Highly selective etching of silicon nitride over silicon ...

Highly selective etching of silicon nitride over silicon and silicon dioxide

B. E. E. Kastenmeier, P. J. Matsuo, and G. S. Oehrleina)
Department of Physics, The University of Albany, State University of New York, Albany, New York 12222

͑Received 29 January 1999; accepted 27 August 1999͒

A highly selective dry etching process for the removal of silicon nitride ͑Si3N4) layers from silicon
and silicon dioxide (SiO2) is described and its mechanism examined. This new process employs a
remote O2 /N2 discharge with much smaller flows of CF4 or NF3 as a fluorine source as compared
to conventional Si3N4 removal processes. Etch rates of Si3N4 of more than 30 nm/min were achieved
for CF4 as a source of fluorine, while simultaneously the etch rate ratio of Si3N4 to polycrystalline
silicon was as high as 40, and SiO2 was not etched at all. For NF3 as a fluorine source, Si3N4 etch
rates of 50 nm/min were achieved, while the etch rate ratios to polycrystalline silicon and SiO2 were
approximately 100 and 70, respectively. In situ ellipsometry shows the formation of an
approximately 10-nm-thick reactive layer on top of the polycrystalline silicon. This oxidized
reactive layer suppresses etching reactions of the reactive gas phase species with the silicon.
© 1999 American Vacuum Society. ͓S0734-2101͑99͒05706-1͔

I. INTRODUCTION Si3N4 typically was about 30 nm/min, that of SiO2 3 nm/min.
Typical etch rate values for SiO2 and Si3N4 in the afterglow
The stripping of silicon nitride mask material after the of NF3 /O2 discharges were 60 and 80 nm/min, respectively,
local oxidation of silicon ͑LOCOS͒ is a possible source of too high to achieve selective etching of Si3N4 relative to
device damage during integrated circuit ͑IC͒ fabrication. The SiO2.
pad oxide can suffer degradation during the overetch. Fur-
thermore, etchants can reach the underlying Si substrate Etching of Si4,7 always occurred at rates much faster than
through imperfections in the pad oxide, and etch the sub-
strate at a significant rate. This effect leaves behind craters in those of Si3N4. Etch rates as high as 300 nm/min for
the substrate, and is referred to as ‘‘pitting.’’ Current dry CF4 /O2 /N2 and 700 nm/min for NF3 /O2 gas mixtures were
processes used for the Si3N4 stripping step favor this undes- measured. These high etch rates can be explained with the
ired effect, since they typically etch Si much faster than
Si3N4.1–4 Therefore, a process which etches Si3N4 selectively high F density and the spontaneous reaction of the Si surface
over both SiO2 and Si is desirable.
with F atoms. The etch rate of Si varies linearly with the
In previous work by this group2,5 and by Blain et al.,3 the fluorine density.6 A decrease of the Si etch rate is consis-
etching of Si3N4 in the afterglow of CF4 /O2 /N2 and NF3 /O2
discharges has been investigated. The flow of the fluorine tently observed for additions of 20% or more of O2 to the
source, CF4 or NF3, was kept at constant values for most CF4 or NF3 plasma. This effect is due to the oxidation of the
experiments, and O2 and N2 were added in varying amounts. Si surface in the presence of O and/or O2. The oxidation
The etch rate of Si3N4 was found to be correlated to the makes the Si surface very similar to that of SiO2 during
density of nitric oxide ͑NO͒ in a linear relationship for both etching, and in the limit of very high flows of O2, the etch
gas mixtures. No correlation was observed between the rates of both materials become equal.
Si3N4 etch rates and the density of atomic fluorine. The F
atom concentration, determined from the polycrystalline sili- Based on the above observations, a new process for the
con etch rate2,6 and mass spectrometry, was found to be
higher by at least a factor of 20 than necessary to sustain the etching of Si3N4 was developed, that provides high selectivi-
measured Si3N4 etch rates. We concluded that the arrival of ties to both SiO2 and Si. A mixture of O2 and N2 was used as
NO is the etch rate limiting step in Si3N4 etching, and F the primary discharge gas, to which small amounts of a fluo-
atoms are available in abundance for Si3N4 etching. rine source ͑CF4 or NF3) were admitted. Three mechanisms
contributing to a high Si3N4 /Si selectivity are exploited by
The etch rates of SiO2 ͑Refs. 2, 5͒ for the same param- choosing an O2 /N2 based chemistry, rather than a fluorine
eters were found to be independent of the NO concentration, based one.
but followed the F radical density very well. The CF4 based
chemistry produces the desired ratio between the etch rates ͑1͒ Decreased F atom density. The density of atomic fluo-
of SiO2 and Si3N4. With O2 and N2 added to CF4, ratios of 10
or slightly higher could be obtained easily. The etch rate of rine in the reaction chamber is very low as compared to

