etch rate by roughly sevenfold to a maximum at an O2 /CF4 ratio of 0.15. The addition
of small amounts of N2 ~N2 /CF450.05! can again double this etch rate maximum. Strong changes
in surface chemistry were also seen as a result of N2 addition to CF4 /O2. Real-time ellipsometry and
atomic force micro-roughness measurements reveal that nitrogen addition at low O2 /CF4 ratios
leads to the smoothing of surfaces, but to increased oxidation at high O2 /CF4 ratios. Based on etch
rate data and gas phase species analysis, we propose that NO plays an important role in the overall
etching reaction. Variable tube lengths separated the reaction chamber from the discharge. These
tubes were lined with either quartz or Teflon liners. In general, etch rates diminished with quartz
tube length. At the longer transport tube lengths ~e.g., 125 cm!, using a Teflon lining material
strongly increases the etch rate for pure CF4 /O2 discharges as compared to the quartz. For
discharges containing N2, the etch rate is more than doubled. This can be explained by the low
recombination rate of atomic fluorine on Teflon and the subsequent high density of F atoms that
reach the process chamber, even for long transport tube lengths. In situ ellipsometric measurements
reveal postplasma surface modifications for certain etching chemistries. Comparisons of these
results to x-ray photoemission measurements reveal a dependence of the stability of the
postprocessing surface reaction layer on the etching conditions and hence the thickness and
composition of the layer, i.e., whether the layer is comprised of volatile ~SiFx -like! or involatile
(SiOy -like! species. Thicker, more SiOy -like reaction layers create a barrier for the diffusion and
subsequent desorption of the volatile products and a postplasma removal of a portion of the reaction
layer is observed. Thinner, more SiFx -like layers leave a fluorine deficient surface in the postplasma
stage which results in increased tendency to postplasma layer growth. The etching of silicon is not
always limited by the arrival rate of atomic fluorine for our processing conditions. © 1997
American Vacuum Society. @S0734-2101~97!00804-X#
I. INTRODUCTION
There is currently a significant interest in remote plasma
chemical dry etching ~CDE! due to several advantages over
competing etching technologies,1–5 e.g., exposure of samples
directly to the plasma environment or photostimulation processes such as ultraviolet ~UV! induced etching. In contrast
to a direct plasma etch, a remote plasma allows for better
control of the processing parameters and hence the environment of the substrate.4 Since the lifetime of ions is much less
than the gas travel time from the plasma to the sample, the
etching mechanism is purely chemical. Therefore damage
effects, such as gate oxide charging and physical degradation, can be eliminated.4,5 Surface smoothing, critical in the
manufacturing of high quality semiconductor devices, is also
possible in CDE.2 A chemical dry etcher is easily incorporated into a vacuum cluster process environment where there
is an advantage for mask removal, e.g., a Si3N4 oxidation
a!
Electronic mail: [email protected]
Electronic mail: [email protected]
b!
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J. Vac. Sci. Technol. A 15(4), Jul/Aug 1997
barrier, where CDE can eliminate the need for the conventional wet processing step used in this removal.6
There has been a great deal of study on the role of oxygen
addition to a CF4 plasma.1,2,4,7–13 The conclusion that O2
addition into the CF4 discharge increases the atomic fluorine
concentration due to the oxidation of CFx molecules has
been established. Also, the primary etch product of poly-Si
etching is SiF4. 2,10,14–16 Therefore, barring the formation of a
prohibitive reactive layer on the substrate surface, e.g., an
oxidized SiOx Fy layer, the connection to the etch rate enhancement observed in the CF4 /O2 system has been made
clear. However, there has been only little study of the role
nitrogen plays when injected into the CF4 /O2 plasma.17,18 In
order to determine the role of nitrogen, we studied etch rates,
the surface reaction layer, and the gas phase processes using
CF4 /O2 /N2 remote plasmas. Diagnostic tools utilized include
in situ ellipsometry, mass spectrometry, optical emission
spectroscopy, and x-ray photoelectron spectroscopy via ultrahigh vacuum ~UHV! transport from the process chamber.
Also of interest is the transport tube configuration and lining
0734-2101/97/15(4)/1801/13/$10.00
©1997 American Vacuum Society
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FIG. 1. Schematic of the chemical downstream etching tool used in this work.
material used to flow the excited gas from the discharge to
the reaction chamber.
II. EXPERIMENTAL APPARATUS AND PROCEDURE
In Fig. 1 the ultrahigh vacuum-compatible microwave
based etching reactor used in this work is shown. A microwave plasma is produced in a modified ASTeX DPA-38 microwave plasma applicator. This device is equipped with a
38 mm outside diameter ~o.d.! wall quartz tube to which the
process gases are fed. The plasma is separated from the processing chamber by tubing of variable length and lining material. For the current work, these lining materials were
quartz and Teflon and the lengths varied from 0 to 125 cm.
The etching experiments were performed in a cylindrical
processing chamber ~inside diameter 27 cm, height 30 cm!
with water cooled walls. The system is pumped using a Balzers 2200 l/s corrosive service turbomolecular pump backed
by a roots blower and vane pump. The pressure was measured with a MKS Baratron capacitance manometer. For the
experiments described here, the pressure was kept at 500
mTorr ~the pressure used in commercial equipment, e.g., the
Shibaura CDE-80!. A throttle valve in the bypass line was
used to control the pumping speed and maintain this pressure
in the chamber. Although, in general, the poly-Si etch rate
increases with microwave power, the microwave power was
maintained at 400 W, which was a good compromise be-tween process optimization and modest erosion rate of the
quartz applicator tube.17
For the etching and surface analysis experiments, 1 in. by
1 in. square samples were prepared from 200 mm wafers
covered with 250 nm poly silicon on 100 nm SiO2 on Si and
125-mm-diam single-crystal silicon wafers. These samples
J. Vac. Sci. Technol. A, Vol. 15, No. 4, Jul/Aug 1997
were HF dipped to remove the native oxide layer and were
then mounted on 125 mm silicon carrier wafers using thermal glue. The mounted samples were then placed on an electrostatic chuck ~dc bias of 600 V! with a helium backside
pressure of 5 Torr. The electrostatic chuck temperature was
set to 10 °C. The wafer temperature was measured with a
fluoroptic probe which contacts the backside of the wafer.
