Materials Science in Semiconductor Processing 5 (2002) 5–10
Suppression of oxidation-induced stacking fault generation
in argon ambient annealing with controlled oxygen
and the effect upon bulk defects
Toshiharu Suzuki*
New Display Device Division, Core Technology Development Center, Sony Corporation Core Technology and Network Company,
4-16-1 Okada, Atsugi-shi, Kanagawa 243-0021, Japan
Abstract
The effect of oxygen partial pressure during annealing in argon on the suppression of the generation and growth of
oxidation-induced stacking faults (OSFs) was investigated by precisely and widely controlling the oxygen partial
pressure. Similarly to the case of OSF in the surface region, the generation of OSFs in the deep region of substrate was
effectively suppressed by an annealing with the oxygen partial pressure below the critical value of 6 10 3 atm. The
formation of precipitation was also decreased throughout the substrate. Mechanism by which the generation of OSFs
was suppressed has been attributed to the oxidation mechanism in these oxygen partial pressures. It is suggested that an
electric field is built across the oxide layer when the oxygen partial pressure is below the critical value. The electric field
causes silicon interstitials and impurities, such as metals, to drift from the substrate to the outer surface of the oxide.
During the annealing the origins of OSFs, such as Si–O clusters and metallic impurities, are eliminated and any
mechanical damage is annealed out before climbing to the nuclei of the OSFs. It is suggested that Si interstitials and
metallic impurities deep in the substrate are also eliminated by the diffusion, when their concentration at the surface
region is decreased. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Oxidation-induced stacking fault; Bulk defect; Suppression of defect; Preliminary annealing in argon; Oxygen
concentration; Oxidation mechanism
1. Introduction
Stacking faults generated during the oxidation of
silicon often decorate metallic impurities at the partial
dislocation, and this causes deterioration of the electrical
characteristics of silicon devices, such as an increase in
the leakage current of metal-oxide–semiconductor
(MOS) transistors and the image characteristics of
charge-coupled devices (CCDs). In recent MOS LSIs
miniaturized beyond 0.18 mm, the thickness of the gate
oxide must be scaled down to o3 nm, the thickness at
which direct tunneling makes a non-negligible contribution to the leakage current through the gate oxide. The
leakage current through defects or impurities in the thin
*Fax: +81-46-227-2135.
E-mail address: toshiharu.suzuki@jp.sony.com (T. Suzuki).
oxide, however, can be greater than the pure tunneling
current at that regime [1]. The number of defects and
impurities that occur in the tunnel oxide of the
coming generation of MOS transistors must be minimized. Because these defects and impurities mainly
originate from the crystalline defects and impurities
of the substrate [2,3], it is essential that the formation
of crystalline defects during the oxide growth will
be suppressed in the coming generation of MOS
technology.
The mechanisms responsible for the generation and
growth of oxidation-induced stacking faults (OSFs)
have been investigated for several decades. The sites of
metallic impurities [4], oxygen precipitates or Si–O
clusters [5,6], and mechanical damage [7,8] have been
identified as the origins of OSFs. OSFs grow through the
coalescence of silicon interstitials [5,7], which are
1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 4 4 - 6
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T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10
generated at the oxide–substrate interface during oxidation [9]. Many researchers have attempted to suppress
the generation of OSFs by performing a preliminary
annealing of the substrate wafers in an inert ambient,
such as nitrogen or argon, before oxidizing them
[10–12]. This author annealed wafers in an argon
ambient that included oxygen and found that the
suppression of OSF generation in the surface region
was effective only when the oxygen partial pressure was
lower than the critical partial pressure of 6 10 3 atm,
and that the difference in ability to suppress OSFs
between below and above the critical oxygen partial
pressure can be attributed to the difference in oxidation
mechanisms [13]. The oxidation-related species, such as
silicon interstitials, are transported from the oxide–
substrate interface to the oxide surface by the electric
field when oxygen partial pressures are lower than the
critical value. This decreases the interstitial concentration at the interface and results in the suppression of the
generation and growth of OSFs.
When the surface concentration of interstitials is
decreased, the concentration of interstitials in the depth
of substrate must be lowered by a diffusion mechanism.
Thus, the generation of stacking faults in the deep region
of a substrate can be suppressed. This article reports the
results of an investigation in which the suppression
effect of argon annealing with reduced oxygen partial
pressure on the generation of OSFs in the depths of the
substrate was studied.
