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1Design, Fabrication, and Characterization of
Scandium Aluminum Nitride-Based
Piezoelectric Micromachined
Ultrasonic Transducers

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Abstract— This paper presents the design, fabrication,and characterization of piezoelectric micromachined ultrasound transducers (PMUTs) based on scandium aluminum
nitride (Scx Al1−x N) thin films (x = 15%). ScAlN thin film was
prepared with a dual magnetron system and patterned by areactive ion etching system utilizing chlorine-based chemistrywith an etching rate of 160 nm/min. The film was characterizedby X-ray diffraction, which indicated a crystalline structure
expansion compared with pure AlN and a well-aligned ScAlN
film. ScAlN PMUTs were fabricated by a two-mask process
based on cavity SOI wafers. ScAlN PMUTs with 50- and 40-µm
diameter had a large dynamic displacement sensitivity measured
in air of 25 nm/V at 17 MHz and 10 nm/V at 25 MHz,
twice that of AlN PMUTs with the same dimensions. The peak
displacement as a function of electrode coverage was characterized, with maximum displacement achieved with an electroderadius equal to 70% of the PMUT radius. Electrical impedance
measurements indicated that the ScAlN PMUTs had 36% greater
electromechanical coupling coefficient (k2t ) compared with AlN
PMUTs. The output pressure of a 7 × 7 ScAlN PMUT array
was 0.7 kPa/V at ∼1.7 mm away from the array, which isapproximately three times greater that of an 8 × 8 AlN PMUT
array with the same element geometry and fill factor measured at
the same distance. Acoustic spreading loss and PMUT insertion
loss from mechanical transmit to receive were characterized witha 15 × 15 ScAlN PMUT array via hydrophone and laser Doppler
vibrometer.
[17509-2017]

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Fig. 1.

Schematic cross-section of PMUT.

pling, lower manufacturing cost and lower power consumption
compared to conventional bulk ultrasonic transducers.
Piezoelectric micromachined ultrasonic transducers (PMUT)
have been rapidly developed in recent years due to the progressof piezoelectric thin films. Aluminum nitride (AlN) has
been widely used for piezoelectric MEMS device fabrication
because it is available from a number of MEMS foundries andis compatible with CMOS manufacturing [7]–[9]. However,compared to lead zirconate tianate (PZT), a piezoelectricmaterial which requires high annealing temperature and is
not process-compatible with CMOS, AlN has relatively low
piezoelectric coefficient (e31, f ), which leads to low sensitivity
and low electromechanical coupling (kt2 ) [9]–[11].
Scandium (Sc) alloying has been proposed recently as ameans to increase the e31, f of AlN, while maintaining processcompatibility with existing AlN based manufacturing [12].
Most of the previously-reported work on ScAlN focused on
bulk acoustic wave (BAW) resonators or surface acoustic
wave (SAW) devices which utilize the longitudinal piezoelectric mode and require high stiffness to achieve high frequency
operation and high quality factor (Q) [12], [13]. However,studies also found that with the increase of Sc concentrations,the stiffness of the thin film decreased and the dielectric
constant increased [14], [15]. In this paper, we present flexural
PMUT devices which use the transverse piezoelectric mode
and where the reduced stiffness of ScAlN may provide abenefit over conventional AlN.

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Qi Wang, Student Member, IEEE, Yipeng Lu, Member, IEEE, Sergey Mishin,
Yury Oshmyansky, and David A. Horsley, Senior Member, IEEE

Index Terms— Piezoelectric micromachined ultrasoundtransducers(PMUT),
piezoelectricfilms,
piezoelectrictransducers.

I. I NTRODUCTION
ANY applications have been developed based on
micromachined ultrasonic transducers (MUTs) inrecent years, such as medical imaging [1]–[3], gesturesensors [4], ultrasonic fingerprint sensors [5], and bodycomposition sensors [6]. MUTs have a better acoustic cou-

M

Manuscript received November 7, 2016; revised May 23, 2017; accepted
May 26, 2017. This work was supported by Berkeley Sensor and Actuator industrial members. Subject Editor A. Seshia. (Corresponding author:
Qi Wang.)
Q. Wang, Y. Lu, and D. A. Horsley are with the Department of Mechanical and Aerospace Engineering, University of California at Davis, Davis,
CA 95616 USA (e-mail: qixwang@ucdavis.edu).
S. Mishin and Y. Oshmyansky are with Advanced Modular Systems, Inc.,
Goleta, CA, USA.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2017.2712101

II. M ATERIALS AND M ETHODS
A cross-section schematic of a PMUT is shown in Fig. 1.
The PMUT was composed of a 1 μm thick ScAlN

1057-7157 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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Fig. 2. An optical microscope image of a 7 × 49 PMUT array. The individual
PMUTs are 50 μm diameter and the array pitch is 70 μm.

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Fig. 3. Cross-sectional SEM image of a ScAlN PMUT. The Mo and ScAlN
show good columnar structure indicating a highly c-axis oriented film.

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piezoelectric layer, a 200 nm Mo layer as bottom electrodeand a 2.5 μm thick silicon membrane. Devices were alsofabricated using pure AlN with identical film thicknesses andgeometries in order to provide a comparison between ScAlN
and AlN PMUTs. The fabrication process used custom cavity
SOI (CSOI) substrates wherein vacuum cavities are formed
beneath the Si device layer of the CSOI wafer [10]. This
process avoids the need for through-wafer etching or sacrificial release layers and eliminates the possible squeeze-film
damping between the PMUT membrane and the Si substrate.
150 mm diameter CSOI wafers (IceMOS Technologies) were
manufactured with 2 μm deep cavities patterned with diameters from 40 μm to 50 μm and both individual PMUTs and 2D
arrays of PMUTs were defined on the wafer. A 7 × 49 arraycomposed of PMUTs with 50 μm diameter and 70 μm pitch
is shown in Fig. 2.
The Mo and Scx Al1−x N (x = 15%) layers were sputteredin an Advanced Modular Systems (AMS) cluster tool with
AlN deposition chambers and ion beam trimming module. The
system used a standard dual conical magnetron with an AC
deposition source operating at 40 kHz and power varying from
3 to 10 kW. The ScAlN deposition process was in deep poisonmode using targets composed of Al and Sc pieces. High purity
research grade argon and nitrogen process gases were used forthe deposition. The base pressure of the process is ∼5 mTorr
and the process temperature is ∼400 °C. Compared to
Al-Sc alloy target and multiple targets of Al and Sc, multiple
piece targets are easy to make and practical for high volume
production. Locally adjusted magnetic field for target pieces
of both Al and Sc guaranteed a constant thin film composition
over the entire target life. Substrate rotation was utilized
to compensate for the variation of the sputtering yield for
different materials and composition non-uniformity across thesubstrate.
The CSOI wafers were cleaned by ion milling first inorder to achieve a good interface for the following thin film
deposition. A 30 nm thick ScAlN film was first depositedon the CSOI as a seed layer in order to achieve a good
crystalline structure of the subsequent Mo and ScAlN layers.
Then a 200 nm thick molybdenum (Mo) layer was sputtered

TABLE IRIE PARAMETERS

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as the bottom electrode in a different chamber in the system
without breaking vacuum. Finally, 1 μm thick ScAlN was
sputtered on the Mo layer. A cross-section scanning electron
microscope (SEM) image of a PMUT, Fig. 3, shows thedense columnar structure of the ScAlN film and Mo bottom
electrode.
Following deposition of the ScAlN layer, vias were opened
to contact the Mo bottom electrode. AlN films are often
etched using heated Microposit MF-319, a positive photoresist
developer mainly composed of tetramethylammonium hydroxide (TMAH). However, experiments showed that the ScAlN
etch rate in MF-319 was ∼50 nm/min at 60 °C to 70 °C,
approximately 4 times slower than that of AlN thin films at
the same etching temperature. For this reason, reactive ion
etching (RIE) in a transformer coupled plasma (TCP) etcher
was studied using a combination of Cl2 and BCl3 gases with
He used as diluent to improve etch uniformity. A 6.5 μm thickg-line photoresist (OCG 825 35S, Fujifilm) was spin coated,patterned, and hard baked for 16 hours to be used as a mask.
An etch rate of 160 nm/min was achieved with the recipe
shown in the Table I with an etching selectivity of 0.4 to themask. Following the via etch, a 200 nm thick aluminum (Al)layer was evaporated and patterned by a lift-off process toform the top electrode and contact pads for the top and bottom
electrodes.

