sensors
Article
The Characterization of Surface Acoustic Wave
Devices Based on AlN-Metal Structures
Lin Shu 1 , Bin Peng 1, *, Chuan Li 1 , Dongdong Gong 1 , Zhengbing Yang 2 , Xingzhao Liu 1 and
Wanli Zhang 1
1
2
*
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and
Technology of China, Chengdu 610054, China; s89s89s@126.com (L.S.); uestc_lich@hotmail.com (C.L.);
15196609270@163.com (D.G.); xzliu@uestc.edu.cn (X.L.); wlzhang@uestc.edu.cn (W.Z.)
China Gas Turbine Establishment, Jiangyou 621703, China; zbyang668@163.com
Correspondence: bpeng@uestc.edu.cn; Tel.: +86-28-8320-1475; Fax: +86-28-8320-4938
Academic Editor: Stephane Evoy
Received: 26 January 2016; Accepted: 6 April 2016; Published: 12 April 2016
Abstract: We report in this paper on the study of surface acoustic wave (SAW) resonators based
on an AlN/titanium alloy (TC4) structure. The AlN/TC4 structure with different thicknesses of
AlN films was simulated, and the acoustic propagating modes were discussed. Based on the
simulation results, interdigital transducers with a periodic length of 24 µm were patterned by lift-off
photolithography techniques on the AlN films/TC4 structure, while the AlN film thickness was in the
range 1.5–3.5 µm. The device performances in terms of quality factor (Q-factor) and electromechanical
coupling coefficient (k2 ) were determined from the measure S11 parameters. The Q-factor and k2
were strongly dependent not only on the normalized AlN film thickness but also on the full-width at
half-maximum (FWHM) of AlN (002) peak. The dispersion curve of the SAW phase velocity was
analyzed, and the experimental results showed a good agreement with simulations. The temperature
behaviors of the devices were also presented and discussed. The prepared SAW resonators based on
AlN/TC4 structure have potential applications in integrated micromechanical sensing systems.
Keywords: AlN film; TC4; surface acoustic wave; layered structure; simulation
1. Introduction
In recent years, there have been growing demands for surface acoustic waves (SAWs) strain
sensors in structural health monitoring (SHM) due to its potential applications in wireless and passive
measurements. Most SAW strain sensors are prepared with piezoelectric crystal substrates such as
quartz, langasite (LGS, La3 Ga5 SiO14 ), lithium niobate (LiNbO3 ), and zinc oxide (ZnO) [1]. For example,
SAW orthogonal frequency coded (OFC) strain sensors using a LGS substrate were researched by
Wilson [2]. Furthermore, another strain sensor based on a one-port SAW resonator using quartz was
investigated by Stoney [3]. Yet constraints such as mass, volume, and bonding techniques often limit
the usage of SHM sensors in practical applications. The SAW strain sensors must be pasted onto the
measured components with adhesives. This would increase the measurement error and have a risk of
peeling off in harsh environment.
An AlN film SAW sensor integrated with a metal structure has been demonstrated in our previous
work [4]. A layer of AlN film was directly sputtered onto the metal substrate, and a SAW resonator
was fabricated on the AlN film. Compared with conventional SAW sensors, the SAW sensors in this
work can be fabricated directly on the components without any adhesives. This would decrease the
measurement error caused by the adhesives in a harsh environment. Moreover, because the thickness
of AlN film is far less than the wavelength of the acoustic waves, the acoustic waves penetrate the
underlying substrate. In this case, the properties of the acoustic waves are mainly determined by the
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waves penetrate the underlying substrate. In this case, the properties of the acoustic waves are
mainly determined by the substrates and the AlN films. Hence, compared with conventional SAW
substrates and the AlN films. Hence, compared with conventional SAW sensors, the AlN film SAW
sensors, the AlN film SAW devices integrated with a metal structure would be more sensitive to the
devices integrated with a metal structure would be more sensitive to the mechanical deformation of
mechanical deformation of the metal substrates [4].
the metal substrates [4].
In this work, we report on the design, simulation, and fabrication of the AlN film SAW devices
In this work, we report on the design, simulation, and fabrication of the AlN film SAW devices
integrated with metal components systematically. The dependence of the acoustic velocity and the
integrated with metal components systematically. The dependence of the acoustic velocity and the
electromechanical coupling coefficient of the SAW devices on the AlN film thickness are presented
electromechanical coupling coefficient of the SAW devices on the AlN film thickness are presented and
and discussed. Lastly, the temperature behaviors of the devices with different AlN film thicknesses
discussed. Lastly, the temperature behaviors of the devices with different AlN film thicknesses are
are presented and discussed.
presented and discussed.
2.2.Design
and
Simulation
Design
and
Simulation
InIn this
work,we
wedesigned
designed
a one-port
resonator,
which consists
of an interdigital
this work,
a one-port
SAWSAW
resonator,
which consists
of an interdigital
transducer
transducer
(IDT)
and
two
reflector
banks.
The
IDT
contained
101
equal-interval-finger
electrodes,
(IDT) and two reflector banks. The IDT contained 101 equal-interval-finger electrodes, and each
and
each bank
reflector
bank contained
400 short-circuited
gratings.
The finger
of the
IDTs
was
reflector
contained
400 short-circuited
gratings. The
finger width
of the width
IDTs was
6 µm,
yielding
6an
μm,
yielding
an
acoustic
wavelength
()
of
24
μm.
The
acoustic
aperture
W
was
100.
The
IDT
acoustic wavelength (λ) of 24 µm. The acoustic aperture W was 100λ. The IDT was patterned
on
was
patterned
on
the
AlN
films,
which
were
deposited
on
a
TC4
(titanium
alloy,
known
as
the AlN films, which were deposited on a TC4 (titanium alloy, known as Ti-6Al-4V) alloy substrate.