2. Experiment
A Czochralski-grown silicon crystal doped with boron
to a concentration of about 1015 cm 3 was used in the
experiment. The crystal was sliced into a (1 0 0) surface
orientation and polished to a mirror surface. Final
thickness of the wafers was 300 mm. The OSF density on
the surface of the wafers was initially examined by
oxidizing the wafers at 11001C and was found to be on
the orders of 102–105 cm 2.
After the wafers were cleaned with a warm mixture of
sulfuric acid and nitric acid, they were dipped into
diluted hydrofluoric acid (5% HF) to remove the
chemical oxide that had grown during the acid-mixture
treatment. Finally, the wafers were rinsed in deionized
water using ultrasonic agitation and then dried.
Argon gas was chosen as the annealing ambient in
order to avoid the chemical reaction reported to occur
when silicon wafers were annealed in nitrogen [14]. A
resistance-heating furnace was used for the annealing
and oxidation, and the heat-treatment temperature was
11001C. The oxygen partial pressure of the 1-atm
annealing ambient was controlled to a value between
10 6 and 10 1 atm. The air tightness of the gasdistribution system was tested by filling the system with
nitrogen to a pressure of 3 kg cm 2; after 48 h there was
no reduction in the pressure. Some of the annealed
wafers were oxidized in the same furnace by changing
the ambient to dry oxygen in order to grow the OSFs to
a size visible under an optical microscope.
After annealing and oxidation, the oxide was removed
using a buffered HF solution. OSFs were delineated
using a Secco etch solution [15] for 1 min. The surface
density of the OSFs on the samples was evaluated using
an optical microscope. The depth distribution of the
stacking faults generated during annealing and oxidation was obtained by repeating the etching and
measurement of OSFs. Then, the number of OSFs was
differentiated by depth to count the newly delineated
OSFs. Defects and precipitates in the whole region of
the substrate were observed by X-ray section topography using AgKa1 with asymmetric /4 4 4S reflection.
3. Results
Annealing at an oxygen partial pressure below
3 10 5 atm resulted in surface roughening in the form
of anomalous film growth and pitting [16]. The influence
of oxygen partial pressure on OSF generation in the
surface region and the deep region of the substrate was
investigated by annealing wafers for 30 min under
oxygen partial pressures from 3 10 5 to 1 10 1 atm
and then oxidizing them for 120 min. Fig. 1 shows the
oxygen partial pressure dependence of the OSF density
in the surface region observed after the subsequent
oxidation. For oxygen partial pressures higher than
6 10 3 atm, the OSF density was high: from 10 to
105 cm 2. In contrast, the OSF density was o20 cm 2
for the wafer annealed with oxygen partial pressure
below a critical value of 6 10 3 atm. This shows that
annealing in argon with an oxygen partial pressure lower
than the critical value, suppresses the generation of
OSFs in the surface region.
Surface roughening that occurred in the oxygen
partial pressure lower than 3 10 5 atm is comprised
of SO2 film deposition and the formation of square pits
with inverted pyramidal shapes and sometimes accompanying the deposition of a fiber-like material. This
surface roughening has been attributed to the evaporation of SiO before the formation of stable SiO2 in the
extremely low oxygen partial pressures. This mechanism
was verified by considering the stagnant region around
the substrate [16].
The OSF density in the deep region of the substrate
was evaluated by repeating the step etching and
measurement of the OSF number. Fig. 2 shows the
depth variation of the OSF density newly delineated by
every 1.5-mm etching. The measured results for the
substrates oxidized for 120 min with and without
annealing are also shown in the figure. For the substrate
T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10
7
Fig. 1. Oxygen partial pressure dependence of OSF density observed after annealing wafers for 30 min at 11001C under various oxygen
partial pressures and then oxidizing them for 120 min in dry oxygen at the same temperature.
Fig. 2. Depth distribution of OSF after oxidation for 120 min
with and without preliminary annealing in argon with oxygen
of 2 10 3 atm at 11001C for 30 min. OSF densities were
measured after every etching by 1.5 mm and differentiated with
the depth.
annealed with the oxygen partial pressure of
2 10 3 atm was suppressed effectively to a density
o40 cm 2 to the depth of 30 mm. This value is almost
the same level with the surface OSF density of the
annealed substrate in low oxygen partial pressure as
shown in Fig. 1.
