SCANDIUM DOPED ALUMINUM NITRIDE BASED PIEZOELECTRIC
MICROMACHINED ULTRASOUND TRANSDUCERS
Qi Wang1, Yipeng Lu1, Stephanie Fung1, Xiaoyue Jiang2, Sergey Mishin3, Yury Oshmyansky3
David A. Horsley1
1
University of California, Davis, USA
2
University of California, Berkeley, USA
3
Advanced Modular Systems, Inc., USAABSTRACTThis paper presents the design, fabrication and characterizationof piezoelectric micromachined ultrasound transducers (PMUTs)
based on scandium doped aluminum nitride (ScxAl1-xN) thin films(x = 20%). ScAlN thin film was prepared with a dual magnetron
system and patterned by a reactive ion etching system utilizing
chlorine-based chemistry with an etching rate of 160 nm/min. Thefilm was characterized by X-ray diffraction (XRD) which indicateda crystalline structure expansion change compared to pure AlN anda well-aligned ScAlN film. ScAlN PMUTs were fabricated by a 2mask process based on cavity SOI wafers. ScAlN PMUTs with
50 μm diameter had a large dynamic displacement sensitivity of
25 nm/V at 17 MHz in air, twice that of AlN PMUTs with the samedimensions. Electrical impedance measurements indicated that theScAlN PMUTs had 20% greater electromechanical coupling
coefficient (kt2) compared to AlN PMUTs. The output pressure of a7x7 ScAlN PMUT array was 0.7 kPa/V at ~1.7mm away from thearray, which is ~3 times greater that of an 8x8 AlN PMUT arraywith the same element geometry and fill factor measured at the samedistance.

INTRODUCTION
Micromachined ultrasonic transducers (MUTs) have beendeveloped for many applications in recent years, such as medicalimaging [1-2], gesture sensors [3], ultrasonic fingerprint sensors [4],and body-composition sensors [5]. Compared with conventionalbulk ultrasonic transducers, MUTs have a better acoustic coupling,
lower cost for two-dimensional array fabrication, and lower power
consumption. Recently, piezoelectric micromachined ultrasonictransducers (PMUTs) have been rapidly developed due to theprogress of piezoelectric thin films. Aluminum nitride (AlN) has
been widely used for PMUT fabrication because it is available froma number of MEMS foundries and is compatible with CMOS
manufacturing. However, compared to lead zirconate titanate (PZT),
a piezoelectric material which requires high annealing temperatureand is not process-compatible with CMOS, AlN has relatively low
piezoelectric coefficient (e31,f), which leads to low sensitivity andlow electromechanical coupling (kt2) [6].
Recently scandium (Sc) doping has been proposed as a means
to increase the e31,f of AlN, while maintaining process compatibility
with existing AlN-based manufacturing [7]. Studies also found thatwith the increase of Sc doping concentrations, the stiffness of thethin film decreased and the dielectric constant increased [8]. Mostof 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 highstiffness to achieve high frequency operation and high quality factor(Q) [9-10]. In this paper, we present flexural PMUT devices which
use the transverse piezoelectric mode and where the reduced
stiffness of ScAlN may provide a benefit over conventional AlN.
The schematic of each PMUT is shown in Figure 1. The PMUT
was composed of a 1 μm thick ScAlN piezoelectric layer, a 200nm
Mo layer as bottom electrode and a 2.5 μm thick silicon membrane.
The vacuum cavity underneath the silicon membrane eliminates

Figure 1: Schematic of ScAlN PMUT.
possible squeeze-film damping between the PMUT membrane andthe Si substrate.

