EXPERIMENTAL INVESTIGATION OF DAMPING FACTORS IN 20%
SCANDIUM-DOPED ALUMINUM NITRIDE LATERALLY VIBRATINGRESONATORS
Zachary A. Schaffer1, Luca Colombo1, Abhay S. Kochhar1, Gianluca Piazza1,Sergey Mishin2, and Yury Oshmyansky2
1
Carnegie Mellon University, Pittsburgh, USA
2
Advanced Modular Systems, Goleta, USA
ABSTRACTDESIGN AND FABRICATIONThis paper reports experimental investigation ofdamping factors for 20% Scandium-doped Aluminum
Nitride (ScAlN) laterally vibrating resonators (LVRs).
ScAlN films for LVRs are very promising since theyenable greater electromechanical coupling (kt2) thanundoped AlN films (~ 2X for 20% doping), but havegenerally exhibited lower quality factors (Qs) [1,2]. This
work is the first to study what damping factors impact theperformance of these devices and provide preliminary
design guidelines to attain high Qs. Different ScAlN LVR
geometries are analyzed and devices tested in air, undervacuum, and at cryogenic temperatures (11 K). Anchor
losses and thermoelastic damping (TED) are evaluated andcompared to AlN LVRs. Similarities in thermal scaling ofQs in ScAlN resonators indicate that the source of TED issimilar to AlN devices.

For this work, LVRs with different anchor dimensions
and consistent plate geometries were fabricated using both
AlN and 20% Sc-doped AlN piezoelectric films. The LVR
is composed of a 1 μm thick piezoelectric film with a 100nm thick Pt bottom electrode and 100 nm thick interdigital
Al top electrodes setup in a two-port configuration. Anchor
length (La) and anchor width (Wa) are varied as a functionof the LVRs acoustic wavelength (λ). LVRs with λs of 17
μm and 34 μm for the S0 mode (longitudinal in-planevibrations), are used. These wavelengths correspond tocenter frequencies around 250 and 500 MHz (exact value
depends on the materials forming the stack). Resonatorgeometric parameters can be seen in Figure 1 and Table 1.

INTRODUCTION
Over the past decade piezoelectric MEMS resonators
have been successfully integrated into a range of RF
filtering applications [3]. Performance in these filters aredefined by insertion loss and filter bandwidth, which areset primarily by kt2 and Q and have limits ultimately set by
fundamental material properties of the piezoelectric andmetal layers within the resonator stack. This has driven
interest in new piezoelectric materials. ScAlN is one suchmaterial. It possesses the unique advantage of being sputter
deposited in a manner similar to AlN films, while having
an up to 4 times improvement in kt2 —up to 8% at 40% Sc
concentration [1] and 4.5% at 20% Sc concentration [4] forlaterally vibrating resonators (LVRs). ScAlN offers similar
coupling improvements for other resonance modes, and istherefore being explored at various concentrations for awide range of uses including bulk acoustic wave resonators[5], ultrasonic transducers (PMUTs) [6], and energy
harvesting
cantilevers
[7].
These
preliminary
investigations into ScAlN LVRs have shown Qs that are
below those recorded for AlN films and have not made any
inroads in clarifying what the sources of damping in theseresonators are.
In this work, we compare damping mechanisms
between AlN and 20% Sc-doped AlN LVRs operating at
approximately 250 and 500 MHz. Specifically, Qs fromvarious anchor geometries are compared for both materials
to show general trends and optimal anchor designs, andthermal scaling of Q is examined by measuring devices at
293 and 11 K. Similarities between thermal scaling of Q in
AlN and ScAlN devices indicate thermoelastic damping
scales in the same way in the two classes of devices, hintingthat the losses are not intrinsic to the piezoelectric film.

Figure 1: SEM image of LVR with varied parameterslabeled.

ScAlN and AlN resonators were both fabricated using
a six-step process shown in Figure 2. Effort was made toensure similar fabrication results for accurate comparison.
Notable differences between AlN and ScAlN process flows
are changes in the release trench etch process (step 4) due
to ScAlN etch compatibility, and changes in top electrodemetal deposition tool due to tool availability.