a͒Electronic mail: [email protected] CF4 /O2 /N2 and NF3 /O2 gas mixtures. The Si etch rate de-
creases in the same way as the F density, since they are

linearly correlated. The etch rate of Si3N4, on the other hand,
should not be affected as strongly, because F is available in

abundance in the conventional gas mixtures and the etching

of Si3N4 in those cases is not limited by the arrival rate of F.

3179 J. Vac. Sci. Technol. A 17„6…, Nov/Dec 1999 0734-2101/99/17„6…/3179/6/$15.00 ©1999 American Vacuum Society 3179

3180 Kastenmeier, Matsuo, and Oehrlein: Highly selective etching of silicon nitride 3180

FIG. 1. Schematic of the chemical downstream etcher used in this investi- port tube to the cylindrical reaction chamber. The length of
gation. The gases are fed into the sapphire applicator, where a microwave the tube was fixed at 75 cm, and the inside was lined with a
discharge is ignited. The species effluent from the plasma travel through Teflon liner. Samples of size 1 in. by 1 in. are glued on a 5
tubing of variable length and lining material to the reaction chamber. The in. carrier wafer, which is placed on an electrostatic chuck in
sample is placed on the center of an electrostatic chuck. A quadrupole mass the reaction chamber. The materials used for this investiga-
spectrometer is mounted on the chamber on top of the sample, and mono- tion are low-pressure chemical vapor deposition ͑LPCVD͒
chromatic ellipsometry is used to determine etch rates. Si3N4 and thermally grown SiO2. Surface modifications of
Si were studied using crystalline Si and polycrystalline Si,
͑2͒ Oxidation of the Si surface. The second mechanism etch rates of silicon were determined using polycrystalline
that suppresses the Si etch rate is the oxidation of the Si silicon. The temperature of the sample is monitored with a
surface in the presence of O and O2 in the gas phase. The fluoroptic probe which contacts the backside of the sample.
oxidized Si surface is very inert to the attack of F atoms, It was kept constant at 10 °C for all experiments. Helium at a
therefore, the Si etch rate is significantly decreased. In addi- pressure of 5 Torr was fed between the surface of the elec-
tion to oxygen species, NO, which is produced in the O2 /N2 trostatic chuck and the carrier wafer in order to obtain a good
discharge, was found to contribute to the oxidation of Si heat conduction. Etch rates are measured in situ by mono-
surfaces.4 The surface of Si3N4 is only slightly oxidized dur- chromatic ellipsometry ͑wavelength 632.8 nm͒. Some ana-
ing etching in the presence of O and O2, not enough to in- lytical experiments were performed in order to gain a quali-
fluence the etch rates.5,8 tative understanding of the mechanisms responsible for
achieving the high selectivity. A fiber optic cable for optical
͑3͒ Si3N4 etch rate enhancement by NO. The etch rate of emission experiments of the discharge was mounted on the
Si3N4 is proportional to the density of NO. The O2 /N2 chem- housing of the applicator. The spectrograph used in this in-
istry produces a significant amount of NO in its afterglow, vestigation is a 30 cm optical multichannel analyzer. A quad-
which allows for high Si3N4 etch rates. The Si etch rate is rupole mass spectrometer is mounted on top of the reaction
less influenced by NO addition, and in the surface oxidation chamber such that the distances from the discharge to the
limited regime no enhancement of the etch rate is observed.4 sample and to the orifice are the same. Some samples were
moved to the surface analysis chamber through the ultrahigh
This process also provides selectivity of Si3N4 over SiO2. vacuum ͑UHV͒ wafer handling system without exposure to
The etch rate of SiO2 in the absence of ion bombardment air. Results of similar analytical experiments on a CDE sys-
depends only on the F density, and is expected to be smaller tem are reported in greater detail in previous
than the Si etch rate. publications.2–5,7