Ti-6Al-4V)
alloyillustration
substrate. of
The
illustration
of the
one-port in
SAW
resonator
is presented
in
The schematic
theschematic
one-port SAW
resonator
is presented
Figure
1. Because
the IDTs are
Figure
1.
Because
the
IDTs
are
periodic
in
nature,
one
period
cell
of
the
IDT
electrode
is
sufficient
to
periodic in nature, one period cell of the IDT electrode is sufficient to model the SAW resonator as a
model
as a whole.
The extends
height aoffew
thewavelengths
simulation down
cell only
extends
whole.the
TheSAW
heightresonator
of the simulation
cell only
to the
bottoma offew
the
wavelengths
down
to
the
bottom
of
the
substrate,
because
the
SAW
has
almost
died
outelectrode
at the
substrate, because the SAW has almost died out at the lower boundary. Since the length of the
lower
boundary.
Since
the edge
length
of the
is farcan
larger
than itsand
width,
edge effects
of the
is far larger
than its
width,
effects
of electrode
the electrodes
be ignored,
the model
geometry
can
electrodes
can
be
ignored,
and
the
model
geometry
can
be
reduced
to
a
periodic
cell
[5].
The
be reduced to a periodic cell [5]. The geometry of the SAW structure used in the simulation is shown
geometry
in Figure of
1. the SAW structure used in the simulation is shown in Figure 1.
Figure 1. Schematic illustration of the one-port surface acoustic wave (SAW) resonator and the
Figure 1. Schematic illustration of the one-port surface acoustic wave (SAW) resonator and the
modeling periodic cell.
modeling periodic cell.
We analyzed the
wave
characteristics
in the
structure
using COMSOL
software
We
theacoustic
acoustic
wave
characteristics
inAlN/TC4
the AlN/TC4
structure
using COMSOL
2
2
to determine
the velocity
electromechanical
coupling coefficient
(k ) for the(kacoustic
waves.
The
software
to determine
theand
velocity
and electromechanical
coupling coefficient
) for the
acoustic
material
constants
AlN andof
TC4
areand
listed
Table
1. in Table 1.
waves.
The
materialofconstants
AlN
TC4inare
listed
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Table 1. The material parameters used in the simulation.
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Table 1. The material parameters used in the simulation.
Material
AlN [6]
Ti * [7]
Material3 )
density(kg/m
density(kg/m3)
AlN [6]
3260
345
125
120
395
118
110
−0.48
−0.45
1.55
9
11
Ti *[7]
3260
4510
4510
C11
345
162.2
C11
162.2
C12
125
91.8
C12
91.8
C13
120
69
elastic constant (GPa)
C13
69
C33
395
180.6
elastic constant (GPa)
C33
180.6
C44
118
46.7
C44
46.7
C66
110
35.2
C66
35.2
e15
´0.48
...
e15
…
2)
e31
´0.45
...
piezoelectric
constant
(c/m
2
…
piezoelectric constant (c/m ) e31
e33
1.55
...
e33
…
ε11
9…
...
ε11
relative permittivity
relative permittivity
ε33
11…
...
ε33
* *We
use
thethe
material
parameter
of TiofinTi
the
because
the material
parameter
of the Ti-6Al-4V
We
use
material
parameter
insimulation
the simulation
because
the material
parameter
of the (TC4)
alloy
is close(TC4)
to Ti. alloy is close to Ti.
Ti-6Al-4V
mode
acousticwave
wavewas
wasidentified
identified with
with an
an Eigen
Eigen mode
TheThe
mode
of of
acoustic
mode simulation;
simulation;thus,
thus,the
theacoustic
acoustic
wave
velocity
was
calculated
by
wave velocity was calculated by
v v=
“ λf
(1) (1)
where
λ isthe
wavelength
of the
acoustic
wave
propagation,
andand
f is fthe
result
from
where
is the
wavelength
of the
acoustic
wave
propagation,
is Eigen-frequency
the Eigen-frequency
result
thefrom
simulation.
The calculated
acoustic wave
velocity
a function
the normalized
thickness of
the simulation.
The calculated
acoustic
wave as
velocity
as aoffunction
of the normalized
AlNisfilm
(tAlN/)
is shown
Figure
2a.tAlN
Here,
the thickness
tAlN is the of
thickness
offilms.
the AlN
thethickness
AlN filmof(tthe
shown
in Figure
2a.inHere,
the
is the
the AlN
The
AlN /λ)
films.simulation
The typical
simulation
diagrams
AlN filmare
thicknesses
are also
presented
typical
diagrams
with
differentwith
AlNdifferent
film thicknesses
also presented
in Figure
2a. in
The
Figure 2a. The electromechanical
coupling
coefficient
k2 structure
for the AlN/TC4
structure
is calculated
electromechanical
coupling coefficient
k2 for the
AlN/TC4
is calculated
by [8]:
by [8]:
v0 ´−vm
k2 “ 2 ˆ
(2)
= 2 × v0
(2)
where
thethe
v0 vand
vmvare
phase
boundaryconditions
conditionsofofthe
theAlN
AlNsurface
surface
where
0 and
m are
phasevelocities,
velocities,when
when the
the electrical
electrical boundary
2
2
areare
assumed
to to
bebe
electrically
free
shownininFigure
Figure2b.
