A Piezoelectric MEMS Microphone Based on Lead 

Zirconate Titanate (PZT) Thin Films 

by Ronald G. Polcawich 

ARL-TR-3387 November 2004 

Approved for public release; distribution unlimited. 
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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-3387 November 2004 

A Piezoelectric MEMS Microphone Based on Lead 

Zirconate Titanate (PZT) Thin Films 

Ronald G. Polcawich 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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14. ABSTRACT 

Piezoelectric microelectromechanical (MEMS) scale acoustic sensors have potential applications in a wide variety of 

applications including hearing aids, surveillance, and heart monitoring. For each of these systems and many others, the acoustic 

sensors must be miniaturized and have low power requirements. A piezoelectric-based microphone can provide a solution to 

these requirements, since it offers the ability to passively sense without the power requirements of condenser or piezoresistive 

microphone counterparts. This research effort reports on the design and fabrication of a piezoelectric PbZr0.52Ti0.48O3 (PZT) 

based acoustic sensor. A circular clamped membrane consisting of a dielectric for structural support and a piezoelectric actuator 

has been fabricated on a silicon wafer via silicon deep reactive ion etching (DRIE). Sensors ranging from 500 to 2000 microns 

in diameter have been fabricated and characterized with the use of scanning laser Doppler vibrometry and calibrated acoustic 

tone sources. The PZT sensors exhibited a sensitivity of 97.9 to 920 nV/Pa, depending on geometry. 

15. SUBJECT TERMS

PZT, acoustic Sensor, MEMS, microphone 

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Ronald G. Polcawich 

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iii

Contents 

List of Figures iv

List of Tables v

1. Introduction 1

2. Microphone Fabrication 3

3. Acoustic Testing 6

4. Results and Discussions 8

5. Conclusion 16

6. References 17

Distribution List 19 
iv

List of Figures 

Figure 1. Schematic description of a MEMS photoacoustic resonant cell. ....................................2 

Figure 2. Processes for the deposition of (a) PECVD dielectrics, (b) DC magnetron sputtered 

metals, and (c) sol-gel deposited PZT thin films. ......................................................................3 

Figure 3. Fabrication process flow for creating a PZT membrane microphone: (a) Starting 

wafer, (b) sacrificial titanium, (c) Ar ion mill of Ti/Pt/PZT/Pt, (d) PZT wet etch, (e) oxide 

passivation deposition, (f) etching of passivation, (g) Ti/Au contact, (h) silicon DRIE, and 

(i) front view of released PZT membrane..................................................................................4 

Figure 4. Illustration of the device die, after the Bosch process, with as many as four 

individual PZT microphones per die..........................................................................................6 

Figure 5. Images of the acoustic test chamber used for testing the PZT microphone. (The 

BNC mounts provide electrical contact to the packaged microphone whereas the acrylic 

plate on top provides a connection for the acoustic tube.).........................................................7 

Figure 6. Schematic representation of experimental test setup for analysis of the PZT and 

calibrated B&K microphones. ..................................................................................................8

Figure 7. (a) Optical surface micrograph of a PZT membrane actuator with center 80% 

electroded and (b) optical surface micrograph of a PZT actuator with outer 20% 

electroded...................................................................................................................................9 

Figure 8. (a) An SEM image of a silicon DRIE release, (b) SEM image of the surface profile 

of a released membrane, and (c) SEM image illustrating the SiO2 passivation. .....................10 

Figure 9. Laser Doppler vibrometry (LDV) results (a) mode shapes of fundamental and third 

harmonic vibration and (b) frequency response from a 500-μm diameter membrane with 

80% sensor coverage................................................................................................................11 

Figure 10. Resonant frequency versus the inverse membrane radius confirming membrane 

behavior rather than clamped plate behavior of the PZT microphones..................................12 

Figure 11. Acoustic response of a B&K and a 750-μm diameter PZT MEMS microphone 

with 20% sensor coverage for (a) B&K Pistonphone 250-Hz tone at 124 dB and (b) ACO 

Pacific 1-kHz tone at 104 dB...................................................................................................13 

Figure 12. Sensitivity versus radius of microphone including both predicted values and the 

amplified experimental results for the 20% and 80% coverage sensors.................................14 

Figure 13. Abnormal polarization electric field hysteresis loop for a 1000-μm diameter 

PZT microphone. .....................................................................................................................15 

Figure 14. Optical micrograph of a 80/20 combined acoustic sensor...........................................15 
v

List of Tables 

Table 1. Basic performance characteristics of the three most investigated MEMS 

microphones (1). ........................................................................................................................1 

Table 2. ZnO (13) and PZT thin film properties and microphone sensitivity. ..............................2 
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

Miniature acoustic sensors are crucial for a wide range of military and commercial applications. 

