Ternary films about 200 nm thick of composition Ru20Si15O65 have been synthesized by reactive rf magnetron sputtering of a Ru1Si1 target in an argon-oxygen gas. As-deposited, the films are X-ray-amorphous. Their atomic density is 8.9 × 10^22/cm^3 (5.1 g/cm^3), and their electrical resistivity is in the range of 2 mOmega cm. After annealing in dry oxygen at 600°C for 30 min, micron-sized grains of RuO2 grow out of the film and volatile RuO4 escapes. The significance of these results is discussed

Gasser, S. M.

Kolawa, E.

Nicolet, M.-A.

Ruiz, R.

English

Caltech Authors - Main

1546 Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999)
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.Instability of Amorphous Ru-Si-O Thin Films under Thermal Oxidation
S. M. Gasser,z R. Ruiz, E. Kolawa, and M.-A. Nicolet
California Institute of Technology, Pasadena, California 91125, USA
Ternary films about 200 nm thick of composition Ru20Si15O65 have been synthesized by reactive rf magnetron sputtering of a
Ru1Si1 target in an argon-oxygen gas. As-deposited, the films are X-ray-amorphous. Their atomic density is 8.9 3 10
22/cm3
(5.1 g/cm3), and their electrical resistivity is in the range of 2 mV cm. After annealing in dry oxygen at 6008C for 30 min, micron-
sized grains of RuO2 grow out of the film and volatile RuO4 escapes. The significance of these results is discussed. 
© 1999 The Electrochemical Society. S0013-4651(98)07-010-4. All rights reserved.
Manuscript received July 6, 1998.Ternary thin films of the type TM-Si-N, where TM are transition
metals of the Ti, V, and Cr groups, have properties that are both sci-
entifically interesting and practically useful. Depending on their
mode of deposition, on their composition, and on their postanneal-
ing treatment, they can be structurally amorphous down to high-
resolution transmission electron microscopy (TEM-amorphous),
nanostructured so as to appear amorphous under X-ray diffraction
(X-ray-amorphous), or polycrystalline with varying grain sizes.
They can be single-phase or two-phase structured, electrically insu-
lating or conducting. A recent compilation of the relevant literature
is given in Ref. 1-3. Uses where one or several unique properties of
these films are exploited have been demonstrated in applications as
various as a primary mask for X-ray lithography,4 thin-film diffusion
barriers in semiconductor metallizations, 5-8 ultrahard coatings, 9-12
and micromachining.13
Motivated by these successes, we have asked whether analogous
ternary thin films of the type TM-Si-O could be synthesized with
properties that might be similarly useful and interesting. This paper
presents the first results and answers to this question. 
Specifically, we asked if amorphous ternary films of Ru-Si-O
could be obtained by reactive sputtering a Ru1Si1 target. This choice
follows the line of thought that leads to amorphous TM-Si-N films.
These alloys may be viewed as a combination of a metallically con-
ducting binary transition metal nitride (such as TiN, that has a simple
B1 crystal structure) with Si3N4, an insulating nitride with a predilec-
tion for an amorphous structure, to produce a TM-Si-N film (such as
Ti-Si-N). RuO2 is a metallically conducting oxide that has a simple
C4 structure, and SiO2 is an insulating oxide that typically forms
amorphous films. In analogy with TiN and Si3N4, RuO2 and SiO2
may thus also yield amorphous ternary Ru-Si-O. Underlying this
approach is the concept that a combination of species that normally
solidify in different crystalline structures will frustrate the system and
induce the formation of an amorphous phase. And, in addition, the
combination of a predominantly metallically bonded species with one
that is predominantly covalently bonded will similarly favor that out-
come. The validity of that approach is underscored by the results
obtained with the TM-Si-N alloys, some of which remain X-ray-
amorphous up to heat-treatments of 9508C for 30 min in vacuum.5
In the present investigation we address two particular questions
about the Ru-Si-O alloy system: (i) Is it possible to obtain amor-
phous films by reactive sputtering of a ruthenium silicide target in an
oxidizing ambient, as has been successfully done with the reactive
sputtering of early transition metal silicides in a nitrogen-carrying
ambient? (ii) If so, how stable are the amorphous films thus obtained
upon thermal annealing? For the latter investigation, we have chosen
thermal treatments in an oxidizing ambient because ruthenium can
form the volatile compound ruthenium tetroxide (RuO4). In contrast
to annealing in vacuum, this treatment tests the stability of the result-
ing film both structurally as well as chemically under conditions
near those encountered in practice by coatings. Amorphous conduct-
z E-mail: sgasser@cco.caltech.eduing oxides may offer practical advantages in applications in corro-
sive media.
