J. Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.
2995
Schliwinski, ibid., 68, 3532 (1990).
19. T. V. Herak and D. J. Thompson, ibid., 67, 6347 (1990)
22. P. Lange, J. App!. Phys., 66, 201 (1989)
E. A. Irene, J. Vac. Sci. Technol., B5, 530 (1987).
21. W. J. Datrick, G. C. Schwartz, J. D. Chapple-Sokol, R.
24. T Fukada and T. Akahori, in Extended Abstracts of
20. G. Lucousky, M. J. Manitini, J. K. Srivastava, and
Carruthers, and K. Olsen, This Journal, 139, 2604
(1992).
23. B. Fowler and E. O'Brien, J. Vac. Sci. Technol., B12,
441 (1994)
ISSDM, p. 158 (1993).
Infrared Spectroscopy Study of Chemical Oxides Formed by a
Sequence of RCA Standard Cleaning Treatments
Carlos R. Inomata
Fujitsu AMD Semiconductor Limited, Process Engineering Department, Monden Machi, Aizuwakamatsu,
Fukushima 965, Japan
Hiroki Ogawa,* Kenji Ishikawa,' and Shuzo Fulimura
Fujitsu Limited, Process Development Division, 4-1-1 Kamikodanaka, Nalcarahara-ku, Kawasaki 211, Japan
ABSTRACT
Chemical oxides formed on silicon surfaces by the wet cleaning agents [H2S04-H2O2 (SPM), HNO3, NH4OH-H202-H2O
(SC1), HC1-H202-H2O (SC2) solutions] of the RCA standard cleaning procedure were characterized by infrared reflection
absorption spectroscopy (IR-RAS) by the longitudinal optical phonon and transverse optical phonon modes arising from
Si-O stretching vibration. These oxides were found to each have their own distinct IR-RAS spectrum. By using these wet
cleaning agents in sequence, we found that the chemical oxide IR-RAS spectrum corresponds to the spectrum of one of
the single treatments. That is, SC2 has no effect on chemical oxides formed by other treatments. Otherwise, we found the
last treatment to prevail, except in the case of HNO3 following SPM, where SPM prevailed. Thus, we found the SC2 cleaning solution to be the least effective at removing previous oxides. SC1, SPM, and HNO3 were all very effective at removing previous oxides.
Infroduction
In semiconductor device manufacturing processes, wet
chemical treatments based on the RCA cleaning process1
are widely used to obtain a contamination-free silicon surface. These chemical treatment processes are required not
only for the removal of contamination on the surface, but
also to control the physical and chemical states of the surface.2 Various films in semiconductor manufacturing are
becoming thinner with higher device integration. Thus,
the surface state after wet chemical cleaning will influence the quality of the film grown afterward. In fact, Ohmi
et al. have shown that the silicon surface roughness
depends on the concentration of ammonia (NH4OH) in the
SC1 (NH4OH-H2O2-H2O) cleaning solution and that such
surface roughness influences the device performance.34
Additionally, the electrical properties of metal oxide semiconductor (MOS) diodes depend on the various wet chemical treatments used to form their oxide films.5 This suggests that the chemical structure of chemical oxide formed
during wet chemical treatments influences the device performance. It is, therefore, important to understand the
physical and chemical structure of the silicon surface after
wet chemical treatments.
The chemical structure of a silicon surface after treatment in hydrofluoric acid (HF) solution is well defined.
The HF solution which removes the silicon dioxide (SiO2)
films covers the silicon surface with hydrogen.6-8 In particular, the (111) Si surfaces are atomically flat and hydrogen
terminated when treated in buffered HF (BHF) or ammonium fluoride (NH4F) solution." On the other hand, other
cleaning solutions'2 including SC1, SC2 (HC1-H202-H2O),
SPM (H2SO4-H202), and HNO3 form chemical oxides on the
silicon surface. Hattori and co-workers have studied the
chemical structures of these chemical oxides using x-ray
photoelectron spectroscopy (XPS) and Fourier transform
infrared attenuated total reflection spectroscopy (FTIRATR). They discovered a peak related to the Si—H bonds in
Si 2p photoelectron spectra and have reported that the
*Electrochemical Society Active Member.
chemical oxides are characterized by the amounts of Si—H
bonds in the chemical oxide and/or chemical oxide/silicon
interface."-'3 In addition, the use of SC1 as the first or last
process in a sequence of two wet cleaning treatments was
found to result in an SCI-formed chemical oxide film.
