sensors
Article
Effects of AlN Coating Layer on High Temperature
Characteristics of Langasite SAW Sensors
Lin Shu 1 , Bin Peng 1, *, Yilin Cui 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.); g3ingmtg@gmail.com (Y.C.);
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: Vittorio M. N. Passaro
Received: 20 July 2016; Accepted: 30 August 2016; Published: 6 September 2016
Abstract: High temperature characteristics of langasite surface acoustic wave (SAW) devices coated
with an AlN thin film have been investigated in this work. The AlN films were deposited on the
prepared SAW devices by mid-frequency magnetron sputtering. The SAW devices coated with AlN
films were measured from room temperature to 600 ◦ C. The results show that the SAW devices
can work up to 600 ◦ C. The AlN coating layer can protect and improve the performance of the
SAW devices at high temperature. The SAW velocity increases with increasing AlN coating layer
thickness. The temperature coefficients of frequency (TCF) of the prepared SAW devices decrease
with increasing thickness of AlN coating layers, while the electromechanical coupling coefficient
(K2 ) of the SAW devices increases with increasing AlN film thickness. The K2 of the SAW devices
increases by about 20% from room temperature to 600 ◦ C. The results suggest that AlN coating layer
can not only protect the SAW devices from environmental contamination, but also improve the K2 of
the SAW devices.
Keywords: langasite; AlN coating layer; high temperature; temperature coefficient of frequency
1. Introduction
Nowadays, a wireless and battery-free sensor which can stably operate in harsh environments is
in great demand for many industries, particularly in aerospace, automotive, and energy industries.
Sensing of physical properties such as temperature, strain, and vibration are useful for dynamic
and static health monitoring and maintenance of the target components. Sensors based on surface
acoustic wave (SAW) technology are attractive due to its characteristics of passivity, wireless, high
sensitivity, high precision, and good reproducibility. In a typical SAW device, a surface mechanical
wave is generated and propagates in a piezoelectric substrate due to an electro-mechanical coupling
effect. The characteristics of SAW sensors are strongly dependent on the piezoelectric substrate.
For example, the operation temperature is strongly dependent on the phase transition temperature
of the piezoelectric substrate. Among various piezoelectric materials, lanthanum gallium silicate
(La3 Ga5 SiO14 , LGS, langasite) has been thought as one of the most attractive materials in high
temperature application due to the absence of a phase transition from room temperature up to its
melting temperature (1450 ◦ C), good temperature stability, and large coupling constant K2 (0.34%),
which is two times larger than that of ST-quartz (0.14%) [1]. Peng [2] and Thiele [3] fabricated a high
temperature SAW gas sensor with LGS materials. Another LGS SAW gas sensor for harsh environments
was reported by Greve et al. [4]. We also reported SAW strain sensors based on different cuts of LGS
substrate at high temperature [5].
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It was found that an obvious signal attenuation or an unexpected shift of the resonance frequency
of the SAW sensor appears after long-term measurement at high temperature [6]. This is caused not only
by the degradation of the metal electrodes, but also surface contamination in harsh environments [7].
So, a protective layer is required to improve the stability of the SAW sensor. Some works about
protective coatings for SAW sensors have been reported by Cunha [8] and Freudenberg [9]. However,
to the best of our knowledge, there are no reports on the thickness effects of the coatings on the
performance of SAW sensors.
◦
◦
◦
In thisSensors
paper,
2016,SAW
16, 1436 sensors based on an LGS substrate with Euler angles of (0 , 138.5
2 of 7 , 26.6 ) are
fabricated. AlN thin film is deposited on the entire surface of the SAW devices as a protective layer.
It was found that an obvious signal attenuation or an unexpected shift of the resonance
We chose AlN
film of
asthe
a protective
layer on
SAW
devices
due to its
excellent
properties,
such as high
frequency
SAW sensor appears
after
long-term
measurement
at high
temperature
[6]. This is
◦
hardness (>11
GPa),
high
temperature
electrical
resistivity
(>1010 Ω·cm),
caused
not only
by the
degradation ofstability
the metal (>2000
electrodes,C),
but high
also surface
contamination
in harsh
environments
[7].
So,
a
protective
layer
is
required
to
improve
the
stability
of
the
SAW
sensor.
Some
good thermal conductivity (>100 W/(m·K)), and good corrosion resistance [10]. The influences of AlN
works about protective coatings for SAW sensors have been reported by Cunha [8] and Freudenberg
film thickness
on SAW velocity and electromechanical coefficients have been explored in this work.
