Preparation of highly textured AlN films using Mo and Ti electrode
for integrated AlN-based film bulk acoustic resonators
Cheng-Hsien Chou1, Yung-Chen Lin1, Jin-Hua Huang1, Nyan Hwa Tai1 and I-Nan
Lin2
1
Department of Material Science and Engineering, National Tsing-Hua University,
Hsin-Chu 300, Taiwan, R. O. C.
2
Department of Physics, Tamkang University, Tamsui 251, Taiwan, R. O. C.
inanlin@mail.tku.edu.tw
Abstract:
Effect of buffer layer on the characteristics of the AlN thin films deposited on
SiNx/Si substrate was systematically examined. Among the buffer layers examined,
both Mo and Ti buffer layers can not only greatly enhance the (002) preferred
orientation of the films, but also improve the smoothness of the AlN films, whereas
the Al thin films contain large grains microstructure and resulting in rough surface
and wide distribution of (002) preferred orientation of the films. AlN thin films with
smooth surface with (r.m.s.< 6 nm) and narrow distribution of grains’ orientation
(rocking curve < 3.8°), which is suitable for fabricating the devices. A thin
film-bulk-acoustic wave-resonator with resonance frequency around 1.7 GHz was
fabricated from thus obtained AlN thin films.
Keywords: AlN, RF Sputtering, buffer layer, FBAR.
I. Introduction
Due to its high acoustic velocity and excellent thermal conductivity at room
temperature, AlN films are very promising material for the fabrication of acoustic
wave1-3 devices in the giga-hertz frequency range. Moreover, aluminum nitride (AlN)
possesses very attractive property such as high resistivity, high breakdown voltage
and highest acoustic velocity among all piezoelectric materials4-8. It has great
potential for developing a device operating at very high frequency. However,
deposition of high quality AlN on silicon-nitride substrate is difficult due to the
complicated interface reaction between substrates and AlN films. In this work, we
provide a solution for depositing high quality AlN on silicon-nitride substrate, which
is the usage of buffer layer for enhancing the growth quality of AlN on diamond films.
II. Experimental
Aluminum, Molybdenum or Titanium films were prepared by RF magnetron
sputtering of an metal target in a gas ratio of Ar=30 sccm. The AlN thin films were
reactive RF magnetron sputtered from Al target in a Ar/N2 gas mixture ( Ar/N2
=20sccm/10sccm ). The optimized parameters of growing high quality thin films were
systematically adjusted. The AlN thin films were deposited with a target power of 300
W, a pressure of 2 m torr and base pressure of 3×10-7 torr. The structure and
morphologies of these thin films were examined using X-ray diffractometer (Shimazu
6000) and scanning electron microscopy (Jeol, JSM 6300), respectively. The surface
roughness was measuring using atomic force microscopy (PARK).
III. Results and discussion
Directly deposition of AlN thin film on SiNx substrate is difficult. Randomly
oriented AlN is usually resulted, which can be ascribed to the large discrepancy in
lattice parameter of the substrate and films. Moreover, the adhesion of AlN thin films
directly deposited on SiNx surface is not sufficient to hold the AlN thin films, which
occasionally induced the pill-off or cracking of the AlN films as the films is thicker.
Fortunately utilization of buffer layer would markedly improve the texture
characteristics of the AlN thin films. Figure1(a) to 1 (c) illustrated that (002) AlN can
be obtained when a thin crystalline Al, Mo or Ti film was precoated on
SiNx/Si-substrates as buffer layer. The texture quality of thin films was analyzed by
rocking curve in X-ray diffraction. Figure 2 & inset shows that among the 3 buffer
layer used, the Ti-films results in best texture characteristics, i.e., narrowest rocking
curve, whereas the Al-films results in broadest distribution of (002) AlN grains the
rocking curve(θ=3.98) of Al-buffered AlN
is too large to be recorded.
Presumably, the main factor altering the texture characteristics of the (002) AlN
films is the granular structure of the buffer layer. SEM micrograph shown in Fig. 3(a)
and inset indicates that the RF-sputtered Al films contain grains about 200 nm, which
results in large grains (~400 nm) for the AlN films and large distribution in orientation
of the AlN grains. In contrast, Figs. 3(b) and 3(c) reveals that both the Mo and Ti thin
films contain very fine grains and very smooth surface. The AlN/Mo/SiNx possess
1
much smoother surface than the AlN/Ti/SiNx thin films, which is intimately related to
the smoother surface of the buffer layer, ( shown as inset of the corresponding
figures ). Nevertheless, both Mo and Ti films exhibit good buffering effect, which
results in AlN thin films with smooth surface (r.m.s.< 6 nm) and highly preferred
orientation (rocking-curve<3.8°).
It should be noted that the Al, Mo and Ti layer can also enhance the growth of
(002) textured AlN thin films, even when they are of amorphous form (not shown).