This was inferred from the amount of Si—H bonds in the
oxide and at oxide/silicon interface.'3 This was ascribed to
SC1 etching the chemical oxide formed by a previous
treatment, and also to oxidation by SC1.'3 In the case
where SC1 was used as the first treatment, the lack of
Si—H bonds in the oxide was found to hinder the transport
of other oxidizing species, resulting in the preservation of
the SC1-formed chemical oxide.'3
In this study, we characterize the chemical oxides
formed in the four most widely used wet cleaning treatments, SPM, HNO3, SC1, and SC2. We used FTIR reflection absorption spectroscopy (RAS),'4" which is sensitive
to the longitudinal optical (LO) phonon vibrational mode
and transverse optical (TO) phonon vibrational mode arising from Si—O stretching vibrations. We can directly
obtain information regarding Si—O bonds. We show that
each chemical oxide has a distinct RAS spectrum. To verify agreement with the results obtained by Hattori et a!.,
we also observed the JR absorption arising from Si—H
vibrations using FTIR-ATR. Since the wet cleaning
process in practical device manufacturing is usually combined with various chemical solutions, we tried to categorize the type of chemical oxide resulting from the use of
more than one wet chemical treatment in a cleaning
sequence. In addition, since each cleaining solution leaves
an oxide with a characteristic IR-RAS spectrum, we can
rank the various cleaning solutions in order of their ability to remove previously formed oxides.
Experimental
The samples we used for RAS measurements were 90 X
45 x 0.525 mm Czochralski-grown (CZ) p-type (100) Si
with lapped underside surfaces and a resistivity of 10 fl
cm. In the ATR measurements, the samples were 0.625 x
20 >< 50 mm 30 — 40 fi cm p-type CZ (100) Si polished on
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2996
J. Electrochem. Soc., Vol.143, No.9, September 1996 The Electrochemical Society, Inc.
both sides and with 45° bevels polished off on each of the
short sides. Both types of samples were cut from 4 in,
wafers. Prior to cutting the wafers, 100 nm sacrificial
oxide films were grown in dry 02 at 1000°C and annealed
in Ar at 1000°C for 1 h.
We formed chemical oxide films using the treatments
given in Table I after removing the sacrificial oxide films
in 2.5 M HF solution and terminating the silicon surface
a)
C)
C
Ct,
.0
with hydrogen. The treatments were performed either
aU)
singly or as a sequence of two different treatments. Except
for HNO3, all chemical solutions were freshly prepared
20 s before sample immersion. Following chemical oxidation, we rinsed the samples with ultrapure deionized water
for io mm and then dried them in N2 for 10 mm. Samples
were allowed to cool for 10 mm following N2 drying.
We obtained JR absorption spectra using a JEOL JIB6500 FTIR spectrometer. In the RAS measurements, we set
the angle of incidence of p-polarized JR radiation to 80°
from the normal to the sample surface. The detector was
liquid N2 cooled mercury-cadmium-tellurium (MCT), the
resolution was 8 cm', and the counting time was 15 mm.
Following wet chemical treatment, chemical oxides were
formed on both the polished mirror side and on the lapped
underside of the wafers. We removed the oxide from the
.0
C
2400
Fig. 1. IR absorption spectra arising from Si-H stretching vibrations obtained for chemical oxides formed by SC i, 5C2, and HNO3
treatments.
treatments have Si—H bonds in the oxides. However, the
underside using HF since this oxide film influences the
SC1 treatment does not produce Si—H bonds in the oxide.