A microelectromechanical (MEMS) scale acoustic sensor can easily be implemented into a 

sensor suite used for unattended remote sensing applications or as an inexpensive hearing aid for 

the hearing impaired. Other potential applications include use in acoustic signal localization, 

physiological monitoring, and as an integral component in a MEMS scale photoacoustic 

spectrometer. 

There have been numerous efforts in developing a MEMS microphone. The three main 

approaches investigated have been capacitive, piezoresistive, and piezoelectric (1-7). Most past 

research has focused on complementary metal oxide semiconductor (CMOS) integration so that 

on-chip amplifier circuitry can be implemented along with the microphone. Capacitive and 

resistive microphones have been examined the most because of their relative ease integrating 

with microelectronics. Additionally, substantial efforts have been put forth attempting to 

integrate a zinc oxide (ZnO) piezoelectric microphone with CMOS (8-10). 

Table 1 illustrates several performance specifications for the three main microphone 

technologies. A piezoelectric-based microphone can offer two main advantages: no required 

input power and a wide dynamic range. A passive acoustic sensor is ideal for the low cost, 

disposable sensors required by the military for remote sensing. Although the piezoelectric 

sensors may be less sensitive than their capacitive and piezoresistive counterparts, they can be 

very advantageous in situations not necessarily governed by sensor sensitivity. For example, in 

an open environment, low frequency, 1/f noise, can dominant the spectrum and provide a noise 

floor of 30 to 40 dB sound pressure level (SPL) (re 20 μPa). During the same conditions, typical 

battlefield sounds can be much greater than 100 dB SPL (e.g., a brick of C4 explosive at 30 m is 

165 dB and a rifle shot at the shooter’s ear is 156 dB). 

Table 1. Basic performance characteristics of the three most investigated 

MEMS microphones (1). 

Capacitive Piezoresistive Piezoelectric

Sensitivity 

(◊V/Pa)

Good 

400 to 1000

Low 

0.1 to 100

Medium 

10 to 500

Input Power Required Required None

Dynamic 

Range Narrow Relatively Wide Wide
2

For a passive piezoelectric microphone, there are several options for the thin film sensor material 

including ZnO, aluminum nitride (AlN), and lead zirconate titanate (PZT). Our research focused 

on developing a PZT microphone because of its large piezoelectric coefficient and low dielectric 

loss compared to ZnO. The microphone was designed for use in a MEMS scale photoacoustic 

spectroscopy system (11,12). The MEMS microphone was integral in the miniaturization of the 

photoacoustic resonant cell allowing for improved trace gas sensing through a reduction in path 

length between the photoacoustic event and the acoustic sensor (see figure 1). 

Figure 1. Schematic description of a MEMS photoacoustic resonant cell. 

A general comparison of ZnO and PZT thin films is available in table 2. With a rudimentary 

equation for microphone sensitivity, it is quite clear that a microphone using PZT will not yield 

the most sensitive microphone. Although PZT has a transverse piezoelectric coefficient an order 

of magnitude larger than ZnO, the gain in performance is reduced because of its 100-fold 

increase in dielectric constant. One potential benefit of using PZT over ZnO is the lower 

dielectric loss typically exhibited in PZT. With this understanding, this report outlines the 

research to date on the development of PZT MEMS microphone for potential use in a prototype 

MEMS photoacoustic spectrometer and for use as a remote acoustic sensor. 

Table 2. ZnO (13) and PZT thin film properties 

and microphone sensitivity. 