[9]. However, to the best of our knowledge, there are no reports on the thickness effects of the coatings
Additionally,
theperformance
temperature
coefficients
of frequency and quality factor of the SAW sensors have
on the
of SAW
sensors.
◦ C,substrate
this paper,
SAW
sensors basedto
on600
an LGS
with
Euler angles
(0, 138.5, 26.6)of
areSAW device
been measuredInfrom
room
temperature
thereby
enabling
theofoptimization
fabricated. AlN thin film is deposited on the entire surface of the SAW devices as a protective layer.
design for operation in harsh environments.
We chose AlN film as a protective layer on SAW devices due to its excellent properties, such as high
hardness (>11 GPa), high temperature stability (>2000 C), high electrical resistivity (>1010 Ω·cm),
2. Experimental
Setup
good thermal conductivity (>100 W/(m·K)), and good corrosion resistance [10]. The influences of AlN
film thickness on SAW velocity and electromechanical coefficients have been explored in this work.
The LGS substrate with the Euler angles (0◦ , 138.5◦ , 26.6◦ ) used in this paper were purchased
Additionally, the temperature coefficients of frequency and quality factor of the SAW sensors have
from SICCAS,
Shanghai.
Thetemperature
SAW sensors
had
a typical
one-port
SAW
resonator
been measured
from room
to 600 C,
thereby
enabling the
optimization
of SAW
device structure
design for
in harsh environments.
which consisted
ofoperation
an interdigital
transducer (IDT) and two reflector banks. Each IDT contained
101 equal-interval-finger electrodes with finger widths of 6 µm, yielding an acoustic wavelength
2. Experimental Setup
λ of 24 µm. Each reflector bank contained 400 short-circuited gratings. The aperture was 100 λ.
The LGS substrate with the Euler angles (0, 138.5, 26.6) used in this paper were purchased
The electrodes,
which consisted of a 10 nm-thick Ti adhesion layer and a 100 nm-thick Au film, were
from SICCAS, Shanghai. The SAW sensors had a typical one-port SAW resonator structure which
patterned by
lift-off
techniques
onreflector
LGS substrates.
Thecontained
protective
AlN films were
consisted
of photolithography
an interdigital transducer
(IDT) and two
banks. Each IDT
101 equalinterval-finger
electrodes
with finger
widths
of 6 μm, yielding an
 of 24 μm. sputtering,
deposited on
the surface
of the SAW
sensor
by mid-frequency
(40acoustic
kHz) wavelength
reactive magnetron
Each reflector bank contained 400 short-circuited gratings. The aperture was 100 . The electrodes,
except for on
the electrode pad. The sputtering target was an aluminum disk with purity of 99.999%,
which consisted of a 10 nm-thick Ti adhesion layer and a 100 nm-thick Au film, were patterned by
whereas argon
purity of 99.999%
was
as plasma
and nitrogen
purity ofon99.999% was
lift-offwith
photolithography
techniques
on used
LGS substrates.
Thegas
protective
AlN filmswith
were deposited
employed the
as reactive
gas.
Before
the (40
sputtering
chamber
was
evacuated
surface of the
SAW
sensordeposition,
by mid-frequency
kHz) reactive
magnetron
sputtering,
exceptto
for1 × 10−4 Pa.
on the electrode
pad. The sputtering
target
was an
purity
of 99.999%,into
whereas
After a 15-minute
pre-sputtering
on the Al
target,
N2aluminum
and Ar disk
werewith
both
introduced
the chamber at
argon with purity of 99.999% was used as plasma gas and nitrogen with purity of 99.999% was
a fixed flow
ratio of 52 sccm:78 sccm. The sputtering power was 1900 W. The depositing rate of AlN
employed as reactive gas. Before deposition, the sputtering chamber was evacuated to 1 × 10−4 Pa.
overlayers After
was aabout
12 nm/min.
Inon
this
the
AlN
films was
range of 50 nm
15-minute
pre-sputtering
thework,
Al target,
N2thickness
and Ar wereof
both
introduced
into in
thethe
chamber
at
a
fixed
flow
ratio
of
52
sccm:78
sccm.
The
sputtering
power
was
1900
W.