The tendency of these results roughly correspond to those
obtained by Hattori et al.'3
Figure 2 shows the Si—H absorption spectra of chemical
oxides formed in various wet chemical sequences obtained
RAS spectra.14 RAS oxide spectra in this study thus correspond to the oxide film on the polished mirror surface of
the wafer used in device manufacture. Following infrared
measurement of the oxide film, we stripped the chemical
oxide film using 2.5 M HF solution and carried out in-
by JR-ATE measurement. The spectra are divided into
frared measurement again on the bare silicon surface. The
second spectrum was subtracted from the first to give the
reflectance absorption difference spectra. In the ATE
measurements, JR radiation from the interferometer was
focused at normal incidence onto the input bevel, was
internally reflected about 80 times, exited the output
bevel, and was then refocused and collected onto the pho-
todetector. The number of accumulation scans was 250
and the resolution was 4 cm1. The sample served as the
ATE prism itself. Following ATE measurement of the
chemical oxide on a sample, we treated the sample with
2.5 M HF solution to remove the chemical oxide and leave
a bare, H-terminated Si surface. We then immersed the
sample in SPM solution to form a chemical oxide lacking
in Si—H bonds.'2 This SPM chemical oxide was then used
for JR reference measurement to obtain the Si—H adsorption difference spectrum.13
Results and Discussion
fR-AIR—Figure 1 shows the Si—H adsorption spectra of
chemical oxides formed in three kinds of wet chemical
treatments obtained by IR-ATR measurement. Here, we
used the ATE spectrum obtained for chemical oxide
formed in SPM solution for the reference spectrum since
the amount of Si—H bonds of that native oxide is negligible.13 The infrared absorption arising at 2250 cm' have
been assigned to Si—H bonds in the oxide and the absorption bands arising from 2000 to 2200 cm were assigned to
Si—H bonds at the oxide/Si interface.13 Since the area!
intensity of ATE spectrum is proportional to the amount of
Si—H bonds, the number of Si—H bonds at the oxide/Si
interface is highest in the case of the SC2 treatment and is
almost the same as in the case of the SC1 and HNO3 treatments. The chemical oxides formed by the SC2 and HNO3
three blocks and correspond to the treatments listed on the
-
right of the figure. In the following, "+" denotes a
sequence of two wet treatments, i.e., "SPM + SC2"
denotes treatment in SPM, followed by rinsing, treatment
in SC2, rinsing, and then N3 drying. In Fig. 2, the treatment labeled SPM + SC2 shows that the use of SC2 treatment following SPM treatment resulted in the formation
of a chemical oxide characteristic of the SPM-formed
chemical oxide indicated by the lack of absorption by
Si—H bonds. Since SPM-formed chemical oxides were used
for reference spectra,13 the treatment consisting of SPM +
5C2 did not result in a chemical oxide that differs in the
amount of Si—H bonds from SPM treatment alone. Both
SPM + SC2 and SPM only treatments resulted in a chemical oxide with negligible Si—H bonding. Note that among
the four different treatments used in this study, SC2-
formed chemical oxides had the largest amounts of Si—H
bonds at the oxide/Si interface. There seems to be a slight
trough at 2100 cm' in the spectrum, which means that the
amount of Si—H bonds at the chemical oxide/silicon interface is smaller than in the case of SPM-formed chemical
oxides. However, this difference is negligible. We therefore
SPMi-SC2
—'. SC2+SPM
—__A...-....-----—--—-.-...-_._..--———--—---.
HNO+SPM
j
HNO3
HNO3÷SC2
SC2+HNO3
SC1+HNO3
Table I. chemical treatments examined.
SPM
HNO3
SC1
Composition (M)
8.0 H2S04
1.8 H301
9.5 HNO3
1.1 NH4OH
1.3 11102
SC2
1.4 HC1
1.3 H102
SCI+SPM
SPM+HNO3
,
Treatment
1900
Wavenumber (cm-1)
SC1
Temperature
Time
170°C
10mm
75°C
60°C
10 mm
60°C
10 mm
10 mm
SC1 +S62
SC2+SC1
SPM÷SC1
2x1031
HNO3+SC1
_____________________________
1900
2400
Wavenumber (cm-1)
Fig. 2. ATR spectra of chemical oxides formed by various wet
chemical sequences.
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2997
.1 Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.
considered that SC2 treatment after SPM had no effect on
the chemical structure of the SPM-formed chemical oxide.