2b.
assumed
electrically
freeand
andshorted,
shorted,respectively.
respectively. The
The simulated
simulated kk isisshown
(a)
(b)
Figure
2. (a)
Dependence
thesimulated
simulatedacoustic
acoustic wave
wave velocity
velocity on
ofof
the
Figure
2. (a)
Dependence
ofofthe
on the
thenormalized
normalizedthickness
thickness
the
AlN film and the typical mode schematic diagram of the acoustic waves with different thicknesses
AlN film and the typical mode schematic diagram of the acoustic waves with different thicknesses of
of the AlN film. The color corresponds to the displacement amplitude; (b) Dependence of the
the AlN film. The color corresponds to the displacement amplitude; (b) Dependence of the simulated
simulated electromechanical coupling coefficient of the devices on the normalized thickness of the
electromechanical coupling coefficient of the devices on the normalized thickness of the AlN film.
AlN film.
From Figure 2a, it can be found that different acoustic wave propagation modes occur in the
AlN/TC4 bilayer with different AlN film thicknesses. In Figure 2a, we observe that the phase
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From Figure 2a, it can be found that different acoustic wave propagation modes occur in the
AlN/TC4 bilayer with different AlN film thicknesses. In Figure 2a, we observe that the phase velocity
is dispersive; that is, it is dependent on the normalized film thickness [9]. There are three regions,
which are marked as (I), (II), and (III). In region (I), because the tAlN /λ is very small, the particle
displacements extend far into the substrate, causing the phase velocity to approach the value in bare
substrate. The acoustic wave velocity in region (I) is in the range 3000–3200 m/s, which is very close to
the acoustic wave velocity in Ti (2958 m/s [7]). This suggests that the acoustic wave is excited in AlN
film and mainly propagates in the TC4 substrate in region (I). In region (III), the motion of particles
is mainly concerned in the vicinity of AlN film, causing the acoustic wave velocity to approach the
value in the layered AlN film. Both the Rayleigh acoustic wave and the leaky acoustic wave exist
when the tAlN is comparable to λ. With a further increase in AlN film thickness, the leaky wave
disappears, and only the Rayleigh SAW occurs in the thick AlN film. The acoustic wave velocity is
about 5100–5200 m/s in region (III), which is close to the acoustic wave velocity in AlN film (about
5100–5600 m/s [10,11]). This result confirms that the acoustic wave mainly propagates in the AlN
film and barely scatters into the TC4 substrate in region (III). These conclusions also explain why the
SAW velocity increases with an increasing tAlN /λ in regions (I) and (III). In addition, we find that the
Rayleigh wave cannot be excited when 0.2 < tAlN /λ < 0.5, corresponding to region (II). We think this
is due to the low k2 value in region (II) related to the specific AlN/TC4 layered structure, as shown
in Figure 2b. In Figure 2b, the k2 increases firstly with an increase of tAlN /λ and reaches the relative
maximum value of 0.81% when the tAlN /λ is about 0.08%. The k2 decreases with the further increase
of tAlN /λ. The maximum k2 of the layered structure is influenced by electrical boundary conditions
and the material constant [12]. In our simulation model, the interface between the AlN film and the
metal substrate is electrically shorted, which leads to a large k2 relative to the open electric conditions
of the interface. We can find that the k2 approaches 0 when the AlN film thickness is close to 0. This is
because the piezoelectricity of the system in fact disappears. Thus, electrical boundary conditions on
the surface do not influence the mode of the acoustic wave. These results are similar with the results
reported in [12].
The propagation loss for the layered acoustic devices can be calculated by [13]:
α“
8.686π f r
Q fg
f g “ v g {λ
vg “
dVp
Bω
“ Vp ` k
Bk
dk
(3)
(4)
(5)
where fr is the resonance frequency, Q is the quality factor, vg is the group velocity of the SAW, vp is the
phase velocity of the SAW, and k is the relative wave number of the SAW, which can be obtained by
k = 2π*tAlN /λ.
In the calculation, the Q is obtained by calculating the admittance of the layered structure. The
admittance Y of the device can be calculated by [5]:
Y “ jωQi {Vi
(6)
where ω is the angular frequency, Qi is the complex charge in the electrodes, and Vi is the potential.
The calculated propagation loss in regions (I) and (III) is presented in Figure 3.
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(a)
(a)
(b)
(b)
Figure 3. (a) The simulated propagation loss in region (I); (b) The propagation loss in region (III).
Figure
(b) The
The propagation
propagationloss
lossininregion
region(III).
(III).
Figure3.3.(a)
(a)The
Thesimulated
simulatedpropagation
propagation loss in region (I); (b)
3. Fabrication
Fabrication
3.3.Fabrication
We fabricated the SAW devices with the AlN/TC4 structure when tAlN/ < 0.2. The thin AlN
AlN/
<<0.2.
The
AlN
Wefabricated
fabricated
theSAW
SAW
deviceswith
withthe
theAlN/TC4
AlN/TC4 structure
structure
when
tAlN
We
the
when
/λ
0.2.
Thethin
thin
AlN
films
were
prepared
via devices
middle-frequency
magnetron
sputtering
on tthe
TC4
substrate.
The
films
were
prepared
via middle-frequency
magnetron
on
the
TC4
substrate.
The
films
were
prepared
middle-frequency
magnetron
the TC4
substrate.
The dimension
dimension
of thevia
substrate
was 20 mm
× 20
mmsputtering
× 0.8sputtering
mm,onand
all of
the
substrates
were
of was
the
substrate
was
20ˆmm
20and
mm
×of0.8
and all
the tosubstrates
were
ofdimension
the
substrate
20
mm
ˆ 20
mm
0.8 mm,
alldeposition
the mm,
substrates
were
mechanically
mechanically
polished
before
sputtering.
A× two-step
process
wasofused
depositpolished
AlN
mechanically
polished
before
sputtering.
A process
two-step
process
was onto
used
to TC4
deposit
AlN
films
onto the
TC4
substrate.
The growth
is deposition
studied
in [14]
in detail.
By controlling
the
before
sputtering.
