The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015
DOI Number: 10.6567/IFToMM.14TH.WC.OS19.017
Sensitivity Calibration of a Lead-Zirconate-Titanate (PZT)
Thin-Film Intracochlear Microphone
Chuan Luo1
Tsinghua University
Beijing, China
M. Staley2
Siemens Wind Service
Orlando, United States
Abstract: An intracochlear microphone is a critical
component to enable totally implantable cochlear
prostheses. In this study, the intracochlear microphone
takes the form of a piezoelectric diaphragm with an area
. A
of 0.8 mm by 0.8 mm and a thickness of roughly 2
tuning fork generates a pure-tone sound at 4096 Hz
exciting the piezoelectric diaphragm. In the meantime, a
laser Doppler vibrometer measures the vibration
amplitude at the center of the diaphragm. Also, a charge
amplifier measures the charge generated by the
piezoelectric diaphragm.
The measured sensitivity,
defined as the charge-to-displacement ratio, is around 53
pC/ . For a purpose of comparison, a finite element
analysis is conducted to estimate the sensitivity. The
simulation shows that the piezoelectric diaphragm has a
sensitivity of 30 pC/ . The close agreement between the
numerical and experimental results assures the reliability
of the measured sensitivity.
Keywords: Intracochlear microphones, Lead-zirconate-titanate, thin
films, diaphragms, sensitivity calibration.
I. Introduction
Normal ears rely on hair cells in cochlea to transduce
pressure waves into neural signals that generate the sense
of hearing. For patients with severe hearing loss, the hair
cells are so badly damaged that the mechano-electric
transduction is largely lost. In this case, a cochlear
implant, also known as a neural prosthesis, is introduced
to treat patients with severe hearing loss.
Basically, a cochlear implant is an electrode array
surgically inserted in scala tympani of cochlea. The
electrode array stimulates the auditory nerve electrically,
bypassing the mechano-electrical transduction mechanism
(i.e., the hair cells) in cochlea. To perform these
functions, cochlear implants have a behind-the-ear unit
that houses a microphone and a speech processor.
A latest development in cochlear implants is to make
the implants totally implantable [1-5]. In other words,
there will be no behind-the-ear unit. Therefore, the
microphone must be buried inside the body. Totally
implantable cochlear prostheses have the advantage of no
bulky external components, more natural hearing via
auditory pathways, more versatile speech processing
algorithms, and reduced surgical time and complexity
[1-5]. Nevertheless, recent clinical trials using invisible
microphones implanted under the skin reveal increased
body noise interference and reduced microphone
sensitivity [4].
1
luochuan@mail.tsinghua.edu.cn
mckenzie.staley@siemens.com
3
gzcao@u.washington.edu
4
ishen@u.washington.edu
2
G. Z. Cao3
University of Washington
Seattle, United States
I. Y. Shen4
University of Washington
Seattle, United States
One way to bypass the current difficulty encountered in
the invisible microphones is to use an intracochlear
microphone, i.e., a microphone housed inside the cochlear
[6]. Since a good percentage of patients qualified for
cochlear implants do not have conduction hearing loss, a
microphone inside the inner ear will not only be a viable
approach but also provide easy integration with the
electrode array.
Piezoelectric microphones are good candidates for
intracochlear applications because of their high bandwidth
and sensitivity especially in aqueous environments. In
fact, many hydrophones are piezoelectric based. Piezoelectric microphones usually take the form of a piezoelectric diaphragm [7-11], which is relatively easy to
deflect thus capable of producing more charge and a
higher sensitivity. There are, however, many challenges.
One major challenge is size constraints. Most micromachined piezoelectric microphones have a size of 2-4
mm [7-11]. These microphones are all too big to fit into
cochlea, where cross section of scala tympani is roughly 1
mm by 0.5 mm. Another challenge is to maintain a high
sensitivity while keeping the size of the microphones
small enough to fit in the cochlea. As a piezoelectric
diaphragm is scaled down in size, its sensitive is reduced
accordingly [12].
Yet another challenge is calibration of the microphones
given their small size. Ideally, a direct calibration is
preferred. For example, an acoustic wave excites the
piezoelectric diaphragm to generate a voltage output of
the microphone. At the same time, the pressure of the
acoustic wave is measured via a reference microphone.
By comparing the voltage output and the pressure from
the reference microphone, one can obtain the sensitivity
of the microphone in V/Pa [7-11].