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WANG et al.: DESIGN, FABRICATION AND CHARACTERIZATION OF SCANDIUM ALUMINUM NITRIDE-BASED PMUTs

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Fig. 5.
(a) left: SEM image of FIB milled trench on ScAlN surface,right: EDX results over the line scan; (b) EDX results.

Fig. 4. (a) Normal coupled XRD measurement of ScAlN and AlN films inlog scale; (b) Rocking curve measurement of the ScAlN and AlN (002) peakin linear scale.

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III. R ESULTS

Fig. 6. Measured resonance frequency and dynamic displacement at resonance for ScAlN PMUT with 50 m diameter and 2.5 m nominal Si thickness.

A. ScAlN film characterization

The ScAlN crystalline structure was studied using X-ray
diffraction (XRD). Fig. 4(a) shows a comparison of the XRD
peaks of pure AlN and ScAlN thin films on Mo electrodewith 1 μm thickness. The (002) peak and small (100) peak of
ScAlN were shifted to a slightly lower angle compared withthat of AlN, indicating an expansion of the crystalline lattice
according to Bragg’s law. The rocking curve of the ScAlN(002) peak was also measured and is shown in Fig. 4(b). The
full-width-half-maximum (FWHM) of the (002) peak is 1.6°
for the AlN film and 1.9° for the ScAlN film, indicating thatthe c-axis of the ScAlN film is well aligned and predicting
good piezoelectric properties [16].
A focused ion beam (FIB, Scios Dual Beam SEM/FIB
system) was utilized to open a trench of approximate dimension 20 μm wide × 20 μm long × 10 μm deep on
ScAlN thin film surface. The scandium concentration wasmeasured on the cross section of ScAlN via energy dispersive
X-ray spectroscopy (EDX, Oxford Instrument) at 15 keVbeam energy. The EDX line-scan data is shown in Fig. 5.

The results show a consistent scandium concentration of
x = 15 at% throughout the thickness of the film. Note that thex-axis position of the EDX intensity in Fig. 5(a) is not exact
due to the sample tilt in the SEM.

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B. Dynamic characterization

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The frequency response of ScAlN PMUTs and AlN PMUTs
with the same geometry were tested in air using a laser
Doppler vibrometer (LDV, OFV 512 and OFV 2700, Polytec)
in conjunction with a network analyzer (E5061B, Agilent
Technology). LDV measurements were collected on 16 ScAlN
PMUTs with 50 μm diameter selected from locations across
one wafer, resulting in a 17.5 ± 1.5 MHz natural frequency,
22 ± 4 nm/V peak displacement sensitivity at resonance,
and an average quality factor of Q = 140 in air. The
die to die variation in resonant frequency was within 10%
and the variation in amplitude was ∼20%. The results areshown in Fig. 6. Cross-section SEM images showed that

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values obtained from ScAlN BAW devices with similar
Sc composition [19]. This formula also confirms that themeasured variation in natural frequency across the wafer isconsistent with the measured variation of the Si device layer
thickness.
To extract an estimate of the transverse thin-film
piezoelectric coefficient (e31, f ) from the frequency response
data, we normalized the peak displacement by the quality
factor, yielding an average value of ds = d p /Q = 180 pm/V.
ds is related to the transverse piezoelectric coefficient e31, f
via [20]:
D · Idof

ds = −rtp
2 e31, f (tsi + tm + 2 − z n ) · I p (r )

(3)

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where tsi is the thickness of Si substrate, tm is the thicknessof bottom electrode, t p is the thickness of ScAlN film, z n is
the distance from the middle of the ScAlN film to neutral
axis, and I p (r ) and Id are integrals related to the piezoelectric
bending moment and modal stiffness of the PMUT, both ofwhich depend on the assumed vibration mode shape of thePMUT, u(r ), red 2 u(re ) du(re )I p (re ) =(re+) · dre
(4)
dre2
dre
0
 1 2
d u(r ) 1 du(r ) 2
)
Id =
[(
+
dr 2
r dr
0
1 du(r ) d 2 u(r )−2(1 − υ)
]r dr
(5)
r dr
dr 2

Fig. 7. LDV measurement results for (a) 50 μm diameter and (b) 40 μm
diameter ScAlN and AlN PMUTs.

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the Si thickness of these samples varied from 2.40 μm to
2.93 μm. Fig. 7 compares the LDV results of ScAlN and AlN
PMUTs with 50 μm diameter and 40 μm diameter. The peak
displacement of the ScAlN PMUTs are more than two times
as large as that of the AlN devices.
The difference in the resonance frequency of ScAlN and
AlN PMUTs is due to the stiffness reduction from Sc alloying.
The resonant frequency of a circular PMUT can be computed
from

D
1.63

f = 2
(1)
r
ρi ti
where r is the PMUT radius, D is rigidity, ρ and t are thedensity and thickness of the Si, Mo, and ScAlN layers. The
density of Sc0.15 Al0.85 N is estimated to be 3430 kg/m3, extrapolated from the density of Sc0.4 Al0.6 N and AlN [15], [16]. The
rigidity D can be expressed as
D=

n
3 )
1  E i2 (z i3 − z i−13i=1

1 − υi2(2)where E i is the Young’s modulus and νi is the Poisson’s
ratio of the material, z i is the distance of the i -th layer
top surface from the neutral axis. The Poisson’s ratio of
Sc0.15 Al0.85 N in this paper is assumed to be 0.23 [15], [18].
Using (1), the Young’s modulus of ScAlN was estimated to be
200 GPa ± 15 GPa, which is consistent with the reported

where υ is Poisson’s ratio. I p (re ) is a function of re , the radiusof the circular top electrode normalized to the PMUT radius.
Using u(r ) = (1 − r 2 )2 as the assumed mode shape for
the 01 vibration mode of a circular membrane, (4) yields
I p = −1 at re = 70% and Id = 10.67. Substituting
these values along with the geometrical parameters into (3)yields an estimate of e31, f ∼ 1.6 C/m2 which is ∼60%
higher than that of AlN. Our estimated value is consistent
with the value extrapolated from [12] measured via a doubleside beam interferometry (DBI) and slightly higher than thevalue extrapolated from [21] measured via a cantilever energy
harvester.
Equation (3) also allows the optimum electrode radius forpeak displacement to be identified. The estimated material
properties including Young’s modulus and e31, f were usedin (3) to compute the theoretical displacement with re varying
from 30% to 90%. The results are compared to experimental
measurements of PMUTs with varying electrode diametersin Fig. 8, demonstrating good agreement between model andexperiment, with the maximum displacement observed withelectrode radius from 70% to 80% of the PMUT radius.
The difference between theoretical and experimental results
at 80% and 90% electrode coverage may be due to inexact
boundary conditions (the model assumes perfect clamping at
the membrane boundary while some flexing occurs in thislocation in the real device) or misalignment of the electrodeto the silicon membrane (when the electrode covers nearly thewhole membrane, an off-center electrode will be partly locatedon the anchor).