During processing, the silicon wafers heated up by approximately 2 °C.
In situ ellipsometry using a SOFIE rotating compensator
ellipsometer in the polarizer-compensator-sample-analyzer
~PCSA! configuration with a 632.8 nm He/Ne laser source
beam was employed to monitor real-time film etching and
deposition. Optical emission spectroscopy ~OES! and mass
spectrometry were used for real-time plasma and chamber
gas phase diagnostics. The etching chamber is also connected via a UHV central wafer handler to both a load-lock
and a multitechnique surface analysis chamber. Processed
specimens were transported under vacuum to the multitechnique surface analysis system for x-ray photoemission
spectroscopy ~XPS! measurements.
III. EXPERIMENTAL RESULTS
A. Etch rates
1. Etch rates versus gas composition in CF4 /O2 /N2
The dependence of the poly-Si etch rate on the O2 /CF4
ratio, with and without N2, is displayed in Fig. 2. An experimental error of less than 10% can be assumed for these values and any discussed trend should be considered beyond
this error. Etch rates were studied as a function of added
O2 and N2. For pure CF4, the etch rate was 20 nm/min. By
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FIG. 2. Poly-Si etch rates vs O2 /CF4 for several N2 /CF4 ratios.
FIG. 3. Poly-Si etch rates vs N2 /CF4 for O2 /CF4 ratios of ~a! 0.00, ~b! 0.15,
and ~c! 0.75.
adding O2 to CF4, the etch rate increased to 150 nm/min at
an O2 /CF4 flow ratio of 0.15. For more O2-rich CF4 /O2 gas
mixtures, the etch rate decreased again. Mechanisms for this
enhancement of the etch rate and the subsequent decrease
have been discussed in the literature.1–4,9–11,19 It has been
established that admixing small amounts of O2 to a pure
CF4 discharge results in an increase of atomic fluorine production due to oxidation of the CFx radicals. Higher flows of
O2 quench this fluorine production. When 20 sccm N2 was
injected in addition, the etch rate could be increased once
more by a factor of 2 relative to values without N2. Adding
more than 20 sccm N2 resulted in no further increase of the
poly-Si etch rate at the etch rate maximum. It caused, however, a more rapid decrease of the etch rate at high O2 /CF4
ratios. Figure 3 illustrates this dependence by means of the
poly-Si etch rate versus N2 composition in the discharge for
important O2 /CF4 ratios. Under these considerations we proceeded to use 20 sccm of N2 (N2 /CF450.05) as the standard
feed for gas mixtures containing nitrogen.
rate seen without N2 in the discharge. The etch rates for
N2 /CF450.00 dip slightly with quartz tube length.
For the oxygen rich discharge and zero tube length, the
etch rate is greater if N2 is added. The etch rate without N2
increases with tube length, yet, for CF4 /O2 /N2, the etch rate
drops off slightly. At 125 cm tube length, the etch rates with
and without N2 are approaching the same value.
3. Etch rates versus lining material and geometry
2. Etch rates versus tube length
In particular, it is the transport tube lining material which
impacts the gas phase effects in the etching process most
dramatically. A reactive versus an inert lining medium can
significantly deplete etching agents. In Fig. 5~a!, the influence on etch rates of a 125 cm Teflon liner compared to a
quartz liner are shown. The same maximum in etch rate is
observed as with the quartz liner, yet the values are dramatically enhanced.
In panel ~b! of Fig. 5, the influence of a 90° bend in the
transport tube, positioned adjacent to the applicator housing,
The separation distance and design of the transport region
encompass some important process parameters. The separation distance, lining, and geometry play a major role in
which reactive species survive and reach the processing
chamber.2,4,8 Figure 4 shows the dependence of the Poly-Si
etch rates on quartz lined transport tube length for ~a! no
O2 in the discharge, ~b! a fluorine rich discharge (O2 /CF4
5 0.15), and ~c! an oxygen rich discharge (O2 /CF450.75)
all with and without 20 sccm N2 (N2 /CF450.05).
For pure CF4, a linear decrease of the etch rate with tube
length is observed. Nitrogen addition to pure CF4 increases
the etch rate by almost a factor of 2 for zero tube length, but
a rapid decrease in the etch rate occurs within the first 30 cm
of the tube length, after which it drops below that measured
for pure CF4. It then remains roughly constant. This behavior
is also seen for Si3N4 and SiO2 etch rates.17,18
For an O2 /CF4 ratio of 0.15 and at zero tube length, the
etch rate is again increased by a factor of 2 upon injection of
N2. The etch rate then drops off, again relatively quickly for
the first 50 cm, and then starts to level off near to the etch
FIG. 4. Poly-Si etch rate vs quartz lined transport tube length. Panel ~a!
represents an O2 /CF4 ratio of 0.00, ~b! 0.15, and ~c! 0.75.