Experimental
Silicon wafers covered with thermally grown SiO2 of 120 nm
thickness served as substrate for this study. Thin films of Ru-Si-O
were deposited by reactive rf (13.56 MHz) sputtering with argon as
sputtering gas and oxygen as reactant in a cryopumped vacuum sys-
tem of 5 3 1027 Torr base pressure. A net forward sputtering power
of 300 W was applied on a Ru1 Si1 target 3 in. in diam, and the sam-
ples were centrally positioned 13 cm below the circular target. No
bias was applied, neither was the sample actively heated or cooled,
and the substrate temperature is estimated to stay below 1008C dur-
ing the deposition. Selected samples were patterned by drawing
stripes on the oxide surface with a conventional felt-tip pen and lift-
off with acetone after the deposition to measure the thickness of the
sputtered layers with a stylus profilometer. The samples were then
annealed in an open quartz-tube furnace and a dry oxygen flow of
80 cm3/min at 600 and 8008C. Elemental depth profiles were mea-
sured with 2.0 MeV 4He21 backscattering spectrometry, and atomic
composition were extracted from them with an overall accuracy
level of approximately 5%. Sheet resistance measurements were per-
formed with a four-point probe. Structural information was obtained
by Debye-Scherrer X-ray diffraction with a collimated Co Ka beam
incident on the sample at a glancing angle of 128 and an Inel posi-
tion-sensitive detector. Scanning electron micrographs were taken
on a Cambridge Stereoscan 250 Mk2 with a Kevex energy-disper-
sive spectrometer. The beam spot for energy-dispersive X-ray map-
ping has a diameter of 83 nm. To test whether some volatile rutheni-
um tetroxide (RuO4) evolves, an oxygen-saturated Ru-Si-O film was
sealed into a Pyrex test tube in ambient air and annealed at 6008C for
60 min. The test tube was subsequently positioned in a HP 8452A
diode array spectrophotometer in such a way that the beam traversed
the test tube walls, any deposits on them, and the enclosed gas with-
out seeing the sample that was lying on the bottom of the sealed test
tube. The absorption spectrum was measured before and after the
heat-treatment. 
Results
Partial flow rates of oxygen from 0 to 10% were employed for the
reactive sputtering of the Ru-Si-O thin films from the Ru1Si1 target.
Depending on the oxygen partial flow, sputtering rates between 130
and 250 Å/min are achieved. The atomic concentration of oxygen in
the film reaches a saturation value of about 65 atom % at a partial
flow rate of oxygen above 6%. This value is expected if RuO2 and
SiO2 are to be the only and stable binary reaction products of the
sputtering process. SiO2 is well known to favor the formation of
amorphous films. We observe that our films are X-ray amorphous
when the oxygen atomic composition nears saturation, above 55%. 
The samples that are used for further investigations in this study
are all oxygen-saturated. After deposition, their atomic density is
8.9 3 1022/cm3 (5.1 g/cm3), and their electrical resistivity is in the
range of 2 mV cm. Figure 1 shows the X-ray diffraction spectra of a
Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999) 1547
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.200 nm thick Ru20Si15O65 film before and after annealing in dry
oxygen at 6008C for 30 min. No diffraction peaks are observed from
the as-deposited sample, but clear peaks emerge after oxidation.
