Effect of deposition temperature on surface acoustic wave

Effect of deposition temperature on surface acoustic wave
JOURNAL OF APPLIED PHYSICS 98, 096102 共2005兲
Effect of deposition temperature on surface acoustic wave velocity
of aluminum nitride films determined by Brillouin spectroscopy
M. B. Assouar
Laboratoire de Physique des Milieux Ionisés et Applications, UMR 7040 CNRS, Université
Henri-Poincaré Nancy 1, Bld. Des Aiguilletes-BP 239, F-54506 Vandoeuvre-lès-Nancy Cédex, France
R. J. Jiménez Riobóo and M. Vila
Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, E-28049 Madrid, Spain
P. Alnot
Laboratoire de Physique des Milieux Ionisés et Applications, UMR 7040 CNRS, Université
Henri-Poincaré Nancy 1, Bld. Des Aiguilletes-BP 239, F-54506 Vandoeuvre-lès-Nancy Cédex, France
共Received 6 June 2005; accepted 23 September 2005; published online 3 November 2005兲
Brillouin spectroscopy has been used to study the effect of the deposition temperature on the surface
acoustic wave 共SAW兲 propagation velocity of aluminum nitride 共AlN兲 films. The results show a
dependence of the SAW propagation velocity on the growth temperature of AlN films. The highest
value of acoustic velocity was obtained for the film elaborated without heating. Structural
characterization of the AlN films synthesized at various deposition temperatures was carried out by
x-ray diffraction. These analyses pointed out that the deposition temperature influences the standard
deviation of 共002兲 AlN film preferred orientation. The growth temperature clearly influences the
acoustical and crystalline properties of AlN thin films. © 2005 American Institute of Physics.
关DOI: 10.1063/1.2121927兴
Aluminum nitride 共AlN兲 has been considered as an attractive thin film piezoelectric material for integrated circuit
共IC兲 compatible surface acoustic wave 共SAW兲 devices. This
compatibility requires a deposition process at relatively low
temperatures.1 Then, the study of acoustical properties of
AlN films as a function of deposition temperature becomes
crucial to develop AlN films of high crystallinity and acoustical quality. High resolution Brillouin spectroscopy 共HRBS兲
was used in order to obtain information about the surface
acoustic waves propagation velocity in AlN films elaborated
at different growth temperatures. Even though there are some
HRBS works on AlN elastic properties,2–7 the study of the
SAWs properties is still a rarity8 and no works about the
determination of temperature effects on SAW propagation
velocity by Brillouin spectroscopy were carried out.
In this work, c-axis oriented aluminum nitride thin films
on 共100兲 silicon substrates were deposited by the reactive rf
magnetron sputtering technique at various substrate holder
temperatures 关without heating 共WH兲 up to 400 °C兴 with the
same thickness 共1.3 ␮m兲. The aluminum target 共purity
99.99%兲 diameter was 107 mm and 6.35 mm thick. The
deposition chamber was pumped down to a base pressure of
1 ⫻ 10−7 mbar by a turbomolecular pump prior to the introduction of the argon-nitrogen gas mixture for AlN thin film
production. The gas discharge mixture was Ar/ N2 and the
total pressure was kept constant at 5 ⫻ 10−3 mbar. The nitrogen percentage in the Ar/ N2 gas mixture was 60% and the rf
power delivered by the rf generator was 170 W. In order to
perform a better comparison between the different samples,
the deposition time of AlN films was adjusted to obtain
1.3 ␮m thick films for various deposition temperatures.
In order to study the growth thermal conditions on the
SAW propagation velocity 共from now on SAW velocity兲,
0021-8979/2005/98共9兲/096102/3/$22.50
HRBS was the experimental technique chosen. The experimental set up was already described elsewhere.9 It can be
summarized as follows: The light source was a 2060 Beamlok Spectra Physics Ar+ ion laser provided with an intracavity temperature stabilized single-mode and single-frequency
z-lok étalon 共␭0 = 514.5 nm兲. The scattered light was analyzed using a Sandercock-type 3 + 3 tandem Fabry-Pérot
interferometer.10 The incident polarization direction was chosen to be in -plane 共p polarization兲 while no polarization
analysis of the scattered light was made. The typical values
for finesse and contrast were 150 and 109, respectively.
AlN films are very transparent materials thus making
extremely difficult to obtain information of SAWs velocity
by means of HRBS. A successful way to enhance the ripple
scattering mechanism11 is to deposit a very thin metallic film
on the transparent sample.8,12–14 It has been shown that the
thin metallic film will reproduce the main features of the
SAWs of the transparent material, as in the case of AlN and
synthetic diamond.8,14 A 40 nm thin Al film was deposited on
each of the different AlN film samples via dc magnetron
sputtering. The Brillouin spectroscopy was made for all the
samples at backscattering with a sagittal angle of 55°. As far
as we were only interested in the temperature evolution of
the surface acoustic wave velocity, we only needed to fix one
acoustic wave vector and follow its changes. In this case
kh = 0.8 共k is the scattering wave vector and h the thickness
of the Al thin film兲. All the recorded spectra are of similar
quality and the inset in Fig. 1 shows the typical spectrum for
a growth temperature of 250 °C, where the Rayleigh mode
共SRM兲 and a higher order Sezawa mode 共PSM兲 can be seen.
The SAW propagation velocity can be obtained straightforward from the Brillouin frequency shift 共f兲:
98, 096102-1
© 2005 American Institute of Physics
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096102-2
J. Appl. Phys. 98, 096102 共2005兲
Assouar et al.