A
two-step
deposition
process
was used
to deposit
AlN
films
the
substrate.
films
onto
the
TC4
substrate.
The
growth
process
is
studied
in
[14]
in
detail.
By
controlling
theof
deposition
time,
the
thickness
of
the
AlN
film
was
adjusted
from
1.5
μm
to
3.5
μm,
yielding
a
t
AlN
/
The growth process is studied in [14] in detail. By controlling the deposition time, the thickness
from
0.0625
to
0.1458.
On
the
top
of
the
AlN
films,
a
one-port
SAW
resonator
was
patterned
via
deposition
the thickness
AlN
filmµm,
wasyielding
adjusteda from
1.5from
μm 0.0625
to 3.5 μm,
yielding
tAlN/
the
AlN filmtime,
was adjusted
fromof
1.5the
µm
to 3.5
tAlN /λ
to 0.1458.
Onathe
top
lift-off
photolithography
techniques.
The
electrodes
consisted
of
a
10-nm-thick
Ti
adhesion
layer
from
0.0625
to
0.1458.
On
the
top
of
the
AlN
films,
a
one-port
SAW
resonator
was
patterned
via
of the AlN films, a one-port SAW resonator was patterned via lift-off photolithography techniques.
and aphotolithography
100-nm-thick Au film.
Photos ofThe
the electrodes
devices are consisted
shown in Figure
4.
lift-off
techniques.
of a 10-nm-thick
Ti adhesion layer
The electrodes consisted of a 10-nm-thick Ti adhesion layer and a 100-nm-thick Au film. Photos of the
and a 100-nm-thick
film.4.Photos of the devices are shown in Figure 4.
devices
are shown inAu
Figure
Figure 4. (a) The photograph of the SAW devices deposited onto AlN/TC4 bilayer; (b) The detailed
picture of the electrodes; (c) The cross-section SEM of the AlN/TC4 structure.
Figure
bilayer;(b)
(b)The
Thedetailed
detailed
Figure4.4.(a)
(a)The
Thephotograph
photographofofthe
theSAW
SAWdevices
devicesdeposited
deposited onto
onto AlN/TC4
AlN/TC4 bilayer;
The
crystal
structures
of
the
AlN
films
were
characterized
by
X-ray
diffraction
(XRD)
(Cu-Kα,
picture
structure.
pictureofofthe
theelectrodes;
electrodes;(c)
(c)The
Thecross-section
cross-sectionSEM
SEM of
of the
the AlN/TC4
AlN/TC4 structure.
Bede-D1). The degree of c-axis orientation of the AlN films was characterized by the full width at
half
maximum
(FWHM)of
the
AlN
(002)
diffraction
peak. The
characterization
of
the (Cu-Kα,
SAW
The
crystalstructures
structures
ofofthe
the
AlN
films
were
characterized
by
diffraction
The
crystal
AlN
films
were
characterized
by X-ray
X-ray
diffraction(XRD)
(XRD)
(Cu-Kα,
resonatorThe
wasdegree
performed
by measuring
S11ofparameters
as a was
function
of frequency
using
a width
vector at
Bede-D1).
of
c-axis
orientation
the
AlN
films
characterized
by
the
full
Bede-D1). The degree of c-axis orientation of the AlN films was characterized by the full width
network analyzer
(VNA, Agilent AlN
E5071b, Agilent
Technologies Inc.,
Santa Clara, CA, USA)
andSAW
a
athalf
halfmaximum
maximum(FWHM)
(FWHM)ofofthe
the AlN(002)
(002)diffraction
diffractionpeak.
peak.The
Thecharacterization
characterizationofofthe
the SAW
microwave
micro-prober.
resonator was performed by measuring S11 parameters as a function of frequency using a vector
resonator was performed by measuring S11 parameters as a function of frequency using a vector
networkanalyzer
analyzer(VNA,
(VNA, Agilent
Agilent E5071b,
E5071b, Agilent
network
Agilent Technologies
Technologies Inc.,
Inc.,Santa
SantaClara,
Clara,CA,
CA,USA)
USA)and
anda a
4. Results and Discussion
microwave micro-prober.
microwave micro-prober.
Typical XRD-spectra of the AlN films on the TC4 substrate is presented in Figure 5a. The
thickness
of the
AlN film is 3.5 μm, corresponding to tAlN/ of 0.1458. The FWHM value of the X-ray
Results
and
Discussion
4.4.Results
and
Discussion
rocking curve for the (002) oriented thin AlN film is only 3.3, which indicates that the AlN film is
Typical
XRD-spectra
of the
onTC4
the substrate
TC4 substrate
is presented
in 5a.
Figure
5a. The
Typical
XRD-spectra
of the
AlNAlN
filmsfilms
on the
is presented
in Figure
The thickness
thickness
of
the
AlN
film
is
3.5
μm,
corresponding
to
t
AlN/ of 0.1458. The FWHM value of the X-ray
of the AlN film is 3.5 µm, corresponding to tAlN /λ of 0.1458. The FWHM value of the X-ray rocking
rocking curve for the (002) oriented thin AlN film is only
3.3, which indicates that the AlN film is
curve
for the (002) oriented thin AlN film is only 3.3˝ , which
indicates that the AlN film is highly
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highly c-axis oriented on the TC4 substrate. The FWHM values of the AlN film with different
highly
c-axis
on
the TC45b.
substrate.
The
FWHM
values
of the
AlN
filmofwith
c-axis
oriented
the TC4in
substrate.
The
values
ofthat
the
AlN
with
different
thicknesses
are
thicknesses
areonoriented
shown
Figure
ItFWHM
can be
found
the film
FWHM
value
the different
AlN films
thicknesses
inoffound
Figure
5b. the
It can
be found
FWHM
value of the
films
shown
in Figure
5b.shown
It can be
FWHM
value that
of thethe
AlN
films decreases
withAlN
the increase
decreases
withare
the
increase
tAlN/.that
with the increase of tAlN/.
of tdecreases
AlN /λ.