JVST A - Vacuum, Surfaces, and Films
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FIG. 5. Poly-Si etch rate vs O2 /CF4. In panel ~a! the results are for a quartz
vs Teflon liner. In ~b! the curves represent a straight 125 cm quartz tube vs
one with a 90° bend adjacent to the applicator.
FIG. 6. Actinometry determined concentration of atomic fluorine in the discharge as a function of O2 /CF4. Ar was used as the actinometer using the
7500 Å peak. The fluorine signal was monitored with the 7037 Å peak.
is evaluated and compared with a straight tube of equivalent
length ~125 cm!. A significant increase in the etch rate is
observed with the bent transport tube.
Upon inclusion of oxygen into the CF4 /N2 discharge, we see
significant NO production which then maintains its concentration through greater O2 /CF4 ratios.
B. Gas phase diagnostics
C. Surface analysis
1. OES
Figure 6 depicts the relative change in concentration of
atomic fluorine produced in the plasma, by means of Ar actinometry, as a function of the O2 /CF4 ratio both with and
without nitrogen. The 7037 Å peak was used to monitor the
fluorine signal, while the Ar 7500 Å peak was used for the
actinometry.20–22 As expected, injection of small amounts of
O2 into the discharge results in a higher concentration of
atomic fluorine. Then, as the level of oxygen is further increased, this fluorine production is suppressed. Inclusion of
nitrogen into the plasma results in an increase of the atomic
fluorine produced only slightly above the experimental error
of 10%.
1. Real-time ellipsometry
The modifications of single-crystal silicon for different
CDE processes were followed in real-time ellipsometry. Figure 10 shows real-time ellipsometry data obtained during
microwave induced chemical dry etching of single-crystal
silicon. A low proportion of O2 in CF4 was used for this
experiment (O2 /CF450.15), and initially no N2 was present.
The ellipsometric angles Delta and Psi are plotted versus
each other and versus time. At time ~a! the CF4 /O2 microwave discharge is ignited. It took about 20 s before the re-
2. Mass spectrometry
Figure 7 shows the plasma on-plasma off value for the 19
amu F signal. Panel ~a! represents no oxygen in the discharge, panel ~b! is for O2 /CF450.15, and panel ~c! is for a
high oxygen content in the plasma (O2 /CF450.75). As expected, there is an increase in the amount of available fluorine in the etching region upon injection of oxygen into the
discharge. However, for gas compositions containing nitrogen, the free fluorine density in the etching region is less
than that without the N2. For a pure CF4 discharge, the fluorine signal decreases significantly with increasing quartz tube
length, but for all other gas compositions, this effect is not
seen.
The effect of N2 addition on the production of atomic
oxygen is seen in Fig. 8. Clearly, injection of nitrogen into
the discharge enhances the dissociation of molecular oxygen.
Figure 9 shows the NO concentration in the processing
chamber as a function of the O2 /CF4 ratio in the discharge.
J. Vac. Sci. Technol. A, Vol. 15, No. 4, Jul/Aug 1997
FIG. 7. The F concentration in the chamber as determined by mass spectrometry as a function of quartz lined transport tube length. Panel ~a! represents a discharge void of oxygen. Panel ~b! is an O2 /CF4 ratio of 0.15 and
in ~c!, O2 /CF450.75.
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FIG. 8. Normalized O and O2 concentrations as determined by mass spectrometry illustrating the enhanced dissociation of molecular oxygen upon
N2 inclusion to the CF4 /O2 discharge.
FIG. 9. NO concentration in the reaction chamber vs O2 /CF4 as determined
by mass spectrometry.
flected power could be minimized and the discharge stabilized. This is represented in the region between ~a! and ~b!.
Once the reflected power is minimized, both Delta and Psi
decrease. In general, the decrease of Delta indicates the formation of a progressively thicker modified layer on the unperturbed silicon, but the corresponding decrease in Psi between points ~b! and ~c! is inconsistent with the growth of a
transparent film on the Si surface, e.g., due to the formation
of a SiFx Oy reaction layer,23,24 and suggests that in this case
it is the formation of surface roughness. At time ~c!, 20 sccm
of N2 is injected in addition to the CF4 /O2 and the surface
roughness formed during the CF4 /O2 discharge is rapidly
removed. Microroughness measurements determined from a
tapping mode atomic force microscope ~AFM! on Si~100!
surfaces after plasma treatment using a low proportion of
O2 with and without nitrogen supports this interpretation of
the ellipsometric angles C and D. In fact, the low oxygen
treatments containing nitrogen result in a decrease of the
calculated rms roughness by almost a factor of 2. At point
~d! the microwave plasma is extinguished, but the sample
remains exposed to the long lived species produced by the
discharge. The formation of another layer takes place now.
The time constant for this formation is just under 10 s, which
is of the same order as the residence time in the reactor. Note
that here Psi increases, whereas before, at time ~b!, Psi decreased. This suggests that the postplasma effect is actually
an increase in the reaction layer thickness. At time ~e! the
chamber is evacuated using the turbomolecular pump and the
layer no longer changes in thickness.
Figure 11 shows real-time ellipsometry data, which are
similar to that shown in Fig. 10, but were obtained using a
high proportion of O2 in CF4 ~O2 /CF4 5 0.75). At time ~a!
the microwave discharge fed with CF4 /O2 is ignited. The
strong decrease of Delta and increase of Psi indicate the formation of a rather thick reaction layer on the unperturbed
silicon. At time ~b! 20 sccm of N2 are injected in addition.
The N2 injection results in an increase of the thickness of the
reaction layer in this case. At time ~c! and N2 flow is stopped
and the original thickness of the reaction layer characteristic
of only CF4 /O2 is reestablished. At point ~d! N2 is injected
once more and the reaction layer thickness increases again.