In Sec. II the setup of the remote plasma etching tool and The experiments were conducted with a sapphire applica-
the experimental procedure are described. Subsequently, etch tor at 1000 W of microwave power and a chamber pressure
rates of Si3N4, poly-Si, and SiO2 are reported for both CF4 of 600 mTorr. Flows of O2 and N2 were kept constant at 800
and NF3 as sources of F. Gas phase experiments ͑Ar acti- and 110 sccm, respectively, for most experiments. These pa-
nometry and mass spectrometry͒ are conducted to gain infor- rameters are referred to as the standard conditions.
mation about the concentration of reactive species. The sur-
face of poly-Si suring etching is examined by ellipsometry, III. RESULTS AND DISCUSSION
and the close correlation between the decrease of the etch
rate and the formation of a reactive layer on the Si is dem- The Si3N4 etch rates were measured as a function of CF4
onstrated. addition to a O2 /N2 plasma ͑see top panel of Fig. 2͒. The
etch rate increases linearly with the flow of CF4. The param-
II. EXPERIMENT eter for the two curves in the top panel of Fig. 2 is the
amount of oxygen fed into the discharge. A lower flow of O2
The remote plasma etching tool has been described before ͑300 sccm͒ yields an etch rate about 15 nm/min higher than
in detail.2,4,5 Figure 1 shows a schematic of the apparatus the flow of O2 at standard conditions ͑800 sccm͒. The highest
used for the experiments. Mixtures of O2, N2, and the fluo- etch rates, obtained with 46 sccm of CF4, are 24 nm/min for
rine source ͑ CF4 or NF3) are excited using an ASTEX 2.45 800 sccm of O2, and 39 nm/min for 300 sccm of O2.
GHz microwave applicator with a sapphire coupling tube.
The species produced in the plasma travel through a trans- Nitrogen trifluoride was used as an alternative fluorine
source to CF4. The plasma chemistry of NF3 is significantly
different than that of CF4.5,9,10 The threshold for electron
impact dissociation of NF3 is 0 eV,11,12 that of CF4 is 12.6
eV.13 This results in a higher degree of dissociation of NF3 in
a discharge. In fact, in the high density microwave dis-
charges employed for the work reported here, 100% disso-
ciation is typically achieved.5,14 The dissociation of CF4 for

J. Vac. Sci. Technol. A, Vol. 17, No. 6, Nov/Dec 1999

3181 Kastenmeier, Matsuo, and Oehrlein: Highly selective etching of silicon nitride 3181

FIG. 3. Etch rates of poly-Si for CF4 and NF3 as source of fluorine at
standard conditions. The samples were HF dipped before the experiments to
remove the native oxide layer. The initial high etch rate is suppressed as an
oxygen rich reaction layer forms during etching.

FIG. 2. Etch rates of Si3N4 as a function of the flow of CF4 ͑top panel͒ and mates for the decay time. All etch rates reported in the fol-
NF3 ͑bottom panel͒. lowing are ‘‘final’’ rates, measured after 300 s of etching.

