Design, Optimization and Performance Assessment of Single Port Film Bulk Acoustic Resonator through Finite Element Simulation

Abstract

In this paper, the study is supported by design, FEA simulation, and practical RF measurements on fabricated single-port-cavity-based acoustic resonator for gas sensing applications. In the FEA simulation, frequency domain analysis was performed to enhance the performance of the acoustic resonator. The structural and surface morphologies of the deposited ZnO as a piezoelectric layer have been studied using XRD and AFM. The XRD pattern of deposited bulk ZnO film indicates the perfect single crystalline nature of the film with dominant phase (002) at 2θ = 34.58°. The AFM micrograph indicates that deposited piezoelectric film has a very smooth surface and small grain size. In the fabrication process, use of bulk micro machined oxide (SiO2) for the production of a thin membrane as a support layer is adopted. A vector network analyzer (Model MS2028C, Anritsu) was used to measure the radio frequency response of the resonators from 1 GHz to 2.5 GHz. As a result, we have successfully fabricated an acoustic resonator operating at 1.84 GHz with a quality factor Q of 214 and an effective electromechanical coupling coefficient of 10.57%.

Full text

Citation: Patel, R.; Adhikari, M.S.;
Tripathi, S.K.; Sahu, S. Design,
Optimization and Performance
Assessment of Single Port Film Bulk
Acoustic Resonator through Finite
Element Simulation. Sensors 2023,23,
8920. https://doi.org/10.3390/s
23218920
Academic Editor: Guofeng Chen
Received: 12 June 2023
Revised: 26 July 2023
Accepted: 24 October 2023
Published: 2 November 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
sensors
Article
Design, Optimization and Performance Assessment of Single Port
Film Bulk Acoustic Resonator through Finite Element Simulation
Raju Patel 1, Manoj Singh Adhikari 2, Shailendra Kumar Tripathi 3and Sourabh Sahu 4,*
1School of Electronics Engineering (SENSE), Vellore Institute of Technology (VIT), Chennai 600127, India;
raju.patel@vit.ac.in
2School of Electronics & Electrical Engineering, Lovely Professional University, Phagwara 144411, India;
manoj.space99@gmail.com
3Department of Electronics & Communication Engineering, KL University, Guntur 522502, India;
shailendra.amu@gmail.com
4
Department of Electronics & Communication Engineering, Gyan Ganga Institute of Technology and Sciences,
Jabalpur 482003, India; sourabhsahu@ggits.org
*Correspondence: sourabh.gbits@gmail.com; Tel.: +91-707-315-5620
Abstract:
In this paper, the study is supported by design, FEA simulation, and practical RF mea-
surements on fabricated single-port-cavity-based acoustic resonator for gas sensing applications.
In the FEA simulation, frequency domain analysis was performed to enhance the performance of the
acoustic resonator. The structural and surface morphologies of the deposited ZnO as a piezoelectric
layer have been studied using XRD and AFM. The XRD pattern of deposited bulk ZnO film indicates
the perfect single crystalline nature of the film with dominant phase (002) at 2
θ
= 34.58
◦
. The AFM
micrograph indicates that deposited piezoelectric film has a very smooth surface and small grain
size. In the fabrication process, use of bulk micro machined oxide (SiO
2
) for the production of a thin
membrane as a support layer is adopted. A vector network analyzer (Model MS2028C, Anritsu) was
used to measure the radio frequency response of the resonators from 1 GHz to 2.5 GHz. As a result,
we have successfully fabricated an acoustic resonator operating at 1.84 GHz with a quality factor Q of
214 and an effective electromechanical coupling coefficient of 10.57%.
Keywords:
MEMS; single port acoustic resonator; zinc oxide; structural morphologies; vector
network analyzer
1. Introduction
Modern mobile communication systems have become increasingly pervasive, driving
the need for efficient power utilization, spectrum optimization, and high-quality perfor-
mance. However, the existing integrated circuit (IC) technology has encountered limitations
in reducing the size of RF components required for these systems. Despite this, advance-
ments in micro-fabrication processes have facilitated the production of these components,
even within the constraints of current silicon CMOS technology [
1
–
6
]. Surface acoustic
wave (SAW) and quartz crystal microbalance (QCM) resonators are capable of operating in
the frequency range of up to 500 MHz. In contrast, microwave ceramic resonators can oper-
ate in the GHz range but suffer from bulkiness. These resonators are typically integrated as
discrete devices with high insertion losses on the silicon IC, resulting in a larger system
area and subsequent deterioration in temperature sensitivity and power handling [7,8].
To address these challenges, researchers have been exploring innovative solutions to
enhance the performance and integration of RF components within mobile communication
systems. By leveraging novel fabrication techniques and materials, it becomes possible to
develop miniaturized and efficient components that can meet the demands of modern wire-
less applications. One such approach involves the utilization of nanoscale materials, such as
carbon nanotubes (CNTs) and graphene, which exhibit exceptional electrical properties and
Sensors 2023,23, 8920. https://doi.org/10.3390/s23218920 https://www.mdpi.com/journal/sensors