At ~e! the N2 supply is shut off, and finally at point ~f! the
microwave discharge is extinguished and the chamber is
evacuated using the turbomolecular pump. We note that the
reaction layer decreases in thickness in this case once the
microwave discharge is extinguished.
Each of the major parameter settings were investigated
using this technique in order to determine the thickness of
the reaction layers. For a low proportion of O2 in the feed
gas, surface roughness made an accurate determination of the
film thickness impossible and so, for these settings, XPS
interpretation was utilized. However, for O2 /CF450.75, the
ellipsometric method was the best suited technique for determining reaction layer thicknesses and these results are presented in Fig. 12. Panel ~a! represents the reaction layer during etching while panel ~b! represents the postplasma
reaction layer thickness. In all cases there is a trend towards
increasing reaction layer thickness with quartz tube length.
This trend is enhanced upon injection of N2 into the discharge. Also easily seen here is the consistent desorption of
volatile species after the plasma has been extinguished and
the processing chamber pumped.
It is interesting to directly compare the postplasma effects
seen for this oxygen rich processing environment to that of
the fluorine rich environment. Figure 13 displays these differences as Delta versus time plots. Curve ~a! represents a
nitrogen free discharge with an O2 /CF4 ratio of 0.15. In ~b!
the O2 /CF4 ratio is also 0.15 but the N2 /CF4 ratio has been
increased to 0.05. Curves ~c! and ~d! represent O2 /CF4 ratios
of 0.75 both without and with N2, respectively. In the fluorine rich regime, inclusion of nitrogen is a necessary condition for the growth of a postplasma reaction layer. If a refractive index is known, the Delta and Psi versus time data
can be converted into thicknesses using an algorithm derived
from McCrackin’s FORTRAN program.25 Accordingly, the
thickness of this layer was measured to be approaching 1.0
JVST A - Vacuum, Surfaces, and Films
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FIG. 10. Low oxygen real-time ellipsometry. At time ~a! the microwave power is turned on. At ~b! the plasma is stabilized and surface roughness forms.
Twenty sccm of N2 is injected at time ~c! which removes this roughness. At time ~d! the plasma is extinguished, but surface passivation results in a modified
surface. Finally at ~e! the chamber is evacuated using the turbo-molecular pump and the surface dynamics cease.
nm. In the oxygen rich processing regime, postplasma desorption occurs for gas flows both with and without nitrogen,
although a more significant loss is observed for discharges
void of N2. Measured reductions in layer thickness are just
over 2.0 nm for the nitrogen free discharge and, for a plasma
containing nitrogen, a slightly less dramatic 1.7 nm.
2. Poly Si versus c-Si surface comparison
A careful comparison of the surfaces of poly-Si and Si
~100! after chemical dry etching using XPS showed that the
two kinds of surfaces showed basically the same modifications as a result of CDE for the different process conditions.
These results are presented in Fig. 14. This prompted us to
interpret changes of the poly-Si etch rates as a function of
process conditions using surface analysis results which were
in part obtained using single-crystal Si. For a pure CF4 discharge, there were, however, surface chemistry modifications
observed between Si ~100! and poly-Si samples. These differences are illustrated in Fig. 15~a!. However, as seen in
Fig. 15~b!, when nitrogen is added to the feed gas, this relatively increased reactivity of fluorine with the surface disappears and the two surfaces become virtually identical to
XPS. This slightly different surface modification of singlecrystalline as opposed to polycrystalline silicon in the form
of the increased reactivity of fluorine with c-Si could be
attributed to the more effective relaxation in the Si ~100!
surface than the poly Si.
J. Vac. Sci. Technol. A, Vol. 15, No. 4, Jul/Aug 1997
3. Gas composition effects
Etched specimens like those used for Figs. 10 and 11
were transferred into the multitechnique surface analysis system for x-ray photoemission measurements. The survey
spectra showed Si related peaks, and gas mixture dependent
F 1s and O 1s peaks. Carbon 1s emission was negligible for
most etching conditions, except for pure CF4 plasmas. However, even in that case it was very weak. Nitrogen 1 s emission was looked for but found to be very weak for all
samples, independent of the amount of N2 used in the etching
experiment. This shows that, even though nitrogen plays a
profound role in the etching of silicon, it is not incorporated
in a stable reaction layer. Apparently any chemical groupings
incorporating nitrogen are not stable on the surface but escape into the gas phase. Figure 16~a! shows Si(2p) spectra
obtained with single crystalline Si surfaces after CDE treatment using CF4 /O2 without and with 20 sccm N2 injection,
respectively. The CF4 flow was fixed at 400 sccm.17 In all
spectra we note major peaks at either 100 eV binding energy
or 105 eV binding energy, or both. The component at 99.7
eV is due to Si bonded Si, whereas the intensity at higher
binding energy is due to Si bonded to F and O. We see that
as the proportion of O2 in the gas mixture is increased, the
intensity of the peak near 105 eV binding energy increases,
because a thicker SiFx Oy reaction layer is formed in this
case. A comparison of the curves representing gas flows with
and without N2 in Fig. 16~a! shows that injection of N2 into
the discharge promotes a greater steady state thickness of the
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FIG. 11. High oxygen real-time ellipsometry. At time ~a! the discharge is ignited and a thick reaction layer grows. At ~b!, 20 sccm of N2 is injected into the
plasma and the thickness of this layer is increased. The nitrogen is switched on and off until at time ~f! the discharge is extinguished and the thickness of the
reaction layer is observed to decrease.