Conversely, the treatment labeled SC2 + 5PM shows that
the chemical oxide formed in SC2, which contained the
largest amount of Si—H bonds at the oxide/Si interface,
was replaced by a chemical oxide lacking in Si—H bonds,
and which corresponds to an SPM-formed chemical oxide.
5C2 treatment thus had no effect on SPM treatment in
terms of the amount of Si—H bonds in the resulting chemical oxide. Since 5C2- and SPM-formed chemical oxides
had the largest and smallest amounts of Si—H bonds,
respectively, the type of oxide according to the amount of
Si—H bonding obtained is most clearly seen in this
sequence.
The treatment labeled SC1 + SPM shows that the SC1formed chemical oxide did not prevail after SPM use since
no Si—H bonds were detected by this sequence. The treatments labeled SPM + HNO3 and HNO3 + SPM also show no
resulting Si—H bonds despite the large amounts of Si—H
bonds in the oxide and at the oxide/Si interface obtained by
a single HNO3 treatment. The top five treatments in Fig. 2,
therefore, resulted in chemical oxides that present negligible Si—H bond infrared absorbance spectra and that conespond to the spectrum of an SPM-formed chemical oxide.
The treatment labeled HNO3 shows the infrared absorbance spectrum from Si—H bonds in the oxide and at the
oxide/Si interface obtained for this chemical oxide.
Although 5C2 IR-ATH spectra are not shown, 5C2-formed
chemical oxides resulted in roughly the same amount of
Si—H bonds in the oxide as with HNO3 treatment, but the
amount of Si—H bonds at the oxide/Si interface were greater
C)
0
C
t
Ct
C)
'4a)
1
Wavenumber (cm-1)
800
Fig. 3. IR-RAS specfra arising from 10 (-1200 cm1) and TO
(—1050 cmj modes of Si-0 sketching vibrations obtained for
chemical oxides formed by 5PM, SC 1, SC2, and HNO3 freaftuents.
for 5C2 than for HNO3 treatment, judging by the nearly
double peak height and peak area in the range from 2000 to
2200 cm for SC2. The treatments labeled HNO3 + 5C2 and
5C2 +
therefore, resulted in an amount of Si—H
bonds at the interface that was closer to an HNO3-formed
chemical oxide than to an 5C2-formed chemical oxide.
HNO3,
The treatment labeled SC1 shows the infrared
absorbance spectrum of Si—H bonds at the oxide/Si inter-
face obtained for this chemical oxide. The treatments
labeled 5C2 + SC1, SPM + SC1, and HNO3 + SC1 show
interface Si—H bond infrared absorbance spectra which
correspond to the amount of interface Si—H bonds
obtained for single SC1 treatment. This is ascribed to
etching of the previous chemical oxide by the SC1 along
with oxidation by SC1. Using SC1 as the last treatment
therefore resulted in an SC1-formed chemical oxide.13
Less conclusive are the spectra labeled SC1 + HNO3 and
SC1 + 5C2. The former treatment shows that the amount
of Si—H bonds in the oxide and at the interface is nearly
the same as that of the HNO3-formed chemical oxide. The
latter treatment shows that the amount of Si—H bonds at
the interface approaches that found in an SCI-formed
chemical oxide, although the Si—H bond amount in the
oxide was not negligible for this sequence.
The treatment labeled SC1 +
SPM
differs from the
results from Hattori et at. in that treatment resulted in an
SC1-formed chemical oxide. This could be due to the lower
temperature (80 to 90°C) for SPM used by Hattori et at.
IR-RAS.—Figure 3 shows the lB absorption spectra of
the LO and TO phonon modes arising from Si-O stretching vibration by lB-HAS of the chemical oxides formed by
the separate wet treatments, 5PM, HNO3, SC1, and 5C2.