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TABLE IIC OMPARISON OF P IEZOELECTRIC M ATERIAL P ROPERTIES

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Fig. 8. Theoretical and experimental results of PMUT static displacementwith different electrode radius.

where f a and fr are the anti-resonant and resonant frequency
respectively. The extracted kt2 was 1.9% for ScAlN PMUTs,
consistent with the value calculated using the model presented
in [20]. For AlN PMUTs, the extracted kt2 was 1.4%. Thisvalue is higher than the value calculated from the model (modeled AlN kt2 = 0.8%). One source of error is that we subtracted
the parasitic capacitance of the bond pads and probe setup,
and the subtracted parasitic capacitance may have been larger
than the true value. Comparing only the two extracted values,
we find that the extracted kt2 of the ScAlN PMUT is 36%
greater than that of AlN. The relative dielectric permittivity(ε Sc Al N ) of ScAlN was also estimated from the impedance
test as ∼12 which is around 20% higher than that of pure
AlN. The estimated dielectric permittivity is consistent withthe value reported in [12].
We also calculated the electromechanical coupling factor using an alternative method to the impedance method
described above. The 31 electromechanical coupling coeffi2 ∝ e2
cient is defined as k31
31, f /ε33 [22], [23]. For pulseecho performance, this metric can be interpreted as follows– the square of the piezocoefficient appears in the numerator because both the TX and RX operations require energy
conversion between the electrical and mechanical domains,while the dielectric constant is in the denominator because
the RX charge is converted to a voltage by dividing by thecapacitance. AlN, ScAlN, and PZT are compared in Table II
using the extracted material parameters reported here. Notethat while PZT is superior to AlN, the figure-of-merit for
ScAlN is 30% greater than that of PZT. One caveat tothis conclusion is that the presence of parasitic capacitance(e.g. due to bond-pads or cables between the PMUT and thereceive amplifier) will greatly degrade the RX signal amplitudeof a ScAlN or AlN PMUT due to the much lower dielectric
constant of these materials. For example, a 50 μm diameter
PMUT with re = 70% and a 1 μm thick ScAlN layer has
a capacitance of 0.1 pF, so the presence of a 1 pF bondpad capacitance will reduce the RX voltage by a factor of 11(=0.1 pF /1.1 pF). In comparison, a PZT PMUT of the same
size has 100 times greater capacitance, so a 1 pF parasitic
capacitance would have negligible effect on the RX voltage.

Fig. 9. Impedance measurement results for 50 μm diameter (a) ScAlN PMUT
and (b) AlN PMUT.

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C. Electrical characterization
Impedance measurements of ScAlN and AlN PMUTs,
Fig. 9, were performed in air using a GSG RF probe calibrated
with an impedance substrate standard (Cascade Microtech).
The electromechanical coupling factor kt2 was calculated by:
fr f a − fr
4 fa
fa

π
kt2 =

2

(6)

D. Acoustic characterization
An array of ScAlN PMUTs was immersed in nonconductive fluid (Fluorinert FC-70, 3M) and the output
acoustic pressure was measured with a 70 μm diameter needlehydrophone (Precision Acoustic, Inc.). The results are shown

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Fig. 10.

Pressure measurement results for 7 × 7 ScAlN PMUT array.
TABLE III

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Fig. 11.

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Acoustic pressure measured from a 15 × 15 PMUT array.

in Fig. 10. A 7 × 7 ScAlN PMUT array was driven by
four 9 MHz 11 Vpp pulses from a function generator (Rigol,
DG-4102). The measured pressure generated by the ScAlN
PMUT array was detected at ∼2.5 μs after the pulse generation, which corresponds to ∼1.7 mm from the PMUT surface
to the hydrophone. The peak-to-peak pressure detected was∼8 kPa, which was 30% larger than ∼6 kPa pressure generated
from a 8 × 8 AlN PMUT array driven at 25 Vpp, suggesting
3x greater transmit efficiency from the ScAlN array. The
acoustic transmitting performance of ScAlN, AlN and PZT

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Fig. 12. LDV measurement of pulse-echo from a ScAlN PMUT in the centerof 15 × 15 array with different Fluorinert heights of 4 mm, 3 mm, 2 mm
and 1.2 mm.

PMUT arrays were compared as shown in Table III. The
normalized output pressure represents the acoustic pressure
output from a 1 mm2 PMUT array area at a distance ∼1.5 mm
away from PMUT surface under 1 V driving voltage. The
normalized pressure output of the ScAlN PMUTarray presented

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estimated to be SRX = 0.25 nm/kPa. Since the PMUT is a−1
reciprocal transducer, STX = SRX, as expected.
IV. C ONCLUSION

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here is ∼55% of the PZT array presented in [10]. Considering
the much smaller capacitance of ScAlN, we expect that thepulse-echo performance of the PZT and ScAlN arrays should
be comparable, because the ScAlN array should have higher
receive sensitivity.
The output acoustic pressure of a 15 × 15 ScAlN PMUT
array was measured via hydrophone at different distances to
PMUT surface. The measured peak-to-peak pressure versus
distance is shown in Fig. 11. The result shows that the pressure
decays inversely with the radial distance from the array [24]:ACKNOWLEDGMENT

The authors thank UC Berkeley Marvel Nanolab for theadvice and help on the fabrication process, and IceMOS
Technology for providing cavity SOI wafers.

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Fig. 13. Measured insertion loss from TX to RX vibration amplitudes as afunction of round-trip pulse-echo distance.

The results presented here demonstrate that ScAlN PMUTs
have better performance than PMUTs made with AlN. Using
15% Sc, the transmit amplitude was increased by a factorof two relative to PMUTs made with pure AlN, consistent
with a 60% increase in the transverse piezoelectric coefficient,e31, f . The PMUT fabrication process is nearly unchanged byintroducing ScAlN. While wet etching of Sc0.15 Al0.85 N in
TMAH proceeds at a much slower etch rate than pure AlN, a
Cl2 /BCl3 plasma etch was demonstrated to achieve an etch rateof 120 nm/min for Sc0.15 Al0.85 N. We expect that increasing the
Sc concentration would further improve PMUT performance,since other work has shown that the piezoelectric coefficientsof ScAlN increase as the Sc concentration is increased up to
40%. While RF devices, such as BAW filters, may suffer due
to the reduced stiffness (and therefore lower acoustic velocity)that occurs as the Sc concentration is increased, this reduced
stiffness does not degrade the performance of PMUTs.

P (r ) = P0 R0 x −1

(7)where R0 is the Rayleigh distance and P0 is the theoreticalsurface pressure. A fit of (7) to the experimental data gives
R0 = 2.9 mm and P0 = 20.6 kPa. Given that the peak-topeak displacement is 5 nm, the value of P0 gives a transmit
sensitivity of STX = 4 kPa/nm.
The dynamic displacement of a 15 × 15 ScAlN PMUT arraydriven with 11 Vpp and immersed in fluid was measured via
LDV, Fig. 12. An ∼5 nm displacement of the center ScAlN
PMUT was measured during the transmit (TX) excitation.
PMUT vibration due to received (RX) echoes returning fromthe fluid-air surface were also visible in these experiments. The
Fluorinert-air surface was varied from 4 mm to 1.2 mm, andthe plots in Fig. 12 show the echoes return 11 μs, 8 μs, 5 μs
and 3 μs after the TX pulse is sent. The corresponding roundtrip distances calculated from these pulse echo measurements
are consistent with the Fluorinert height using c = 750 m/s
as the speed of sound in Fluorinert. In Fig. 12(c) and (d),a second echo can be observed due to the short liquid distanceand large output pressure. The vibration amplitude of the firstreceived echo relative to the transmit vibration amplitude fits
the acoustic spreading model from (7),d R X (x)/dT X = R0 x −1(8)This model is plotted along with the experimental data in dB
units in Fig. 13. Comparing the experimental pressure measurements from Fig. 11 with the RX vibration amplitudes
shown in Fig. 12, the receive sensitivity of the array is