SiFx Oy reaction layer. This is consistent with the real-time
ellipsometry data of Fig. 11. Figure 16~b! qualitatively
shows the fluorination of the reaction layers by means of
F(1s) spectra obtained from XPS. For gas compositions void
of oxygen, the F(1s) signal is enhanced with inclusion of
N2 in the feed gas.
The oxygen content in the surface reaction layer is qualitatively shown in Fig. 16~c!. Inclusion of nitrogen in the feed
gas only serves to increase the oxidation of the reaction
layer. The most dramatic enhancement of the O(1s) signal is
FIG. 12. Ellipsometrically determined reaction layer thicknesses vs quartz
lined transport tube length for a high proportion of O2 in the discharge.
Panel ~a! represents steady state etching conditions while ~b! is the postplasma thickness.
JVST A - Vacuum, Surfaces, and Films
seen at an O2 /CF4 ratio of 0.15. For both discharges devoid
of oxygen and with a high proportion of O2, the O(1s) signal is minimally affected by inclusion of nitrogen, although
this effect is enhanced with increasing quartz tube length.
Analysis of the XPS spectra gives a reliable idea of the
reaction layer thicknesses resulting from low oxygen processing, where surface roughness makes the use of real-time
ellipsometry obsolete for this purpose. Using the assumption
FIG. 13. Changes in the reaction layer thickness due to the postplasma effect
represented by the time evolution of the ellipsometric variable Delta. Curves
~a! and ~b! represent an O2 /CF4 ratio of 0.15 without and with 20 sccm of
N2, respectively. Curves ~c! and ~d! represent an O2 /CF4 ratio of 0.75 without and with 20 sccm of N2, respectively.
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FIG. 14. Comparisons of Si (2 p) spectra for c-Si vs poly Si for O2 /CF4
ratios of ~a! 0.00, ~b! 0.15, and ~c! 0.75. Curves are shown without admixed
nitrogen.
that the photoelectron escape probability drops off proportional to 1/e for each inelastic mean free path of travel
through the reaction layer, the thickness, d r is obtained by
d r 5l ln@~ I re /I el! k 21 11 # ,
where l is the inelastic mean free path of Si(2p) photoelectrons in the reaction layer, k is a constant which depends on
the ratio of the Si number density in the reaction layer and
the substrate, and on the ratio of the attenuation lengths of
the Si(2p) photoelectrons in the reaction layer and the substrate, and I re and I el are the intensities of the reacted and
elemental Si(2p) components, respectively.23,26 A k value of
0.6 was assumed which gave good agreement to the ellipsometrically determined postplasma reaction layer thicknesses.
The deviation from the SiO2-like k value of 0.3 considered
elsewhere23 is attributed to the stoichiometrically incomplete
FIG. 15. Comparisons of Si (2 p) spectra for c-Si and poly Si illustrating the
effect N2 addition has on eliminating surface differences. Panel ~a! represents a nitrogen free plasma, while in ~b! the N2 /CF4 ratio is 0.05.
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FIG. 16. XPS spectra obtained with an electron emission angle of 15° qualitatively illustrating the effects of gas composition on the silicon surface.
Panel ~a! are the Si (2p) spectra, ~b! F (1s), and ~c! are O (1s).
SiFx Oy reaction layers formed under our conditions. Figure
17 shows the thickness of the reaction layer as a function of
O2 /CF4 in the discharge both with and without added nitrogen. The thicknesses resulting from high oxygen processing
have been included from the previously shown ellipsometry
data ~see Fig. 12! for completeness. An increase in the reaction layer thickness is seen upon injection of nitrogen into
the discharge.
The stoichiometry of a reaction layer could certainly account for the opposing trends in the etch rates and the thickness of the SiFx Oy reaction layer. A more highly fluorinated
layer would be both more responsive to each F atom arriving
at the surface ~i.e., a higher probability that an impinging F
atom could create volatile SiF4!, and subsequently more effective at removing Si atoms at the Si-reaction layer interface. For these gas flows, it is justified to consider both the
postplasma reaction layer thickness and the steady state reaction layer thickness during etching as close to zero and it
should follow that the stoichiometry of this layer should not
be considered significant to the etching dynamics. Even so,
photoemission data like those seen in Fig. 16 were curve
fitted to extract quantitative numbers. The constraints on this
curve fitting procedure have been described elsewhere.23
Subsequently, quantitative values for the stoichiometry of
the layer could be determined. Figure 18 presents these results for O/Sireacted and F/Sireacted versus increasing O2 content in the feed gas. Nitrogen addition to a pure CF4 plasma
only slightly enhances the oxidation of the near surface reaction layer ~XPS emission angle of 75° to the surface normal!, but, as oxygen is included in the discharge, this effect
is enhanced. It is clear that the stoichiometry of the reaction
layer cannot explain the etch rates simply in terms of the
higher fluorination. The observed increase in oxidation
should slow the reaction of available fluorine with the surface of the reaction layer ~the vacuum-reaction layer interface! and the subsequent lack of fluorine and associated radi-
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FIG. 17. The thickness of the surface reaction layer vs the O2 /CF4 ratio as
determined by XPS. The ellipsometrically determined thicknesses for
O2 /CF450.75 are included for comparison.
cals reaching the etching interface should result in a
decreased etch rate. The observation of an actual etch rate
enhancement is evidence for a reactive oxygen containing
species.