Sensors 2023,23, 8920 2 of 17
mechanical strength. These materials have shown promise in enabling the development of
compact and high-performance RF components. For instance, CNT-based resonators have
demonstrated superior frequency stability and reduced power consumption compared to
conventional technologies [
9
–
12
]. Another avenue of research focuses on the integration of
RF components using advanced packaging techniques. System-on-chip (SoC) technology,
which combines multiple functionalities on a single chip, offers a potential solution for
reducing the system area and mitigating insertion losses. Additionally, three-dimensional
(3D) integration techniques provide a means of vertically stacking multiple layers of RF
components, thereby further reducing the overall system footprint [13–15].
Furthermore, the emergence of new materials, such as piezoelectric polymers and fer-
roelectric thin films, holds promise for the development of compact and high-performance
resonators. These materials exhibit excellent electromechanical properties and can be
integrated directly into silicon substrates, enabling the monolithic integration of RF com-
ponents [
16
–
19
]. Researchers have made significant advancements in the fabrication of
Film Bulk Acoustic Resonators (FBARs) using Microelectromechanical Systems (MEMS)
technology and compatible Piezoelectric (PZE) materials. Several PZE materials, including
Zinc Oxide (ZnO), Aluminium Nitride (AlN), Gallium Nitride (GaN), magnesium-doped
zinc oxide (MgxZn1-xO), and PZT, have been successfully employed in the fabrication of
FBARs [
20
–
24
]. Additionally, researchers have explored the use of ferroelectric materials
such as barium strontium titanate (BaxSr1-xTiO
3
, BST), strontium titanate (SrTiO
3
, STO),
and barium titanate (BaTiO
3
, BTO) for FBAR fabrication. Among the various PZE materials
investigated, AlN and ZnO have emerged as promising candidates for thin film FBAR
fabrication due to their desirable properties. In particular, zinc oxide has shown significant
potential as a PZE material for FBARs, primarily attributed to its high coupling coefficient
within the 1–2 GHz frequency range [25–28].
The choice of zinc oxide as a PZE material for FBAR fabrication is based on several
advantageous characteristics that it possesses. Firstly, its high coupling coefficient ensures
efficient energy transfer between electrical and mechanical domains, leading to enhanced
resonator performance. Additionally, zinc oxide exhibits excellent piezoelectric properties,
including a high electromechanical coupling factor, low mechanical losses, and a stable
frequency response. These attributes are crucial for achieving accurate frequency control
and high Q-factor in FBAR devices. Zinc oxide thin films can be readily deposited using
various techniques such as sputtering, chemical vapor deposition (CVD), and atomic layer
deposition (ALD). This versatility in deposition methods enables the fabrication of FBARs
on a wide range of substrates, including silicon, glass, and flexible materials [29–32].
In recent years, environmental pollution problems have increased extensively due
to encroachment on technology. All living organisms are exposed to tremendous risks
as a result of atmospheric pollution; hence, it is imperative the pollution level of gases,
and, consequently, their effect on change in the climate, be monitored. MEMS-based gas
sensor is one of the most promising applications currently available. Until now, several
attempts have been made for gas sensing using acoustic wave resonators, mostly with
quartz crystal microbalance (QCM), cantilever, and surface acoustic wave (SAW) devices
with remarkable results. Since the sensitivity of acoustic sensors increases with the square
of the resonant frequency, high-frequency acoustic resonators are preferred. QCM sensors
are well-established gravimetric sensors which have been in use since 1972. However,
their LOD lies in the order of a few nanograms, rendering it inadequate for the detection
of low concentrations (<100 nM). To lower the LOD to the pictogram and femtogram
range, higher resonant frequency sensors are needed, a phenomenon which have boosted
the development of SAW and FBAR resonators based on PZE thin films operating in the
GHz range. SAW-based resonators have a range of up to 1 GHz due to the pitch spacing
limitation of surface interdigitated electrodes (IDT). By contrast, the FBAR-based resonators
are made for high-frequency and have a range of up to a few GHz. The FBAR-based sensor
shows a much higher sensitivity in the field of gravimetric sensors, as well as in biological