The direct calibration method described above is very
costly. It often requires capital investment, such as an
anechoric chamber [11] or a plane wave tube [8]. Also,
the piezoelectric diaphragm has a very tiny surface area.
Technically, the reference microphone does not exactly
measure the pressure acting on the diaphragm.
In this paper, we investigate the feasibility of using a
piezoelectric diaphragm as an intracochlear microphone.
Specifically, we develop strategies to address the three
challenges described above.
Figure 1 shows a piezoelectric diaphragm studied in
this paper. The diaphragm is 0.8 mm by 0.8 mm in size,
. The diaphragm is
and its thickness is roughly 2
located at the tip of a cantilever probe. The diaphragm
can serve as an acoustic actuator when voltage is applied.
Conversely, the diaphragm can serve as a microphone.
When an acoustic load is applied to the diaphragm, a
charge amplifier
connected Lead-Zirconiumto the electrodes can measure
novel small-scale
piezoelectric
the
generated
charge.
As
an
uators to be implanted in the inneradded
ear. convenience,
The PZT such a
piezoelectric diaphragm has been successful fabricated
re wave directly
stimulating
in the
and thoroughly
studied as perilymph
an acoustic actuator
[13,14]. It
Togetherhas
with
a
shortened
electrode,
the
PZTfor animal
also been implanted in guinea pig cochlea
testsacoustic
[15]. Therefore,
the piezoelectric
electric and
stimulation
(CEAS)diaphragm
of the in Fig.
meets
the size
small enough to fit in
s research,1 we
have
twoconstraint,
goals to i.e.,
achieve.
cochlea. Therefore, it can readily be tested as an intracochlear microphone for its sensitivity in the feasibility
ate a piezoelectric
acoustic actuator to fit inside
study.
lop an actuator probe with 1 mm wide, 10 mm
the tip of the
Driving Voltage
Piezoelectric
from Amplifier
gm serving as
Diaphragm
0.8
mm
x
0.8
mm
is applied, the
erate acoustic
The designed
.8 mm, and is
8.5-9 mm
tire actuator is
0.4 mm
(outside cochlea)
together with
1-1.5 mm
1 mm (inside cochlea)
trodes of the
and analyzed. Finally, we conduct a finite element
analysis to estimate the sensitivity to assure the measured
sensitivity.
II. Specimen Preparation
The design and fabrication of the specimen is published
in [13,14] with great detail. Only key features are
summarized here. The diaphragm consists of a layer of
silicon base materials (silicon oxide and nitride) of about
thick and a layer of lead-zirconate-titanate (PZT) of
1
thick; see Figs. 2 and 3. There are three
roughly 1
electrodes. One is a bottom electrode providing the
electric ground (not visible in Fig. 3). There are also a
center electrode and an outer electrode (separated by a
yellow polygon in Fig. 3).
Fig. 1 Proposed PZT probe
Fig. 1. Piezoelectric diaphragm as an
Pressure acoustic actuator and a microphone
Guinea Pig
Wave
Cochlea
, the tip of the
mm) will To
be bypass the challenge encountered in direct
calibration, we adopt an indirect calibration procedure as
cochlea before
follows. An acoustic wave is first generated to excite the
The remaining
diaphragm. At the same time, the diaphragm displacement
side cochlea
to
Auditory to give the
and the charge generated are measured
Brainstem
e piezoelectric
sensitivity in the form of a charge-to-displacement
ratio
Response
e, the actuator
). Then one can convert the displacement
(i.e.,
Piezoelectric
sensitivity
ing the basilar
Probe Tip from to the pressure sensitivity V/Pa by
Round Window
to Pa, one can
simple calculations.
To convert from
piral of cochlea
use the stiffness of the diaphragm obtained from a finite
Fig. 2 Feasibility
element analysis,
for example.study
To convert from pC to V,
one can use the capacitance of the piezoelectric
nction of the
intracochlear
acoustic
diaphragm.
With thispiezoelectric
approach, we can
use no anechoric
chamber.
Moreover,
the
displacement
measurement is
eve this goal, we will acutely implant the actuator
directly
from
the
piezoelectric
diaphragm.
ea of guinea pigs through the round window; see
It is, however, diaphragm
important to generate
stable acoustic
vibrate the piezoelectric
to generate
excitations in the indirect calibration process. Ideally, the
m response
is measured to prove the feasibility
acoustic excitations should contain all frequencies (e.g., a
random noise), so that the measured response can be
averaged to enhance the signal-to-noise ratio and the
measured sensitivity will be valid for a wide range of
frequencies. In our preliminary studies, however, we find
that the PZT diaphragm is very sensitive to electroy in agingmagnetic
seniorsinterference.
and people
work longmeans to
Anywho
electromagnetic
ority of persons
hearing
loss have
generatewith
acoustic
excitations
(e.g., sensorispeakers) severely
interferes the piezoelectric measurements. As a result, the
calibration process is limited only to mechanical
excitations. Three means that mechanically generate
acoustic excitations are tested: an air jet via a 1nozzle, an
air horn, and a tuning fork. Only the tuning fork gives
repeatable results.