similar discharge parameters varies between 40% and 60%. Figure 4 shows the etch rates of silicon as a function of
Therefore, the production of free fluorine radicals from NF3 CF4 ͑top panel͒ and NF3 addition ͑bottom panel͒. Again, the
is higher than from CF4. Moreover, the concentration of NO parameter of the two curves in the top panel is the amount of
in the afterglow of the discharge might be affected by the O2 fed into the plasma. Strong variations of the etch rate with
additional N atom feed. respect to the O2 flow are observed. The etch rate is signifi-
cantly suppressed if 800 sccm of O2 are used. No etch rate
The etch rates of Si3N4 as a function of the flow of NF3 suppression is found at a low flow ͑300 sccm͒ of O2. At 30
are shown in the bottom panel of Fig. 2. The measurements sccm of CF4, for example, the etch rate for 300 sccm of O2 is
were performed under standard conditions, i.e., the flow of 19 times larger than that for 800 sccm. The Si etch rate is
O2 was kept constant at 800 sccm. The etch rates of Si3N4 proportional to the flow of NF3 at small flows up to 20 sccm.
are proportional to the flow of NF3, and significantly higher At higher NF3 flows, the etch rate assumes a plateau value at
if NF3 is used instead of CF4. An addition of 46 sccm of CF4, 0.5 nm/min.
e.g., yields an etch rate of 24 nm/min, whereas the etch rate
for the same flow of NF3 is twice as high ͑49 nm/min͒. The etch rates of silicon dioxide are plotted in Fig. 5. The
top panel shows etch rates when CF4 is used as a fluorine
The etch rates of polycrystalline silicon were found to source. Etching occurs only at the low flow of O2 ͑300
depend strongly on the initial surface conditions of the
sample, and on the etch time. These etch rate variations all FIG. 4. Etch rate of poly-Si vs the flow of CF4 ͑top panel͒ and NF3 ͑bottom
can be explained with the presence of a native oxide layer on panel͒. The ellipsometric determination of an etch rate for 300 sccm of O2
the Si surface, and the formation of a reactive layer during and flows of CF4 higher than 30 sccm was not possible, probably due to the
etching. A layer of native oxide on a polycrystalline silicon formation of roughness on top of the poly-Si.
film suppresses the etching almost completely. As an ex-
ample, the etch rate of poly-Si at standard conditions and 30
sccm of CF4 is as low as 0.07 nm/min with the native oxide
layer present. If the native oxide is removed immediately
before processing by dipping the sample in HF for 3 s, the
etch rate is 0.49 nm/min, a sevenfold increase. In Fig. 3 the
etch rates of poly-Si are plotted as a function of time. The
native oxide layer was removed before the experiments by
HF dipping. Both samples were treated at standard condi-
tions, with CF4 and NF3 as a source of fluorine. The etch rate
of a ‘‘clean’’ Si surface ͑no native oxide and no reactive
layer͒ can be as high as 20 nm/min. As etching proceeds, the
reactive layer forms on the Si and impedes etching reactions.
The final etch rates are 0.50 nm/min for both CF4 and NF3.
The etch rate decreases faster if CF4 is used as fluorine
source. Extrapolation of the curves to the x axis, using the
initial slopes, yields 12 s for CF4, and 40 s for NF3 as esti-

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3182 Kastenmeier, Matsuo, and Oehrlein: Highly selective etching of silicon nitride 3182

FIG. 5. Etch rates of SiO2. At the high O2 flow with CF4 added, the etch FIG. 7. Ratio of the Si3N4 and SiO2 etch rates, determined from the data
rates were too small to be detected by ellipsometry ͑Ͻ0.05 nm/min͒. shown in Fig. 2 and Fig. 5. The curve for the high O2 flow in the top panel
is missing, since the selectivity at these conditions is extremely high.

sccm͒, and etch rates never exceed 1 nm/min. The etch rates with increasing flow of NF3. A selectivity value of 100 is
reached at standard conditions.
at 800 sccm of O2 were too small to be detected by ellipsom-
etry ͑Ͻ0.5 nm/min͒. In the bottom panel, the etch rates for In Fig. 7, the ratio of the Si3N4 and SiO2 etch rates is
shown. These data are obtained from Fig. 2 and Fig. 5. In the
800 sccm of O2 and NF3 as a fluorine source are shown. The case of CF4 as fluorine source ͑top panel͒, the curve for the
etch rates increase linearly for NF3 flows up to 30 sccm, and high O2 flow is missing. These values are extremely high
then level out at 0.8 nm/min. ͑Ͼ500 or infinity͒ due to the unmeasurably small SiO2 etch
rates at these conditions. The selectivities for NF3 as a fluo-
The Si3N4 /Si etch rate ratio obtained from Fig. 2 and Fig. rine source ͑bottom panel͒ are at around 60 for intermediate
4 is shown in Fig. 6. If CF4 is used as a fluorine source ͑top
panel͒, the highest selectivity is achieved at the high O2 flow flows of NF3, and have an increasing tendency at higher
and intermediate flows of CF4. At these conditions, the se- flows.
lectivity assumes a peak value of around 40. The selectivity
The selectivities reported here are improved significantly
for the low O2 flow is much less due to the higher Si etch
rates. If NF3 is used ͑bottom panel͒, the selectivity increases as compared to those obtained in earlier work. In CF4 based
etching of Si3N4, a typical selectivity to the underlying pad
oxide of 10 is achieved. For a NF3 based chemistry, which is
predominantly used for chamber cleaning after deposition

processes, this value is less than half as big. In both chem-

istries, Si is etched at a significantly faster rate than Si3N4. In
Table I, the Si3N4 etch rates and selectivities to SiO2 and Si
are summarized for these processes, together with the data

presented above.