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[7] D. A. Horsley et al., “Piezoelectric micromachined ultrasonic transducers in consumer electronics: The next little thing?” in Proc. IEEE 29th
Int. Conf. Micro Electro Mech. Syst. (MEMS), Jan. 2016, pp. 145–148.
[8] R. Ruby and P. Merchant, “Micromachined thin film bulk acoustic
resonators,” in Proc. 48th. IEEE Int. Freq. Control Symp., Jun. 1994,
pp. 135–138.
[9] J. Zou, C.-M. Lin, Y.-Y. Chen, and A. P. Pisano, “Theoretical studyof thermally stable SiO2 /AlN/SiO2 Lamb wave resonators at hightemperatures,” J. Appl. Phys., vol. 115, no. 9, p. 094510, Mar. 2014.
[10] Y. Lu and D. A. Horsley, “Modeling, fabrication, and characterization ofpiezoelectric micromachined ultrasonic transducer arrays based on cavity
SOI wafers,” J. Microelectromech. Syst., vol. 24, no. 4, pp. 1142–1149,
2015.
[11] Q. Wang, H. Oguchi, M. Hara, and H. Kuwano, “Investigation of
dominant factors to control c-axis tilt angle of aln thin films for efficientenergy harvesting,” in Proc. IEEE 27th Int. Conf. Micro Electro Mech.
Syst. (MEMS), Jan. 2014, pp. 636–639.

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Sergey Mishin received the M.Phys. degree (Hons.)from the Russian National Research University ofElectronic Technology in 1986.
His main research interests include cold discharge,
high density plasma, magnetron plasma, excimer
laser systems, and the application of plasma discharge and lasers in the semi-conductor industry.
He founded Advanced Modular Systems, Inc. in
2000. Advanced Modular Systems, Inc. is responsible for developing the first production worthy
cluster tool for high volume piezoelectric thin films.
By its second year, Advanced Modular Systems, Inc. was presented with the
Supplier of the Year Award by Agilent Technologies, Inc. Since founding
his company, he has been focused on the development of new equipment
for the manufacture and treatment of piezoelectric thin films. As part of thisresearch and development, he has had an opportunity to work closely withindustry leaders and high-class research laboratories. He has also authoredor co-authored numerous papers, and holds patents on the deposition andtrimming technologies used in FBAR/BAW/SAW and MEMS application.

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[12] R. Matloub et al., “Piezoelectric Al1−x Scx N thin films: A semiconductor compatible solution for mechanical energy harvesting and sensors,”
Appl. Phys. Lett., vol. 102, no. 15, p. 152903, Apr. 2013.
[13] M. Moreira, J. Bjurström, I. Katardjev, and V. Yantchev, “Aluminumscandium nitride thin-film bulk acoustic resonators for wide band
applications,” Vacuum, vol. 86, no. 1, pp. 23–26, Jul. 2011.
[14] K. Hashimoto, S. Sato, A. Teshigahara, T. Nakamura, and K. Kano,“High-performance surface acoustic wave resonators in the 1 to 3 GHz
range using a ScAlN/6H-SiC structure,” IEEE Trans. Ultrason.,Ferroelectr., Freq. Control, vol. 60, no. 3, pp. 637–642, Mar. 2013,
doi: 10.1109/TUFFC.2013.2606[15] A. Konno et al., “Determination of full material constants of ScAlN
thin film from bulk and leaky Lamb waves in MEMS-based samples,”in Proc. IEEE Int. Ultrason. Symp., Sep. 2014, pp. 273–276.
[16] H. P. Loebl, M. Klee, C. Metzmacher, W. Brand, R. Milsom, and P. Lok,
“Piezoelectric thin AlN films for bulk acoustic wave (BAW) resonators,”
Mater. Chem. Phys., vol. 79, nos. 2–3, pp. 143–146, Apr. 2003.
[17] J.-M. Wagner and F. Bechstedt, “Properties of strained wurtzite GaN
and AlN: Ab initio studies,” Phys. Rev. B, Condens. Matter, vol. 66,
no. 11, p. 115202, Sep. 2002.
[18] A. V. Sotnikov, H. Schmidt, M. Weihnacht, E. P. Smirnova,
T. Y. Chemekova, and Y. N. Makarov, “Elastic and piezoelectric
properties of AlN and LiAlO2 single crystals,” IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, vol. 57, no. 4, pp. 808–811, Apr. 2010.
[19] K. Umeda, H. Kawai, A. Honda, M. Akiyama, T. Kato, and T. Fukura,“Piezoelectric properties of ScAlN thin films for piezo-MEMS devices,”in Proc. IEEE 26th Int. Conf. Micro Electro Mech. Syst. (MEMS),
Jan. 2013, pp. 733–736.
[20] M. Dubois and P. Muralt, “PZT thin film actuated elastic fin micromotor,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 45, no. 5,
pp. 1169–1177, Sep. 1998.
[21] P. M. Mayrhofer et al., “ScAlN MEMS cantilevers for vibrationalenergy harvesting purposes,” J. Microelectromech. Syst., vol. 26, no. 1,
pp. 102–112, 2016.
[22] P. Muralt and J. Baborowski, “Micromachined ultrasonic transducers
and acoustic sensors based on piezoelectric thin films,” J. Electroceram.,vol. 12, nos. 1–2, pp. 101–108, 2004.
[23] K. Smyth and S.-G. Kim, “Experiment and simulation validated analytical equivalent circuit model for piezoelectric micromachined ultrasonictransducers,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 62,
no. 4, pp. 744–765, Apr. 2015.
[24] D. T. Blackstock, Fundamentals of Physical Acoustics. Hoboken, NJ,
USA: Wiley, 2000.

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Qi Wang (S’15) received the B.S. degree inmechanical engineering and automation from the
Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2011, and the M.S. degree
in nanomechanics from Tohoku University, Sendai,
Japan, in 2013.
He is currently pursuing the Ph.D. degree inmechanical engineering with the Department of
Mechanical and Aerospace Engineering, Universityof California at Davis (UCD), Davis. He is alsoa Graduate Student Researcher with the Berkeley
Sensor and Actuator Center, UCD. His research interests include piezoelectricthin films and MEMS sensors and actuators.

Yipeng Lu received the B.S. degree in materials science and engineering from Jilin University,
Changchun, China, in 2007; the M.S. degree inmicroelectronics from Shanghai Jiao Tong University, Shanghai, China, in 2010, and the Ph.D. degree
in mechanical engineering from the University ofCalifornia at Davis (UCD), Davis, CA, USA, in
2015. He joined the Berkeley Sensor and Actuator
Center, UCD, as a Graduate Student Researcher.
He was a Digital Hardware Engineer at Huawei
in 2011. He is currently a Senior Engineer with
Qualcomm. His research interests include MEMS sensors and actuators.

Yury Oshmyansky received the B.S. and M.S.
degrees from the Colorado School of Mines. He wasat Hewlett-Packard and subsequently Vitesse,
Motorola, SFI, Agilent, Avago, and Advanced Modular Systems, Inc. He is currently the Director ofprocess development with Advanced Modular Systems, Inc. He is also a Chemical Engineer. He holds
many patents, and has published numerous papers in
the field of manufacturing of the FBAR filters andrelated technologies.