The
depositing
rate
of
AlN 150, Veeco,
to 200 nm. The thicknesses of the AlN films were measured by a profile meter (Dektak
overlayers was about 12 nm/min. In this work, the thickness of AlN films was in the range of 50 nm◦ to
New York, NY, USA). Before measurement, all of the samples were annealed at 600 C for 30 min
200 nm. The thicknesses of the AlN films were measured by a profile meter (Dektak 150, Veeco, New
in pure N2York,
to improve
stability.
The
thermal
process
only improve
NY, USA).their
Beforethermal
measurement,
all of the
samples
were annealing
annealed at 600
C for 30can
minnot
in pure
2
the electrical
of the
Au/Ti
electrodes
at high
temperature
[11],
eliminate
voids in
N properties
to improve their
thermal
stability.
The thermal
annealing
process can
notbut
onlyalso
improve
the
electrical
of the the
Au/Ti
electrodes
at high temperature
[11], but also
eliminate
voids in AlN
AlN coatings,
so asproperties
to improve
thermal
stability
of the prepared
LGS
SAW sensor.
The schematic
coatings, so as to improve the thermal stability of the prepared LGS SAW sensor. The schematic
illustrationillustration
of the SAW
resonator and the photos of the prepared SAW device are presented in Figure 1.
of the SAW resonator and the photos of the prepared SAW device are presented in Figure
1.
(a)
(b)
Figure 1. (a) Schematic illustration of one-port surface acoustic wave (SAW) resonator. The upper one
Figure 1. (a)
Schematic
illustration
ofisone-port
surface
acoustic
wave
(SAW)
resonator.
The upper one
is the
top view and
the lower one
the sectional
view. LGS:
langasite.
(b) Photo
of the
prepared SAW
resonator.
SEM photo
electrodes.
is the top view
andInset:
the lower
oneofisthe
the
sectional view. LGS: langasite. (b) Photo of the prepared SAW
resonator. Inset: SEM photo of the electrodes.
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The prepared SAW devices were electrically characterized in air atmosphere using a vector
The prepared SAW devices were electrically characterized in air atmosphere using a vector
network analyzer (VNA, Agilent E5071C, Agilent Technologies Inc., Santa Clara, CA, USA).
network analyzer (VNA, Agilent E5071C, Agilent Technologies Inc., Santa Clara, California, USA).
The reflection scattering parameters (S11 ) of the prepared SAW devices were recorded and processed
The reflection scattering parameters (S11) of the prepared SAW devices were recorded and processed
by a computer. In the experiment, the prepared SAW devices were continuously measured from room
by a computer. In the experiment, the prepared SAW devices were continuously measured from room
temperature to 600 ◦ C for a period of 800 min. The resonance frequency of the SAW devices was
temperature to 600 C for a period of 800 min. The resonance frequency of the SAW devices was
recorded in 30 s intervals.
recorded in 30 s intervals.
3. Results and Discussion
3. Results and Discussion
The measured frequency responses of the prepared SAW resonators with different thickness of
The measured
responses of the prepared SAW resonators with different thickness of
AlN coating
layers (tfrequency
AlN ) are shown in Figure 2a. We can observe a clear resonance peak for every
AlN coating
layers (tAlN
) are shown
in Figure
We can
observe
a clearMHz
resonance
peak
forvaries
every
device.
The resonance
frequency
increased
from 2a.
112.8325
MHz
to 114.9585
when the
tAlN
device.
The
resonance
frequency
increased
from
112.8325
MHz
to
114.9585
MHz
when
the
t
AlN varies
from 0 to 200 nm. The SAW phase velocity v can be calculated by
from 0 to 200 nm. The SAW phase velocity v can be calculated by
v = λ × fr ,
(1)
= × ,
(1)
where
wheref rfr isisthe
theresonance
resonancefrequency
frequencyofofthe
theSAW
SAWdevice
deviceobtained
obtainedfrom
fromthe
theS11
S11curve.
curve.The
Thecalculated
calculated
SAW
velocities
as
a
function
of
t
are
shown
in
Figure
2b.
We
can
find
that
the
SAW
SAW velocities as a function of AlN
tAlN are shown in Figure 2b. We can find that the SAWvelocities
velocities
increase
increasewith
withincreasing
increasingtAlN
tAlN.. This
This is
is due
due to
to AlN
AlN guiding
guiding layers
layers offering
offering higher
higheracoustic
acousticwave
wavevelocity
velocity
(about
5100–5600
m/s
[12,13])
than
the
LGS
substrate
(about
2700
m/s
[14]).
