Spectroscopic ellipsometry characterization of the optical properties and thermal stability of Zr O2 films made by ion-beam assisted deposition

Article abstract of DOI:10.1063/1.2811955

The optical properties, interface structure, and thermal stability of the Zr O2 films grown on Si(100) were investigated in detail. A 2 nm thick interfacial layer (IL) is formed at the Zr O2 -Si interface for the as-grown Zr O2. The optical constants of Z

Full text of DOI:10.1063/1.2811955

Spectroscopic ellipsometry characterization of the optical properties and thermal
stability of Zr O 2 films made by ion-beam assisted deposition
C. V. Ramana, S. Utsunomiya, R. C. Ewing, U. Becker, V. V. Atuchin, V. Sh. Aliev, and V. N. Kruchinin
Citation: Applied Physics Letters 92, 011917 (2008); doi: 10.1063/1.2811955
View online: http://dx.doi.org/10.1063/1.2811955
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APPLIED PHYSICS LETTERS 92, 011917 ͑2008͒
Spectroscopic ellipsometry characterization of the optical properties
and thermal stability of ZrO2 films made by ion-beam assisted deposition
C. V. Ramanaa͒
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA and
Department of Metallurgical and Materials Engineering, University of Texas, El Paso, Texas 79968, USA
S. Utsunomiya, R. C. Ewing, and U. Becker
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
V. V. Atuchin and V. Sh. Aliev
Institute of Semiconductor Physics, SB RAS, Novosibirsk 90, 630090, Russia
V. N. Kruchinin
Institute of Catalysis, SB RAS, Novosibirsk 90, 630090, Russia
͑Received 7 August 2007; accepted 23 October 2007; published online 7 January 2008͒
The optical properties, interface structure, and thermal stability of the ZrO2 films grown on Si͑100͒
were investigated in detail. A 2 nm thick interfacial layer ͑IL͒ is formed at the ZrO2–Si interface for
the as-grown ZrO2. The optical constants of ZrO2 films and IL correspond to amorphous-ZrO2 and
amorphous-SiO2, respectively. The oxidation and IL growth at 900 °C, as a function of annealing
time, exhibit a two-step behavior with a slow and a fast growth-rate zones. The transition from a
zone of slow to fast rate is attributed to structurally modified ZrO2 facilitating the faster oxygen
transport to the ZrO2/Si interface. © 2008 American Institute of Physics.
͓DOI: 10.1063/1.2811955͔
Zirconia ͑ZrO2͒ is an important material with a potential
for a wide range of technological applications.1–12 The
outstanding chemical stability, electrical and mechanical
properties, high dielectric constant, and wide band gap of
ZrO2 make it suitable for several industrial applications in
the field of electronics, magnetoelectronics, and
optoelectronics.1–4,6–12 ZrO2 is employed in superplastic
structural ceramics that demonstrate excellent strength and
fracture toughness.5 Zirconia is also considered to be an ex-
cellent nuclear waste form for the direct geological disposal.5
Most importantly, ZrO2 is considered as a promising dielec-
tric to replace SiO2 in advanced metal oxide semiconductor
devices in gate stack.7–11 However, it is well known that the
electrical and optical properties of ZrO2 thin films are highly
dependent on the film-substrate interface structure, morphol-
ogy, and chemistry, which are in turn controlled by the film-
fabrication technique, growth conditions, and postdeposition
processes.1–4,6–15
The chemical reaction between ZrO2 and Si substrate
during fabrication or postdeposition annealing process result-
ing in the formation of interfacial layer ͑IL͒, with silicon
oxide ͑SiO2͒, or metallic Zr clusters, or Zr-silicate ͑ZrSixOy͒,
or a combination of these, has been found in most of the
earlier investigations, although the proposed mechanism var-
ies greatly.6–11,13–15 Interfacial chemical compounds from the
interfacial reaction suppress the effective dielectric constant
and degrade the device performances. As such, elucidation of
the ultramicrostructure, and electronic properties of ZrO2
films has recently been the subject of numerous investiga-
tions. However, the existing data and reports on the optical
properties of ZrO2 films are meager. In this context, the
present investigation has been performed to understand the
optical properties of ZrO2 films grown on n-type Si͑100͒
substrates using ion-beam assisted sputter deposition. Spec-
troscopic ellipsometry ͑SE͒, which is known to be a sensitive
and nondestructive method for the structure and optical char-
acterization of thin-films,16,17 has been employed in combi-
nation with the reflection high energy electron diffraction
͑RHEED͒ and high-resolution transmission electron micros-
copy ͑HRTEM͒ to study the optical properties and thermal
stability of the ZrO2/Si system. The qualitative and quanti-
tative information, based on the SE and HRTEM results, on
the thermal stability, interfacial oxide layers, and optical
properties of ZrO2 films grown on Si is reported in this letter.