They can all be assigned to RuO2. Peaks of SiO2 are not detected. 
The annealing also dramatically changes the backscattering spec-
tra of the films. Figure 2 compares 2 MeV He21 backscattering spec-
tra of films annealed at 8008C for 5, 15, and 30 min with the spec-
trum of the as-deposited film. One interpretation of these spectra is
that ruthenium has diffused deeply into the substrate. Another is that
thick columnar features containing ruthenium have grown on or
through the surface. A decrease of the total counts in the ruthenium
signal upon annealing is indicated and leads to the suspicion that
some ruthenium might have been lost during the oxidation process.
Ruthenium tetroxide (RuO4) starts to volatilize noticeably14 at 78C,
has a very low melting point15 of 25.58C and a boiling point16 of
129.68C. If RuO4 were formed during the oxidation, it would be
volatile and lead to the loss of ruthenium from the film. 
We proved the formation of RuO4 by light absorption spectro-
scopy. Small samples were sealed under ambient air in a Pyrex
(borosilicate glass) test tube, and the test tube was annealed at 6008C
Figure 1. Diffraction spectra of a 200 nm thick Ru20Si15O65 film deposited
on a 120 nm thick SiO2 covered Si wafer, taken with Co Ka X-rays before
and after oxidation at 6008C for 30 min.
Figure 2. 2.0 MeV 4 He21 backscattering spectra of a 200 nm thick
Ru20Si15O65 film deposited on a 120 nm thick SiO2-covered Si wafer, before
and after oxidation at 8008C for 5, 15, and 30 min.for 30 min. The amount of oxygen enclosed in the test tube was suf-
ficient to form RuO4 with all the ruthenium contained in the thin
film. Signs of material removal from the Ru-Si-O thin film are al-
ready seen visually by noticing that after annealing the walls of the
test tube are coated with a diffuse milky layer. Figure 3 shows the
absorption spectra, taken at room temperature, of the test tube before
and after annealing. A clear absorption peak can be seen after an-
nealing with its maximum around 290 nm. A main absorption max-
imum at 310 nm for RuO4 (oxidation state d
0)17 has been published
by Wells et al.,18 while the reflectance spectrum for RuO2 (oxidation
state d4)17 is reported to exhibit a broad band from 430 to 700 nm,
centered at 585 nm. 19 Accounting for the width of the peak and the
resolution of the spectrometer, our result clearly proves the presence
of RuO4 on the walls of the test-tube and thus the loss of ruthenium
from the film during annealing in dry oxygen at 6008C. 
A scanning electron micrograph (SEM) of the surface of a
Ru20Si15O65 thin film, after oxidation at 6008C for 30 min, is shown
in Fig. 4. Predominantly columnar-shaped crystallites with dimen-
sions of about 5 mm in length and 1 mm in diameter can be observed,
sticking out of a flat surface and perhaps partly embedded in it. Sur-
face mapping by two-dimensional energy-dispersive analysis of in-
Figure 3. Light absorption spectra of the walls of a test tube that contained a
200 nm thick Ru20Si15O65 film deposited on a 120 nm thick SiO2-covered Si
wafer, obtained at room temperature before and after annealing at 6008C for
30 min.
Figure 4. Scanning electron micrograph of a 200 nm thick Ru20Si15O65 film
deposited on a 120 nm thick SiO2-covered Si wafer, after oxidation at 6008C
for 30 min.
1548 Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999)
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.duced X-rays reveals that all the ruthenium is concentrated in the
crystallites while silicon is found only in the surrounding matrix.
Oxygen can be observed everywhere. Together with the X-ray data,
these facts indicate that the crystallites are RuO2. The remainder is a
film of SiO2, possibly containing some residual ruthenium oxide.