Several experiments were performed to investigate quan-

titatively the importance of the three mechanisms mentioned

above for achieving high Si3N4 /SiO2 and Si3N4 /Si selectivi-
ties while maintaining a high Si3N4 etch rate. As described in
previous work,2,5 argon actinometry and quadrupole mass

TABLE I. Typical values of the Si3N4 etch rate and the selectivities to SiO2
and Si. See Refs. 2 and 4 for the work on CF4 /O2 /N2, and Refs. 5 and 7 for
NF3 /O2.

CF4 /O2 /N2 NF3 /O2 O2 /N2 /CF4 O2 /N2 /NF3

FIG. 6. Ratio of the Si3N4 and Si etch rates, determined from the data shown Si3N4 etch rate ͑nm/min͒ 30 80..350 10..20 30..50
in Fig. 2 and Fig. 4. Sel Si3N4 /SiO2 10 0.7–4 Ͼ500 60
Sel. Si3N4 /Si 0.1–0.6 0.1–0.8
30 80–100

J. Vac. Sci. Technol. A, Vol. 17, No. 6, Nov/Dec 1999

3183 Kastenmeier, Matsuo, and Oehrlein: Highly selective etching of silicon nitride 3183

FIG. 8. Variation of the F atom concentration in the discharge region, as FIG. 9. Signal intensity of the NOϩ peak at amu 30 in the reaction chamber.
determined by Ar actinometry. See the text for the parameters used for this
measurement. This is in contrast to CF4 and NF3 based processes, in which
small amounts of O2 and/or N2 are added to the F-containing
spectrometry were used to measure the change of the con- gas. In these cases, F atoms are available in abundance, and
centration of species relevant for etching. For the actinom- the Si3N4 etch rates were found to be limited by the NO
etry measurements,15,16 argon at a constant flow rate of 60 density.2,5
sccm was injected into the discharge. Figure 8 shows the
intensity ratio of the F emission at 703.7 nm and the Ar line As mentioned above, a thick reactive layer on top of the
at 811 nm. This ratio is proportional to the F atom density in Si surface inhibits etching reactions. The formation of this
the discharge. The top panel shows the ratio for CF4, the layer was monitored in situ during the etching by monochro-
bottom panel for NF3 as a source of fluorine. The ratio is matic ellipsometry. Figure 10 shows the time evolution of
approximately 4 times higher if NF3 is used as compared to
CF4, indicating a higher F atom concentration in the case of FIG. 10. Evolution of the ellipsometric variables ⌿ and ⌬ during the etching
NF3. This is consistent with the observation that NF3 is dis- of a film of polycrystalline silicon ͑total thickness 250 nm͒ on a SiO2 layer
sociated to a higher degree than CF4 in microwave ͑100 nm͒ on a Si͑100͒ substrate. The experiments were performed under
discharges.5 For the present experiments, mass spectrometry standard conditions, with addition of 30 sccm of CF4 ͑top panel͒ or 30 sccm
measurements of the NFϩ3 peak ͑amu 71͒ for plasma-on and of NF3 ͑bottom panel͒. The time interval between two data points is 1.1 s.
plasma-off states show the complete destruction of NF3. The Simulations of ⌿/⌬ for different reactive layer thickness ͑0, 10, and 12.5
corresponding measurement of the CFϩ3 intensity ͑amu 69͒ nm͒ on top of the poly-Si are included. An increase in ⌬ is equivalent to the
suggests a dissociation of the CF4 of about 20%–30%, if it is removal of the poly-Si and increase in ⌿ means the growth of the oxidized
assumed that the contribution of CF3 radicals produced in the reactive layer.
discharge of the signal intensity of the peak at amu 69 is
negligible.

A significant amount of NO is present in the reaction
chamber during the experiments reported here. The intensity
of the NOϩ peak at amu 30 is shown in Fig. 9. In the case of
CF4 ͑top panel͒, the signal intensity decreases by almost
50% as a small amount of CF4 ͑15 sccm͒ is added to the
O2 /N2 discharge, and then decreases further as more CF4 is
injected. The same initial decrease is observed for NF3 added
to O2 /N2 ͑bottom panel͒, but then, in contrast to the CF4
case, the NO density increases as more NF3 is injected.