GroundGroundGroundGroundused, and for the ScAlN devices Al was used. Both weresputter deposited and patterned via liftoff. Note that these
additional metal layers were placed only on the routing
lines/pads and not on the resonant body of the device. Sixthand lastly, (step 6) devices were released using XeF2
etching to undercut the Si substrate. XeF2 etching times
were kept consistent between AlN and ScAlN devices toavoid difference in anchor losses due to undercut amount[8].
SEM inspection of devices (Fig 3,4) confirms ion
milling successfully prevented the side wall residuals seenin previous ScAlN fabrication [4]. A high stress gradient
was observed in the ScAlN film, causing bowing of theresonator plate.

Figure 2: LVR step by step fabrication process top downand cross-sectional views.
A high resistivity Si wafer was used as a starting
substrate. Resistivity and planarity were confirmed using
DC probe electrical measurements and a Tencor Flexus
stress measurement system, respectively. First, (step 1) a Pt
bottom electrode was patterned using liftoff. 100 nm of Pt
with a 10 nm Cr adhesion layer was DC sputtered, and thenpatterned through liftoff using photoresist with acetone
sonication. Cr was used instead of Ti as an adhesion layer
for compatibility with subsequent BHF etching. Second,(step 2) a 1 μm piezoelectric layer was deposited. AlN wasdeposited using a Tegal AMS AlN Sputtering System.
ScAlN was deposited by AMS using a dual magnetron
sputter system. Sc concentration was confirmed to be 20%
by Energy-Dispersive X-ray Spectroscopy. Third, (step 3)
vias were etched to connect the bottom Pt electrode. A 2.5
μm SiO2 layer was deposited by PECVD for use as a hardmask. 50 nm Cr was sputtered on top and patterned using aCl-based RIE process with photoresist serving as a softmask. A F-based RIE process was then used to transfer theCr layer pattern to the SiO2. The patterned SiO2 served asa hard mask for a Cl-based RIE for both AlN and ScAlNwafers. The etch was performed in steps, with opticalinspection to stop etching when the Pt bottom electrodewas exposed. Afterwards, the SiO2 was removed using
BHF. Fourth, (step 4) a trench was etched to define therelease and device area. For AlN, this was done using thesame SiO2 hard masked Cl-based RIE process detailed forthe via etch. The Cl-based RIE process used for AlN
exhibited a 4 times lower etch rate of the ScAlN (50nm/min vs. 200 nm/min). Changes to the etch recipe
resulted in deposition of residuals on the sidewalls of theetched trench and high sidewall roughness. Instead, ion
milling with a photoresist soft mask was used. Fifth, (step5) top metal electrodes were deposited and patterned withliftoff. For the AlN devices, 100 nm of Al alloyed with Cu
and Si were deposited using a CVC Connexion sputter
system. For the ScAlN devices, 100 nm of pure Al wassputtered using a Perkin Elmer 8L deposition system. Fifth,(step 5) a thicker, 200 nm, top metal layer was deposited toallow better contact for measurement probes, and thicker
coating of via sidewalls. For the AlN devices gold was

Figure 3: SEM image of device before XeF2 releaseshowing sidewall profile from ion milling.

Figure 4: SEM image of device after XeF2 releaseshowing lack of etch residuals and uniform sidewall.

RESULTS
Measurement Procedure
Devices were measured using an Agilent vector
network analyzer and cryogen-free low temperature probestation (CRX-VF Lakeshore) with GSG probes connectedto the two ports of the device as labeled previously in
Figure 2. Measured S-parameters were converted, and fit
to Modified Butterworth Van Dyke (MBVD) models. Qs
were measured as the frequency at an admittance -3dBfrom the series resonance peak divided by the series
resonance frequency; terminal loading (Rs) was not deembedded. Qs from air to vacuum increased by less than3%, indicating air damping was not a major factor. Devices
with different anchor geometries were then measured invacuum, to compare anchor damping. Optimal anchor
geometries (those with minimal anchor damping) werethen measured at 11 K to compare thermal scaling of device
Qs. All devices used to compare thermal scaling had Rs
values that were less than 10% of the motional resistance(Rm) of the devices.

Anchor Losses
Anchor losses are caused by elastic wave scatteringfrom the resonator into its surrounding substrate. Vibrationof the resonator plate exerts force on the anchors, causing
displacement in them, which is then transmitted to thesubstrate. Qs of devices with anchor length (La) and anchor
width (Wa) ranging from 0.25-1.0 λ are compared. TheAlN LVRs showed little effect from La, but stronger
dependence on Wa, with highest Qs at Wa = 0.5λ—
followed closely by 0.25 λ—and lower Qs for larger anchor
widths. This trend matches previous studies on anchor
losses in AlN LVRs [9]. ScAlN devices did not follow thistrend. Instead, ScAlN devices with wider anchors exhibited
higher Qs, with Wa = 1λ being best. A comparison of Wa
trends can be seen in Figure 5, and Qs of 500 MHz ScAlN
devices vs anchor geometry in Figure 6.

