Investigation of 20% Scandium-doped Aluminum
Nitride Films for MEMS Laterally Vibrating
Resonators
Luca Colombo, Abhay Kochhar, Changting Xu,Gianluca Piazza
Electrical and Computer Engineering Department
Carnegie Mellon University
Pittsburgh, Pennsylvania (USA)
Abstract — This paper reports on the investigation of 1 µm
thick films of 20% Scandium-doped Aluminum Nitride (ScAlN)
for the making of piezoelectric MEMS laterally vibrating
resonators (LVRs). The ScAlN films, which can be sputterdeposited such as undoped Aluminum Nitride (AlN) films, wereused to demonstrate high performance resonators. These devices
showed quality factor (Qs) in excess of 1000 in air centeredaround 250 and 500 MHz and enhanced electromechanicalcoupling (kt2) in the range of 3.2-4.5%. This kt2 is double the valueof what has been achieved on similar resonators made out of AlN
films. A 3-dB Qs of 1300 has been recorded both for 1-port and 2port resonators at 250 and 500 MHz, while a maximum Qs of1500 has been recorded for a 1-port resonator at 500 MHz. Alongwith experimental results from actual devices, this work also
reports the etching characteristics of the piezoelectric material
under Cl2/BCl3 chemistry to attain high selectivity and straightsidewall with a SiO2 hard mask. More broadly, enhancement ofresonators design and fabrication process, suppression ofspurious modes and increase in the concentration of Sc(theoretically up to 40%) will lead to significant performance
improvements for many classes of piezoelectric MEMS, especiallytunable filters.
Index Terms — MEMS, AlN, ScAlN, Resonators, LVR,fabrication, Scandium-dopedI. INTRODUCTION
RF filtering technologies based on piezoelectric MEMS
resonators have been widely investigated and successfully
deployed over the past decade due to their size andoutstanding performance. Especially for mobile phonefiltering applications, systems based on thin film bulk acoustic
wave resonators (FBARs) and surface acoustic wave (SAW)
devices have been proposed and commercialized on a large
scale, setting the threshold for any future improvement of thistechnology. The challenge to enhance filter’s performance interms of lower insertion losses and wider bandwidth is still anactive research topic, especially spurring the interest for newclasses of materials such as doped-AlN [1] and thin films ofLithium Niobate (LN) [2]. Research is currently focused inovercoming fabrication constraints and device performance forboth materials, which are still far from the theoreticalmaximum.

Sergey Mishin, Yury Oshmyansky
Advanced Modular Systems, Inc.
Goleta, California (USA)Different solutions based on lateral vibrating resonators(LVRs) or contour mode resonators (CMR) have beenpreviously proposed to allow multi-frequency fabrication on asingle die and achieving a good Figure of Merit (FOM=kt2·Qs)
at the same time, which respectively limits FBARs and SAW
devices. Despite encouraging Qs achieved with AlN-based
systems, the electromechanical coupling (kt2) of these devices
remains very limited (1-2%), preventing the construction ofwideband filters. Furthermore, the low dielectric constant(εr=9) of AlN needs arraying for application requiring highstatic capacitances (nominally C0 of a resonator). X-cut [3] andY-cut LN [4] alternatives have also been proposed, due to highεr (43) and kt2 (10-30%). Although results indicates good
performance, the increased fabrication complexity and cost ofthin film transfer still limits LN resonators large-scale
implementation.
A possible solution for both performance enhancement andfabrication complexity is the use of sputtered Scandium-doped
films of AlN. The most promising study reports values of kt2
up to 8% for 40% Sc-doped AlN [1][5], roughly 4 times
higher than non-doped AlN [6]. Despite lower kt2 and εrcompared to LN resonators, ScAlN devices may show higher
quality factor, Qs, leading to higher overall FOM. Finally,
ScAlN devices allow better system integration compatibility
with CMOS and easier fabrication due to thin film sputtering.
This paper describes the design and fabrication of 20% Scdoped MEMS LVRs resonators by building off the knowledge
developed through the design and fabrication of similar AlN
devices. The main goals of this work are to demonstrate: 1)
higher kt2 (~2x) compared to non-doped AlN, 2) ease of
fabrication process compared to lithium niobate thin films, 3)fabrication of multi-frequency devices (250 MHz – 500 MHz)on the same substrate.
II. RESONATOR DESIGN AND FABRICATION
A. General layout and geometries
The resonators described in this work are made ofinterdigital electrodes (IDT) on top of a suspended plate of
ScAlN thin film (1 µm). A patterned Pt bottom electrode of
100 nm deposited on 10 nm of Cr adhesion layer is present andgrounded through laterally etched vias connected to groundpads. The presence of this bottom electrode increases theresonator static capacitances, C0, and allows the fabrication of
two-port networks. Different finger pitches, plate sizes andnumber of Al top electrodes have been selected for the test
based on the previously developed AlN resonators designs.
Device center frequencies were designed to range from 200
MHz to 500 MHz based on the density and Young’s modulus
reported in literature for 20% Sc-doped AlN films [7]. Since
the characteristics of the films were not known at the time of
the tape-out, the overall design is not fully optimized for
ScAlN. The plate’s length was varied between 130 µm and 200µm, the width between 50 µm and 100 µm with a number ofelectrodes equal to 6. IDT coverage was kept constant at 50%.
Starting from this configuration, some 1-port devices have been
included in the layout. For this particular class of devices thebottom electrode is floating and vias are not present. Contact
pads with 150 µm pitch in a ground-signal-ground (GSG)
configuration are used for measurements.