The ZrO2 films were deposited on n-type single-
crystalline Si͑100͒ substrates using an ARC-12M sputtering
device using Zr metal as a target. The Si substrate was sub-
jected to chemical cleaning procedure and treatment in di-
luted HF to remove the native oxide layer on the surface. An
argon ͑Ar͒/oxygen ͑O2͒ mixture was introduced into the
vacuum chamber, which was initially pumped down to a
pressure of 10−6 Torr, during film deposition. The pressure in
the chamber was kept at 10−3 Torr during deposition, and the
films were made at a low substrate temperature ͑TS͒ of
70 °C. TEM measurements were performed on the cross-
sectional ZrO2/Si specimens using a JEOL JEM2010F at an
accelerating voltage of 200 kV. RHEED measurements were
performed using an EFZ4 device ͑Carl Zeiss͒ on the as-
grown and finally annealed ZrO2 samples to assess their sur-
face structure. The technical details of HRTEM, RHEED,
and cross-sectional sample preparation procedures are de-
scribed elsewhere.18–20 SE measurements were made on
ZrO2 films using a Spectroscan ellipsometer in the spectral
range of 250 nmϽ␭Ͻ900 nm at an incidence angle of 70°.
Annealing of the ZrO2/Si samples was performed in air us-
ing a conventional furnace. Two sets of annealing experi-
ments were performed: ͑a͒ in the temperature range of
600–900 °C at a constant annealing time of 1 h and ͑b͒
a͒
Author to whom correspondence should be addressed. Tel.: 734-763-5344.
FAX: 734-763-4690. Electronic mail: ramanacv@umich.edu.
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0003-6951/2008/92͑1͒/011917/3/$23.00 92, 011917-1 © 2008 American Institute of Physics
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011917-2
Ramana et al.
Appl. Phys. Lett. 92, 011917 ͑2008͒
FIG. 1. HRTEM image of ZrO2 films grown on Si͑100͒ substrate. The
formation of an amorphous interfacial layer is evident in the micrograph.
The estimated thickness of the amorphous interface layer is about 2 nm.
high-temperature annealing at 900 °C as a function of an-
nealing time.
FIG. 3. ͑Color online͒ The spectral dependence of the optical constants, n
and k, of ZrO2 films derived from SE data. The curves shown indicate the
corresponding phase as amorphous ZrO2. Inset shows the dispersion of re-
fractive index obtained for the IL, which matches with that of amorphous
SiO2.
The RHEED analysis indicates the amorphous nature of
as-grown ZrO2 films. The HRTEM micrograph of the
ZrO2/Si cross-sectional structure is shown in Fig. 1. The
existence of an IL between the ZrO2 and Si is evident in the
HRTEM image, which indicates that the reaction between
the Si substrate and growing ZrO2 film readily takes place
even without a high-TS. HRTEM results indicate the amor-
phous nature of the IL, which is about 2 nm thick ͑Fig. 1͒.