The backscattering spectra of Fig. 2 are consistent with this picture.
SEMs of as-deposited films on the same scale as Fig. 4 are feature-
less. The electrical resistivity of that film is too high to be measur-
able by a four-point probe.
Discussion and Conclusion
The synthesis of amorphous ternary films of composition
Ru20Si15O65 is demonstrated and confirms the idea that the com-
bination of metallically conducting RuO2 with covalently bonded
SiO2 leads to the formation of amorphous ternary Ru-Si-O thin
films. Their electrical resistivity of about 2 mV cm (bulk resistivity
of RuO2: 35 mV cm) together with the X-ray amorphous structure
makes such films acceptable candidates for applications with mod-
erate temperature requirements. 
RuO2 is used for thin-
21 and thick-film resistors,22,23 as well as
for coatings on electrodes.24,25 Interest has recently risen for RuO2
films as electrodes for ferroelectric barium-strontium-titanate
(BST),26,27 and lead-zirconate-titanate (PZT),28,29 thin films to make
nonvolatile memory elements. Films of RuO2 have also been con-
sidered for metallizations in integrated circuits.30-35 The values of
the heats of formation of the transition metal oxides are comparable
to those of the transition metal nitrides and indicate good thermal
stability.36 The surface morphology of RuO2 films is reported to re-
main smooth after rapid thermal annealing up to 9008C for 10 s in
argon, nitrogen, and oxygen.37
On the other hand, Green et al. report that RuO2 films dispropor-
tionate and decompose to ruthenium and volatile RuO4 upon anneal-
ing in vacuum at 9008C for 2 h, according to the reaction 2 RuO2 r
Ru 1 RuO4.
34 Using the process RuO4 r RuO2 1 O2, Yuan et al.
synthesized RuO2 films from RuO4 by chemical vapor deposition.
38
In our present situation, annealing of the Ru20Si15O65 films occurred
in an open system and a dry oxygen ambient at atmospheric pressure
where the supply of oxygen was unlimited. Rather than the decom-
position of RuO2 films to Ru and volatile RuO4, we observe the for-
mation of volatile RuO4 and growth of RuO2 crystallites, which is
the reverse of the process described by Yuan et al. The growth of
RuO2 particles in bulk glasses was studied previously by Nakano
et al.: starting with 10 vol % RuO2 grains of around 15 nm average
size dissolved in glass, extensive growth is observed only at 9508C
with a particle size of 60 nm after 1 h.39 In contrast, our oxygen-sat-
urated ternary Ru-Si-O thin films separate into RuO2 crystals and a
surrounding SiO2 matrix much more rapidly, accompanied by the
evolution of volatile RuO4. As indicated through the backscattering
spectrum, major changes are observable already after 5 min at
8008C. A reaction process based on the gaseous phases of oxygen
and RuO4 appears to be most likely. Atomic oxygen has been found
to etch RuO2 to form volatile RuO4 products.
17 Other reaction
processes might also be involved, since the catalytic power of ruthe-
nium complexes is well known and used in many other fields such
as solar energy conversion,40 coordination chemistry,41,42 and bio-
chemical applications.43,44
The answers to the two questions posed initially are therefore
that (i) it is possible to obtain X-ray-amorphous Ru20Si15O65 alloys
by reactive sputtering of RuSi, but (ii) the alloys are unstable in an
oxidizing ambient, at a time/temperature exposure of 30 min at
6008C, or less. Our results from treatment of Ru20Si15O65 films in
oxidizing ambient do not preclude that the same films could be ther-
mally stable in an inert ambient or vacuum. This aspect would
deserve further attention.Acknowledgments
Dr. Martina Hüber established the connection with Dr. Harry
Gray, who suggested light absorption spectroscopy to identify RuO4.
The authors express their appreciation to them for helpful discus-
sions. Technical assistance from Rob Gorris is gratefully acknowl-
edged. Financial support for this study was provided by the Center
for Integrated Space Microsystems at JPL/NASA and is gratefully
acknowledged. 