The HAS spectrum of the chemical oxide is characterized
by the frequency (cm') and full-width-half-maximum
(FWHM) line of the LO peak at around 1200 cm'. The
FWHM line of the LO peak is smallest in the case of 5PM-
formed chemical oxides and largest in the case of 5C2formed chemical oxides. Moreover the frequency of the
LO peak for SPM-formed chemical oxides is highest
among the four treatments, and approaches the frequency
of LO peaks obtained for thin thermally oxidized SiO2
films (—1250 cm'). Of the four treatments, SPM-formed
chemical oxides thus resemble thin thermally oxidized
Sb2 most closely. On the other hand, the HAS spectra of
5C2-formed chemical oxides differ most significantly
from thin, thermally oxidized Si02 spectra in terms of the
FWHM of the LO peak. This means that the large
reflectance is in the region between 1100 and 1200 cm'
with respect to the LO peak height. This region has been
attributed to the increased roughness at the Si/5i02 interface and/or on the 5i02 surface.16 The area of the TO
trough at around 1050 cm is proportional to the oxide
film thickness.14"5 Thus SPM and HNO3 are considered to
be strong Si oxidants and SC1 and 5C2 are considered to
be weak Si oxidants. We found the four wet treatments:
SPM, NHO3, SC1, and 5C2, each to have their own distinct
JR-HAS spectrum.
The spectrum obtained for the SPM treatment is shown
at the top of Fig. 4, and that for HNO3 treatment at the
bottom. The SPM spectrum shows an LO peak at
—1220 cm' and the HNO3 spectrum shows an LO peak at
—1200 cm'. The spectrum labeled SPM + HNO3 shows an
LO peak at —1220 cm', meaning that the SPM + HNO3
spectrum is closer to the SPM spectrum than to the HNO3
spectrum. Furthermore, the FWHM line of the LO peak of
the spectrum labeled SPM + HNO3 is closer to the SPM
spectrum than to the HNO3 spectrum. Therefore, HNO3
treatment of the chemical oxide formed in SPM had no
effect on the SPM-formed chemical oxide. So, the treatment consisting of SPM + HNO3 is equivalent to SPM
treatment alone, from the infrared reflectance absorption
spectra of the chemical oxides. The center of Fig. 5 shows
the spectrum obtained for a chemical oxide formed by a
sequence consisting of HNO3 + SPM. The above arguments suggest that the spectrum labeled HNO3 + SPM is
closer to the SPM spectrum than to the HNO3 spectrum. In
this case, SPM treatment after HNO3 treatment resulted in
a chemical oxide whose spectrum corresponds to the spectrum of SPM treatment alone. It is worth noting that SPM,
HNO3, SPM + HNO3, and HNO3 + SPM treatments formed
oxides of similar thicknesses by the area of the TO troughs
of the spectra.
We also measured chemical oxides formed by other com-
bination treatments, SC1 + SPM, SPM + SC1, SC2 +
SPM, SPM + 5C2, HNO3 + 5C2, 5C2 + HNO3, SC1 +
HNO3, HNO3 + SC1, SC1 + 5C2, and 5C2 + SC1 and the
results are shown in Fig. 6-8. We then found that the HAS
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2998
J. Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.
a)
C-)
C
Ct
4J
C)
ci
4a)
Wavenumber (cm1)
1400
Fig. 4. lR-RA5 spectra of 5PM, 5PM + HNO3, and HNO3 treatments.
spectra obtained from oxides formed by combination treatments are categorized according to the position, FWHM,
and intensity of LU and TO phonon modes into three kinds
of spectra, i.e., 5PM, Sd, and HNO3 alone. These 5PM
spectra are shown in Fig. 6. Treatment by SPM combina-
tions gives similar HAS spectra to the spectrum of SPM
alone, except for treatment by 5PM + Sd. SPM + SCi
treatment is attributed to the SC1 alone, like the spectrum
in Fig. 7. This is associated with etching by the SC1 treatment of the chemical oxide formed in SPM and to oxidation along with etching by SC1,'3 thus resulting in a chem-
ical oxide characteristic to Sd treatment by the HAS
spectrum. From these results, we found that the SPM
WaVenumber (cm1)
800
Fig. 6. RAS spectra of 5PM alone obtained for various wet chemical sequences.
spectra obtained from oxide formed in SCI are collected in
Fig. 7. When the SC1 treatment performs subsequent to
any treatments, the spectra correspond to that obtained
from oxide formed by SC1 treatment alone. As stated
above, this reason is due to the etching effect of Sd. The
spectrum obtained from SC1 + SC2 also gives the spectrum obtained by SC1 treatment alone, which indicates
that the SC1 treatment prevails over 5C2 treatment.