More
acoustic
(about 5100–5600 m/s [12,13]) than the LGS substrate (about 2700 m/s [14]). More acousticwave
wave
energy
energywill
willbe
beconcentrated
concentratedininthe
theguiding
guidinglayers
layerswith
withincreasing
increasingtAlN
tAlN..
(a)
(b)
Figure 2. (a) S11 parameters of the SAW devices coated with AlN layers. The thickness of the AlN films
Figure
(a) S150
ofnm,
the SAW
devices coated
with AlN of
layers.
The thickness
ofvelocity
the AlN on
films
11 parameters
is 0, 502.nm,
nm, and 200
respectively.
(b) Dependence
experimental
phase
the
isthickness
0 nm, 50 of
nm,
150
nm,
and
200
nm,
respectively.
(b)
Dependence
of
experimental
phase
velocity
on
AlN coating layers. The line is drawn as a guide for the reader.
the thickness of AlN coating layers. The line is drawn as a guide for the reader.
The SAW devices without and with AlN coatings were measured from room temperature to
◦ C,
600 The
°C, and
S11 results
of the
devices
are shown
in Figurefrom
3. It room
can betemperature
found that to
the
sharp
SAWthe
devices
without
andSAW
with AlN
coatings
were measured
600
resonance
exists
at 600
°C, indicating
both of
SAW
devices
canthe
work
upresonance
to 600 °C.
and
the S11 peak
resultsstill
of the
SAW
devices
are shownthat
in Figure
3. the
It can
be found
that
sharp
◦
◦
We
also
found
that
the
resonance
frequency
decreases
with
increasing
temperature.
can
be
peak still exists at 600 C, indicating that both of the SAW devices can work up to 600 C.ItWe
also
interpreted
as resonance
that both the
LGS substrate
and
theincreasing
AlN coating
layer have Ita can
negative
temperature
found
that the
frequency
decreases
with
temperature.
be interpreted
as
coefficient
SAW
velocity
[14,15],
which
results
a decrease
of the
fr of the SAW
devices
with
that
both theofLGS
substrate
and
the AlN
coating
layerinhave
a negative
temperature
coefficient
of SAW
increasing
temperature.
FrominFigure
3a,b, it
observed
that
the height
the resonance
peak of
velocity
[14,15],
which results
a decrease
ofcan
the be
f r of
the SAW
devices
with of
increasing
temperature.
the SAW
device
coatings
atof
high
while
thatSAW
of devices
AlN
From
Figure
3a,b,without
it can beAlN
observed
thatdecreases
the height
thetemperature,
resonance peak
of the
devicewith
without
coatings
do not
attenuate
at high
temperature.
suggests
SAW do
device
is indeed
AlN
coatings
decreases
at high
temperature,
whileThis
thatresult
of devices
with that
AlNthe
coatings
not attenuate
by the AlNThis
coating
at high
temperature.
Withisthe
measured
S11 curve,
the
admittance
atprotected
high temperature.
resultlayer
suggests
that
the SAW device
indeed
protected
by the
AlN
coating
(conductance
and susceptance)
of measured
the SAW devices
can
beadmittance
calculated,(conductance
which is shown
the inset of
layer
at high temperature.
With the
S11 curve,
the
andin
susceptance)
Figure
3b. The
admittance
will next be
usedistoshown
calculate
theinset
electromechanical
coupling
coefficient
of
the SAW
devices
can be calculated,
which
in the
of Figure 3b. The
admittance
will
2
2
(K ).be used to calculate the electromechanical coupling coefficient (K ).
next
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(a)
(a)
(b)
(b)
Figure
3. SS11 results
Figure
resultsof
of(a)
(a)the
thebare
bareLGS
LGS SAW
SAW devices
devices without
without AlN
AlN coatings;
coatings; and
and (b)
(b) the
the SAW
SAW devices
devices
Figure 3.
3. S11
11 results
of
(a)
the
bare
LGS
SAW
devices
without
AlN
coatings;
and
(b)
the
SAW
devices
◦
coated
with
a
150
nm-thick
AlN
film
at
20,
50,
100,
200,
300,
400,
500,
and
600
°C.
Inset:
measured
coated
with
a
150
nm-thick
AlN
film
at
20,
50,
100,
200,
300,
400,
500,
and
600
C.