FABRICATIONScAlN thin film deposition
ScxAl1-xN thin film (x = 20%) of 1 μm thickness was sputteredon 6 inch cavity SOI wafers (IceMOS Technology) in an Advanced
Modular Systems (AMS) cluster tool with AlN depositionchambers and ion beam trimming module. The system used astandard dual conical magnetron with an AC deposition source. TheScAlN deposition process was in deep poison mode using targets
composed of Al and Sc pieces. Locally adjusted magnetic field fortarget pieces of both Al and Sc guaranteed a constant thin film
composition over the entire target life. Substrate rotation wasutilized to compensate for the variation of the sputtering yield for
different materials and composition non-uniformity across thesubstrate.
A 30 nm thick ScAlN was firstly deposited on the cavity SOI
as a seed layer. Then a 200 nm thick molybdenum (Mo) layer wassputtered as the bottom electrode in a different chamber in thesystem without breaking the vacuum. Finally, 1 μm thick ScAlN
was sputtered on the Mo layer.
ScAlN PMUT fabrication
ScAlN PMUTs were fabricated based on a simple 2-mask
process with cavity SOI wafers as reported in [6]. The process
requires etching the ScAlN film to open vias to the Mo bottomelectrode. An initial study of wet etching the ScAlN thin film using
heated positive photoresist developer (Microposit MF-319) mainlycomposed of tetramethylammonium hydroxide (TMAH) showedthat the ScAlN etch rate was ~50 nm/min at 60°C to 70°C,
approximately 4 times slower than that of AlN thin films with thesame thickness and at the same etching temperature. Then reactive
ion etching (RIE) was studied with a combination of Cl2, BCl3 andHe gases in a transformer coupled plasma (TCP) etcher. He gas isused as diluent to improve etch uniformity. A 6.5 μm thick g-line
photoresist (OCG 825 35S, Fujifilm) was spin coated, patterned, andhard 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 1 with anetching selectivity of 0.4 to the mask. A 200 nm thick aluminumTable 1: Parameters for RIE of ScAlN thin film

Parameters
Cl2 flow rate (sccm)
BCl3 flow rate (sccm)
He flow rate (sccm)
TCP RF Power (W)RF Bias Power (W)Values9030100
550150(a)Figure 2: An optical microscope image of a 7x49 PMUT array.
The individual PMUTs are 50 μm diameter and the array pitch
is 70 μm.
(Al) layer was evaporated and patterned by a lift-off process to form
the top electrode and contact pad for the bottom electrode. A PMUT
array composed of 7×49 single PMUTs with 50 μm diameter and 70μm pitch is shown in Figure 2. A cross-section scanning electron
microscope (SEM) image of a PMUT, Figure 3, shows the dense
columnar structure of the ScAlN film and Mo bottom electrode.

CHARACTERIZATIONScAlN thin film characterization
The ScAlN crystalline structure was studied by X-ray
diffraction (XRD). Figure 4(a) shows a comparison of the XRD
peaks of pure AlN and ScAlN thin film with 1 μm thickness. The(002) peak of ScAlN was slightly shifted to a lower angle comparedwith that 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 Figure 4(b). The full-widthhalf-maximum (FWHM) of the peak is 1.7°, indicating that the caxis of the ScAlN thin film is well aligned and predicting good

Figure 3: Cross-sectional SEM image of a ScAlN PMUT.

(b)
Figure 4: (a) Normal coupled XRD measurement of ScAlN andAlN films; (b) Rocking curve measurement of ScAlN (002) peak.
piezoelectric properties.
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) in conjunction with a network analyzer. LDVmeasurements were collected on 16 ScAlN PMUTs across thewafer, resulting in a 17.5 +/- 1.5 MHz natural frequency and 22 +/4 nm/V dynamic displacement sensitivity at resonance. The results
are shown in Figure 5. Cross-section SEM image showed that the Si
thickness varied from 2.40 μm to 2.93 μm. Using equation (1), theobserved variation in natural frequency is consistent with themeasured thickness variation. Figure 6 compares the LDV results ofScAlN and AlN PMUTs. The dynamic displacement sensitivity of

Figure 5: Measured resonance frequency and dynamicdisplacement at resonance for ScAlN PMUTs with 50 μmdiameter and 2.5 μm nominal Si thickness.

(a)Figure 6: LDV measurement results for 50 μm diameter ScAlN
and AlN PMUTs
the ScAlN PMUT is two times as large as that of the AlN device.
The difference in the resonance frequency of ScAlN and AlN
PMUTs is due to the stiffness reduction from Sc doping as
mentioned earlier. The Young’s modulus of ScAlN was estimatedto be 200GPa via equation (1), which is consistent with the reported
values obtained from ScAlN BAW devices [11],
𝑓=