The chemical analyses ͑not shown͒ indicate that the grown
ZrO2 layers are stoichiometric with all of the Zt ions in the
Zr4+ state.19,20
Optical properties and interfacial oxide growth behavior
of ZrO2/Si system have been primarily probed by SE, which
measures the relative changes in the amplitude and phase of
the linearly polarized monochromatic incident light upon ob-
lique reflection from the film surface.16,17 The experimental
parameters obtained by SE are the angles ⌿ ͑azimuth͒ and ⌬
͑phase change͒, which are related to the microstructure and
optical properties, defined by
temperature annealing at 900 °C along with those for films
without annealing. Two important observations derived from
the spectra are ͑a͒ the evolution of the ellipsometric func-
tions ⌿͑␭͒ and ⌬͑␭͒ is remarkably different for the as-
deposited and annealed ZrO2 films, and ͑b͒ the thermal an-
nealing of ZrO2 films at 900 °C results in a noticeable
spectral shift of ⌿͑␭͒ and ⌬͑␭͒ with increasing time of an-
nealing. The dispersion relations of the optical constants, i.e.,
n and k, of as-grown ZrO2 films are derived using a simple
model based on a double-layer system consisting of ͑air͒/
͑isotropic homogeneous layer͒/͑Si substrate͒. The standard
models17,21 employed for amorphous dielectrics have been
used for the determination of n͑␭͒ and k͑␭͒. The spectral
dependences of n and k calculated for ZrO2 films is shown in
Fig. 3. The n͑␭͒ and k͑␭͒ dispersion curves are in good
agreement with those reported for amorphous ZrO2.21 The
extinction coefficient ͑k͒ being low and very close to zero
indicates very small optical loss due to absorption. The dis-
persion relation of n͑␭͒ evaluated for the IL is shown as an
inset in Fig. 3. The n͑␭͒ vs ␭ curve is very similar to that of
amorphous SiO2,22 thus indirectly confirming the SiO2-based
chemical nature of the IL. SE is very sensitive to the IL
formation and the simple model as described for the as-
grown ZrO2/Si structure was not suitable to fit the experi-
mental results in the case of annealed samples. Thus, a more
complicated two-layer model is adopted not only to take the
growing ILs into consideration. The system consists of an
array ͑air͒/͑first isotropic homogeneous layer with thickness
L1͒/͑second isotropic homogeneous layer with thickness
L2͒/͑Si substrate͒, whereby thicknesses L1 and L2 are varied
during fitting. Excellent fitting of ⌿͑␭͒ and ⌬͑␭͒ is obtained
for the annealed ZrO2 films using the model adopted. The
values of L1 and L2 are presented in Fig. 4 along with the
representation of the model adopted. The data are presented
as a function of the square root of the annealing time to
indicate the two-step behavior of the thermal oxidation pro-
cess noticed in the present work. A continuous increase in L2
adjacent to Si substrate with annealing time is due to the
oxidation of the Si substrate by oxygen diffused from air
through the ZrO2 layer. Thus, this IL, which is L2 in SE
␳ = Rp/Rs = tan ⌿ exp͑i⌬͒,
͑1͒
where Rp and Rs are the complex reflection coefficients of
the light polarized parallel and perpendicular to the plane of
incidence, respectively. The spectral dependencies of ⌿ and
⌬ can be fitted with appropriate models to extract the film
thickness and the optical constants, i.e., the refractive index
͑n͒ and extinction coefficient ͑k͒, based on the best fit be-
tween the experimental and simulated spectra.17 The spectral
dependencies of ⌿ and ⌬ determined for ZrO2 films on Si
are shown in Fig. 2 as a function of time of the high-
FIG. 2. ͑Color online͒ The spectral dependences of ⌿ and ⌬ for the as-
grown and annealed ZrO2 films. A significant shift of ⌿ and ⌬ of the
annealed ZrO films when compared to that of the as-grown ZrO2 films is
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evident, which indicates the modification of the ZrO2 /Si interface structure.
model notations, is mostly SiO2 in chemical composition,
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011917-3
Ramana et al.