 ZnO PZT

Piezoelectric 

Coeffidient, d31

(pm/V) 

-5 -50 

Dielectric 

Constant 

10 1000 

Dielectric Loss 5 to 10% 2 to 4 % 

Sensitivity 

(μV/Pa) 

488 68 

MEMS Microphone 

Photoacoustic Event 
3

2. Microphone Fabrication 

A piezoelectric microphone based on PZT thin films was designed in order to investigate its 

potential use as a passive sensing element in a photoacoustic resonant cell. The microphone 

fabrication process began with a double-sided polished silicon wafer and used several different 

types of deposition systems (see figure 2). A plasma-enhanced chemical vapor-deposited 

(PECVD) silicon dioxide thin film (1 μm thick) was deposited with a Plasma-Therm 790 reactor 

using a mixture of SiH4, He, and N2O. The oxide served as the membrane structural layer and 

was chosen to be 1 μm thick. After deposition, the film was annealed in an A.G. Associates 

Heatpulse 610 rapid thermal anneal (RTA) furnace at 700 °C for 60 seconds in a nitrogen 

atmosphere. This annealing removes the trapped hydrogen within the film and causes the film to 

obtain a slightly tensile stress, which aids in producing a planar membrane and a high 

performance microphone. 

Figure 2. Processes for the deposition of (a) PECVD dielectrics, (b) DC magnetron sputtered metals, 

and (c) sol-gel deposited PZT thin films. 

After the oxide is deposited and annealed, a metal electrode was deposited via sputtering to serve 

as the bottom electrode and as a growth template for the piezoelectric actuator. For adhesion to 

the oxide, a thin layer (200 Å) of titanium was first sputtered and was immediately followed by 

platinum deposition (800 Å) without breaking vacuum within a Varian 3190 direct current (DC) 

magnetron sputter deposition system. Following the platinum deposition, the wafers were 

annealed in the RTA furnace at 700 °C for 60 seconds in flowing dry air in order to improve the 

adhesion between the oxide and metal layers and the surface texture of the platinum before 

deposition of the piezoelectric thin film. 

PECVD DC Magnetron Sputtering Sol-Gel Deposition
4

The next fabrication step was to deposit the PbZr0.52Ti0.48O3 thin film. The deposition process is 

a solution spin on process. First, a PZT sol-gel solution was prepared via a modified alkoxide 

process first introduced by Budd, Dey, and Payne (14). This process used lead acetate trihydrate, 

zirconium n-propoxide, and titanium isopropoxide as the precursors and 2-methoxyethanol as the 

solvent. Once the sol-gel solution was prepared and aged, the repetitive deposition process, as 

depicted in figure 3c, began with a portion of the sol dispensed onto a platinized silicon 

substrate. The wafer was then spun at 2,500 rpm for 30 seconds. Next, the wafers were placed 

onto a hotplate at 350 °C for 120 seconds, which causes the film to undergo pyrolysis, thereby 

decomposing all the organics. This process of deposition, spin, and pyrolysis was then repeated 

a total of four times. After the last pyrolysis, the wafer was annealed in a RTA furnace at 700 °C 

for 30 seconds in flowing air in order to crystallize the PZT thin film. The result was 

approximately a 0.25 μm PZT film, and the entire process was continued in order to achieve the 

desired thickness of a 1-μm PZT thin film. 

After the piezoelectric was deposited, a top electrode of platinum (800 Å) was sputter deposited 

onto the wafer surface. The wafers were then annealed in an RTA at 350 °C for 120 seconds in 

flowing air to improve adhesion and reduce any sputtering induced surface damage. 

a) b) c)

d) e) f)

g) h) i) 

Figure 3. Fabrication process flow for creating a PZT membrane microphone: (a) Starting wafer, 

(b) sacrificial titanium, (c) Ar ion mill of Ti/Pt/PZT/Pt, (d) PZT wet etch, (e) oxide 

passivation deposition, (f) etching of passivation, (g) Ti/Au contact, (h) silicon DRIE, and 

(i) front view of released PZT membrane. 
5

The resultant wafer stack at this stage is Si/SiO2/Ti/Pt/PZT/Pt. The first step in the microphone 

fabrication is to define the final actuator dimensions and the location of the electrical contacts. 

Because electrical contact has to be made with the bottom Ti/Pt electrode, a sacrificial titanium 

layer was electron beam evaporated onto the wafer and patterned with a lift-off technique. The 

next step was to pattern the wafer with Clariant AZ 5214E reverse image photoresist. To define 

the final actuator dimensions, argon ion milling was employed to sputter remove the 

Ti/Pt/PZT/Pt stack from most of the wafer. During this process, the sacrificial Ti layer will 

prevent the ion milling of approximately half the PZT layer and all of the Ti/Pt bottom electrode. 