California Institute of Technology assisted in meeting the publication
costs of this aricle.
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Instability of Amorphous Ru-Si-O Thin Films under Thermal Oxidation

1546 Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999)
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.Instability of Amorphous Ru-Si-O Thin Films under Thermal Oxidation
S. M. Gasser,z R. Ruiz, E. Kolawa, and M.-A. Nicolet
California Institute of Technology, Pasadena, California 91125, USA
Ternary films about 200 nm thick of composition Ru20Si15O65 have been synthesized by reactive rf magnetron sputtering of a
Ru1Si1 target in an argon-oxygen gas. As-deposited, the films are X-ray-amorphous. Their atomic density is 8.9 3 1022/cm3
(5.1 g/cm3), and their electrical resistivity is in the range of 2 mV cm. After annealing in dry oxygen at 6008C for 30 min, micron-
sized grains of RuO2 grow out of the film and volatile RuO4 escapes. The significance of these results is discussed. 
© 1999 The Electrochemical Society. S0013-4651(98)07-010-4. All rights reserved.
Manuscript received July 6, 1998.Ternary thin films of the type TM-Si-N, where TM are transition
metals of the Ti, V, and Cr groups, have properties that are both sci-
entifically interesting and practically useful. Depending on their
mode of deposition, on their composition, and on their postanneal-
ing treatment, they can be structurally amorphous down to high-
resolution transmission electron microscopy (TEM-amorphous),
nanostructured so as to appear amorphous under X-ray diffraction
(X-ray-amorphous), or polycrystalline with varying grain sizes.
They can be single-phase or two-phase structured, electrically insu-
lating or conducting. A recent compilation of the relevant literature
is given in Ref. 1-3. Uses where one or several unique properties of
these films are exploited have been demonstrated in applications as
various as a primary mask for X-ray lithography,4 thin-film diffusion
barriers in semiconductor metallizations, 5-8 ultrahard coatings, 9-12
and micromachining.13
Motivated by these successes, we have asked whether analogous
ternary thin films of the type TM-Si-O could be synthesized with
properties that might be similarly useful and interesting. This paper
presents the first results and answers to this question. 
Specifically, we asked if amorphous ternary films of Ru-Si-O
could be obtained by reactive sputtering a Ru1Si1 target. This choice
follows the line of thought that leads to amorphous TM-Si-N films.
These alloys may be viewed as a combination of a metallically con-
ducting binary transition metal nitride (such as TiN, that has a simple
B1 crystal structure) with Si3N4, an insulating nitride with a predilec-
tion for an amorphous structure, to produce a TM-Si-N film (such as
Ti-Si-N). RuO2 is a metallically conducting oxide that has a simple
C4 structure, and SiO2 is an insulating oxide that typically forms
amorphous films. In analogy with TiN and Si3N4, RuO2 and SiO2
may thus also yield amorphous ternary Ru-Si-O. Underlying this
approach is the concept that a combination of species that normally
solidify in different crystalline structures will frustrate the system and
induce the formation of an amorphous phase. And, in addition, the
combination of a predominantly metallically bonded species with one
that is predominantly covalently bonded will similarly favor that out-
come. The validity of that approach is underscored by the results
obtained with the TM-Si-N alloys, some of which remain X-ray-
amorphous up to heat-treatments of 9508C for 30 min in vacuum.5
In the present investigation we address two particular questions
about the Ru-Si-O alloy system: (i) Is it possible to obtain amor-
phous films by reactive sputtering of a ruthenium silicide target in an
oxidizing ambient, as has been successfully done with the reactive
sputtering of early transition metal silicides in a nitrogen-carrying
ambient? (ii) If so, how stable are the amorphous films thus obtained
upon thermal annealing? For the latter investigation, we have chosen
thermal treatments in an oxidizing ambient because ruthenium can
form the volatile compound ruthenium tetroxide (RuO4). In contrast
to annealing in vacuum, this treatment tests the stability of the result-
ing film both structurally as well as chemically under conditions
near those encountered in practice by coatings. Amorphous conduct-
z E-mail: sgasser@cco.caltech.eduing oxides may offer practical advantages in applications in corro-
sive media.