4. Transport tube length effects
Figure 19 shows the thickness of the reaction layer versus
quartz tube length for ~a! a pure CF4 discharge and CF4 with
N2 inclusion and ~b! discharges both with and without nitrogen and a O2 /CF4 ratio of 0.15. For discharges devoid of
oxygen, the behaviors of the reaction layers are similarly,
both with and without admixed nitrogen, constant with increasing quartz tube length. When both N2 and a low proportion of O2 ~O2 /CF450.15) are admixed into the CF4 discharge, the resulting reaction layer thickness is consistently
higher than that which results from a CF4 /O2 discharge devoid of nitrogen. Again, the thickness stays constant with
increasing quartz tube length.
Comparisons with etch rates reveal no correlation between retarded rates and constant reaction layer thicknesses,
FIG. 19. The thickness of the surface reaction layer vs quartz lined transport
tube length. Panel ~a! is for a discharge without oxygen, while in ~b! the
O2 /CF4 ratio is 0.15.
therefore these trends cannot account for the etching mechanism.
Therefore, the increase in the surface reaction probability
~upon injection of N2 into the discharge! must be promoted
by another mechanism. Figure 20~a! shows the stoichiometry
of the reaction layer in the form of O(1s)/F(1s) XPS peak
areas versus quartz lined transport tube length for discharges
without admixed oxygen. The reaction layer maintains a constant O/F ratio and no correlation with the etch rate data can
be seen. Figure 20~b! also shows that stoichiometry of the
reaction layer remains roughly the same at 0 cm as it is for
the 125 cm tube length. Therefore, the dominant etching
mechanism must again be due to some unconsidered enhancement to the surface reaction probability. In the high
oxygen regime (O2 /CF450.75), the surface reaction layer
thickness becomes the dominant mechanism by which the
etch rate is limited. Comparing the etch rates with the thickness of the surface reaction layer as determined by real-time
ellipsometry for the gas feed containing N2, one sees good
agreement. Again, the fluorine concentration is approximately constant with length. For the discharge devoid of nitrogen, the surface levels of fluorine and oxygen stay constant, as can be seen from the O(1s)/F(1s) emission signals
in Fig. 20~c!, although, as will be discussed next, these high
oxygen setting surface reaction layers are not stable after the
plasma has been extinguished and therefore the postplasma
surface analysis may not be truly indicative of what is taking
place during etching.
5. Lining material effects
FIG. 18. The stoichiometry of the reaction layer vs O2 /CF4 flows in terms of
the ~a! O (1s) and ~b! F (1s) peak areas over the reacted Si peak area.
JVST A - Vacuum, Surfaces, and Films
Figures 21~a!–21~c! shows Si(2p), F(1s), and O(1s)
XPS spectra for gas flows containing nitrogen. The curves
represent surfaces processed using either a quartz or a Teflon
liner. The Si(2p) peaks indicate slightly more reactivity
when the quartz liner is used. For gas feeds containing nitrogen but no oxygen, use of a Teflon liner reduces the ob-
1810
Matsuo et al.: Role of N2 addition on CF4 /O2 CDE
1810
FIG. 20. The stoichiometry of the reaction layer vs quartz lined transport
tube length in terms the O (1s) to F (1s) ratio. Panel ~a! represents an
O2 /CF4 ratio of 0.00, ~b! 0.15, and ~c! 0.75.
FIG. 21. XPS spectra reveal surface modifications resulting from the transport tube lining material. Panel ~a! shows the Si (2p) spectra, ~b! the F
(1s) spectra, and ~c! the O (1s) spectra. All data were taken at an electron
emission angle of 15° and were processed with a N2 /CF4 ratio of 0.05.
served fluorination of the reaction layer. As the level of O2 in
the discharge is increased to a moderate flow, the fluorination of the surface layer approaches the same value for both
the quartz and Teflon liners. For a high proportion of oxygen
in the feed gas, use of the Teflon liner provides for a significantly more fluorinated reaction layer than for that seen with
quartz. Except for a nitrogen containing gas flow with a
O2 /CF4 ratio of 0.15, a higher oxidation of the surface reaction layer is seen when a quartz liner is used, although these
modifications are much more subtle than those observed for
the F(1s) spectra.
mechanism is the survival of reactive species during travel to
the etching interface. This is accomplished by the decreased
reactivity of the F species with the transport tube and chamber lining due to the passivation of these surfaces. The third
mechanism is an enhancement in the surface reaction probability, e.g., due to the dynamics of the surface reaction/
passivation layer or the availability of an energy that enhances the reaction rate.
6. Transport tube geometry effects
A. Gas composition effect and enhanced silicon
etching
A bend in the 125 cm quartz lined transport tube also
significantly affected the etch rates. Figures 22~a!–22~c!
compare XPS spectra for a straight geometry and one bend
positioned adjacent to the applicator. All curves represent
gas flows containing nitrogen. Unless a high fraction of oxygen is included in the gas feed, the bend clearly results in a
more reactive surface. With no oxygen in the discharge, both
the F(1s) and O(1s) signal are increased, although the increase in the oxygen spectrum is much more pronounced.
When the O2 /CF4 ratio is 0.15, the bend results in a higher
oxidation of the reaction layer. At this time, more experiments are necessary to elucidate the mechanism by which the
bend affects the etching.
IV. DISCUSSION OF RESULTS
For the etching of Si under the gas chemistries used in
this work, it has been established that the primary etch product is SiF4 . 2,10,4–16 Therefore, the etch rates are determined
by the rate at which this SiF4 is produced and can leave the
surface. Under this consideration, three mechanisms for the
etch rate enhancement were delineated and compared with
the experimental results. The first mechanism is enhanced
atomic fluorine production in the discharge. The second
J. Vac. Sci. Technol. A, Vol. 15, No. 4, Jul/Aug 1997
Figure 9 shows the NO concentration in the processing
chamber as a function of the O2 /CF4 ratio in the discharge.