Figure 8 shows the spectrum for HNO3 treatment alone.
treatment prevails over other three kinds of treatment. The
a)
0
C
ct
4-,
0
a)
U)
'4-
C-)
a)
C
4'
C-,
a)
800
1
1400
Wavenumber (cm')
WaVenumber (cm-1)
800
Fig. 5. IR-RAS spectra of 5PM, HNO3 + 5PM, and HNO3 treatments.
Fig. 7. MS spectra of SCI alone obtained for various wet chemical sequences.
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J. Electrochem. Soc., Vol. 143,No. 9,September 1996 O The Electrochemical Society, Inc.
1400
2999
800
Wavenumber (cm-l )
Fig. 9. Three kinds of IR-RAS spectra of three-step sequences.
1400
800
Wavenumber (cm-1)
Fig. 8. RAS spectra of HN03 alone obtained for various wet
chemical sequences.
From this figure, we found that the HNO, treatment prevails over the SC1 and SC2 treatments, except SPM treatment. However, this is not the case for HNO, + SC1,
because the SC1 treatment has an etching effect. The
results in Fig. 5-7 are summarized in Table 11. In general,
the last treatment was found to prevail, except in the case
of HNO, following SPM, where SPM was found to prevail.
We found SC2 to have no effect on chemical oxides formed
by other treatments. This classification roughly corresponds to the IR-ATR study in Fig. 2.
IR-RAS of three-step sequences.-Figure 9 shows the IRRAS spectra of chemical oxides obtained by sequences
consisting of three different treatments. We verified the
above results for two-step sequences. In the spectrum
labeled SC1 + SPM + HNO, (dotted line), the initial SC1formed oxide would have been replaced by an oxide corresponding to SPM treatment. HNO, treatment of this oxide
would have no effect on the SPM-formed oxide, and the
final oxide would then correspond to a chemical oxide
formed by SPM treatment alone. The spectrum labeled SC1
+ SPM + HNO, shows an LO peak at -1220 cm-', which
Table II. Summary of two step wet clean sequences.
corresponds to an SPM-formed chemical oxide IR-RAS
spectrum. In the spectrum labeled SC1 + HNO, + SPM
(broken line), the initial SC1-formed oxide would have
been replaced by an oxide corresponding to HNO, treatment. SPM treatment of this oxide would then replace the
oxide by an oxide corresponding to SPM treatment alone.
The spectrum labeled SC1 + SPM + HNO, corresponds to
an SPM-formed chemical oxide IR-RAS spectrum.
In the spectrum labeled SPM + HNO, + SC1 (solid line),
the initial oxide formed by SPM treatment would not be
affected by HNO, treatment. Treatment by SC1 of this
oxide would etch this oxide, and etching with oxidation by
SC1 would result in an oxide characteristic to SC1 treatment alone. Certainly, the spectrum labeled SPM +
HNO, + SC1 corresponds to the IR-RAS spectrum of
treatment by SC1 alone.
Conclusions
The chemical oxides formed in various wet chemical
treatments and by two or more step sequence treatments
were characterized using FTIR-ATR and FTIR-RAS.
IR-ATR results can be used to characterize the chemical
oxide by the Si-H bond absorbance in the oxide and at the
SiO,/Si interface, as reported by Hattori et al. This is
extended to the twelve two-way combinations. These twostep sequences of the four wet clean treatments, SPM
HNO,, SC1, and SC2 resulted in IR-ATR spectra that were
divided into three groups, such as SPM, HNO,, SC1, and
alone.
From the IR-RAS results from the four wet treatments,
we found SPM, HNO,, SC1, and SC2 to each have their
own distinct IR-RAS spectrum. The SPM-formed chemical
oxides had spectra which most closely resembled that of
thin, thermally oxidized SiO,. In contrast, the SC2-formed
oxide spectra were significantly different from the other
treatments. The use of more than one wet cleaning treatment resulted in a chemical oxide whose IR-RAS spectrum
corresponded to the spectrum of one of the single treatments. In general, we found SC2 to have no effect on
chemical oxides formed by other treatments. Otherwise,
the last treatment was found to prevail, except in the case
of HNO, following SPM, where SPM was found to prevail.