David A. Horsley (M’97) received the B.S., M.S.,
and Ph.D. degrees in mechanical engineering fromthe University of California at Berkeley, Berkeley,CA, in 1992, 1994, and 1998, respectively. He has
been a Co-Director of the Berkeley Sensor andActuator Center since 2005. He held researchand development positions at Dicon Fiberoptics,
Hewlett Packard Laboratories, and Onix Microsystems. He is currently a Professor with the Department of Mechanical and Aerospace Engineering,
University of California at Davis, Davis, CA. His
research interests include microfabricated sensors and actuators with applications in ultrasonics and physical sensors. He was a recipient of the NSF
CAREER Award and the UC Davis College of Engineering’s Outstanding
Junior Faculty Award.

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AUTHOR QUERIESAUTHOR PLEASE ANSWER ALL QUERIESPLEASE NOTE: We cannot accept new source files as corrections for your paper. If possible, please annotate the PDF
proof we have sent you with your corrections and upload it via the Author Gateway. Alternatively, you may send us
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AQ:1 = Please provide the postal code for “Advanced Modular Systems, Inc.”
AQ:2 = Please provide descriptions of all labeled subparts for Fig. 12.
AQ:3 = The term “AMSystems” has been changed to “Advanced Modular Systems, Inc.” Please confirm.

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

1Design, Fabrication, and Characterization of
Scandium Aluminum Nitride-Based
Piezoelectric Micromachined
Ultrasonic Transducers

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Abstract— This paper presents the design, fabrication,and characterization of piezoelectric micromachined ultrasound transducers (PMUTs) based on scandium aluminum
nitride (Scx Al1−x N) thin films (x = 15%). ScAlN thin film was
prepared with a dual magnetron system and patterned by areactive ion etching system utilizing chlorine-based chemistrywith an etching rate of 160 nm/min. The film was characterizedby X-ray diffraction, which indicated a crystalline structure
expansion compared with pure AlN and a well-aligned ScAlN
film. ScAlN PMUTs were fabricated by a two-mask process
based on cavity SOI wafers. ScAlN PMUTs with 50- and 40-µm
diameter had a large dynamic displacement sensitivity measured
in air of 25 nm/V at 17 MHz and 10 nm/V at 25 MHz,
twice that of AlN PMUTs with the same dimensions. The peak
displacement as a function of electrode coverage was characterized, with maximum displacement achieved with an electroderadius equal to 70% of the PMUT radius. Electrical impedance
measurements indicated that the ScAlN PMUTs had 36% greater
electromechanical coupling coefficient (k2t ) compared with AlN
PMUTs. The output pressure of a 7 × 7 ScAlN PMUT array
was 0.7 kPa/V at ∼1.7 mm away from the array, which isapproximately three times greater that of an 8 × 8 AlN PMUT
array with the same element geometry and fill factor measured at
the same distance. Acoustic spreading loss and PMUT insertion
loss from mechanical transmit to receive were characterized witha 15 × 15 ScAlN PMUT array via hydrophone and laser Doppler
vibrometer.
[17509-2017]

Fig. 1.

Schematic cross-section of PMUT.

pling, lower manufacturing cost and lower power consumption
compared to conventional bulk ultrasonic transducers.
Piezoelectric micromachined ultrasonic transducers (PMUT)
have been rapidly developed in recent years due to the progressof piezoelectric thin films. Aluminum nitride (AlN) has
been widely used for piezoelectric MEMS device fabrication
because it is available from a number of MEMS foundries andis compatible with CMOS manufacturing [7]–[9]. However,compared to lead zirconate tianate (PZT), a piezoelectricmaterial which requires high annealing temperature and is
not process-compatible with CMOS, AlN has relatively low
piezoelectric coefficient (e31, f ), which leads to low sensitivity
and low electromechanical coupling (kt2 ) [9]–[11].
Scandium (Sc) alloying has been proposed recently as ameans to increase the e31, f of AlN, while maintaining processcompatibility with existing AlN based manufacturing [12].
Most of the previously-reported work on ScAlN focused on
bulk acoustic wave (BAW) resonators or surface acoustic
wave (SAW) devices which utilize the longitudinal piezoelectric mode and require high stiffness to achieve high frequency
operation and high quality factor (Q) [12], [13]. However,studies also found that with the increase of Sc concentrations,the stiffness of the thin film decreased and the dielectric
constant increased [14], [15]. In this paper, we present flexural
PMUT devices which use the transverse piezoelectric mode
and where the reduced stiffness of ScAlN may provide abenefit over conventional AlN.

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Qi Wang, Student Member, IEEE, Yipeng Lu, Member, IEEE, Sergey Mishin,
Yury Oshmyansky, and David A. Horsley, Senior Member, IEEE

Index Terms— Piezoelectric micromachined ultrasoundtransducers(PMUT),
piezoelectricfilms,
piezoelectrictransducers.

I. I NTRODUCTION
ANY applications have been developed based on
micromachined ultrasonic transducers (MUTs) inrecent years, such as medical imaging [1]–[3], gesturesensors [4], ultrasonic fingerprint sensors [5], and bodycomposition sensors [6]. MUTs have a better acoustic cou-

M

Manuscript received November 7, 2016; revised May 23, 2017; accepted
May 26, 2017. This work was supported by Berkeley Sensor and Actuator industrial members. Subject Editor A. Seshia. (Corresponding author:
Qi Wang.)
Q. Wang, Y. Lu, and D. A. Horsley are with the Department of Mechanical and Aerospace Engineering, University of California at Davis, Davis,
CA 95616 USA (e-mail: qixwang@ucdavis.edu).
S. Mishin and Y. Oshmyansky are with Advanced Modular Systems, Inc.,
Goleta, CA, USA.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2017.2712101

II. M ATERIALS AND M ETHODS
A cross-section schematic of a PMUT is shown in Fig. 1.
The PMUT was composed of a 1 μm thick ScAlN

1057-7157 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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Fig. 2. An optical microscope image of a 7 × 49 PMUT array. The individual
PMUTs are 50 μm diameter and the array pitch is 70 μm.

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piezoelectric layer, a 200 nm Mo layer as bottom electrodeand a 2.5 μm thick silicon membrane. Devices were alsofabricated using pure AlN with identical film thicknesses andgeometries in order to provide a comparison between ScAlN
and AlN PMUTs. The fabrication process used custom cavity
SOI (CSOI) substrates wherein vacuum cavities are formed
beneath the Si device layer of the CSOI wafer [10]. This
process avoids the need for through-wafer etching or sacrificial release layers and eliminates the possible squeeze-film
damping between the PMUT membrane and the Si substrate.
150 mm diameter CSOI wafers (IceMOS Technologies) were
manufactured with 2 μm deep cavities patterned with diameters from 40 μm to 50 μm and both individual PMUTs and 2D
arrays of PMUTs were defined on the wafer. A 7 × 49 arraycomposed of PMUTs with 50 μm diameter and 70 μm pitch
is shown in Fig. 2.
The Mo and Scx Al1−x N (x = 15%) layers were sputteredin an Advanced Modular Systems (AMS) cluster tool with
AlN deposition chambers and ion beam trimming module. The
system used a standard dual conical magnetron with an AC
deposition source operating at 40 kHz and power varying from
3 to 10 kW. The ScAlN deposition process was in deep poisonmode using targets composed of Al and Sc pieces. High purity
research grade argon and nitrogen process gases were used forthe deposition. The base pressure of the process is ∼5 mTorr
and the process temperature is ∼400 °C. Compared to
Al-Sc alloy target and multiple targets of Al and Sc, multiple
piece targets are easy to make and practical for high volume
production. Locally adjusted magnetic field for target pieces
of both Al and Sc guaranteed a constant thin film composition
over the entire target life. Substrate rotation was utilized
to compensate for the variation of the sputtering yield for
different materials and composition non-uniformity across thesubstrate.
The CSOI wafers were cleaned by ion milling first inorder to achieve a good interface for the following thin film
deposition. A 30 nm thick ScAlN film was first depositedon the CSOI as a seed layer in order to achieve a good
crystalline structure of the subsequent Mo and ScAlN layers.
Then a 200 nm thick molybdenum (Mo) layer was sputteredFig. 3. Cross-sectional SEM image of a ScAlN PMUT. The Mo and ScAlN
show good columnar structure indicating a highly c-axis oriented film.