Sensors 2023,23, 8920 3 of 17
and chemical sensing and has contributed to substantial progress in applications of gas
sensing [33–41].
One of the main issues with FBAR technologies is the propagation of lateral (Lamb)
waves, which result in the creation of undesirable spurious (transverse) modes and higher
harmonic modes. These stimulated spurious modes have the ability to produce new lateral
modes in true FBAR devices because of the discontinuous edges and bounds. Due to
energy conversion into these erroneous modes, these transverse modes can reduce the
k2
e f f
and quality factor as well as generate disruptive and undesirable ripples in the filter’s
pass band. FBARs are of great interest for sensor and RF filter applications owing to their
superior performance characteristics as compared to QCM and SAW resonator. Researchers
in this field earlier developed the frame-like FBARs to remove spurious modes in order to
attain superior qualities. Unwanted higher harmonic modes are another factor that affects
how well FBAR functions but have not been well researched because there is inadequate
theoretical understanding of the structure [42–45].
In our previous work the performance parameter of the FBAR is proposed using
an active area optimization technique [
4
]. With the incorporation of the active area the
spurious modes and higher order harmonic modes are suppressed with the confinement of
the acoustic signals at the central part due to the tuning of the resonance frequency. This
methodology is comparatively easier to fabricate with less complications.
In this article, we performed the 2D FEA simulation for computing the thickness of
oxide (SiO
2
), bottom electrode, piezoelectric layer and top electrode to design the FBAR
with 1.76 GHz frequency. 2D FEA simulation permits to perform the frequency domain
analysis for finding the different resonance modes. The main objective of this analysis
is to obtain the resonant frequency, anti-resonant frequency, quality factor, and coupling
coefficient of the designed resonator. Further, active area optimization technique has
been used for obtaining the perfect confinement of acoustic signal by suppressing the
spurious modes and higher harmonic modes. Optimize design parameter was used to
design the mask of the resonator using L-edit Pro v13.00 software. In the fabrication
process, the first bulk piezoelectric (ZnO) layer deposition method was optimized using RF
reactive magnetron sputtering on the bottom aluminum electrode for the (002) plane. The
morphology of the deposited piezoelectric film is inspected using XRD, FESEM, and AFM.
The optimized device with an active area of 310
×
310
µ
m
2
and a backside air cavity was
then fabricated using a TMAH setup. It was finally characterized for its RF measurement
using a vector network analyzer. Frequency response of fabricated FBAR were analyzed in
detail with respect to 2D FEA simulation.
2. Design and FEA Simulation of Single-Port Cavity-Based FBAR
In this section, a comprehensive study of FBAR performance characteristic of a cavity-
based structure is delineated using COMSOL Multiphysics 4.4 FEM tool. The interaction
between the electrical potential and displacement is represented by the following piezo-
electric constitutive equations [1].
Ti=cE
ijSj−eijEj(1)
where Ti, cij, Sj, eij, and Ej are the stress, stiffness constant, strain, and piezoelectric stress
components, respectively.
Di=εS
ijEj+eijSj(2)
where Di is the electric displacement and
ε
ij is the permittivity constant. Further, the
superscript E is calculated at a constant electric field and the superscript S is calculated at a
constant strain.
In back-trench-(cavity)-based FBAR, the cavity is formed by etching substrate under
the resonating structure, as shown in Figure 1. In this configuration, the acoustic wave
is reflected due to a mismatch in acoustic impedance when the bottom Al electrode is