There is evidence for a NO assisted etching mechanism in
the case of Si3N4 etching using CF4 /O2 /N2. A comparison of
NO mass spectrometry results with the observed etch rates
for Si3N4 exhibits a powerful correlation.17,18
In Fig. 23 we correlate the etching behavior of poly Si
with the NO chamber concentration. Upon increasing the
O2 /CF4 ratio above 0.15 ~for discharges containing nitrogen!, the increased thickness in the surface reaction layer and
the resulting increase in the time constant for diffusion
through this barrier27 retard the etch rate. This NO trend
exhibits a higher correlation with etch rates than the concentration of atomic fluorine. This thickening of the reaction
layer is not seen in the case of Si3N4 etching17,18 and this
explains the near perfect agreement with the NO concentration in that case.
Based upon the sum of evidence seen for a mechanism by
which the surface reaction probability is enhanced, we can
propose an etching mechanism which is supported by both
gas phase and surface analysis data. For discharges without
nitrogen, the etching is limited by the effectiveness of the
attack of the atomic fluorine and the rate at which the volatile
1811
Matsuo et al.: Role of N2 addition on CF4 /O2 CDE
1811
FIG. 23. Poly-Si etch rate vs the concentration of NO in the processing
chamber. A dependence similar to Fig. 24 is apparent.
FIG. 22. XPS spectra reveal surface modifications resulting from the transport tube lining geometry. Panel ~a! shows the Si (2p) spectra, ~b! the F
(1s) spectra, and ~c! the O (1s) spectra. All data were taken at an electron
emission angle of 15° and were processed with a N2 /CF4 ratio of 0.05.
SiF4 can be formed and removed from the surface. When the
discharge gas consists of pure CF4, the concentration of
atomic fluorine is itself the limiting agent. When small
amounts of O2 are added, the oxidation of the CF4 daughter
species allows for higher F concentrations and the etch rate is
enhanced. At this point there is sufficient fluorine available
to explain the highest observed etch rate. Addition of nitrogen into the discharge again increases this etch rate by another factor of 2, even though the arrival rate of atomic fluorine does not increase. The addition of N2 results in the
strong production of NO and atomic oxygen. The mechanism
by which the NO enhances the poly-Si etch rate could be as
follows: once oxygen is included in the discharge ~and the
fluorine concentration becomes more than sufficient!, a reaction layer composed of Si, O, and F is formed. NO arrives at
the surface and attacks the Si–O bonds. The formation of
NO2 is favorable and this removal of oxygen creates an open
bond for whatever may come its way. In the fluorine rich
regime, the probability of this bond being attacked by F is
greater than reoxidation and the etch rate will in response
increase. This is consistent with the observed decrease in
surface roughness when N2 is added to the fluorine rich discharge, since the surface roughening is typically due to microscopic etch masks, i.e., SiO2 on the Si.
In the oxygen rich processing regime, there is a significant
increase in the dissociation of molecular oxygen upon N2
injection. This explains the increased oxidation seen in the
reaction layer as well as the sharp decrease in etch rate as
compared to those seen for CF4 /O2 processing. More work is
needed to isolate the role of N2 and to determine if it is one
of the metastable states of NO which play an important role
in the etching behavior of poly Si.
At the 0 cm tube setting and for discharges absent of
JVST A - Vacuum, Surfaces, and Films
oxygen, injection of nitrogen into the plasma increases the
atomic fluorine concentration in the discharge only slightly.
However, this effect is not seen downstream in the chamber.
Recall, in fact, that the 19 amu F signal is reduced upon
injection of nitrogen ~see mass spectrometry results!. So
even at the 0 cm tube setting, this increased level of free
fluorine in the discharge has been recombined before it enters the reaction chamber and the mass spectrometer. This
leaves us with only the third mechanism, the increased surface reaction probability, as an explanation of the observed
etch rate enhancement.
Surface roughness makes it difficult to obtain accurate
reaction layer thicknesses by means of ellipsometry when no
oxygen, or only a low proportion of oxygen, is fed into discharges devoid of nitrogen. However, these surfaces become
smoothed when nitrogen is included into the discharge, and,
hence, real-time ellipsometry measurements become feasible
and suggest that it is an extremely thin reaction layer which
is formed. Analysis of XPS determined thicknesses ~to be
discussed later! suggests that this thin postplasma reaction
layer increases slightly in thickness with N2 inclusion at this
parameter setting. This thickening of the surface layer and
the enhanced etch rate verifies the onset of a more reactive
surface upon N2 injection into the discharge.
In Fig. 24 we plot the poly-Si etch rate versus the concentration of atomic fluorine in the chamber for discharges both
with and without nitrogen. The fluorine concentration was
determined by mass spectrometry using the results found in a
reference plasma-off condition subtracted from those measured during actual processing conditions. For the curve representing the discharge devoid of nitrogen, we see the familiar drop in the etch rate as surface effects begin to dominate
the etching process. But when compared to the curve representing the CF4 /O2 /N2 plasma, it becomes immediately clear
that, even in the low fluorine regime, there is more than
enough F available to account for almost a tripling of the
etch rate. Then both etch rates drop off in response to the
1812
Matsuo et al.: Role of N2 addition on CF4 /O2 CDE
1812
FIG. 24. Poly-Si etch rate vs the concentration of atomic fluorine in the
processing chamber. Both the abundance of available F and the effects of
the thickening surface reaction layer are illustrated in this figure.
surface reaction layer until, finally, at a high oxygen proportion, the behavior of the two curves becomes more similar.