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as the bottom electrode in a different chamber in the system
without breaking vacuum. Finally, 1 μm thick ScAlN was
sputtered on the Mo layer. A cross-section scanning electron
microscope (SEM) image of a PMUT, Fig. 3, shows thedense columnar structure of the ScAlN film and Mo bottom
electrode.
Following deposition of the ScAlN layer, vias were opened
to contact the Mo bottom electrode. AlN films are often
etched using heated Microposit MF-319, a positive photoresist
developer mainly composed of tetramethylammonium hydroxide (TMAH). However, experiments showed that the ScAlN
etch rate in MF-319 was ∼50 nm/min at 60 °C to 70 °C,
approximately 4 times slower than that of AlN thin films at
the same etching temperature. For this reason, reactive ion
etching (RIE) in a transformer coupled plasma (TCP) etcher
was studied using a combination of Cl2 and BCl3 gases with
He used as diluent to improve etch uniformity. A 6.5 μm thickg-line photoresist (OCG 825 35S, Fujifilm) was spin coated,patterned, and hard baked for 16 hours to be used as a mask.
An etch rate of 160 nm/min was achieved with the recipe
shown in the Table I with an etching selectivity of 0.4 to themask. Following the via etch, a 200 nm thick aluminum (Al)layer was evaporated and patterned by a lift-off process toform the top electrode and contact pads for the top and bottom
electrodes.

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Fig. 5.
(a) left: SEM image of FIB milled trench on ScAlN surface,right: EDX results over the line scan; (b) EDX results.

Fig. 4. (a) Normal coupled XRD measurement of ScAlN and AlN films inlog scale; (b) Rocking curve measurement of the ScAlN and AlN (002) peakin linear scale.

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III. R ESULTS

Fig. 6. Measured resonance frequency and dynamic displacement at resonance for ScAlN PMUT with 50 m diameter and 2.5 m nominal Si thickness.

A. ScAlN film characterization

The ScAlN crystalline structure was studied using X-ray
diffraction (XRD). Fig. 4(a) shows a comparison of the XRD
peaks of pure AlN and ScAlN thin films on Mo electrodewith 1 μm thickness. The (002) peak and small (100) peak of
ScAlN were shifted to a slightly lower angle compared withthat of AlN, indicating an expansion of the crystalline lattice
according to Bragg’s law. The rocking curve of the ScAlN(002) peak was also measured and is shown in Fig. 4(b). The
full-width-half-maximum (FWHM) of the (002) peak is 1.6°
for the AlN film and 1.9° for the ScAlN film, indicating thatthe c-axis of the ScAlN film is well aligned and predicting
good piezoelectric properties [16].
A focused ion beam (FIB, Scios Dual Beam SEM/FIB
system) was utilized to open a trench of approximate dimension 20 μm wide × 20 μm long × 10 μm deep on
ScAlN thin film surface. The scandium concentration wasmeasured on the cross section of ScAlN via energy dispersive
X-ray spectroscopy (EDX, Oxford Instrument) at 15 keVbeam energy. The EDX line-scan data is shown in Fig. 5.

The results show a consistent scandium concentration of
x = 15 at% throughout the thickness of the film. Note that thex-axis position of the EDX intensity in Fig. 5(a) is not exact
due to the sample tilt in the SEM.
B. Dynamic characterization
The frequency response of ScAlN PMUTs and AlN PMUTs
with the same geometry were tested in air using a laser
Doppler vibrometer (LDV, OFV 512 and OFV 2700, Polytec)
in conjunction with a network analyzer (E5061B, Agilent
Technology). LDV measurements were collected on 16 ScAlN
PMUTs with 50 μm diameter selected from locations across
one wafer, resulting in a 17.5 ± 1.5 MHz natural frequency,
22 ± 4 nm/V peak displacement sensitivity at resonance,
and an average quality factor of Q = 140 in air. The
die to die variation in resonant frequency was within 10%
and the variation in amplitude was ∼20%. The results areshown in Fig. 6. Cross-section SEM images showed that

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values obtained from ScAlN BAW devices with similar
Sc composition [19]. This formula also confirms that themeasured variation in natural frequency across the wafer isconsistent with the measured variation of the Si device layer
thickness.
To extract an estimate of the transverse thin-film
piezoelectric coefficient (e31, f ) from the frequency response
data, we normalized the peak displacement by the quality
factor, yielding an average value of ds = d p /Q = 180 pm/V.
ds is related to the transverse piezoelectric coefficient e31, f
via [20]:
D · Idof

ds = −rtp
2 e31, f (tsi + tm + 2 − z n ) · I p (r )

(3)

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where tsi is the thickness of Si substrate, tm is the thicknessof bottom electrode, t p is the thickness of ScAlN film, z n is
the distance from the middle of the ScAlN film to neutral
axis, and I p (r ) and Id are integrals related to the piezoelectric
bending moment and modal stiffness of the PMUT, both ofwhich depend on the assumed vibration mode shape of thePMUT, u(r ), red 2 u(re ) du(re )I p (re ) =(re+) · dre
(4)
dre2
dre
0
 1 2
d u(r ) 1 du(r ) 2
)
Id =
[(
+
dr 2
r dr
0
1 du(r ) d 2 u(r )−2(1 − υ)
]r dr
(5)
r dr
dr 2

Fig. 7. LDV measurement results for (a) 50 μm diameter and (b) 40 μm
diameter ScAlN and AlN PMUTs.

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the Si thickness of these samples varied from 2.40 μm to
2.93 μm. Fig. 7 compares the LDV results of ScAlN and AlN
PMUTs with 50 μm diameter and 40 μm diameter. The peak
displacement of the ScAlN PMUTs are more than two times
as large as that of the AlN devices.
The difference in the resonance frequency of ScAlN and
AlN PMUTs is due to the stiffness reduction from Sc alloying.
The resonant frequency of a circular PMUT can be computed
from

D
1.63

f = 2
(1)
r
ρi ti
where r is the PMUT radius, D is rigidity, ρ and t are thedensity and thickness of the Si, Mo, and ScAlN layers. The
density of Sc0.15 Al0.85 N is estimated to be 3430 kg/m3, extrapolated from the density of Sc0.4 Al0.6 N and AlN [15], [16]. The
rigidity D can be expressed as
D=

n
3 )
1  E i2 (z i3 − z i−13i=1

1 − υi2(2)where E i is the Young’s modulus and νi is the Poisson’s
ratio of the material, z i is the distance of the i -th layer
top surface from the neutral axis. The Poisson’s ratio of
Sc0.15 Al0.85 N in this paper is assumed to be 0.23 [15], [18].
Using (1), the Young’s modulus of ScAlN was estimated to be
200 GPa ± 15 GPa, which is consistent with the reported

where υ is Poisson’s ratio. I p (re ) is a function of re , the radiusof the circular top electrode normalized to the PMUT radius.
Using u(r ) = (1 − r 2 )2 as the assumed mode shape for
the 01 vibration mode of a circular membrane, (4) yields
I p = −1 at re = 70% and Id = 10.67. Substituting
these values along with the geometrical parameters into (3)yields an estimate of e31, f ∼ 1.6 C/m2 which is ∼60%
higher than that of AlN. Our estimated value is consistent
with the value extrapolated from [12] measured via a doubleside beam interferometry (DBI) and slightly higher than thevalue extrapolated from [21] measured via a cantilever energy
harvester.
Equation (3) also allows the optimum electrode radius forpeak displacement to be identified. The estimated material
properties including Young’s modulus and e31, f were usedin (3) to compute the theoretical displacement with re varying
from 30% to 90%. The results are compared to experimental
measurements of PMUTs with varying electrode diametersin Fig. 8, demonstrating good agreement between model andexperiment, with the maximum displacement observed withelectrode radius from 70% to 80% of the PMUT radius.
The difference between theoretical and experimental results
at 80% and 90% electrode coverage may be due to inexact
boundary conditions (the model assumes perfect clamping at
the membrane boundary while some flexing occurs in thislocation in the real device) or misalignment of the electrodeto the silicon membrane (when the electrode covers nearly thewhole membrane, an off-center electrode will be partly locatedon the anchor).