Sensors 2023,23, 8920 4 of 17
exposed to the air through the backside cavity, which is considered to be insignificant
acoustic impedance.
Figure 1. 2D schematic of the single-port cavity-based FBAR.
The proposed design of cavity-based FBAR is shown in Figure 1. An SiO
2
layer, as
an electrical insulator, is deposited on the Si substrate (0.9
µ
m thick) and lies between the
Si substrate and the bottom electrode. The resonating structure consists of a PZE layer
made of zinc oxide (ZnO) that is sandwiched between two metal electrodes and is mounted
on the SiO
2
layer. For this design, a 1.5
µ
m thick PZE layer was chosen, with 0.15
µ
m
thick electrodes.
The 2D FEM geometry of cavity-based FBAR consists of the following layers: a silicon
substrate, SiO
2
, a bottom Al electrode, ZnO, and an Au electrode at the top. In this geometry,
the top Au electrode consists of intermittent layers with a free verge. A fixed (ux = uz = 0)
border constraint was used at the right and left sides of the geometry.
To simulate the effect of absorption and propagation of the elastic wave in the adjacent
region, the lateral dimensions of the resonator are increased by adding a 50
µ
m wide
perfectly matched layer (PML) on both sides of all the layers except for the top electrode.
A two-dimensional FEA simulation allows for the frequency domain analysis to be per-
formed to identify the different resonance modes. The selection of different materials for
the resonator with thickness, PML layer, and fixed boundary conditions is performed in
this step. Further, mapped meshing was utilized for the finite element COMSOL simulation
of the resonator. The 2D mapped meshing structure of FBAR is illustrated in Figure 2. The
main objective of this analysis is to obtain the resonant frequency, anti-resonant frequency,
quality factor, and coupling coefficient for the designed resonator.
When a voltage is applied to the piezoelectric layer, the centrosymmetry of the piezo-
electric crystal structure breaks and the effect of converse piezoelectricity is induced. The
generated mechanical wave results in a mechanically induced polarization due to the direct
piezoelectric effect. When such a mechanically induced polarization is out of phase with
the dielectric polarization by 180
◦
, parallel resonance and net polarization, and hence the
net current, occur, with both being minimized. The displacement profile, phase, impedance,
and quality factor responses of the cavity-based FBAR were illustrated using frequency-
domain finite element analysis. The frequencies corresponding to the minimum and
maximum values of the electrical impedance Z were considered as resonant frequency fr
and anti-resonant frequency fa, respectively. To evaluate the FBAR performance, the value
of the quality factor, Q, at resonant and anti-resonant frequencies, the effective electrome-

Sensors 2023,23, 8920 5 of 17
chanical coupling coefficient,
k2
e f f
, and the figure of merit (FoM) are deduced from the
standard equations [1,2]:
Qfx=fx
2



dφz
d f 


fx
(3)
k2
e f f =π
2fr
fa
tanπ
2fr
fa ≈π
22fa−fr
fa(4)
FoM1=k2
e f f ∗Q(5)
FoM2=fa∗Q(6)
where
φ
z is the phase of the electrical impedance, Z, f
x
is the resonant frequency, f
r
is the
anti-resonant frequency, and fais the acoustic resonator [1,2].
Sensors 2023, 23, x FOR PEER REVIEW 5 of 18
Figure 2. Two-dimensional mapped meshing structure of FBAR.
When a voltage is applied to the piezoelectric layer, the centrosymmetry of the pie-
zoelectric crystal structure breaks and the effect of converse piezoelectricity is induced.
The generated mechanical wave results in a mechanically induced polarization due to the
direct piezoelectric effect. When such a mechanically induced polarization is out of phase
with the dielectric polarization by 180°, parallel resonance and net polarization, and hence
the net current, occur, with both being minimized. The displacement profile, phase, im-
pedance, and quality factor responses of the cavity-based FBAR were illustrated using
frequency-domain finite element analysis. The frequencies corresponding to the mini-
mum and maximum values of the electrical impedance Z were considered as resonant
frequency fr and anti-resonant frequency fa, respectively. To evaluate the FBAR perfor-
mance, the value of the quality factor, Q, at resonant and anti-resonant frequencies, the
effective electromechanical coupling coefficient, 𝑘
, and the figure of merit (FoM) are
deduced from the standard equations [1,2]:
𝑄