The effect of the annealing on the defect generation in
the whole region of a substrate was analyzed by X-ray
section topography. In Fig. 3(a)–(c) images of the
section topography of an annealed substrate, an
annealed and oxidized substrate, and an only oxidized
substrate are shown, respectively. The oxidation time
after annealing was for 4 h. Topographic image of the
annealed substrate, Fig. 3(a) shows striped patterns
without any speckles. The topographic image in Fig. 3(c)
of the substrate oxidized without preliminary annealing
shows many speckles in the region deeper than the socalled denuded zone of about 10 mm. On the contrary,
the image in Fig. 3(b), which is the image of annealed
and oxidized substrate, shows very few speckles. These
speckles are considered to be oxide precipitates [17,18].
Annealing in argon with the oxygen partial pressure
lower than the critical value suppresses both the OSF
generation and formation of the precipitates in the depth
of substrate, effectively, even for an annealing time this
short.
4. Discussion
without preliminary annealing, the density of OSFs was
about 1 103 cm 2, almost the same density as that
obtained from the preliminary substrate examination.
The generation of the bulk OSFs of the substrate
Many researchers have reported that annealing in an
inert ambient can suppress the generation of OSFs in
one case [11,12], and that surface roughening and the
increase in OSF density were induced by a pre-oxidation
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T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10
Fig. 3. Section topography of silicon wafers: (a) annealed in
argon with 2 10 4 atm oxygen for 30 min at 11001C, (b)
annealed in argon with 2 10 4 atm for 30 min and oxidized for
4 h at 11001C, and (c) oxidized for 4 h at 11001C.
annealing in nitrogen in another case [10,14]. The results
of this experiment clarified that the generation of OSFs
at the surface region of the substrate is suppressed when
the substrates are annealed in argon with an oxygen
partial pressure that is lower than the critical value, as
shown in Fig. 1. Furthermore, the scarcity of OSFs after
subsequent oxidation suggests that argon annealing
under oxygen partial pressures lower than the critical
value can eliminate the nuclei and embryos of OSFs [13].
The contradictory results reported in previous reports
[10–12,14] may have been brought about by the nocontrolled oxygen partial pressure and the nitridation
reaction on the surface of the substrates.
When the oxygen partial pressure is higher than the
critical value, the generation and growth of OSFs are
considered to obey the mechanism reported previously
[19–21], in which the interstitials generated through the
reaction of oxygen and silicon climb to be OSFs.
The results of the previous experiments done by this
author [13] shown that the oxidation modes above and
below the critical oxygen partial pressure are different.
Analysis based on the Cabrera–Mott model [22]
suggested that the drift of the ions of the oxidationrelated species through the electric field in the oxide is
dominant in the low oxygen partial pressures. This
mechanism has also been suggested by Kamigaki et al.
[23] for the oxygen partial pressure of 1 10 3 atm.
Furthermore, the oxidation-related species in this region
are considered to be the ionized excess silicon in the
oxide rather than oxygen. As stated by Cabrera and
Mott in Ref. [22], the oxidation rate is independent of
oxygen partial pressure when the oxide grows as a result
of the outward movement of the oxidation-related
species. This must be the case for the results in the
reports [13,24] because of the very weak oxygen partial
pressure dependence of oxide thickness.
A model, which explains both the oxide growth and
the suppression of OSF generation in the surface region
at the oxygen partial pressures lower than the critical
value, has also been proposed in the previous paper [13].
A fraction of the oxygen adsorbed on the oxide surface
is ionized at elevated temperatures. At the same time, the
oxidation-related species, such as silicon interstitials, are
ionized at the substrate–oxide interface and they drift
through the oxide. Then the species are oxidized at the
surface region of the oxide resulting in oxide growth.
Along with this process, metallic impurities, when there
are any in the surface region of substrate, are thought to
drift to the oxide surface in the form of ions. The mostly
contained metallic impurities in the substrate, such as
Fe, Ni, and Cu, are considered to be ionized easily
because their ionization energies are smaller than that of
silicon [27]. Consequently, the nuclei or the embryos of
OSFs are eliminated from the surface region of the
substrates. It is supposed that under the conditions in
which the silicon interstitials and metallic impurities are
sucked from the substrate, the other origins of OSFs,
i.e., oxygen precipitates or Si–O clusters and mechanical
damage, are annealed out during this annealing before
climbing to be OSFs or growing into nuclei of OSFs.