Experimental
Silicon wafers covered with thermally grown SiO2 of 120 nm
thickness served as substrate for this study. Thin films of Ru-Si-O
were deposited by reactive rf (13.56 MHz) sputtering with argon as
sputtering gas and oxygen as reactant in a cryopumped vacuum sys-
tem of 5 3 1027 Torr base pressure. A net forward sputtering power
of 300 W was applied on a Ru1 Si1 target 3 in. in diam, and the sam-
ples were centrally positioned 13 cm below the circular target. No
bias was applied, neither was the sample actively heated or cooled,
and the substrate temperature is estimated to stay below 1008C dur-
ing the deposition. Selected samples were patterned by drawing
stripes on the oxide surface with a conventional felt-tip pen and lift-
off with acetone after the deposition to measure the thickness of the
sputtered layers with a stylus profilometer. The samples were then
annealed in an open quartz-tube furnace and a dry oxygen flow of
80 cm3/min at 600 and 8008C. Elemental depth profiles were mea-
sured with 2.0 MeV 4He21 backscattering spectrometry, and atomic
composition were extracted from them with an overall accuracy
level of approximately 5%. Sheet resistance measurements were per-
formed with a four-point probe. Structural information was obtained
by Debye-Scherrer X-ray diffraction with a collimated Co Ka beam
incident on the sample at a glancing angle of 128 and an Inel posi-
tion-sensitive detector. Scanning electron micrographs were taken
on a Cambridge Stereoscan 250 Mk2 with a Kevex energy-disper-
sive spectrometer. The beam spot for energy-dispersive X-ray map-
ping has a diameter of 83 nm. To test whether some volatile rutheni-
um tetroxide (RuO4) evolves, an oxygen-saturated Ru-Si-O film was
sealed into a Pyrex test tube in ambient air and annealed at 6008C for
60 min. The test tube was subsequently positioned in a HP 8452A
diode array spectrophotometer in such a way that the beam traversed
the test tube walls, any deposits on them, and the enclosed gas with-
out seeing the sample that was lying on the bottom of the sealed test
tube. The absorption spectrum was measured before and after the
heat-treatment. 
Results
Partial flow rates of oxygen from 0 to 10% were employed for the
reactive sputtering of the Ru-Si-O thin films from the Ru1Si1 target.
Depending on the oxygen partial flow, sputtering rates between 130
and 250 Å/min are achieved. The atomic concentration of oxygen in
the film reaches a saturation value of about 65 atom % at a partial
flow rate of oxygen above 6%. This value is expected if RuO2 and
SiO2 are to be the only and stable binary reaction products of the
sputtering process. SiO2 is well known to favor the formation of
amorphous films. We observe that our films are X-ray amorphous
when the oxygen atomic composition nears saturation, above 55%. 
The samples that are used for further investigations in this study
are all oxygen-saturated. After deposition, their atomic density is
8.9 3 1022/cm3 (5.1 g/cm3), and their electrical resistivity is in the
range of 2 mV cm. Figure 1 shows the X-ray diffraction spectra of a
Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999) 1547
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.200 nm thick Ru20Si15O65 film before and after annealing in dry
oxygen at 6008C for 30 min. No diffraction peaks are observed from
the as-deposited sample, but clear peaks emerge after oxidation.
They can all be assigned to RuO2. Peaks of SiO2 are not detected. 