Figure 5: Average Qs of 250 and 500 AlN and 20% Scdoped AlN LVRs vs Anchor Width

Thermoelastic Damping
By measuring Qs of the same resonators at 293 and 11K, a thermally scaling damping term (𝑄𝑡ℎ𝑒𝑟𝑚 ) andthermally invariant damping term (𝑄𝑡𝑖 ) can be extracted
from the following relations.
111
1
1=+and≈(1)
𝑄293𝐾

𝑄𝑡ℎ𝑒𝑟𝑚

𝑄𝑡𝑖

𝑄11𝐾

𝑄𝑡𝑖

Based on previous studies of damping in AlN devices,
thermally scaling damping is caused by TED in the metalelectrodes [10]. Thus 𝑄𝑡ℎ𝑒𝑟𝑚 is equated to 𝑄𝑇𝐸𝐷 . Thisapproximation is not perfect, as many other properties
change between 293 and 11 K. As an example, the modeshape of the resonator changes slightly with temperature[11], meaning anchor losses scale slightly withtemperature. This was experimentally observed in thiswork as a 0.5% decrease in series resonance frequency at
11K. However, most approximation errors equally impact
ScAlN and AlN devices, making the approximation valid
for material comparison. From 293 to 11 K, the measured
electrical resistivity decreased by approximately 40% for
the top electrodes on these AlN and ScAlN devices. For anRs/Rm ratio of less than 10%, the 293 to 11 K temperaturechange would increase Qs by less than 6% and is therefore
not responsible for the increase in Qs observed below.
A subset of resonators with high Qs, as to not belimited by anchor damping, and low Rs/Rm ratios, as to not
be impacted by changes in electrical loading were selectedfor measurement at 11 K. Very similar 𝑄𝑇𝐸𝐷 values wereobserved in AlN and ScAlN devices, which implies that
20% Sc-doped AlN does not have a worse TED than AlN.
Table 2 shows a summary of extracted 𝑄𝑇𝐸𝐷 values foreach material type and frequency, while Figure 7 shows agraph of all Qs of measured devices at 293 and 11K,
separated by material type and frequency then ordered by
Q a 293K. Characteristic responses of AlN and ScAlN
devices at 293 and 11 K, with Wa and La each = 0.75λ, areshow in Figure 8, plotted with frequencies normalized.
Table 2: Extracted QTED values

Figure 6: Plot of Qs vs Anchor Geometry for 500 MHz
ScAlN LVRs

Figure 7: Plot of LVR Qs at 293 and 11 K divided by frequency and material type and ordered by Q at 293 K.

ACKNOWLEDGEMENTS
This research has been sponsored by Intel’s University
Research Office.
Authors would like to thank the staff of Carnegie Mellon
University’s Nanofabrication facility for their kind andcontinuous support.

REFERENCES

Figure 8: Y11/Y12 responses of 250 MHz (A) and 500
MHz (B) AlN and 20% Sc-Doped AlN LVRs at 293 and 11K, anchor length and width = 0.75 λ.

CONCLUSIONSThis work compared damping mechanisms in LVRs
made from AlN and 20% Sc-doped AlN films. ScAlN films
offer a promising solution for filtering applications due
their combination of high kt2 (4.5%), CMOS compatibility,and potentially high Qs. Anchor geometry was shown toscale differently in these ScAlN LVRs, with wider anchors
improving Qs. This will be investigated further. More
importantly, thermal scaling of Qs in ScAlN LVRs wasshown to be similar to AlN LVRs, implying that Sc-dopingof the AlN film does not add an extra TED component andreduce Q.
Further experimentation is necessary to validate Scdoped AlN. Extraction of material properties and athorough understanding of damping mechanisms in thesefilms are necessary to begin accurate simulation ofresonators, and produce optimized devices. Deposition
techniques must also be developed to produce low stressgradient films and test higher Sc concentrations. Until
optimized devices with high Sc concentration films areproduced the ultimate limits of this material may remain
unknown.
The presented results are encouraging, and we believethat with device optimization, ScAlN devices with Qs
similar to AlN can be produced.

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CONTACT*Z.A. Schaffer, zschaffer@cmu.edu