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Fig. 8. Theoretical and experimental results of PMUT static displacementwith different electrode radius.

where f a and fr are the anti-resonant and resonant frequency
respectively. The extracted kt2 was 1.9% for ScAlN PMUTs,
consistent with the value calculated using the model presented
in [20]. For AlN PMUTs, the extracted kt2 was 1.4%. Thisvalue is higher than the value calculated from the model (modeled AlN kt2 = 0.8%). One source of error is that we subtracted
the parasitic capacitance of the bond pads and probe setup,
and the subtracted parasitic capacitance may have been larger
than the true value. Comparing only the two extracted values,
we find that the extracted kt2 of the ScAlN PMUT is 36%
greater than that of AlN. The relative dielectric permittivity(ε Sc Al N ) of ScAlN was also estimated from the impedance
test as ∼12 which is around 20% higher than that of pure
AlN. The estimated dielectric permittivity is consistent withthe value reported in [12].
We also calculated the electromechanical coupling factor using an alternative method to the impedance method
described above. The 31 electromechanical coupling coeffi2 ∝ e2
cient is defined as k31
31, f /ε33 [22], [23]. For pulseecho performance, this metric can be interpreted as follows– the square of the piezocoefficient appears in the numerator because both the TX and RX operations require energy
conversion between the electrical and mechanical domains,while the dielectric constant is in the denominator because
the RX charge is converted to a voltage by dividing by thecapacitance. AlN, ScAlN, and PZT are compared in Table II
using the extracted material parameters reported here. Notethat while PZT is superior to AlN, the figure-of-merit for
ScAlN is 30% greater than that of PZT. One caveat tothis conclusion is that the presence of parasitic capacitance(e.g. due to bond-pads or cables between the PMUT and thereceive amplifier) will greatly degrade the RX signal amplitudeof a ScAlN or AlN PMUT due to the much lower dielectric
constant of these materials. For example, a 50 μm diameter
PMUT with re = 70% and a 1 μm thick ScAlN layer has
a capacitance of 0.1 pF, so the presence of a 1 pF bondpad capacitance will reduce the RX voltage by a factor of 11(=0.1 pF /1.1 pF). In comparison, a PZT PMUT of the same
size has 100 times greater capacitance, so a 1 pF parasitic
capacitance would have negligible effect on the RX voltage.

Fig. 9. Impedance measurement results for 50 μm diameter (a) ScAlN PMUT
and (b) AlN PMUT.

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C. Electrical characterization
Impedance measurements of ScAlN and AlN PMUTs,
Fig. 9, were performed in air using a GSG RF probe calibrated
with an impedance substrate standard (Cascade Microtech).
The electromechanical coupling factor kt2 was calculated by:
fr f a − fr
4 fa
fa

π
kt2 =

2

(6)

D. Acoustic characterization
An array of ScAlN PMUTs was immersed in nonconductive fluid (Fluorinert FC-70, 3M) and the output
acoustic pressure was measured with a 70 μm diameter needlehydrophone (Precision Acoustic, Inc.). The results are shown

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Fig. 10.

Pressure measurement results for 7 × 7 ScAlN PMUT array.
TABLE III

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Fig. 11.

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Acoustic pressure measured from a 15 × 15 PMUT array.

in Fig. 10. A 7 × 7 ScAlN PMUT array was driven by
four 9 MHz 11 Vpp pulses from a function generator (Rigol,
DG-4102). The measured pressure generated by the ScAlN
PMUT array was detected at ∼2.5 μs after the pulse generation, which corresponds to ∼1.7 mm from the PMUT surface
to the hydrophone. The peak-to-peak pressure detected was∼8 kPa, which was 30% larger than ∼6 kPa pressure generated
from a 8 × 8 AlN PMUT array driven at 25 Vpp, suggesting
3x greater transmit efficiency from the ScAlN array. The
acoustic transmitting performance of ScAlN, AlN and PZT

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Fig. 12. LDV measurement of pulse-echo from a ScAlN PMUT in the centerof 15 × 15 array with different Fluorinert heights of 4 mm, 3 mm, 2 mm
and 1.2 mm.

PMUT arrays were compared as shown in Table III. The
normalized output pressure represents the acoustic pressure
output from a 1 mm2 PMUT array area at a distance ∼1.5 mm
away from PMUT surface under 1 V driving voltage. The
normalized pressure output of the ScAlN PMUTarray presented

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WANG et al.: DESIGN, FABRICATION AND CHARACTERIZATION OF SCANDIUM ALUMINUM NITRIDE-BASED PMUTs

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estimated to be SRX = 0.25 nm/kPa. Since the PMUT is a−1
reciprocal transducer, STX = SRX, as expected.
IV. C ONCLUSION

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here is ∼55% of the PZT array presented in [10]. Considering
the much smaller capacitance of ScAlN, we expect that thepulse-echo performance of the PZT and ScAlN arrays should
be comparable, because the ScAlN array should have higher
receive sensitivity.
The output acoustic pressure of a 15 × 15 ScAlN PMUT
array was measured via hydrophone at different distances to
PMUT surface. The measured peak-to-peak pressure versus
distance is shown in Fig. 11. The result shows that the pressure
decays inversely with the radial distance from the array [24]:P (r ) = P0 R0 x −1

(7)where R0 is the Rayleigh distance and P0 is the theoreticalsurface pressure. A fit of (7) to the experimental data gives
R0 = 2.9 mm and P0 = 20.6 kPa. Given that the peak-topeak displacement is 5 nm, the value of P0 gives a transmit
sensitivity of STX = 4 kPa/nm.
The dynamic displacement of a 15 × 15 ScAlN PMUT arraydriven with 11 Vpp and immersed in fluid was measured via
LDV, Fig. 12. An ∼5 nm displacement of the center ScAlN
PMUT was measured during the transmit (TX) excitation.
PMUT vibration due to received (RX) echoes returning fromthe fluid-air surface were also visible in these experiments. The
Fluorinert-air surface was varied from 4 mm to 1.2 mm, andthe plots in Fig. 12 show the echoes return 11 μs, 8 μs, 5 μs
and 3 μs after the TX pulse is sent. The corresponding roundtrip distances calculated from these pulse echo measurements
are consistent with the Fluorinert height using c = 750 m/s
as the speed of sound in Fluorinert. In Fig. 12(c) and (d),a second echo can be observed due to the short liquid distanceand large output pressure. The vibration amplitude of the firstreceived echo relative to the transmit vibration amplitude fits
the acoustic spreading model from (7),d R X (x)/dT X = R0 x −1(8)This model is plotted along with the experimental data in dB
units in Fig. 13. Comparing the experimental pressure measurements from Fig. 11 with the RX vibration amplitudes
shown in Fig. 12, the receive sensitivity of the array is

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ACKNOWLEDGMENT

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The authors thank UC Berkeley Marvel Nanolab for theadvice and help on the fabrication process, and IceMOS
Technology for providing cavity SOI wafers.

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Fig. 13. Measured insertion loss from TX to RX vibration amplitudes as afunction of round-trip pulse-echo distance.

