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„Thin Film Electroacoustic Devices“
Klausurtagung in Pappenheim, 16. – 19. Feb. 2004, Vortrag von Matthias Bickermann
the only means to extract energy out of the medium is dielectric resonance.
1. Introduction
Mobile communication applications have boosted
the development of frequency control devices in
the GHz range. Basic passive devices in this area
are SAW and BAW devices used as narrow band
pass filters for frequency selection. In those devices, acoustic waves are excited by an electrical
field, and only waves resonant to the device are
sensed. There are two major concepts to increase
the filter frequency: To further reduce device structure or to employ novel thin-film materials and
acoustic wave modes for higher wave velocities.
Both concepts rely on advances in nanotechnology.
2. Basic Principles
In a piezoelectric medium, the stress T and the
electric displacement D depend linearly on both
the strain S and the electric field E as:
(1)
Tij = cijkl Skl + ekij Ek
(2)
Dj = eijk Sjk + εij Ej
Devices as shown in Fig. 1a are used as narrow
band pass filters. If supplied with a frequency mix,
only the resonant frequency will pass. The resonance is given by the acoustic wave velocity v and
the thickness of the piezoelectric material h. Very
homogeneous thickness and high material quality
(single-crystalline material or textured needles) are
required to obtain good electromechanical coupling kh defined as
(4)
kh = K2 / (1+K2)
The acoustic waves traveling through the device
are referred to as Bulk Acoustic Waves (BAW).
Novel Thin Film Bulk Acoustic Resonators (FBAR)
can also be integrated on standard IC technologies. A pioneer of discrete FBAR devices is Agilent
Technologies, but also Epcos in Germany has
started mass production of those devices recently.
h
where εij, ekij and cijkl are the permittivity, piezoelectric, and stiffness tensors, respectively. In the static
case these equations describe mechanical displacement in the material as a result of application
of a static electrical field. Applying an electromagnetic wave to the material, however, results in the
formation of an acoustic wave having a phase
velocity of
V
in
z
a)
electrodes,
V
y
piezoelectric film
in
h
(3)
v=
(
c
1+ K2
ρ
)
with K =
e2
ε cE
where K2 is called piezoelectric coupling constant
(ρ is the material density). Eq. 3 holds also for any
non-piezoelectric material, but K = 0 in this case.
Note that in piezoelectric materials, v depends on
both material tensors and the direction of propagation of the acoustic wave. In the following we
consider two types of excitations by which acoustic waves can be generated.
3. Piezoacoustic excitation
a) Thickness excitation
If an externally generated electrical field is applied
to a device shown in Fig. 1a, the acoustic wave
and the electric field both will propagate in zdirection. As the electromagnetic wave travels
much faster (about 108 m/s) than the acoustic
wave (103...104 m/s), the piezoelectrically coupled
electric field is assumed to be quasistatic. Hence
the electric displacement vector Dz vanishes, and
b)
d
Fig. 1. a) device operating in thickness excitation mode.
b) device operating in lateral field excitation mode.
b) Lateral field excitation
In this excitation mode, the electric field is in the
plane perpendicular to acoustic propagation as
shown in Fig. 1b. An electric field can excite an
acoustic wave normal to its direction because the
piezoelectric matrix e couples electrical and mechanical fields of different orientations. The internally generated electric displacement vector D is
parallel to the externally generated electric field
vector E, but propagates at acoustic velocity. Lateral field excitation can only exist if the distance
between the electrodes d is much larger than the
thickness of the piezoelectric material h. The resulting wave is called Surface Acoustic Wave
(SAW). The efficiency of such devices is characterized by the piezoelectric coupling constant K2 as
defined in Eq. (3).
Note that SAW stands for a whole zoo of acoustic
wave modes which can be generated by lateral
field excitation. Apart from the "true SAW" Rayleigh
mode described above, higher-order modes
(Sezawa modes) and pseudo-SAWs (bulk modes
which are confined near the surface) can occur.
Their velocities often exceed the velocity of the
Rayleigh mode wave. Excitation of such modes
has been extensively investigated in recent years.
is lower than 2λ, the wave penetrates into the substrate and dispersion rules apply.
4. Thin-film SAW device designs
Fig. 2 shows a sample SAW filter design. The SAW
is generated and sensed by interdigital transducers (IDTs) made of a thin structured Al film. The
IDTs consist of multiple pairs of fingers. This arrangement favors the propagation of a wave resonant to the structure and orientation of the IDT
electrodes. The distance b between two fingers of
the same electrode determines the SAW wavelength λ, which in turn defines the center frequency f of the band pass filter as:
(5)
f=v/λ=v/b
Piezoelectric materials and their acoustic wave
velocities are summarized in Tab. 1. A device with
b = 4 µm (i.e. lithographic structures with 1 µm
resolution) made on LiNbO3 has its center frequency at 1 GHz. To go towards higher frequencies, one could either employ a sub-micron lithography or choose a material with higher acoustic
wave velocity like e.g. AlN or ZnO. As AlN or ZnO
substrates are not available up to now, thin films
are widely used. The films are deposited mostly by
magnetron sputtering (which is a low-cost technique, but the material quality is poor) or by expensive CVD or MBE techniques.
New SAW device designs are proposed and studied extensively in order to further increase the
center frequency at a given photolithographic
resolution. These designs are based on thin-film
layers deposited on substrates with high acoustic
velocity. While AlN is the piezoelectric material
with highest SAW velocity, diamond and SiC exceed In such devices, the acoustic waves transform and generally become near-surface confined
waves with higher acoustic velocity. The "true
SAW" wave is confined within a surface layer of
about 2λ in thickness; if the thickness h of the film
Fig. 2. Schematic diagram of a SAW filter device based on a
IDT/AlN/nanocrystalline diamond/Si structure.
As also shown in Tab.1, diamond and SiC both
exhibit higher acoustic velocities than AlN, which
is the piezoelectric material with highest v known
by now. These properties are used for novel SAW
devices of AlN and ZnO thin films on SIC substrates or diamond films. Fig. 3 shows the basic
design of such a device; Fig. 4 shows the dispersion curves of the AlN/diamond structure in regard
to phase velocity v and electromechanical coupling coefficient K2. Both values can be increased
by over 100% compared to "conventional" AlN
devices. Thin-film growth technology and further
applications are addressed in the oral talk sheets.
Tab. 1: Non-metal materials and cuts with high acoustic velocities. A star (*) denotes non-piezoelectric materials.
Material
Diamond*
SiC*
AlN
ZnO
Sapphire*
Al0.53Ga0.47N
GaN
LiTaO3
LiNbO3
LiTaO3
Quartz
Si
Orientation
(111)
(002)
(002)
(002)
(110)
(002)
(002)
128° YX
YZ
77° YZ
ST-X
(111)
Velocity [m/s]
10,200
6,832
5,790
5,700
5,548
5.060
3.693
3.992
3.488
3.254
3.158
2.650
Literature: G.F.Iriarte, AlN Thin Film Electroacoustic Devices.
Comprehensive summaries of Uppsala Dissertations from the
Faculty of Science and Technology 817 (2003). ISBN 91-5545557-3. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3359.
Fig. 3. Dispersion curves of a
AlN/diamond thin-film SAW structure. left: Phase velocity v (as
calculated and measured), right:
coupling coefficient K2 (as calculated). "Mode 0" denotes the
Rayleigh-like wave mode, "Mode
1" and "Mode 2" are higher-order
Sezawa wave modes.