1.63
𝑟2

𝐷√∑ 𝜌 𝑡(1)𝑖 𝑖where r is the radius, D is rigidity, 𝜌 and t are the density andthickness of the PMUT layers. The rigidity D can be expressed as
𝑡𝑜𝑝 𝑙𝑎𝑦𝑒𝑟
𝐸2𝑧 2
𝐷 = ∫𝑏𝑜𝑡𝑡𝑜𝑚 𝑙𝑎𝑦𝑒𝑟 𝑧 2 𝑑𝑧
1−𝜈𝑧(2)where Ez is the Young’s modulus and νz is the Poisson’s ratio of thematerial at a distance z from the neutral axis. The average quality
factor of the PMUTs is Q = 140. The static displacement sensitivity,
which is the peak displacement normalized by Q, ds = dy/Q, is~180 pm/V. ds is related to the transverse piezoelectric coefficient𝑒31,𝑓 via:
𝑒31,𝑓 𝑍𝑛𝑑𝑠 ∝(3)𝐷
where Zn is the distance from the middle of the piezoelectric layer
to neutral axis. The estimated 𝑒31,𝑓 is ~1.6 C/m2 which is ~60%
higher than that of AlN.
Electrical characterization
Impedance measurements of ScAlN and AlN PMUTs, Fig. 7,
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:
𝑘𝑡2 =𝜋2 𝑓𝑟 𝑓𝑎 −𝑓𝑟
4 𝑓𝑎𝑓𝑎(b)
Figure 7: Impedance measurement results for 50 μm diameter(a) ScAlN PMUT and (b) AlN PMUT.
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 8x8 AlN PMUT arraydriven at 25 Vpp, suggesting 3x greater transmit efficiency from theScAlN array. The dynamic displacement of a 17x17 ScAlN PMUT
array driven with 11 Vpp was measured via LDV, Fig. 9. A ~11nm
displacement of the center ScAlN PMUT was measured during thetransmit (TX) excitation. The plot shows that the PMUT
displacement from the returning echo is also visible 10 µs after theTX pulse is sent. The 6.8 mm round-trip distance calculated fromthis pulse echo measurement is consistent with ~3.4 mm height ofthe Fluorinert above PMUT array. The echo generated deformationof PMUT was ~1 nm indicating a ~20 dB insertion loss from TX
displacement to the RX echo displacement. This result is consistent
with 16 dB spreading loss (which was verified using hydrophone(4)

where fa and fr are the anti-resonant and resonant frequency
respectively. kt2 was 1.8% for ScAlN PMUTs, 20% higher
compared to the AlN PMUT’s 1.47% which was close to the value
expected for a pure AlN device operating in transverse mode. The
relative dielectric permittivity (εScAlN) of ScAlN was also estimatedfrom the impedance test as ~12 which is around 20% higher thanthat of pure AlN.
Acoustic characterization
An array of ScAlN PMUTs was immersed in Fluorinert (FC70, 3M) and the output acoustic pressure was measured with a 70
μm diameter needle hydrophone (Precision Acoustic, Inc.). The
results are shown in Figure 8. A 7x7 ScAlN PMUT array was driven
by 4 9 MHz 11 Vpp pulses from a function generator (Rigol, DG4102). The measured pressure generated by the ScAlN PMUT array
was detected at ~2.5 µs after the pulse generation, which

Figure 8: Pressure measurement results for 7x7 ScAlN PMUTsarrayFigure 9, LDV measurement of pulse-echo from a ScAlN PMUT
in the center of 17x17 Array
Table 2, A comparison of piezoelectric materials properties
2𝑒31,𝑓𝑒31,𝑓Material
ɛ33
E (GPa)(GPa)
𝜀33 𝜀0(C/m2)
PZT [12]-14.0120065
18.5
AlN [12]
-1.05
10.5
390
10.8
ScAlN
-1.6
12200
24.1
(this work)measurements at different distances) and 4 dB of loss due to the 40%
fill-factor of the array.

CONCLUSION
ScAlN PMUTs appear to have important performanceadvantages over AlN PMUTs. The manufacturing process of theScAlN PMUT is nearly identical to that of the earlier AlN device. Asummary of the measured properties of the ScAlN film used here ispresented in Table 2. Importantly, the figure of merit shown in thetable’s last column shows that ScAlN is 1.3 times better than PZTfor PMUTs.

ACKNOWLEDGEMENTThe authors thank UC Berkeley Marvell Nanolab for the advice
and help on the fabrication process, Berkeley Sensor and Actuator(BSAC) industrial members for financial support, and IceMOS
Technology for providing cavity SOI wafers.

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CONTACT*Q. Wang, tel: +1-530-746-1632; qixwang@ucdavis.edu