After removal of the photoresist, another resist pattern was placed onto the wafer in order to 

open windows to the bottom electrode. Within these windows, the structure was 

Si/SiO2/Ti/Pt/PZT. The PZT was then wet etched with H20:HCl:HF (2:1:0.02) in order to expose 

the Pt. Afterwards, the photoresist was removed with photoresist stripper at 85 °C. 

The next step in the fabrication was to deposit a 2500-Å PECVD SiO2 thin film to serve as an 

isolation layer preventing electrical contact between the top and bottom electrical traces. After 

deposition, the film was annealed in an RTA at 350 °C for 120 seconds in flowing N2 and then 

again for 120 seconds in flowing air. This anneal improves the adhesion between the structural 

silicon dioxide and the isolation oxide. The next step was to pattern the oxide around the 

circumference of the actuator. Again, positive photoresist was used and the wafer was placed 

into a LAM1

 590 etching system in which the oxide was etched with CHF3/CF4 plasma. 

To make electrical contact with the top Pt electrode of the actuator, an evaporated 200-Å Ti/ 

2500 Å Au layer was deposited and patterned with a lift-off technique. The Ti/Au was also 

deposited onto the bottom Ti/Pt electrode so that gold wire bonding could be used to package the 

final devices. 

The final fabrication step was a deep reactive ion etch (DRIE) of the silicon substrate in order to 

release the membrane actuator. With a Karl-Suss MA/BA1

 6 mask aligner, a 6-μm thick positive 

photoresist (AZ 9245) was patterned onto the reverse side of the silicon substrate. The silicon 

DRIE was performed with a Unaxis VLR1

 cluster tool configured with an inductively coupled 

plasma (ICP) etch chamber. The silicon DRIE followed the Bosch (15) process using a cyclical 

etching process that consisted of a polymer deposition with a C4F8 plasma followed by an 

isotropic silicon etching with a SF6 plasma. In addition to creating a released membrane, the 

Bosch etch was used to separate each of the device die (see figure 4). 

 1

 not an acronym. 
6

Device die 5 mm 

Figure 4. Illustration of the device 

die, after the Bosch process, 

with as many as four individual 

PZT microphones per die. 

After etching, the remaining photoresist was removed with an oxygen plasma, and the resultant 

microphone die were ready for packaging. The separated device die were subsequently 

packaged with a TO-8 package that was pre-drilled with a circular release hole. The release hole 

allows the membrane to deflect and freely push air out the opening, thereby reducing the 

deleterious effect of squeeze film damping. 

3. Acoustic Testing 

We evaluated the performance of a PZT microphone by placing a packaged microphone die into 

an electrical test chamber (as seen in figure 5) with rubber grommet seals to prevent outside 

noise interference. An acrylic cap was placed on top the packaged microphone die, and a Tygon2

tube was attached between the acrylic cap and the tube driver assembly. The acoustic test 

chamber was configured with BNC3 mounts for electrical connection and a 1/8-inch inner 

diameter (ID) nozzle for connecting to the Tygon tube from the signal source. 

 2

 Tygon® is a registered trademark of Saint Gobain Performance Plastics. 

3

 not an acronym. 
7

Figure 5. Images of the acoustic test chamber used for testing the PZT microphone. 

(The BNC mounts provide electrical contact to the packaged microphone whereas 

the acrylic plate on top provides a connection for the acoustic tube.) 

Two different sound sources were used to characterize the performance of the PZT microphones. 

The first source was an ACO4 Pacific electromagnetic source outputting a 1-kHz tone of either 

94 dB or 104 dB. The second sound source was a B&K4 Pistonphone producing a 124-dB tone 

at either 250 Hz or 325 Hz. As seen schematically in figure 6, the generated acoustic wave was 

detected simultaneously by both a B&K4

 calibrated microphone and the PZT microphone. The 

data were collected with Labview software and subsequently analyzed with a MATLAB5

algorithm. To improve output signal from the PZT microphones, a Stanford Research Systems 

4330 operational amplifier was added during the measurements. 

 4

 not an acronym. 

5

 MATLAB® is a registered trademark of the Math Works. 
8

Network 

Analyzer

B&K 

Mic

MEMS 

Mic

Figure 6. Schematic representation of experimental test 

setup for analysis of the PZT and calibrated 

B&K microphones. 