Appl. Phys. Lett. 92, 011917 ͑2008͒
more, Busch et al. reported the fast interfacial oxidation at
the ZrO2/Si based on oxygen diffusion at the crystalline
boundary.23 A higher oxygen diffusivity ͑103–104 times
faster͒ was also reported for polycrystalline ZrO2 than that in
the bulk crystal. We, therefore, conclude that thermally in-
duced structural modification of the ZrO2 film from amor-
phous to crystalline state causes the transition from slow to
fast interfacial oxidation zone.24
Summarizing the results, ZrO2 films were fabricated on
Si͑100͒ substrates using ion-beam assisted deposition, and
their surface/interface structure and optical properties were
investigated. HRTEM analysis reveals a 2 nm thick IL with a
refractive index profile similar to that of amorphous SiO2 as
determined from SE. The as-deposited ZrO2 films exhibit
profiles of optical constants corresponding to the amorphous-
ZrO2 phase. A two-step behavior is established for the high-
temperature annealing of ZrO2 at 900 °C as a function of
annealing time. The slow-rate growth of IL in zone I is due
to oxygen transport in amorphous ZrO2. The faster-rate
growth in zone II is attributed to the transition from disor-
dered to ordered state of ZrO2. Crystallization and grain
boundary formation in ZrO2 enhance oxygen diffusion
through the layer to the Si substrate and, hence, the oxidation
leading to the interfacial oxide layer at the ZrO2/Si interface.
FIG. 4. ͑Color online͒ The quantitative information obtained on the ZrO2
layers and interfacial SiO2 layer as a function of time of annealing at
900 °C. Linear fits to the data are shown with solid lines. The slow rate of
the IL with almost no change in ZrO2 film thickness in zone I is attributed to
the oxygen diffusion alone. The fast-rate growth rate of the IL along with a
slight decrease in ZrO2 film thickness observed in the zone II is attributed to
the oxygen diffusion coupled with structural transformation of ZrO2 from
amorphous to crystalline state.
but possible Zr-doping cannot be excluded. The growth rate
of IL is controlled by three elemental stages, namely, adsorp-
tion of oxygen on the top surface of ZrO2, diffusion through
the ZrO2 layer, and diffusion through the SiO2 layer to the
oxidation front at the interface SiO2/Si.
The authors at the University of Michigan acknowledge
the support of the National Science Foundation ͑NSF-NIRT,
EAR-0403732͒.
A model can be formulated to explain the observed ef-
fects of high-temperature annealing, stability, and growing
ILs of the ZrO2/Si system. The as-deposited ZrO2 layer is
completely amorphous as confirmed by RHEED and HR-
TEM observations. The IL thickness increases, with the time
of annealing, which is due to the oxidation of the ZrO2/Si
interface. Oxygen needed for this process is provided by the
interaction of the topmost surface layers with air at high
temperature. Subsequently, the diffusion of oxygen through
ZrO2 layer causes further oxidation of the Si substrate layers
into SiO2 or a mixed composition of ZrSixOy. The interesting
point is that the IL oxide growth as a function of annealing
time shows a two-step behavior ͑Fig. 4͒ with a clear distinc-
tion of two regions, which are labeled zone I and zone II.
The growth rate is rather slow in zone I, where the oxygen
transport is, perhaps, controlled by a slow-diffusion process
through the relatively thick amorphous ZrO2 layer at the sur-
face. Continued annealing for more than 3 h shows a transi-
tion of the oxidation behavior into zone II, where the growth
rate of the IL is fast. A slight decrease in thickness of the
ZrO2 layer is noticed corresponding to the fast-rate zone II.
This observation indicates the crystallization of the amor-
phous ZrO2 layer with increasing annealing time. Crystalli-
zation of amorphous ZrO2 results in a drastic shrinkage of
the film volume because of intensive crystal grain formation
with dense atomic packing and structural ordering. As a re-
sult, the interstitial diffusion mechanism of oxygen in ZrO2
becomes less important and oxygen transport is mainly taken
over by high-rate crystal grain boundary diffusion that facili-
tates the faster oxygen transportation to SiO2/Si interface.
Therefore, crystallization of ZrO2 with formation of grain
boundaries is mainly responsible for initiating zone II stage
of the oxidation process. Crystallization of ZrO2 at
700–900 °C as reported in several earlier studies13–15 and
confirmed by the RHEED pattern in the present work pro-
vides evidence that the change in oxidation behavior is due
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