The results presented here demonstrate that ScAlN PMUTs
have better performance than PMUTs made with AlN. Using
15% Sc, the transmit amplitude was increased by a factorof two relative to PMUTs made with pure AlN, consistent
with a 60% increase in the transverse piezoelectric coefficient,e31, f . The PMUT fabrication process is nearly unchanged byintroducing ScAlN. While wet etching of Sc0.15 Al0.85 N in
TMAH proceeds at a much slower etch rate than pure AlN, a
Cl2 /BCl3 plasma etch was demonstrated to achieve an etch rateof 120 nm/min for Sc0.15 Al0.85 N. We expect that increasing the
Sc concentration would further improve PMUT performance,since other work has shown that the piezoelectric coefficientsof ScAlN increase as the Sc concentration is increased up to
40%. While RF devices, such as BAW filters, may suffer due
to the reduced stiffness (and therefore lower acoustic velocity)that occurs as the Sc concentration is increased, this reduced
stiffness does not degrade the performance of PMUTs.

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Sergey Mishin received the M.Phys. degree (Hons.)from the Russian National Research University ofElectronic Technology in 1986.
His main research interests include cold discharge,
high density plasma, magnetron plasma, excimer
laser systems, and the application of plasma discharge and lasers in the semi-conductor industry.
He founded Advanced Modular Systems, Inc. in
2000. Advanced Modular Systems, Inc. is responsible for developing the first production worthy
cluster tool for high volume piezoelectric thin films.
By its second year, Advanced Modular Systems, Inc. was presented with the
Supplier of the Year Award by Agilent Technologies, Inc. Since founding
his company, he has been focused on the development of new equipment
for the manufacture and treatment of piezoelectric thin films. As part of thisresearch and development, he has had an opportunity to work closely withindustry leaders and high-class research laboratories. He has also authoredor co-authored numerous papers, and holds patents on the deposition andtrimming technologies used in FBAR/BAW/SAW and MEMS application.

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[13] M. Moreira, J. Bjurström, I. Katardjev, and V. Yantchev, “Aluminumscandium nitride thin-film bulk acoustic resonators for wide band
applications,” Vacuum, vol. 86, no. 1, pp. 23–26, Jul. 2011.
[14] K. Hashimoto, S. Sato, A. Teshigahara, T. Nakamura, and K. Kano,“High-performance surface acoustic wave resonators in the 1 to 3 GHz
range using a ScAlN/6H-SiC structure,” IEEE Trans. Ultrason.,Ferroelectr., Freq. Control, vol. 60, no. 3, pp. 637–642, Mar. 2013,
doi: 10.1109/TUFFC.2013.2606[15] A. Konno et al., “Determination of full material constants of ScAlN
thin film from bulk and leaky Lamb waves in MEMS-based samples,”in Proc. IEEE Int. Ultrason. Symp., Sep. 2014, pp. 273–276.
[16] H. P. Loebl, M. Klee, C. Metzmacher, W. Brand, R. Milsom, and P. Lok,
“Piezoelectric thin AlN films for bulk acoustic wave (BAW) resonators,”
Mater. Chem. Phys., vol. 79, nos. 2–3, pp. 143–146, Apr. 2003.
[17] J.-M. Wagner and F. Bechstedt, “Properties of strained wurtzite GaN
and AlN: Ab initio studies,” Phys. Rev. B, Condens. Matter, vol. 66,
no. 11, p. 115202, Sep. 2002.
[18] A. V. Sotnikov, H. Schmidt, M. Weihnacht, E. P. Smirnova,
T. Y. Chemekova, and Y. N. Makarov, “Elastic and piezoelectric
properties of AlN and LiAlO2 single crystals,” IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, vol. 57, no. 4, pp. 808–811, Apr. 2010.
[19] K. Umeda, H. Kawai, A. Honda, M. Akiyama, T. Kato, and T. Fukura,“Piezoelectric properties of ScAlN thin films for piezo-MEMS devices,”in Proc. IEEE 26th Int. Conf. Micro Electro Mech. Syst. (MEMS),
Jan. 2013, pp. 733–736.
[20] M. Dubois and P. Muralt, “PZT thin film actuated elastic fin micromotor,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 45, no. 5,
pp. 1169–1177, Sep. 1998.
[21] P. M. Mayrhofer et al., “ScAlN MEMS cantilevers for vibrationalenergy harvesting purposes,” J. Microelectromech. Syst., vol. 26, no. 1,
pp. 102–112, 2016.
[22] P. Muralt and J. Baborowski, “Micromachined ultrasonic transducers
and acoustic sensors based on piezoelectric thin films,” J. Electroceram.,vol. 12, nos. 1–2, pp. 101–108, 2004.
[23] K. Smyth and S.-G. Kim, “Experiment and simulation validated analytical equivalent circuit model for piezoelectric micromachined ultrasonictransducers,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 62,
no. 4, pp. 744–765, Apr. 2015.
[24] D. T. Blackstock, Fundamentals of Physical Acoustics. Hoboken, NJ,
USA: Wiley, 2000.

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Qi Wang (S’15) received the B.S. degree inmechanical engineering and automation from the
Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2011, and the M.S. degree
in nanomechanics from Tohoku University, Sendai,
Japan, in 2013.
He is currently pursuing the Ph.D. degree inmechanical engineering with the Department of
Mechanical and Aerospace Engineering, Universityof California at Davis (UCD), Davis. He is alsoa Graduate Student Researcher with the Berkeley
Sensor and Actuator Center, UCD. His research interests include piezoelectricthin films and MEMS sensors and actuators.

Yipeng Lu received the B.S. degree in materials science and engineering from Jilin University,
Changchun, China, in 2007; the M.S. degree inmicroelectronics from Shanghai Jiao Tong University, Shanghai, China, in 2010, and the Ph.D. degree
in mechanical engineering from the University ofCalifornia at Davis (UCD), Davis, CA, USA, in
2015. He joined the Berkeley Sensor and Actuator
Center, UCD, as a Graduate Student Researcher.
He was a Digital Hardware Engineer at Huawei
in 2011. He is currently a Senior Engineer with
Qualcomm. His research interests include MEMS sensors and actuators.

Yury Oshmyansky received the B.S. and M.S.
degrees from the Colorado School of Mines. He wasat Hewlett-Packard and subsequently Vitesse,
Motorola, SFI, Agilent, Avago, and Advanced Modular Systems, Inc. He is currently the Director ofprocess development with Advanced Modular Systems, Inc. He is also a Chemical Engineer. He holds
many patents, and has published numerous papers in
the field of manufacturing of the FBAR filters andrelated technologies.

David A. Horsley (M’97) received the B.S., M.S.,
and Ph.D. degrees in mechanical engineering fromthe University of California at Berkeley, Berkeley,CA, in 1992, 1994, and 1998, respectively. He has
been a Co-Director of the Berkeley Sensor andActuator Center since 2005. He held researchand development positions at Dicon Fiberoptics,
Hewlett Packard Laboratories, and Onix Microsystems. He is currently a Professor with the Department of Mechanical and Aerospace Engineering,
University of California at Davis, Davis, CA. His
research interests include microfabricated sensors and actuators with applications in ultrasonics and physical sensors. He was a recipient of the NSF
CAREER Award and the UC Davis College of Engineering’s Outstanding
Junior Faculty Award.

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AQ:3

AUTHOR QUERIESAUTHOR PLEASE ANSWER ALL QUERIESPLEASE NOTE: We cannot accept new source files as corrections for your paper. If possible, please annotate the PDF
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AQ:1 = Please provide the postal code for “Advanced Modular Systems, Inc.”
AQ:2 = Please provide descriptions of all labeled subparts for Fig. 12.
AQ:3 = The term “AMSystems” has been changed to “Advanced Modular Systems, Inc.” Please confirm.