For the rest of the paper, we first briefly explain the
design and fabrication of the specimen that contains the
piezoelectric diaphragm. Then we show the experimental
setup that adopts the indirect calibration process to
measure the sensitivity. Experimental results are shown
Fig. 2. A microphone probe with a PZT
diaphragm at the tip
Fig. 3. PZT diaphragm viewed under a
magnifying glass
The PZT diaphragm is fabricated via a sol-gel process
and a deep reactive etch (DRIE) as follows. The silicon
wafer is first treated with oxide and nitride. A Pt-Ti
bottom electrode is deposited. Then PZT sol is coated
and sintered three times to form the PZT thin film. Next,
an Au-Cr top electrode layer is deposited, patterned, and
etched to form the center and outer electrodes. Finally,
the silicon wafer with the PZT thin film is etched via
DRIE from the backside to form the diaphragm and to
release the probes simultaneously. The probes are further
coated with Parylene to prevent the probes from short
circuits in aqueous environments.
The first natural frequency of the PZT diaphragm is
generally between 60 and 90 kHz. The large variation of
Displacement[nm/V]
the natural frequency results from thermal residual
stresses developed in the PZT diaphragm. During the
C. As
fabrication, the PZT needs to be sintered at
the PZT, the electrode, and the silicon wafer cool down to
room temperature, significant thermal stresses develop.
) compared
Since the diaphragm is very thin (cf. 2
), membrane residual
with its large width (cf. 800
stresses will significantly affect the natural frequency of
the diaphragm.
Figure 4 shows a measured frequency response
function when the PZT diaphragm is excited electrically
as an acoustic actuator [14]. A distinct resonance at the
first natural frequency (about 80 kHz) is observed. Below
the first natural frequency, the frequency response of the
PZT diaphragm is virtually flat. Since the audible
frequency range (e.g., less than 15 kHz) is much lower
than the natural frequency, the PZT diaphragm can be
considered as a static structure in the audible frequency
range. This implies that calibration of the diaphragm at a
single audible frequency will suffice.
To O-Scope
PZT
Thin Film
LDV
Top
Electrode
To Charge
Amplifier
Bottom
Electrode
Silicon
Substrate
Diaphragm
Suspension
Sound
Source
Fig. 5. Experimental setup to measure
microphone sensitivity
+
800
Resonance
peak
600
Low frequency
flat zone
400
200
0
0
20
40
60
80
100
Frequency [kHz]
Phase [deg]
200
Fig. 6. Tuning fork used to generate the
acoustic excitations in the experiment
100
0
-100
-200
0
20
40
60
80
100
Frequency [kHz]
Fig. 4. A typical frequency response
function of the PZT diaphragm when
driven electrically [14]
III. Experimental Setup and Measured Results
Figure 5 illustrates the experimental setup, which
includes the following elements. The first element is a
tuning fork as the sound source (Fig. 6). A plastic mallet
is struck on the tuning force to produce a pure-tone
acoustic excitation at 4.096 kHz.
Once the PZT
diaphragm is excited, a laser Doppler vibrometer
measures the velocity at the center of the diaphragm.
Since the excitation frequency is known, the velocity can
be converted to a displacement. At the same time, a
double-end charge amplifier measures the generated
charge from the PZT diaphragm via the center electrode.
The charge amplifier needs to be carefully grounded and
shielded to reduce electromagnetic interference [7]. The
velocity and charge measurements are both fed into a
digital oscilloscope for comparison.
Figure 7 shows the measured response from the digital
oscilloscope. The top trace is the velocity measurement
from the laser Doppler vibrometer, and the bottom trace is
the charge measurement from the charge amplifier. Both
measurements have the same frequency. The velocity
measurement is relatively clean with a good sinusoidal
response. Although the charge measurement presents
some noise, it does follow the velocity measurement in an
out-of-phase manner very consistently at the same
frequency.