Figure
5.
X-ray
diffraction
(XRD)
results
the
3.5
μm
AlN
(002)
peak
Figure
(a)(a)
X-ray
diffraction
(XRD)
results
of 3.5
theµm
3.5thick
μm thick
thick
AlNfilm.
film.Inset:
Inset:
AlN
(002)
peak
Figure
5.5. (a)
X-ray
diffraction
(XRD)
results
of the
AlN AlN
film.
Inset:
AlN
(002)
peak
rocking
rocking
curve
of
AlN
films;
(b)
Dependence
of
full
width
at
half
maximum
(FWHM)
values
of
the
rocking
curve
of
AlN
films;
(b)
Dependence
of
full
width
at
half
maximum
(FWHM)
values
of
curve of AlN films; (b) Dependence of full width at half maximum (FWHM) values of the AlN filmsthe
on
AlN
films
the
normalizedthickness
thickness of AlN films.
AlN
films
onon
the
normalized
the
normalized
thickness
of AlN films. of AlN films.
Figure
6 showsthe
themeasured
measured frequency
frequency responses
responses of
on on
thethe
Figure
shows
of the
theone-port
one-portSAW
SAWresonators
resonators
Figure
66 shows
the
frequency
responses
of
the
resonators
onofthe
AlN/TC4
structures.
Themeasured
thickness of
the AlN films
tAlN varies
fromone-port
1.5 μm toSAW
3.5 μm
with a step
AlN/TC4 structures.
The
of
films
tAlN varies
from 1.5
to 3.5
step of
of
AlN/TC4
The thickness
thickness
of the
the AlN
AlN
films
1.5 μm
µm
3.5 μm
µm with
with
aa step
0.5 μm.structures.
We can observe
clear resonance
peaks
oftAlN
eachvaries
SAWfrom
device.
The to
resonance
frequency
0.5 µm.
μm.We
Wecan
can
observe clear
resonance
of eachdevice.
SAW device.
The resonance frequency
0.5
observe
peakspeaks
of each
The resonance
increases
from
127.90 clear
MHzresonance
to 132.09 MHz
when
theSAW
AlN film thickness
increasesfrequency
from 1.5 increases
μm to
increases
from
127.90
MHz
to
132.09
MHz
when
the
AlN
film
thickness
increases
from
1.5 μm
to
from
MHzspurious
to 132.09
MHzoccur
when
thethe
AlN
film thickness
increasespeaks.
from
1.5is µm
to
3.5that
µm.
Weak
3.5127.90
μm. Weak
peaks
near
Rayleigh-mode
resonance
It
probable
this
3.5
μm.
Weak
spurious
peaks
occur
near
the
Rayleigh-mode
resonance
peaks.
It
is
probable
that
this
spurious
peaks
occur near
Rayleigh-mode
resonance peaks. It is probable that this is due to the
is due to
the defects
of thethe
surface
electrodes [15].
is due to
the surface
electrodes [15].
defects
ofthe
thedefects
surfaceofelectrodes
[15].
Figure 6. S11 parameters of the AlN/TC4 SAW devices. The thicknesses of the AlN films are 1.5 μm,
2.0 μm, 2.5 μm, 3.0 μm, and 3.5 μm, respectively.
Figure 6.
6. SS11
11 parameters
Figure
parameters of
of the
the AlN/TC4
AlN/TC4SAW
SAWdevices.
devices. The
Thethicknesses
thicknessesof
ofthe
the AlN
AlN films
films are
are 1.5
1.5 μm,
µm,
2.0
μm,
2.5
μm,
3.0
μm,
and
3.5
μm,
respectively.
2.0 From
µm, 2.5
µm, 3.0
and
µm,that
respectively.
Figure
6, µm,
it can
be3.5
seen
the resonance frequency of the SAW device shifts from low
frequency to high frequency with the increase in AlN film thickness. Thus, it can be expected that
From
Figure
6, it
it wave
can be
be
seen that
that
the resonance
resonance
frequency
of the
the
SAW
deviceThe
shifts
from low
low
the
surface
acoustic
velocity
increases
with the increase
in AlN
film
thickness.
calculated
From
Figure
6,
can
seen
the
frequency
of
SAW
device
shifts
from
frequency
to
high
frequency
with
the
increase
in
AlN
film
thickness.
Thus,
it
can
be
expected
that
SAW velocities
Equation
(1)the
are increase
shown ininFigure
7. The
simulation
results
are be
alsoexpected
presentedthat
frequency
to high with
frequency
with
AlN film
thickness.
Thus,
it can
the
surface
acoustic
wave
velocity
increases
with
the
increase
in
AlN
film
thickness.
The
calculated
in
Figure
7
for
comparison.
The
SAW
velocity
is
in
the
range
3060–3170
m/s.
One
can
observe
good
the surface acoustic wave velocity increases with the increase in AlN film thickness. The calculated
SAW
velocities
with
Equation
(1)
are
shown
in
Figure
7.
The
simulation
results
are
also
presented
agreement
between
the
experimental
and
simulated
results.
SAW velocities with Equation (1) are shown in Figure 7. The simulation results are also presented
The
prepared
SAW devices
are velocity
characterized
in terms
the Q-factor
and
electromechanical
in Figure
Figure
for
comparison.
The SAW
SAW
velocity
inthe
the
rangeof
3060–3170
m/s.
One
can observe
observe good
good
in
77 for
comparison.
The
isisin
range
3060–3170
m/s.