The amorphous buffer layer behavior similarly with the crystalline are, that is, the Al
amorphous layer results in smallest intensity with widest width for the AlN XRD
peaks, whereas the Ti amorphous results in largest intensity and narrowest width for
the AlN thin XRD peaks. Both th texture characteristics and surface smoothness of the
AlN thin films grown on amorphous buffer layer are slightly inferior to those of the
AlN thin films grown on crystalline buffer layer. These results imply that it is the
nature of chemical reaction rather than the lattice matching of the films and buffer
layer, which determine the texture characteristics of the AlN thin films.
The stoichiometry ratio of the AlN thin films, which is another important factor
influencing the piezoelectric properties of the films, vary with nitrogen-content in the
sputtering atmosphere. The Rutherford-Back-Scattering (RBS) measurements shown
in Fig. 4 indicate that the nitrogen-content in the films increases with the N2/(N2+Ar)
ratio in the sputtering atmosphere and is stabilized for N2/(N2+Ar)>50%, which
results in a film with Al/N ratio closer to stoichiometry, i.e., Al:N=51.5:45.5 mol%.
A AlN thin film bulk acoustic wave resonator (FBAR) was then fabricated out of
the AlN/Mo/SiNx and AlN/Ti/SiNx thin films using bulk micromachining process.
Figures 6(a) and 6(b) show the plane view and cross-sectional view of a resonator,
whereas Fig. 6(c) reveals the frequency response of the resonator with 60 m x 60 m
top & bottom electrodes. This figure indicates that both AlN/Mo/SiNx and
AlN/Ti/SiNx thin films exhibit good resonance characteristics at around 1.7 GHz,
when the thickness of the films are 200 nm for top electrode (Mo), 1m for AlN thin
films 200 nm for bottom electrodes and 1m for SiNx supports. The
electromechanical coupling factor estimated from Fig.6(c) using model, i.e., is kt2~
[(π/2)/ (fsfp)]/tan[(π/2)/ (fsfp)], 4.0~6.0% for these Mo(or Ti) buffered AlN thin
films.
2
IV. Conclusion
Effect of buffer layer on the characteristics of the AlN thin films deposited on
SiNx/Si substrate was systematically examined. Among the buffer layer examined,
both Mo and Ti buffer layers can greatly enhance the (002) preferred orientation of
the films and improve the smoothness of the AlN films, which is ascribed to the
smoother surface for these metallic buffer layer. AlN thin films with smooth surface
with (r.m.s.< 6 nm) and narrow distribution of grains’ orientation (rocking curve <
3.8°), which is suitable for fabricating the thin film-bulk-acoustic wave-resonator
(FBAR), was obtained. A FBAR with resonance frequency around 1.7 GHz was
demonstrated.
V. Acknowledgment
The authors would like to thank the financial support from National Science
Council through the project No. NSC 93-2112-M-032-010.
VI. References
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Figure caption
Fig. 1 X-ray diffraction of AlN thin films deposited on SiNx/Si substrates using (a) Al,
(b) Mo, and (c) Ti as buffer layer.
Fig. 2 Rocking curve of the AlN thin films deposited on SiNx/Si substrates using Al,
Mo and Ti as buffer layer.( the inset shows the variation of full width at half
maximum of XRD with buffer layer )
Fig. 3 SEM microstructure of (a) AlN/Al/SiNx (b) AlN/Mo/SiNx and (c) AlN/Ti/SiNx
thin films ; the insets show the corresponding microstructure of the buffer layer.
Fig. 4 Rutherford-Back-Scattering measurements show the variation of composition
of AlN thin films with N2/(Ar+N2) ratio in RF-sputtering process.
Fig. 5 (a) Planview (b)Cross-sectional view of the FBAR structure, and (c) the
frequency response of the FBAR device made of Mo (200 nm) /AlN (1m)/Mo (200
nm)/SiNx (1m)
4
AlN
(a)
T
Intensity (a.u.)
AlN/Ti/SiNx
AlN: AlN (002)
T: Ti (002)
S: Si (100)
M: Mo (110)
A: Al (111)
Ti/SiNx
M
(b)
S
AlN
AlN/Mo/SiNx
Mo/SiNx
(c)
S
AlN
A
AlN/Al/SiNx
Al/SiNx
30
Fig. 1
35
40
45
50
2 theta
C. H. Chou et. al.
5
55
60
6000
Rocking Curve
0.40
5000
FWHM of AlN(002) on diffenent electrode
Intensity (a.u.)
0
FWHM ( )
0.39
4000
AlN/Ti/SiNx
0.38
0.37
0.36
3000
AlN/Ti/SiNx
AlN/Mo/SiNx
2000
3.98
6.63
1000
AlN/Mo/SiNx
AlN/Al
0
0
0
10
15
20
theta
Fig. 2
C. H. Chou et. al.
6
25
30
Fig. 3
C. H. Chou et. al.
7
60
Al
Atomic Concentration (%)
55
50
45
N
40
5.0
O
2.5
0.0
33.3%
50%
nitrogen/(nitrogen+argon) (%)
Fig. 4 C. H. Chou et. al.
8
66.6%
Fig. 5
C. H. Chou et. al.
9