FIG. 1. Growth temperature dependence of the sound propagation velocity
of the surface Rayleigh mode. Inset: Brillouin scattering spectrum for the
sample growth at 250 °C. The surface Rayleigh mode 共SRM兲 at 15.5 GHz
共VSAW = 4867 m / s兲 and the Sezawa mode at 22.4 GHz 共VSAW = 7037 m / s兲
are clearly seen.
VSAW =
2␲ f
f␭0
.
=
qSAW 2 sin共␣兲
共1兲
␭0 is the laser wavelength in vacuum, qSAW the scattering
wave vector, and ␣ is the scattering angle 共in this case 55°兲.
The final results for the different growth temperatures is
graphically shown in Fig. 1. The tendency is very clear. The
higher the growth temperature the lower the SAW propagation velocity. Only at temperatures near room temperature
FIG. 3. 共a兲 XRD pattern of AlN films deposited at various growth temperatures. 共b兲 AlN films ␻ rocking curve evolution with growth temperature. The
inset shows the rocking curve of AlN film elaborated without heating.
FIG. 2. kh dependence of the propagation velocity of the surface acoustic
modes of the Al thin film on AlN film. The AlN elastic constants used for the
simulations were those from Ref. 8 and the Al ones were as follows: c11
= 110 GPa; c44 = 27 GPa; c12 = 56.5 GPa. The line marks the kh = 0.8 value.
The white circles and the dark squares are the values obtained for samples
growth without heating and at 400 °C, respectively. The corresponding
sound velocities are VSAW = 4985 m / s for the Rayleigh mode and VSAW
= 7109 m / s for the Sezawa mode in the WH case and are VSAW
= 4701 m / s for the Rayleigh mode and VSAW = 6782 m / s for the Sezawa
mode in the 400 °C case. In the case of the WH AlN film the Rayleigh mode
shows a very slight difference of about 1.5% with respect to the value
expected in the simulation.
the results do not show significant changes. This differences
in sound velocity cannot be due to a possible difference in Al
thin film thickness. All the samples were prepared separately
and only stochastic distribution of thickness should be expected. The evolution shown in Fig. 1 clearly discards this
effect.
In order to assess the quality of the without heating
共WH兲 sample, a comparison between the SAW velocities obtained by a simulation program15 and the experimental results for the two extreme temperature samples has been made
and is shown in Fig. 2. From the point of view of the elastic
properties the AlN film obtained without heating is of very
high quality.
In order to study the origin of the difference in elastic
properties, x-ray diffraction 共XRD兲 using Cu K␣ cathode
was employed to determine the crystalline properties of the
AlN thin films. The diffracted intensities were collected in
␪-2␪ scan and ␻ rocking curve scan modes. Figure 3共a兲
shows the XRD patterns of these films deposited on Si共100兲
substrates. We can observe that the preferred orientation of
all the films is the 共002兲 orientation corresponding to the c
axis perpendicular to the surface. The XRD peak intensities
are the same for all the films, which indicate that the growth
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096102-3
J. Appl. Phys. 98, 096102 共2005兲
Assouar et al.
In conclusion, the substrate holder of the rf reactive
magnetron sputtering facility has been warmed up to different temperatures in order to study the influence on the elastic
properties of the growth temperature. X-ray texture studies
performed on these samples show clearly a decrease of the
crystalline quality with increasing growth temperature. The
SAW propagation velocity shows a clear temperature dependence with the high value for the film synthesized without
heating, indicating the influence of the film texture on the
SAW properties of the samples. A SAW device based on the
without heating AlN film was performed and exhibited excellent performances.
FIG. 4. Frequency response of AlN/ Si structure SAW device based on the
WH growth film.
temperature does not influence the preferred orientation.
However, from the ␻ rocking curve scan mode, we have
found that the FWHM of the rocking curve of all the AlN
films varies from 1.56° to 2.8°, with the minimum value for
the AlN film synthesized without heating. Figure 3共b兲 exhibits this evolution and as an inset a typical rocking curve
obtained for this film. This evolution of crystalline orientation quality can be explained by the possibility of formation
of defects in the films at high temperature by promoting the
diffusion of impurities into the films introducing structural
disorder.16 We deduce from these observations that the AlN
film synthesized without heating, presents the better crystalline texture. It is obvious that the structural evolution shown
by the x-ray analyses should influence the acoustic properties
of the samples. An increase of the disorder would imply a
decrease in the elastic properties of the material, i.e., the
sound propagation velocity, as has been clearly seen in the
experimental data of Fig. 1.
The final goal of the AlN films is their application in
SAW based devices.17–19 To assess the suitability of these
films, the one growth without heating was prepared as a
SAW device. The SAW transducers were fabricated on the
AlN film surface by forming an array of aluminum interdigitated electrodes 共IDTs兲. The thickness of the aluminum layer
was 150 nm. It was deposited, patterned, and etched using
the UV lithography technique and wet etching. The individual finger widths were 8 ␮m and spacings were 4 ␮m.
The wavelength of the generated SAW is then 24 ␮m. Figure
4 shows the frequency response of the AlN/ Si structure
SAW device realized using this film. The represented response exhibits the fundamental harmonic of AlN/ Si SAW
device, which have a resonance frequency around 210.75
MHz. This frequency corresponds, taking into account the
wavelength value of 24 ␮m, to a SAW velocity of 5058 m / s.
This result remarks the good crystalline quality of the AlN
thin film elaborated without heating and it shows a concordance with the acoustic quality determined by Brillouin
spectroscopy.
This work was partially supported by the Spanish Ministery of Education and Science 共Project No. MAT-200301880兲, the autonomous community of Madrid 共Project No.
07N/0077/2002兲, and by a bilateral cooperation between
CNRS and CSIC within the frame of the Picasso program
共common project Assouar/Jiménez兲 共France兲 and Acción Integrada 2004FR0008 共Spain兲.
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