4. Results and Discussion 

PZT membrane microphones with a diameter from 500 to 2000 μm were successfully fabricated 

with two main designs initially focused upon. The first consisted of a PZT sensor covering the 

central 80% of the released diaphragm while the second consisted of a PZT sensor covering the 

outer 20% of the released diaphragm (see figures 7a and 7b, respectively). By reducing the 

coverage of the PZT, a maximum output signal can be obtained by the combination of the 

contribution from conflicting strain responses of a deflecting membrane. The 80/20 break down 

of the two microphones was chosen because the inflection point in the strain response of a 

deflecting membrane lies near this location. For each of the two choices, the PZT has been 

removed from outside the sensor region. 
9

a) 

b) 

Figure 7. (a) Optical surface micrograph of a PZT membrane 

actuator with center 80% electroded and (b) optical 

surface micrograph of a PZT actuator with outer 

20% electroded. 

Scanning electron micrographs in figure 8a illustrate the Bosch etch used to create the released 

membrane. The resultant membrane structure consists of an overall positive stress gradient 

resulting in a planar released surface as depicted in figure 8b. The bumps on the surface of the 

device die are the remnant gold wire bonds. The last image in figure 8 shows the oxide 

passivation around the perimeter of the PZT sensor as well as the Ti/Au contact strap. 

500 μm 

500 μm 

Pt on 

PZT 

Ti/Pt 

Top 

Contact

Pt on 

PZT 

Ti/Pt 

Top 

Contact
10

a) 

b) 

c) 

Figure 8. (a) An SEM image of a silicon DRIE release, 

(b) SEM image of the surface profile of a released 

membrane, and (c) SEM image illustrating the 

SiO2 passivation. 

Si DRIE Etch 

Hole 

Ti/Au

SiO2

Passivation 
11

To assess the performance of the PZT membranes, the packaged die were first analyzed with a 

Polytec scanning laser Doppler vibrometer so that the resonance characteristics could be 

obtained. With a pseudorandom 1-Vp-p input signal with a 1-Voffset, the frequency response and 

resonant mode shapes can be obtained for each of the diaphragms tested (see figure 9). Testing 

of various sized membranes confirmed that when driven as actuators, they exhibit a classic 1/r 

frequency dependence, in which r is the membrane radius, thereby proving that the microphones 

operate as membranes and not as clamped plates (see figure 10). 

a) 

b) 

0.2

0.4

0.6

0.8

1

100 200 300 400 500

Velocity (mm/s)

Frequency (kHz)

Figure 9. Laser Doppler vibrometry (LDV) results (a) mode shapes of fundamental 

and third harmonic vibration and (b) frequency response from a 500-μm 

diameter membrane with 80% sensor coverage. 

0 500 nm
12

0 100

1 102

2 102

3 102

4 102

0 1000 2000 3000 4000 5000

Resonant Frequency (kHz)

1/Radius (m-1)

Linear Fit

Figure 10. Resonant frequency versus the inverse membrane 

radius confirming membrane behavior rather than 

clamped plate behavior of the PZT microphones. 

The acoustic response of the microphones was plotted on a power (dB·V) versus frequency plot 

after a fast Fourier transform (FFT) was performed on the voltage output from each sensor. 

Typical plots generated for both sound sources are seen in figures 11a and 11b for a 750-μm 

diameter microphone with 20% sensor coverage. The PZT microphones performed well in these 

early experiments with magnitudes not too dissimilar from the B&K microphone. A clear 

distinction between the PZT and B&K output was the amount of noise in the PZT response. One 

side effect of using an unmatched amplifier was the unintentional amplification of the noise in 

both the data acquistion as well as chamber losses. The noise amplification was also evident in 

the 60 Hz harmonics present in figure 11 at 300 Hz and 1020 Hz. Although the noise floor is 

amplified, it remains in the low to mid 10-6 V range whereas the noise floor for the B&K 

microphone is 10-7 V. 
13

a) 

b) 

Figure 11. Acoustic response of a B&K and a 750-μm diameter 

PZT MEMS microphone with 20% sensor coverage for 

(a) B&K Pistonphone 250-Hz tone at 124 dB and (b) ACO 

Pacific 1-kHz tone at 104 dB. 