The annealing also dramatically changes the backscattering spec-
tra of the films. Figure 2 compares 2 MeV He21 backscattering spec-
tra of films annealed at 8008C for 5, 15, and 30 min with the spec-
trum of the as-deposited film. One interpretation of these spectra is
that ruthenium has diffused deeply into the substrate. Another is that
thick columnar features containing ruthenium have grown on or
through the surface. A decrease of the total counts in the ruthenium
signal upon annealing is indicated and leads to the suspicion that
some ruthenium might have been lost during the oxidation process.
Ruthenium tetroxide (RuO4) starts to volatilize noticeably14 at 78C,
has a very low melting point15 of 25.58C and a boiling point16 of
129.68C. If RuO4 were formed during the oxidation, it would be
volatile and lead to the loss of ruthenium from the film. 
We proved the formation of RuO4 by light absorption spectro-
scopy. Small samples were sealed under ambient air in a Pyrex
(borosilicate glass) test tube, and the test tube was annealed at 6008C
Figure 1. Diffraction spectra of a 200 nm thick Ru20Si15O65 film deposited
on a 120 nm thick SiO2 covered Si wafer, taken with Co Ka X-rays before
and after oxidation at 6008C for 30 min.
Figure 2. 2.0 MeV 4 He21 backscattering spectra of a 200 nm thick
Ru20Si15O65 film deposited on a 120 nm thick SiO2-covered Si wafer, before
and after oxidation at 8008C for 5, 15, and 30 min.for 30 min. The amount of oxygen enclosed in the test tube was suf-
ficient to form RuO4 with all the ruthenium contained in the thin
film. Signs of material removal from the Ru-Si-O thin film are al-
ready seen visually by noticing that after annealing the walls of the
test tube are coated with a diffuse milky layer. Figure 3 shows the
absorption spectra, taken at room temperature, of the test tube before
and after annealing. A clear absorption peak can be seen after an-
nealing with its maximum around 290 nm. A main absorption max-
imum at 310 nm for RuO4 (oxidation state d0)17 has been published
by Wells et al.,18 while the reflectance spectrum for RuO2 (oxidation
state d4)17 is reported to exhibit a broad band from 430 to 700 nm,
centered at 585 nm. 19 Accounting for the width of the peak and the
resolution of the spectrometer, our result clearly proves the presence
of RuO4 on the walls of the test-tube and thus the loss of ruthenium
from the film during annealing in dry oxygen at 6008C. 
A scanning electron micrograph (SEM) of the surface of a
Ru20Si15O65 thin film, after oxidation at 6008C for 30 min, is shown
in Fig. 4. Predominantly columnar-shaped crystallites with dimen-
sions of about 5 mm in length and 1 mm in diameter can be observed,
sticking out of a flat surface and perhaps partly embedded in it. Sur-
face mapping by two-dimensional energy-dispersive analysis of in-
Figure 3. Light absorption spectra of the walls of a test tube that contained a
200 nm thick Ru20Si15O65 film deposited on a 120 nm thick SiO2-covered Si
wafer, obtained at room temperature before and after annealing at 6008C for
30 min.
Figure 4. Scanning electron micrograph of a 200 nm thick Ru20Si15O65 film
deposited on a 120 nm thick SiO2-covered Si wafer, after oxidation at 6008C
for 30 min.
1548 Journal of The Electrochemical Society, 146 (4) 1546-1548 (1999)
S0013-4651(98)07-010-4 CCC: $7.00  © The Electrochemical Society, Inc.duced X-rays reveals that all the ruthenium is concentrated in the
crystallites while silicon is found only in the surrounding matrix.
Oxygen can be observed everywhere. Together with the X-ray data,
these facts indicate that the crystallites are RuO2. The remainder is a
film of SiO2, possibly containing some residual ruthenium oxide.
The backscattering spectra of Fig. 2 are consistent with this picture.
SEMs of as-deposited films on the same scale as Fig. 4 are feature-
less. The electrical resistivity of that film is too high to be measur-
able by a four-point probe.