Once the nuclei or the embryos are annihilated from the
surface region of the substrates, OSFs will hardly be
generated during the subsequent oxidation even if the
oxidation is for a long time. In this situation,
concentrations of the interstitials and metallic impurities
in the bulk of the substrate are decreased due to
diffusion to the surface region.
For the suppression effect of the OSF generation in
the bulk region and the decrease of precipitates in the
whole wafer, annealing for 30 min is sufficient in this
experiment. As the effect reaches a long range, diffusion
of some species, which have large diffusion coefficient,
may be the mechanism responsible for the suppression
of the generation of OSFs and the reduction of
precipitates. Diffusion coefficients of the elements
related to this system are summarized in Table 1.
Diffusion coefficients of the metallic impurities, such as
Fe, Ni, and Cu, are very large [25], and the diffusion
length during an annealing for 30 min at 11001C was
calculated to be more than 900 mm. This diffusion length
T. Suzuki / Materials Science in Semiconductor Processing 5 (2002) 5–10
9
Table 1
Diffusion coefficients of relevant species in the oxidation of Si at 11001C
Species
Diffusion coefficients at 11001C (cm2/s)
Diffusion length for 30 min (cm)
Ref.
Fe
Ni
Cu
Si interstitial
O
Si interstitial
O
Si interstitial
D5 10
D4 10 5
D1.2 10 5
D3.02 10 7
8 10 11
3.91 10 5
1.5 10 10
2.3 10 9
D9.5 10
D2.7 10
D1.5 10
D2.3 10
3.8 10
27 10
5.2 10
2.0 10
.
U. Gosele
[25]
.
U. Gosele
[25]
.
U. Gosele
[25]
.
U. Gosele
[25]
.
U. Gosele
[25]
T. Abe [26]
T. Abe [26]
T. Okino [27]
6
is sufficiently large compared to the 300 mm of the
substrate thickness. When the metallic impurities at
the surface region are removed, those in the depth of the
bulk can easily be eliminated. The diffusion length of
excess Si during the annealing was calculated to be more
than tens of a micrometer [25–27]. Elimination of bulk
stacking faults to the depth of 30 mm is probable. The
diffusion coefficient of oxygen is very small [25,26] and
its diffusion length for 30 min at 11001C was calculated
to be about several micrometers. In the X-ray topography image of the annealed and oxidized substrate,
the speckles had been decreased in the central region of
the substrate, as shown in Fig. 3(b). Thermal diffusion
of oxygen contributed little to the decrease in the
precipitate density. Since the concentration of the
oxygen that was dissolved at the crystal growth step
was about 1.7 1018 cm 3, it cannot be decreased
through diffusion by this annealing condition to the
level at 1.9 1017 cm 3, which is the equilibrium
solubility at 11001C [28]. The diffusion of species other
than oxygen atoms or some complex of atoms is
speculated to be the mechanism responsible for the
decrease in precipitates.
5. Conclusion
Oxidation-induced stacking faults in silicon substrates, that adversely affect the gate-oxide quality in
scaled MOS transistors, were suppressed effectively by
preliminary annealing of the wafer before oxidation in
argon with oxygen partial pressure less than the critical
value at 6 10 3 atm. The suppression of the generation
of OSFs was found to be effective deep into the bulk of
the substrates. At the same time, the precipitates
generated during the long-time oxidation in dry oxygen
were also decreased through the preliminary annealing
in argon with low oxygen partial pressure. The
mechanism of the OSF generation suppression in the
low oxygen partial pressure was attributed to the
difference of the transport mechanisms of oxidationrelated species from the transport mechanism at higher
2
1
1
2
4
1
4
3
oxygen partial pressures than the critical value. The
decrease in OSF in the bulk was also attributed to the
decrease in Si interstitials and metallic impurities in
the surface region, and diffusion from the bulk to the
surface. The decrease in precipitates in the whole region
of the substrates could not be interpreted by diffusion of
oxygen in the substrate.
Acknowledgements
The author extends his immense thanks to Dr.
Y. Hayashi for his encouragement to this work. The
authors also thank Mr. H. Mori for his support to this
work.
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