An examination of the acoustic output from the PZT microphone produced an unamplified 

sensitivity of 97.9 to 920 nV/Pa, depending on the size and configuration of the microphone. 

This sensitivity falls far short from the predicted values that should range in the mid to high 

microvolt per Pascal. The plot in figure 12 compares the predicted sensitivities against the 

experimentally determined values including the amplifier. The amplified sensitivities were 

plotted for ease of viewing both values. Each curve follows a radius squared relationship as 

expected from the area term in the sensitivity equation. Unfortunately, the experimental values 

do not possess the same multiplicative constant for increasing radius. Even with low sensitivities 

of the PZT microphone, they exhibit a current limit of detection of ~ 50 dB SPL, which can 

make the sensor viable as a remote acoustic sensor and useful in the preliminary testing of a 

MEMS photo-acoustic sensor. 
14

0 100

5 102

1 103

2 103

2 103

200 400 600 800 1000 1200

PZT (80% coverage)

PZT (20% coverage)

Predicted (80% coverage)

Predicted (20% coverage)

Sensitivity (

μV/Pa)

Radius (μm)

Figure 12. Sensitivity versus radius of microphone including 

both predicted values and the amplified experimental 

results for the 20% and 80% coverage sensors. 

There are several potential possibilities for the difference between predicted and experimental 

values. One important characteristic to the predicted values concerns the assumption of zero 

residual stress. It is well known from previous research efforts that a composite PZT thin film 

actuator possesses a wide variety of residual stress values with the overall gradient determining 

the behavior of the final device. As previously stated with figure 7, a planar-released membrane 

indicates a positive residual stress gradient in the composite. In addition to the residual stress 

assumption, the sensitivity equation also assumes a minimal load capacitance. There is 

definitely a finite load capacitance in the electrical setup, especially considering the impedance 

mismatch between the PZT sensor and the SRS operational amplifier (op-amp) and the 

extraordinarily long connection cables. The final assumption with the sensitivity equation is 

proper match of both the acoustic and electrical domains. Even though there is a open cavity in 

the TO-5 package, there is an acoustic mismatch between the DRIE release via hole and the open 

air cavity of the test chamber. 

Another possible reason for the diminished output from the PZT microphones was resolved upon 

investigation of the dielectric and ferroelectric properties of the devices. The dielectric 

properties of the microphones were good with dielectric constants of 1021 ± 55 and a dielectric 

loss of 3.5% ± 1%. Unfortunately, the ferroelectric hysteresis loops exhibited a pinched 

hysteresis loop indicative on a non-optimal PZT thin film and may lead to diminished 

piezoelectric properties (see figure 13). It is true that some of the normal hysteretic response can 

be achieved with poling but in order to fully remove the effect of the pinched loop, a thermal 

poling process is required. The packaged microphones were unable to undergo a thermal poling 

process due at the time of this investigation. 
15

Figure 13. Abnormal polarization electric field 

hysteresis loop for a 1000-μm diameter 

PZT microphone. 

Besides optimizing the material properties and the acoustic and impedance matches, one 

possibility for increasing the sensitivity is using electrode shaping and/or design. Combining the 

output signal from the inner and outer regions of a deflecting membrane can be extremely 

beneficial to the performance of a membrane. As presented by Kim (9), differential summation 

from different regions of the acoustic sensor can be used to optimize the voltage output from a 

piezoelectric microphone while minimizing the effects of increased capacitance with sensor size. 

Our first attempts examine the combined output from a PZT sensor with both the outer 20% and 

inner 75% covered with PZT (see figure 14). 

Figure 14. Optical micrograph of a 

80/20 combined acoustic 

sensor. 

Pt on 

PZT 

Ti/Pt

Top 

Contact

Ti/Pt
16

5. Conclusion 

Initial research successfully fabricated a piezoelectric microphone for acoustic sensing. The PZT 

microphones were shown to perform well up to their mechanical resonant frequency and may 

provide a low power means of detecting acoustic signals. These initial microphones exhibit a 

sensitivity of 97.9 to 920 nV/Pa. Although these values are lower than predicted performance, 

further modifications of the microphone, acoustic and electric impedance matching, and 

predicting equations were presented as a means to further enhance the development of a PZT 

MEMS-based microphone. 
17

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