Thus, the SC2 cleaning solution was least effective at
removing previous oxides, whereas SC1, SPM, and HNO,
were all very effective. The results for Si-H IR-ATR and
Si-0 IR-RAS were in agreement with each other.
Acknowledgments
The authors would like to thank H. Tsuchikawa, T.
Ogawa, and K. Imaoka for their encouragement of this
work.
Manuscript submitted May 30, 1995; revised manuscript
received June 20, 1996.
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3000
J. Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.
Fujitsu AMD Semiconductor, Limited, assisted in meeting the publication costs of this article.
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Steady-Stage Growth of NiO Scales on Ceria-Coated
Polycrystalline Nickel
F. Czerwinski* and J. A. Szpunar
Department of Metallurgical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7
W. W. Smeltzer*
Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1
ABSTRACT
The effect of CeO2 ceramic coatings with thicknesses in the range of 14 to 42 nm on the oxidation of high purity polycrystalline nickel at 973 K in 1 atm 02 has been studied. The ceria coatings decrease the Ni oxidation rate after 125 b by
a factor up to 45, which is significantly higher than the reduction achieved during early stages. Growth features, including oxide thickness, surface morphology, and the texture for both the pure and CeO2 modified NiO, demonstrate a definite dependence on the crystallographic orientation of the Ni substrate. Oxides with thicknesses of up to 1 p.m developed
on Ce02-coated Ni consisted of three sublayers. The Ce-rich part, composed of small NiO grains and CeO2 particles, was
located inside the scale and shifted deeper into the scale with increased oxidation time. The growth kinetics and
microstructural evolutions of modified NiO are discussed in terms of the mechanism of inhibition of grain boundary diffusion in NiO by Ce4 ions.
Infroduction
The
requirements of high temperature strength and
formability for many materials are not always compatible
with their corrosion resistance. Therefore, it often becomes
necessary to select a material based on the mechanical
property requirements and to apply a corrosion resistant
coating for protection. A major drawback in the exploitation of any coating is the lack of a reservoir of beneficial
species to renew protection in the case of cracking or spallation. In particular, for coatings composed of reactive ele-
ment oxides, in addition to local failure, the reduced
effectiveness of overall coating with service time is of critical importance.
As has been previously shown,' CeO2 coatings do not
form a protective barrier layer, but disperse and become
incorporated into growing scales to improve their properties. Moreover, inside the scale, the reactive elements do
not statically dope the oxide, but instead actively diffuse
along specific paths.23 Such behavior suggests that after a
certain period of time there will not be enough reactive
element present to protect the entire substrate effectively.
Although there are data in the literature indicating that
300 to 800 nm thick CeO, coatings deposited on Fe-Cr-Ni
alloys are effective at 1098 to 1208 K for up to 6000 h,4
other studies are less optimistic. In some cases the loss of
protection was observed even after a few hours of service.5
Thus a selection of factors which control the effectiveness
of the reactive element inside the native oxides is a major
challenge for the future. To achieve this, the long-term
oxidation behavior of reactive element-bearing systems
should be examined.
In previous studies,'6 the early stages of oxidation of Ni
coated with CeO, particles derived from sol-gel were presented. In this paper, the long-term oxidation behavior of
the Ni-NiO-CeO, system is reported to assess the modification of NiO transport properties by CeO, at the steady
stage and to verify the role of scale microstructure in this
process.
The
Experimental
material used for oxidation was a nickel rod of
99.99% purity supplied by AD. MacKay Inc. Specimens
in the form of disks of 9.5 mm in cRam and 1 mm in thickness were prepared by mechanical polishing, followed by
vacuum annealing at 1173 K and, as a final step, chemical
polishing. In order to improve surface wetness before coat-
ing, polished surfaces were lightly preoxidized at 673 K
for 2 h in flowing oxygen at 1 atm pressure. The same preoxidation procedure was applied for uncoated samples.
Measured oxygen uptake during preoxidation was
* Electrochemical Society Active Member.
7.7 p.g/cm'. Coatings were deposited by dipping the cold
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