One
can
2 to evaluate the performance of the SAW devices. The k2 of the SAW devices
coupling
coefficients
k
agreement between
between the
the experimental
experimental and
and simulated
simulated results.
results.
agreement
can
be deduced
thedevices
following
[16]: in terms of the Q-factor and electromechanical
The
preparedfrom
SAW
areequation
characterized
The prepared SAW devices are characterized in terms of the Q-factor and electromechanical
coupling coefficients
coefficients kk22 to
performance of
the SAW
SAW devices.
devices. The
The kk22 of
SAW devices
devices
coupling
to evaluate
evaluate the
the performance
of the
of the
the SAW
can be
be deduced
deduced from
from the
the following
following equation
equation [16]:
[16]:
can
Sensors
2016,
16, 16,
526526
Sensors
2016,
Sensors
2016,
16, 526
7 of
of
10 10
77of
10
(7)
== 8Ga
(7)
k2 “ 8
(7)
8 f 0 Ct N
whereGGaaisisthe
theradiation
radiationconductance,
conductance,CCt tisisthe
thecapacitance
capacitance
ofan
anIDT
IDTpair,
pair,N
Nrepresents
representsthe
thenumber
number
where
of
2 can be
where
Ga finger
isfinger
the radiation
conductance,
Ct is the
capacitance
of an
pair, N represents
number of
of IDT
IDT
pairs, and
and
the resonance
resonance
frequency.
Then,
theIDT
experimental
calculated
of
pairs,
ff00 isis the
frequency.
Then,
the
experimental
kk2 can
bethe
calculated
2
IDT
finger
pairs,
and
f
is
the
resonance
frequency.
Then,
the
experimental
k
can
be
calculated
using
using
Equation
(7)
with
the
measured
S
11
parameters.
The
Q-factor
is
extracted
by
using
the
phase
0
using Equation (7) with
the measured S11 parameters. The Q-factor is extracted by using the phase
slope
method
[17]
and
defined
as
Equation
(7) with
the
measured
S11 parameters. The Q-factor is extracted by using the phase slope
slope method
[17]
and
defined as
method [17] and defined as
ˇ dφˇ
ωω000ˇˇ ddϕ
φ ˇˇ
Q“ω
(8) (8)
(8)
QQ
ˇ
dωωˇ
222 d
dω
where
ω0ωωis
the
angular
is
the
phase.
where
is
the
angularresonance
resonancefrequency,
frequency,and
andΦ
isthe
thephase.
phase.
where
00is
the
angular
resonance
frequency,
and
is
Figure
Dependence
of experimental
experimental
(dots)
and simulated
simulated
(dash
line) phase
phase velocity
velocity
on the
the
Figure
7.7. Dependence
of
(dots)
and
line)
Figure
7. Dependence
of experimental
(dots)
and simulated
(dash(dash
line) phase
velocity
on the on
thickness
thickness
of
AlN
films.
of thickness
AlN films.of AlN films.
Figure88shows
showsthe
thedependence
dependenceof
ofthe
theQ-factor
Q-factorand
andkk22on
onthe
thenormalized
normalizedthickness
thicknessof
ofAlN
AlNfilm.
film.
Figure
Figure
8
shows
the
dependence
of
the
Q-factor
and
k2 on theofnormalized
thickness
of AlN
film. In
2 of 0.57% and maximum Q-factor
In
Figure
8,
the
maximum
k
1920
are
achieved
in
the
AlN/TC4
2
In Figure 8, the maximum
k of 0.57% and maximum Q-factor of 1920 are achieved in the AlN/TC4
Figure
8, thewhen
maximum
k2/ofis0.57%
Q-factor ofWe
1920
arefind
achieved
in simulated
the AlN/TC4
2
structure
the ttAlN
AlN/
0.0833and
andmaximum
0.1042, respectively.
respectively.
can
that the
the
structure
when the
is 0.0833
and
0.1042,
We can
find that
simulated kk2 2
structure
when
the
t
/λ
is
0.0833
and
0.1042,
respectively.
We
can
find
that
the
simulated
2
AlN
decreases monotonously
monotonouslywith
with the
the increase
increase of
of ttAlN
AlN/.
/. However,
However, the
themeasured
measured kk2 increases
increasesfirstly
firstly and
andk
decreases
2 increases firstly and
2
decreases
monotonously
with
the
increase
of
t
/λ.
However,
the
measured
k
then decreases
decreases with
with the
the increase
increase of
of ttAlN
AlN/.
/. We
WeAlN
think this
this isis because
because the
the kk2 isis dependent
dependent not
not only
only on
on
then
think
2 is dependent not only on the
then
decreases
with
the
increase
of
t
/λ.
We
think
this
is
because
the
k
the
thickness
of
AlN
films
but
also
on
the
quality
of
the
AlN
films.
The
FWHM
of
the
AlN
film
AlN
the thickness of AlN films but also on the quality of the AlN films. The FWHM of the AlN film
thickness
of AlN
films
butthe
also
on the of
quality
of the
AlN
films.
the22AlN
decreased
decreased
rapidly
with
the
increase
of ttAlN
AlN when
when
the
AlN
filmThe
wasFWHM
thinner of
than
μm,film
as shown
shown
in
decreased
rapidly
with
increase
the
AlN
film
was
thinner
than
μm,
as
in
2 would
Figure
5b,
which
shows
that
the
k
increase
with
the
increase
of
t
AlN
because
the
smaller
the
2
rapidly
with
the
increase
of
t
when
the
AlN
film
was
thinner
than
2
µm,
as
shown
in
Figure
Figure 5b, which shows thatAlN
the k would increase with the increase of tAlN because the smaller the5b,
2
2
2 decreases with
FWHM,
thethat
larger
the
k2 [18].