Fig. 7. Velocity and charge measurements
from the digital oscilloscope
Based on the measurements above, we take 23 peak
values to calculate the sensitivity. The average sensitivity
with a standard deviation of 6.7
.
is 53.1
IV. Finite Element Simulations and Discussions
To assure that the measured sensitivity is reasonable,
we further conduct a finite element analysis to see if the
sensitivity falls within the same ballpark range. Figure 8
shows the finite element model that we used to estimate
the sensitivity. Since the finite element model has been
published with great detail in [13,14], it is summarized
only briefly for reference here.
The finite element model is one-eighth of the PZT
diaphragm. The diaphragm is fixed at the outside
boundary simulated by the bulky silicon substrate. For
the other two sides, symmetric boundary conditions are
imposed. The diaphragm has multiple layers, such as
silicon and PZT layers. The PZT layer is modeled via
piezoelectric elements, while the rest is modeled via solid
elements. At the juncture of the diaphragm and the
silicon substrate, there is a transitional area in the form of
trapezoid, which simulates residual silicon that is not
removed by the DRIE process. Moreover, the residual
silicon has a unique geometry. It is circular inside at the
interface with the diaphragm, but it is square outside at
the interface with the silicon substrate. It turns out that the
residual silicon is important in determining mechanical
response and consequently the sensitivity of the PZT
diaphragm [16]. Therefore, it must be included in the
model.
potential can be calculated from the finite element
analysis. The finite element analysis shows that the center
electrode presents a voltage of 0.176 mV and the outer
electrode presents a voltage of 0.0599 mV.
Table 1 shows the area, capacitance, and the sensitivity
of the center and outer electrodes. The capacitance is
calculated from the parallel plate formula given the
dielectric constant of PZT-7A (i.e., 274.15). With the
voltage and capacitance, we can calculate the charge
generated from each electrode. With a displacement of
1.921 nm, the sensitivity for the center and outer
and 31.56
electrodes are found to be 29.84
, respectively.
Fig. 8. Finite element model to estimate
sensitivity of the PZT diaphragm
V. Conclusions
In this paper, we experimentally calibrate the
sensitivity of a PZT thin-film diaphragm of a size 0.8 mm
by 0.8 mm. A tuning fork is a reliable source to generate
acoustic excitations driving the PZT diagram. Via a laser
Doppler vibrometer and a charge amplifier, the measured
.
sensitivity of the PZT diaphragm is around 53
An independent finite element analysis shows that the
.
sensitivity of the PZT diaphragm is around 30
Given uncertainties associated with the PZT diaphragm,
the agreement between the measured and calculated
sensitivities is consider close thus assuring the accuracy of
the calibration.
The one-eighth model is sufficient for the purpose of
this simulation for several reasons. First, a static analysis
is performed on the finite element model, because the
diaphragm behaves like a static structure in the audible
frequency range. Second, a unit pressure of 1 Pa is
applied to the diaphragm. Under these conditions and
geometry, the deflection of the diaphragm with present
symmetry with respect to the centerlines and diagonals.
Therefore, one-eighth model will suffice.
Under the static load of 1 Pa, the corresponding
displacement at the center of the diaphragm is calculated
as 1.921 nm. As shown in Fig. 8, the deflection is not
uniform over the diaphragm. It reaches the maximal
value of 1.921 nm at the center and gradually transition to
zero somewhere in the residual silicon. For the rest of the
paper, we will only refer to the displacement at the center
of the diaphragm.
For the PZT layer, its bottom side is grounded with
zero electric potential. The electrical boundary condition
at the topside of the PZT layer is free, so that an electric
TABLE I. Parameters of the center
and outer electrodes
There are a couple of issues worthy of further
discussions. First, the voltage of 0.176 mV is definitely
small at 1 Pa. Nevertheless, it is better than other intracochlear microphone that has a response in the range of
at 1 Pa [5]. Second, the calculated sensitivity is in the
same order of the measured sensitivity, basically 30
vs. 53
. These two sensitivities agree very
well with each other given many uncertainties in the finite
element models. For example, the exact thickness of the
diaphragm and the size of the residual silicon are often not
known. Even under a scanning electronic microscope,
dimension measurements can easily have 10% variations.
Residual stresses in the diaphragm are also not known
thus affecting stiffness of the diaphragm.
VI. Acknowledgment
This material is based upon work supported by the
National Science Foundation under Grant Nos.
CMMI-1030047 and CBET-1159623. Any opinions,
findings, and conclusions or recommendations expressed
in this material are those of the authors and do not
necessarily reflect the views of the National Science
Foundation.
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