Discussion and Conclusion
The synthesis of amorphous ternary films of composition
Ru20Si15O65 is demonstrated and confirms the idea that the com-
bination of metallically conducting RuO2 with covalently bonded
SiO2 leads to the formation of amorphous ternary Ru-Si-O thin
films. Their electrical resistivity of about 2 mV cm (bulk resistivity
of RuO2: 35 mV cm) together with the X-ray amorphous structure
makes such films acceptable candidates for applications with mod-
erate temperature requirements. 
RuO2 is used for thin-21 and thick-film resistors,22,23 as well as
for coatings on electrodes.24,25 Interest has recently risen for RuO2
films as electrodes for ferroelectric barium-strontium-titanate
(BST),26,27 and lead-zirconate-titanate (PZT),28,29 thin films to make
nonvolatile memory elements. Films of RuO2 have also been con-
sidered for metallizations in integrated circuits.30-35 The values of
the heats of formation of the transition metal oxides are comparable
to those of the transition metal nitrides and indicate good thermal
stability.36 The surface morphology of RuO2 films is reported to re-
main smooth after rapid thermal annealing up to 9008C for 10 s in
argon, nitrogen, and oxygen.37
On the other hand, Green et al. report that RuO2 films dispropor-
tionate and decompose to ruthenium and volatile RuO4 upon anneal-
ing in vacuum at 9008C for 2 h, according to the reaction 2 RuO2 r
Ru 1 RuO4.34 Using the process RuO4 r RuO2 1 O2, Yuan et al.
synthesized RuO2 films from RuO4 by chemical vapor deposition.38
In our present situation, annealing of the Ru20Si15O65 films occurred
in an open system and a dry oxygen ambient at atmospheric pressure
where the supply of oxygen was unlimited. Rather than the decom-
position of RuO2 films to Ru and volatile RuO4, we observe the for-
mation of volatile RuO4 and growth of RuO2 crystallites, which is
the reverse of the process described by Yuan et al. The growth of
RuO2 particles in bulk glasses was studied previously by Nakano
et al.: starting with 10 vol % RuO2 grains of around 15 nm average
size dissolved in glass, extensive growth is observed only at 9508C
with a particle size of 60 nm after 1 h.39 In contrast, our oxygen-sat-
urated ternary Ru-Si-O thin films separate into RuO2 crystals and a
surrounding SiO2 matrix much more rapidly, accompanied by the
evolution of volatile RuO4. As indicated through the backscattering
spectrum, major changes are observable already after 5 min at
8008C. A reaction process based on the gaseous phases of oxygen
and RuO4 appears to be most likely. Atomic oxygen has been found
to etch RuO2 to form volatile RuO4 products.17 Other reaction
processes might also be involved, since the catalytic power of ruthe-
nium complexes is well known and used in many other fields such
as solar energy conversion,40 coordination chemistry,41,42 and bio-
chemical applications.43,44
The answers to the two questions posed initially are therefore
that (i) it is possible to obtain X-ray-amorphous Ru20Si15O65 alloys
by reactive sputtering of RuSi, but (ii) the alloys are unstable in an
oxidizing ambient, at a time/temperature exposure of 30 min at
6008C, or less. Our results from treatment of Ru20Si15O65 films in
oxidizing ambient do not preclude that the same films could be ther-
mally stable in an inert ambient or vacuum. This aspect would
deserve further attention.Acknowledgments
Dr. Martina Hüber established the connection with Dr. Harry
Gray, who suggested light absorption spectroscopy to identify RuO4.
The authors express their appreciation to them for helpful discus-
sions. Technical assistance from Rob Gorris is gratefully acknowl-
edged. Financial support for this study was provided by the Center
for Integrated Space Microsystems at JPL/NASA and is gratefully
acknowledged. 
California Institute of Technology assisted in meeting the publication
costs of this aricle.
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Instability of Amorphous Ru-Si-O Thin Films under Thermal Oxidation

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