[18].When
When
thewith
AlN/
/the
greater
than
the
AlN, ,which
which
which
shows
thethe
k kwould
increase
increase
of 0.1,
t0.1,
because
the smaller
thettAlN
FWHM,
the
FWHM,
the
larger
the
ttAlN
isisgreater
than
kk2 decreases
with the
AlNthe
2
2
2
indicates
that
the
thickness
effect
determines
thek0.1,
k2 and
and
has
anegative
negativewith
effectthe
onthe
performance
of
larger
the k that
[18].
When
the tAlN
/λ determines
is
greater than
thehas
k adecreases
tthe
,
which
indicates
indicates
the
thickness
effect
the
effect
on
performance
of
AlN
2 and
the
devices.
Foreffect
the same
same
reason,the
we kcan
can
find
that
the dependency
dependency
of
the
Q-factor on
onof
the
AlN
/ isis
that
the
thickness
determines
has
a negative
effect onof
the
performance
the
devices.
the
devices.
For
the
reason,
we
find
that
the
the
Q-factor
the
ttAlN
/
2.
similar
to
that
of
k
2
to that
of k we
. can find that the dependency of the Q-factor on the tAlN /λ is similar to that of k2 .
Forsimilar
the same
reason,
Figure8.8.Dependence
Dependence ofthe
theQ-factor
Q-factorand
andkk222on
on thenormalized
normalized thicknessof
ofAlN
AlNfilms.
films.
Figure
Figure
8. Dependence ofofthe
Q-factor and
k onthe
the normalizedthickness
thickness of
AlN films.
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8 of 10
From
thethe
above
results,
it can
be concluded
thatthat
the the
quality
of the
AlN
filmfilm
is anisimportant
factor
From
above
results,
it can
can
be concluded
concluded
quality
of the
the AlN
AlN
an important
important
From
the above
results,
it
be
that the
quality
of
film is
an
in affecting
the
performance
of
the
SAW
devices.
To
explore
the
effects
of
the
FWHM
of
the
AlN
films
factor in
in affecting
affecting the
the performance
performance of
of the
the SAW
SAW devices.
devices. To
To explore
explore the
the effects
effects of
of the
the FWHM
FWHM of
of the
the
factor
on AlN
the characteristics
of the AlN/TC4
SAW
devices,SAW
we prepared
AlN
films onAlN
the TC4
with
films on
on the
the characteristics
characteristics
of the
the
AlN/TC4
devices, we
we
prepared
filmssubstrate
on the
the TC4
TC4
AlN films
of
AlN/TC4
SAW devices,
prepared
AlN films
on
different
FWHMs
by
changing
the
ratio
of
depositing
time
at
the
first
and
second
steps,
while
the
total
substrate
with
different
FWHMs
by
changing
the
ratio
of
depositing
time
at
the
first
and
second
substrate with different FWHMs by changing the ratio of depositing time at the first and second
2 on
2 films
steps,
while
the
total
thickness
was
fixed
to
2.5
μm
[14].
The
dependence
of
the
Q-factor
and
k
on
thickness
was
fixed
to
2.5
µm
[14].
The
dependence
of
the
Q-factor
and
k
the
FWHM
of
AlN
steps, while the total thickness was fixed to 2.5 μm [14]. The dependence of the Q-factor and k2 on
the FWHM
FWHMinof
ofFigure
AlN films
films
is presented
presented in
in Figure
Figure 9.
9.
is presented
9. is
the
AlN
2 on FWHM where the tAlN is 2.5 μm. The lines are drawn as
Figure
9. Dependence
Dependence
of
Q-factor
and
2 on
Figure
9. Dependence
of of
Q-factor
and
k2kkon
FWHM
lines are
are drawn
drawn as
as a
Figure
9.
Q-factor
and
FWHMwhere
wherethe
thetAlN
tAlN is 2.5 µm.
μm. The lines
guide
for reader.
the reader.
reader.
guide
for the
aa guide
for
the
From Figure
Figure 9,
9, it
it can
can be
be found
found that
that the
the Q-factor
Q-factor2 and
and kk22 of
of the
the SAW
SAW devices
devices are
are strongly
strongly
From
From
Figure
9,
it
can
be
found
that
the
Q-factor
and
k
ofand
the kSAW
devices
arethe
strongly
dependent
2 decrease
dependent
on
the
FWHM
of
the
AlN
films.
Both
Q-factor
with
increase
of the
the
on
the
of the
AlN
films. Both
andwith
k2 decrease
with of
thethe
increase
of
2 decrease
on dependent
the FWHM
ofAlN
theFWHM
AlN
films.
Both
Q-factor
andwith
kQ-factor
the increase
FWHM
ofbethe
2 on
FWHM
of
the
film.
The
result
is
agreement
[19].
The
dependency
of
k
FWHM
can
FWHM of the AlN film. The result is agreement with [19]. The dependency
of k2 on FWHM can be
AlN
film. Thebyresult
is agreement with
[19].
The
dependency
of k2 on
FWHM
can
explainedofby
explained
the
piezoelectricity
of
the
AlN
films,
which
strongly
depends
on
thebe
orientation
thethe
explained by the piezoelectricity of the AlN films, which strongly depends on the
orientation
of the
piezoelectricity
of
the
AlN
films,
which
strongly
depends
on
the
orientation
of
the
crystalline
grain.
2
crystalline grain.
grain. In
In general,
general, aa small
small kk2 of
of the
the substrates
substrates would
would increase
increase the
the loss
loss of
of acoustic
acoustic energy
energy
crystalline
In general,
atosmall
k2 of
the substrates
would devices
increasewhen
the loss
of
acoustic
energy
andwith
leadthe
to asame
small
and
lead
a
small
Q-factor
of
the
realized
the
devices
are
fabricated
and lead to a small Q-factor of the realized devices when the devices are fabricated with the same
Q-factor
theThis
realized
devices when
the performance
devices are fabricated
withasthe
sameindesign
This also
designof
[20].
also explained
explained
why the
of the
the devices,
devices,
shown
Figure[20].