The proportionality of both the actinometrically deter-
mined F density in the discharge and the Si3N4 etch rate with
the amount of F-containing gas added suggests that the Si3N4
etch rate is limited by the amount of F atoms available for
etching reactions. No correlation is observed between the
NO density in the reaction chamber and the Si3N4 etch rate.

JVST A - Vacuum, Surfaces, and Films

3184 Kastenmeier, Matsuo, and Oehrlein: Highly selective etching of silicon nitride 3184

the ellipsometric variables ⌿ and ⌬ as a function of time above, it can be concluded that the formation of an etch-
͑dotted curves, top panel for CF4, bottom panel for NF3). A inhibiting reactive layer on top of the Si is the dominant
multilayer stack consisting of poly-Si ͑total thickness 250 mechanism for achieving a high Si3N4 etch rate ratio. The
nm͒ on top of SiO2 ͑100 nm͒ on a Si͑100͒ substrate was etching of the virgin silicon surface proceeds at a rate of
etched. The lines in Fig. 10 show calculations of the ellipso- approximately 20 nm/min. In situ ellipsometry shows the
metric variables ⌿ and ⌬. These can be calculated by assum- formation of a reactive layer on top of the polycrystalline
ing values for the thickness and the optical parameters of silicon during the etch process within a matter of seconds.
each layer of the stack. Also the reactive layer can be in- The etch rate of silicon is decreased to a level comparable to
cluded in those calculations as a film of a certain thickness that of SiO2 after the reactive layer has formed.
on top of the poly-Si. In Fig. 10, the results of these calcu-
lations for three different thicknesses of the reactive layer ͑0, Finally, the authors would like to suggest that imperfect
10, and 12.5 nm͒ are included. Etching of the poly-Si corre- gate oxides can potentially be improved by the process in-
sponds to an increase of the value of ⌬. At the same time, vestigated here. The reactive layer that forms on top of the Si
information about the reactive layer thickness is obtained during etching is highly oxidized with some F content, and
from ⌿. An increase of ⌿ means an increase of the reactive thus similar to the gate oxide. It is possible that the Si un-
layer thickness. derneath voids or imperfections in the gate oxide are oxi-
dized during the overetch period after the Si3N4 removal,
The top panel of Fig. 10 shows the etching of polycrys- thus improving the electrical properties of the gate.
talline Si and formation of the reactive layer under standard
conditions with 30 sccm of CF4 added. Immediately after the ACKNOWLEDGMENTS
plasma is ignited and tuned ͑‘‘start’’͒ the reactive layer
grows at a considerable rate, and Si is removed at the same The authors would like to thank Matt Blain, Gary Powell,
time. The time interval between two measurement points is John Langan, Rob Ellefson, and Lou Fries for their support
1.1 s. The reactive layer assumes a steady state thickness of and fruitful discussions. Marc Schaepkens, Theo Standaert,
approximately 10 nm. At this thickness value the etch rate of and Christian Vo¨lker are thanked for their helpful ideas and
Si is slowed down to 0.4 nm/min. At the moment the dis- technical assistance. We would like to acknowledge Matrix
charge is terminated ͑‘‘end’’͒, the overlayer thickness has Integrated Systems, Sandia National Laboratories, Air Prod-
increased to about 12.5 nm. The etch time from start to end ucts and Chemicals, and Leybold Inficon for financial sup-
was 475 s, and 16.7 nm of Si was removed in that time. After port of this study.
termination of the discharge ͑‘‘postplasma’’͒, more surface
modifications occurred. 1N. Hayasaka, H. Okano, and Y. Horiike, Solid State Technol. 31, 127
͑1988͒.
NF3 was used instead of CF4 for the experiment shown in 2B. E. E. Kastenmeier, P. J. Matsuo, J. J. Beulens, and G. S. Oehrlein, J.
the bottom panel. As in the case of CF4, growth of the reac- Vac. Sci. Technol. A 14, 2802 ͑1996͒.
tive layer and poly-Si etching occur simultaneously. How- 3M. G. Blain, T. L. Meisenheimer, and J. E. Stevens, J. Vac. Sci. Technol.
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J. Vac. Sci. Technol. A, Vol. 17, No. 6, Nov/Dec 1999