deposition (step 2 in Fig. 2) by AMS Inc. Then, a SiO2 hardmask was patterned for the release pit etching (step 3 in Fig. 2).
SiO2 was deposited on the ScAlN surfaces with a TrionPECVD tool. The overall thickness was set at 2.5 µm
according to the etch rate selectivity between SiO2 and ScAlNin chlorine-based RIE for the selected recipe (1:2). In order topattern the SiO2, 50 nm of Cr were sputtered with the Perkin
Elmer 6J and patterned using chlorine-based RIE (Versaline).
AZ 4110 photoresist was used to define the pattern to betransferred in the thin Cr layer. Later, oxide was etched using
fluorine-based RIE (Plasmatherm 790). ScAlN layer wasetched using chlorine-based RIE (Versaline). Compared toAlN, for the same recipe parameters (Pressure: 5 mTorr, ICPpower: 400 W, RF power: 125 W, Cl2 flow: 25 sccm, BCl3
flow: 5 sccm, Ar flow: 70 sccm), the measured etch rate of
ScAlN was 4 times lower (50 nm/min vs 200 nm/min). For thisreason the ICP power was increased to 600 W, the RF power to350 W and the BCl3 flow to 15 sccm to ensure an etch rate of
150 nm/min.

Fig. 1. General layout for a ScAlN resonator.

B. Process flow
The main fabrication steps for ScAlN resonators are shownin Fig. 2. The process flow is based on the same sequencepreviously developed for AlN resonators. Due to the different
behavior of the materials during etching, recipes have been
properly modified and adjusted through direct experimentation
with samples of ScAlN films.
As first step (step 1 in Fig.2), the Pt bottom electrode waspatterned on top of a HR silicon wafer (ρ=15000 Ω·cm) using
DC sputtering (Perkin Elmer 6J) and lift-off (AZ 4110). The
thickness of the metal was set to 100 nm. To ensure adhesion
between the metallic layer and the Si surface, 10 nm of Cr waspre-sputtered with the same tool. Cr is required to make theprocess compatible with hydrofluoric acid (HF) wet etch usedto strip the oxide mask in subsequent steps. Successively,ScAlN was deposited on top of the wafer through sputter

Fig. 2. Fabrication flow for ScAlN devices.

The same step used to define the device structure was also
used to pattern the via (Step 4 in Fig. 2). Another oxide layer
was deposited. The etch step was subdivided into several substeps of 10 second each in order to properly stop the etch on thePt layer. Step (5) involved a double sputtering deposition andlift-off (AZ 4110) of AlSiCu. 100 nm thick of AlSiCu wasdeposited on the ScAlN substrate for the fingers and the base ofthe pad, while another 200 nm were sputtered for the pads toensure good adhesion for wire bonding. As last step (6), thedevices were released using XeF2.

III. RESULTS
A. SEM pictures and fabrication issues
After fabrication and release, the devices were visuallyinspected with an SEM. As shown in Fig. 3, redeposition
effects are present on the resonator surface and on the outer
frame in the form of thin flakes. Furthermore, some out-ofplane bending of the resonator can be noticed. Even if theaverage stress measured in the Sc-doped film was lower than
100 MPa, the stress gradient is sufficient to deform the plates
once released. The sidewalls of the resonator appear smoothand range between 70 and 75 degrees, as shown in Fig. 4.

data, the resonator equivalent electrical parameters wereextracted by fitting the responses to a Butterworth-Van Dyke(BVD) model [8], both for 1 and 2-port devices (Fig. 5).

Fig. 5. BVD models for 1 and 2-port resonators.

Measured responses and fitted results, with equivalent
electrical parameters, are shown in Fig. 6-8 for different
resonators and frequencies (both 1 and 2-port devices).

Fig. 3. SEM picture of 250 MHz 2-port ScAlN resonators. Flakes due toredeposition are visible on the edge of the resonator and on the outer frame.

Fig. 6. Y11 admittance response for 1-port resonator (250 MHz) without deembedding.

Fig. 4. Sidewall detail of a ScAlN resonator.

B. Admittance response of resonators
In order to extract the admittance response of the fabricatedresonators, their S-parameters were measured using a vector
network analyzer (VNA). These set of parameters were thenconverted to Y-parameters. Starting from the Y-parameters

Fig. 7. Y11 admittance response for 1-port resonator (500 MHz) without deembedding. This resonator shows the highest Qs recorded for this type ofdevice.

such as density (ρ), dielectric constant (εr) and stiffness matrix
coefficients.
The results presented herein are very encouraging and webelieve that, through additional work, we will be able to devisethe next generation of sensors and filters for a broad class ofcommercial applications.
V. ACKNOWLEDGMENTS
This research has been sponsored by Intel’s University
Research Office. The material is also based upon work
supported by the Defense Advanced Research Projects Agency(DARPA) under Contract No. HR0011-15-C-0137.
Authors would like to thank the staff of Carnegie Mellon
University’s Nanofabrication facility for their kind andcontinuous support.
REFERENCES

Fig. 8. Y11 and Y12 admittance responses for 2-port resonator (250 MHz)
without de-embedding.

IV. CONCLUSIONSIn this paper 20%-doped Sc AlN films were investigated forthe making of piezoelectric MEMS LVRs. The combination ofhigh kt2 (4.5%) and Qs (1300 for 1 and 2-port resonators)
achieved with ScAlN devices yielded an overall FoM (kt2 Qs)of about 60. Considering the early stage of this investigation of
Sc-doped AlN films, two different approaches can be adopted
to improve the quality of these resonators. The first is toimprove the fabrication process, eliminating redeposition,
improving the sidewall quality and better controlling thestresses of the thin film. We believe that these improvements
would enhance the Qs of the resonators. The second is toincrease the doping concentration of Scandium up to themaximum theoretical optimal value of 40%. A limiting factor
that emerged during the fabrication of ScAlN LVRs is thepresence of spurious modes, which would need to be controlled
for filtering applications.
Further experiments and tests are required to better
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