Inset:
measured
coated with a 150 nm-thick AlN film at 20, 50, 100, 200, 300, 400, 500, and 600 °C. Inset: measured
admittance
(conductance and
and susceptance) of
of the SAW
SAW device when
when T = 600 ◦°C.
admittance
C.
admittance (conductance
(conductance and susceptance)
susceptance) of the
the SAWdevice
device whenTT== 600
600 °C.
The relative resonance frequency change (Δf/f0) is defined as
resonance frequency
frequency change
change (∆f
(Δf/f
The relative resonance
/f00) )isisdefined
definedas
as
∆ ⁄ =f − f 0 ,
,,
∆∆f /⁄f 0 == T
f0
(2)
(2)
(2)
where fT and f0 are the resonance frequency of the SAW devices at temperature T and room
where ffTT and
where
and f00 are
are the
the resonance
resonance frequency
frequency of
of the
the SAW
SAW devices at temperature T and room
temperature, respectively. The Δf/f0 for the SAW devices with different thickness of AlN coating
respectively. The ∆f
Δf/f
coating
temperature, respectively.
/f0 0for
forthe
theSAW
SAW devices
devices with
with different
different thickness
thickness of AlN coating
layers as a function of temperature is presented in Figure 4a. It can be observed that the Δf/f0 is
function of
of temperature
temperature isispresented
presentedininFigure
Figure4a.
4a.ItItcan
canbe
beobserved
observedthat
thatthe
the∆fΔf/f
layers as aa function
/f 00 is
dependent on the AlN film thickness. The absolute value of Δf/f0 decreases with increasing AlN film
dependent on
on the
the AlN
AlNfilm
filmthickness.
thickness.The
Theabsolute
absolutevalue
valueofof∆f
Δf/f
dependent
/f00 decreases
decreases with
with increasing
increasing AlN film
thickness at the same temperature.
thickness at the same temperature.
temperature.
(a)
(a)
(b)
(b)
Figure 4. (a) Relative resonance frequency shift of the prepared SAW devices as a function of
Figure 4. (a) Relative resonance frequency shift of the prepared SAW devices as a function of
temperature.
Temperature
coefficient
of frequency
(TCF)
of the prepared
SAWasdevices.
The
Figure
4. (a)(b)
Relative
resonance
frequency
shift of the
prepared
SAW devices
a function
temperature.
(b)
Temperature
coefficient
of frequency
(TCF)
of the prepared
SAW devices.
The
thickness
of
the
AlN
coatings
is
0
nm,
50
nm,
150
nm,
and
200
nm,
respectively.
The
line
is
drawn
as
of
temperature.
(b)coatings
Temperature
coefficient
of nm,
frequency
of the prepared
SAW
devices.
thickness
of the AlN
is 0 nm,
50 nm, 150
and 200(TCF)
nm, respectively.
The line
is drawn
as
a guide
for the of
reader.
The
thickness
the AlN coatings is 0 nm, 50 nm, 150 nm, and 200 nm, respectively. The line is
a guide
for the reader.
drawn as a guide for the reader.
Experimentally, the temperature coefficient of frequency (TCF) can be extracted from the relative
Experimentally, the temperature coefficient of frequency (TCF) can be extracted from the relative
shifts of the resonance frequency by
shiftsExperimentally,
of the resonance
by coefficient of frequency (TCF) can be extracted from the relative
thefrequency
temperature
shifts of the resonance frequency by
TCF = 1 ∂ f r .
(3)
TCF==
(3)
TCF
..
(3)
f r ∂T
It
can be
observed that
that the absolute
absolute TCF value
value (|TCF|) of
the slope
of the
curve
It
of the
the devices
devices (i.e.,
(i.e.,
It can
can be
be observed
observed that the
the absolute TCF
TCF value (|TCF|)
(|TCF|) of
the
devices
(i.e., the
the slope
slope of
of the
the curve
curve
at
each
temperature)
increases
with
the
increasing
temperature,
as
shown
in
Figure
4b.
For
example,
at
each
temperature)
increases
with
the
increasing
temperature,
as
shown
in
Figure
4b.
For
example,
at each temperature) increases with the increasing temperature, as shown in Figure 4b. For example,
the
|TCF| of
with a
a 200 nm-thick
nm-thick AlN coating
coating layer is
is about 5.2
5.2 ppm/°C
◦ C at
◦ C,
the
of the
the SAW
SAW device
device
at 25
25 °C,
the |TCF|
|TCF| of
the
SAW
device with
with a 200
200 nm-thick AlN
AlN coating layer
layer is about
about 5.2ppm/
ppm/°C
at
25
°C,
while
it
increases
to
34
ppm/°C
at
600
°C.