8, increases
increases
design
[20].
This also
why
performance of
as shown
in Figure
8,
explained
why
the
performance
devices,
as
shown
in
Figure
8,
increases
with
the
increase
of
with
the
increase
of
t
AlN/ when of
t
AlNthe
/
<
0.1.
with the increase of tAlN/ when tAlN/ < 0.1.
The characteristics
characteristics
of the
the prepared
prepared devices
devices were
were measured
measured at
at temperatures
temperatures from
from room
room
tAlN /λ when
tAlN /λ < 0.1. of
The
temperature
to
350
°C.
The
dependence
of
the
resonance
frequency
shifts
of
the
AlN/TC4
SAW
The
characteristics
of
the
prepared
devices
were
measured
at
temperatures
from
room
temperature
temperature to 350 °C. The dependence of the resonance frequency shifts of the AlN/TC4 SAW
˝ C. Theon
resonators
the
temperature
are
plotted
in
Figure
10.
From
Figure
10,
it
can
be
seen
that
thethe
to 350
dependence
of
the
resonance
frequency
shifts
of
the
AlN/TC4
SAW
resonators
resonators on the temperature are plotted in Figure 10. From Figure 10, it can be seen thaton
the
devices
are
characterized
by
a
quasi-linear
sensitivity
to
temperature.
Note
that
the
first-order
temperature
arecharacterized
plotted in Figure
From Figure
10, it can to
be seen
that the devices
are the
characterized
devices are
by a10.
quasi-linear
sensitivity
temperature.
Note that
first-orderby
temperaturesensitivity
coefficienttoof
oftemperature.
frequency (TCF)
(TCF)
values
offirst-order
the realized
realized
SAW devices
devices
increase
slightly
a quasi-linear
Note values
that theof
temperature
coefficient
of frequency
temperature
coefficient
frequency
the
SAW
increase
slightly
withvalues
the temperature
temperature
increasing
(e.g., the
theincrease
TCF changes
changes
from
−80the
ppm/°C
at 20
20 °C
°C increasing
to −101
−101 ppm/°C
ppm/°C
atthe
(TCF)
of the realized
SAW devices
slightly
with
temperature
(e.g.,at
with
the
increasing
(e.g.,
TCF
from
−80
ppm/°C
at
to
350
°C
when
the
AlN
film
thickness
is
1.5
μm).
This
result
is
similar
to
the
previous
reports
on
other
˝
˝
˝
˝
350
°C whenfrom
the AlN
thickness
is 1.5
Thisppm/
result C
is at
similar
to when
the previous
on other is
TCF
changes
´80film
ppm/
C at 20
C μm).
to ´101
350 C
the AlNreports
film thickness
AlN film
acoustic devices
devices [16,21].
[16,21].
acoustic
1.5 AlN
µm).film
This
result is
similar
to the previous reports on other AlN film acoustic devices [16,21].
Figure 10.
10. Temperature
Temperature dependences
dependences of
of the
the resonance
resonance frequency
frequency shifts
shifts of
of the
the prepared
prepared AlN/TC4
AlN/TC4
Figure
Figure
10. Temperature
dependences of the
resonance frequency
shifts of the
prepared
AlN/TC4 SAW
SAW
resonators
with
different
AlN
film
thicknesses.
SAW resonators
with different
AlN
film thicknesses.
resonators
with different
AlN film
thicknesses.
Sensors 2016, 16, 526
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Since the relative AlN thickness tAlN /λ of the devices is very close to 0.15, the SAW is mainly
concerned with the TC4 substrate, as shown in Figure 2a. Thus, the temperature behaviors of the SAW
devices are mainly attributed to the TC4 substrate [16]. However, the temperature behaviors of the
SAW devices are also affected by the AlN film thickness. The devices with thinner AlN films have
larger absolute TCF values than that with thicker AlN films. This is because the AlN has a smaller
coefficient of thermal expansion (CTE) than does TC4. When the AlN film thickness increases, more
acoustic waves will propagate in the AlN films, leading to a decrease in the effective CTE of the layered
AlN/TC4 structure, thus causing a decrease in TCF of the devices [22].
5. Conclusions
SAW resonators deposited on an AlN/TC4 structure were investigated in this work. The acoustic
wave propagation in AlN/TC4 structure was modeled and simulated by finite element simulations.
From the simulation, it was found that there are three regions where the acoustic wave has different
propagation modes. Based on the simulation, SAW resonators were fabricated on the AlN/TC4
structure, and the influences of the AlN films were analyzed. We found that the SAW velocity increased
with the increase in AlN film thickness. The characteristics of the SAW devices were dependent on
the thickness of the AlN film, as well as the quality of the AlN films. The maximum value of Q-factor
reached 1970, and the k2 reached 0.57%. The temperature measurements of the devices show that the
AlN/TC4-layered SAW device is characterized by a large and quasi-linear sensitivity to temperature,
making it suited for sensing applications in high temperature environments. This work may assist us
with a proper design, to fabricate SAW sensors integrated with metal components.
Acknowledgments: This work is financial supported by NSFC (61223002) and Sichuan Youth Science and
Technology Innovation Research Team Funding (No. 2011JTD0006).
Author Contributions: All author participated in the work presented here. Lin Shu contributed the sensor
design and simulation. Chuan Li, and Dongdong Gong contributed to the improvement of AlN film growth
method. Zhengbing Yang contributed to the measurement system. Bin Peng, Wanli Zhang and Xingzhao Liu
made constructive suggestions to the research. All authors read and approved the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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