B. Tube length
For a discharge of pure CF4, as the distance from the
plasma to the etching region is increased, the fall in the arrival of atomic fluorine drops off faster than the poly-Si etch
rate ~see Fig. 25!. This suggests that the etching region is
saturated with atomic fluorine for much of our parameter
space. With N2 injected into the plasma, this trend is reversed and the etch rate quickly drops off while the amount
of atomic fluorine in the etching region exhibits a slow, near
linear drop-off. Therefore it is not the loss of atomic fluorine
during transport from the plasma to the processing chamber
which explains the diminishing etch rates as the transport
tube is lengthened. The thickness of the postplasma reaction
layer also remains quite constant with tube length for this gas
composition ~see Fig. 20! and is therefore eliminated as a
contributor.
The decrease in etch rates seen for the discharges containing nitrogen and a low proportion of O2 cannot result from
changes in the free fluorine density or the thickness of the
surface reaction layer ~see Figs. 4, 7, and 20!.
C. Real-time versus post plasma surface analysis
We attribute the increase in thickness of the surface reaction layer upon the extinguishing of the discharge containing
a low proportion of O2 ~as seen in Fig. 13! to the passivation
of the sample surface by long lived reactive species produced
in the discharge and species desorbed from the reactor walls.
During processing, low oxygen flows admixed to small
amounts of N2 result in a high atomic fluorine concentration.
It is this fluorine attack, made effective by the arrival of NO
related species, which leaves a thin reaction layer through
which all the volatile SiF4 can quickly diffuse through and
J. Vac. Sci. Technol. A, Vol. 15, No. 4, Jul/Aug 1997
FIG. 25. The behavior of the etch rates vs the availability of atomic fluorine
in the processing region vs quartz lined transport tube length. Panel ~a!
represents pure CF4 processing while panel ~b! is for a N2 /CF4 ratio of 0.05.
then escape into the gas phase. It is this surface, deficient in
SiFx compounds, which is susceptible to the postplasma increase in reaction layer thickness.
The removal of a portion of the reaction layer as seen in
Fig. 13 after the chamber is evacuated using the turbomolecular pump must be due to postplasma desorption of volatile species existing within the reaction layer. Comparisons
with stoichiometry determined from XPS show our reaction
layer to be basically a SiFxOy film. Postplasma surface modification must be due to the desorption of fluorine based compounds. Under this consideration, XPS determined stoichiometry needs to be interpreted in such a way as to account
for this escape of fluorine that made up the steady state etching reaction layer. It is of interest to note that the most profound postplasma modification is found for discharges containing a high proportion of oxygen. Oehrlein et al.
investigated this effect under reactive ion etching ~RIE! processing conditions and found that the postplasma desorption
of volatile species from the reaction layer occurred for fluorine rich discharges, and that, for oxygen rich discharges, the
reaction layer was stable upon extinguishing the
discharge.23,24 It is important to note the absence of ion bombardment in our case, and it is possible that the instability of
the reaction layers seen under these processing conditions
results from its thickness. For fluorine rich discharges, ultrathin layers allow passivation of the sample surface and the
dynamics of the reaction layer quickly die once the discharge
is extinguished and the chamber is pumped down. On the
other hand, for oxygen rich discharges, the lag in desorption
due to diffusion constants for the etching reactants and their
products extends this active period of surface modifications
well into the postplasma stage.
1813
Matsuo et al.: Role of N2 addition on CF4 /O2 CDE
V. CONCLUSIONS
Although, in the most general sense, the etch rate limiting
parameter for the chemistries being used in this work is the
availability of atomic fluorine to form the primary etch product of SiF4, we observed that this limit is never realized
under our processing conditions, except for perhaps a pure
CF4 discharge. Instead, there is an enhancement of the net
reaction probability of fluorine with silicon.
Our data are constant with, although not sufficient to establish in detail, the notion of a complex enhancement in the
efficiency of interaction between the atomic fluorine and silicon when, in the presence of oxygen, addition of N2 to the
CF4 /O2 discharge increases the abundance of NO and O atoms. Addition of 5% N2 to a CF4 /O2 discharge doubles the
etch rate, even though the F concentration goes down. Recent measurements using direct downstream injection of nitric oxide support the proposed role of NO and will be discussed in a future work.28
Strong surface chemical changes are observed upon N2
addition, although little nitrogen is incorporated in the reaction layer. The nitrogen is active only as a reactive intermediate. Depending on the O2 /CF4 ratio, i.e., the predominance
of F or O, either thinning or thickness growth of the modified
surface layer can be seen. These changes in surface reactivity
enable control of the surface texture, e.g., the surface roughening of Si, which is seen for low O2 concentrations in
CF4 is absent if CF4 /O2 /N2 gas mixtures are used, especially
O2 rich gas compositions.
For pure CF4 discharges, a simple linear decrease in etch
rate with tube length is seen, whereas for CF4 /N2 and
CF4 /O2 /N2, the decrease is much quicker. A simple wall
passivation model cannot explain this data, since the quantity
that is etch rate limiting depends on the CF4 /O2 /N2 gas composition. The influence of lining material, tube length, and
geometry on the etch rate varies as the gas composition is
altered.
ACKNOWLEDGMENTS
This work was sponsored in part by SEMATECH, Leybold Inficon, and the New York State Center for Advanced
Technology in Thin Films and Coatings. The authors would
like to thank M. Schaepkens and H. J. Sun for their constant
helpfulness. M. Dömling and M. Keller are thanked for their
JVST A - Vacuum, Surfaces, and Films
1813
assistance in analyzing data. N. R. Ruėger is appreciated for
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1