When
temperature
is
higher
than
400
°C,
the
TCF
value
◦
◦
◦
while
C at
at 600
600 °C.
C. When
When temperature
temperature is
is higher
C, the
while it
it increases
increases to
to 34
34 ppm/
ppm/°C
higher than
than 400
400 °C,
the TCF
TCF value
value
approximates to a constant—i.e., the resonance frequency of the SAW devices decreases quasiapproximates to a constant—i.e., the resonance frequency of the SAW devices decreases quasilinearly with increasing temperature. The temperature behaviors of the SAW devices are also
linearly with increasing temperature. The temperature behaviors of the SAW devices are also
dependent on the thickness of the AlN coating layer. It can be found that the |TCF| decreases with
dependent on the thickness of the AlN coating layer. It can be found that the |TCF| decreases with
Sensors 2016, 16, 1436
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approximates
to a constant—i.e., the resonance frequency of the SAW devices decreases quasi-linearly
Sensors 2016, 16, 1436
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with increasing temperature. The temperature behaviors of the SAW devices are also dependent on
the
thickness
the is
AlN
layer.
cancoefficient
be found of
that
the |TCF|
decreases
increasing
tAlNof
. This
duecoating
to the fact
thatItthe
thermal
expansion
(CTE)with
of theincreasing
AlN film
tisAlN
. This is
duethat
to the
factLGS
thatsubstrate
the coefficient
of Therefore,
thermal expansion
(CTE)
of the
AlN
filmfilm-coated
is smaller
smaller
than
of the
[16,17].
the effective
CTE
of the
AlN
than
that of
the LGS
substrate
[16,17].
Therefore,tAlN
the, thereby
effectivecausing
CTE ofa the
AlN film-coated
LGS
LGS SAW
devices
would
decrease
with increasing
reduction
of the |TCF|
of
SAW
devices
would
increasing
tAlN , thereby
causing
a reduction
of the |TCF|
the
the SAW
devices.
Ondecrease
the otherwith
hand,
more acoustic
wave energy
would
be concentrated
in theofAlN
SAW
devices.
On thetAlN
other
hand, in
more
acoustic
would the
be value
concentrated
in the
AlN
films with
increasing
, resulting
a decrease
in wave
|TCF|,energy
approaching
of AlN films
(about
films
with increasing
tAlN , resulting
in aofdecrease
in |TCF|,
approaching
value
of AlN
−30 ppm/°C
[12]), because
the |TCF|
AlN is smaller
than
that of thethe
LGS
material
at films
high
◦ C [12]), because the |TCF| of AlN is smaller than that of the LGS material at high
(about
−30 ppm/
temperature
(about
−48 ppm/°C at 500 °C [5]).
temperature (about −48 ppm/◦ C at 500 ◦ C [5]).
by [18]
[18]
The K22 of the SAW devices can be calculated by
π Gm ((f r ))
,,
K2 ==
4N Bs ((f r ))
(4)
(4)
where N
N is
is the
the number
number of
of IDT
IDT finger
finger pairs
pairs and
and equals
equals to
to 100
100 in
in this
this work,
work, f f00 corresponds
corresponds to the
the
where
resonance frequency, and Gmm(f(fr)r )and
B
s
(f
r
)
are
the
motional
conductance
and
static
susceptance
of
and Bs (f r ) are the motional conductance and static susceptancethe
of
2
2
device
at frat
. The
calculated
K asKa function
of temperature
is plotted
in Figure
5. It 5.
can
that
the
device
f r . The
calculated
as a function
of temperature
is plotted
in Figure
It be
canfound
be found
2 of 2
the K
device
without
AlN
coatings
increases
that
the
K the
of SAW
the SAW
device
without
AlN
coatings
increasesfirstly
firstlywith
withincreasing
increasing temperature
temperature and
2
2
decreases with
withfurther
furtherincrease
increaseofoftemperature.
temperature.We
Wethink
thinkthat
that
the
decrease
high
temperature
decreases
the
decrease
of of
K Katat
high
temperature
is
is due
to the
degradation
of metal
electrode
at high
temperature
Wefind
alsothat
findthe
that
K2 of
the
due
to the
degradation
of metal
electrode
at high
temperature
[19]. [19].
We also
K2the
of the
SAW
SAW devices
withfilms
AlNincreases
films increases
with increasing
temperature,
K2 increases
by
devices
coatedcoated
with AlN
with increasing
temperature,
and theand
K2 the
increases
by about
◦
about
20%room
from temperature
room temperature
600
°C. These
results
also suggest
thatAlN
the film
AlN can
filmbe
can
be used
20%
from
to 600toC.
These
results
also suggest
that the
used
as a
as a good
protective
Moreover,
K2 increases
with increasing
tAlNthe
—i.e.,
thedevice
SAW would
device
good
protective
layer. layer.
Moreover,
the K2 the
increases
with increasing
tAlN —i.e.,
SAW
would
haveperformance
better performance
with thicker
AlN coatings.
However,
with further
increase
the,tthe
AlN,
have
better
with thicker
AlN coatings.
However,
with further
increase
of theoftAlN
the AlN
coatings
would
negative
effects
onproperties
the properties
theSAW
LGS devices
SAW devices
toeffect,
mass
AlN
coatings
would
havehave
negative
effects
on the
of theof
LGS
due todue
mass
effect,
leading
to
reduction
of
the
resonance
strength
of
the
SAW
devices
[20].
leading to reduction of the resonance strength of the SAW devices [20].
2 of
Figure
K2Kof
thethe
SAW
devices
withwith
different
thickness
of AlN
Figure 5.
5. Electromechanical
Electromechanicalcoupling
couplingcoefficient
coefficient
SAW
devices
different
thickness
of
coating
layerslayers
as a function
of temperature.
AlN coating
as a function
of temperature.
4. Conclusions
Conclusions
In this
thiswork,
work,the
thetemperature
temperaturebehaviors
behaviors
LGS
SAW
devices
coated
thin films
In
of of
thethe
LGS
SAW
devices
coated
withwith
AlN AlN
thin films
have
have
been
investigated.
The
AlN
films
with
different
thickness
were
deposited
on
bare
LGS
SAW
been investigated. The AlN films with different thickness were deposited on bare LGS SAW devices by
devices by mid-frequency
magnetron The
sputtering.
The results
show
that the performance
of the
bare
mid-frequency
magnetron sputtering.
results show
that the
performance
of the bare SAW
device
SAW device
without AlN
coatings
to a at
certain
extent at highwhile
temperature,
thecoated
SAW
without
AlN coatings
attenuates
to aattenuates
certain extent
high temperature,
the SAWwhile
devices
devices
show good
performance
evenSAW
at 600
°C. The
SAW velocity
increases
with
AlNcoated
films with
showAlN
goodfilms
performance
even
at 600 ◦ C. The
velocity
increases
with increasing
with increasing
thickness
of layers.
the AlNThe
coating
layers.frequency
The resonance
frequencySAW
of the
prepared
SAW
thickness
of the AlN
coating
resonance
of the prepared
device
decreases
◦ C. The K2toof
device
decreasestemperature,
with increasing
temperature,
from room
temperature
600
Thedevices
K2 of thecoated
SAW
with
increasing
from
room temperature
to 600
the°C.
SAW
devices
with AlNwith
films
increasestemperature,
with increasing
temperature,
means
that these
with
AlNcoated
films increases
increasing
which
means that which
these SAW
devices
have SAW
good
devices have good performance at high temperature. It can be concluded that the AlN coating layer
can not only protect the SAW devices from environmental contamination, but also improve their
performance. The results of this work suggest that the SAW devices with AlN coating layers are
suitable for high temperature sensing applications. However, it should be noted that the thickness of
Sensors 2016, 16, 1436
6 of 7
performance at high temperature. It can be concluded that the AlN coating layer can not only protect
the SAW devices from environmental contamination, but also improve their performance. The results
of this work suggest that the SAW devices with AlN coating layers are suitable for high temperature
sensing applications. However, it should be noted that the thickness of the AlN coating layer should
be optimized if one expects to use the SAW device as a temperature sensor with large K2 and large
|TCF|, because the AlN coatings may reduce the |TCF| of the LGS SAW devices.
Acknowledgments: This work is financially supported by NSFC (61223002).
Author Contributions: All author participated in the work presented here. Lin Shu and Bin Peng conceive
and designed the experiment. Yilin Cui and Dongdong Gong contributed to the improvement of AlN film
growth method. Zhengbing Yang contributed to the measurement system. 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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