=
𝑓

2𝑑𝜙
𝑑𝑓

(3)
𝑘
=󰇡𝜋
2󰇢
𝑓

𝑓

𝑡𝑎𝑛󰇧󰇡𝜋
2󰇢
𝑓

𝑓
󰇨 󰇡𝜋
2󰇢
𝑓
−
𝑓

𝑓
 (4)
𝐹𝑜𝑀=𝑘
∗𝑄 (5)
𝐹𝑜𝑀=
𝑓
∗𝑄 (6)
where 𝝓z is the phase of the electrical impedance, Z, f
x
is the resonant frequency, f
r
is the
anti-resonant frequency, and f
a
is the acoustic resonator [1,2].
The FEM plot depicted in Figure 3 provides insights into the longitudinal displace-
ment at the resonant frequency. The maximum displacement of 2.02 nm was achieved at
the resonant frequency due to the longitudinal mode displacement of the acoustic signal
Figure 2. Two-dimensional mapped meshing structure of FBAR.
The FEM plot depicted in Figure 3provides insights into the longitudinal displacement
at the resonant frequency. The maximum displacement of 2.02 nm was achieved at the
resonant frequency due to the longitudinal mode displacement of the acoustic signal in the
active region of the FBAR. Conversely, the remaining section of the FBAR had a minimum
displacement of 0 nm.
The displacement profile demonstrates the effective confinement of the acoustic signal
within the piezoelectric (PZE) layer, with minimal dissipation of acoustic energy into
the silicon (Si) substrate. This confinement is crucial for achieving efficient resonator
performance and minimizing signal losses. Examining Figures 4and 5, the variation of
impedance and phase with frequency is illustrated for different active area. It confirms
the confinement of acoustic signal by utilizing the active area optimizing techniques. The
frequency response from the results also demonstrates that the optimisation technique
is suitable to suppress higher harmonic and spurious modes. The resonant frequency,
denoted as fr, is determined to be 1.762 GHz, accompanied by a least impedance of 0.11 dB.
This resonant frequency signifies the point at which the FBAR exhibits maximum sensitivity

Sensors 2023,23, 8920 6 of 17
to the input signal, resulting in optimal performance. On the other hand, the anti-resonant
frequency, de-noted as fa, is measured to be 1.82 GHz, correlating with a maximum
impedance of 219.32 dB. The anti-resonant frequency represents the point at which the
FBAR demonstrates minimum sensitivity to the input signal.
Sensors 2023, 23, x FOR PEER REVIEW 6 of 18
in the active region of the FBAR. Conversely, the remaining section of the FBAR had a
minimum displacement of 0 nm.
The displacement profile demonstrates the effective confinement of the acoustic sig-
nal within the piezoelectric (PZE) layer, with minimal dissipation of acoustic energy into
the silicon (Si) substrate. This confinement is crucial for achieving efficient resonator per-
formance and minimizing signal losses. Examining Figures 4 and 5, the variation of im-
pedance and phase with frequency is illustrated for different active area. It confirms the
confinement of acoustic signal by utilizing the active area optimizing techniques. The fre-
quency response from the results also demonstrates that the optimisation technique is
suitable to suppress higher harmonic and spurious modes. The resonant frequency, de-
noted as fr, is determined to be 1.762 GHz, accompanied by a least impedance of 0.11 dB.
This resonant frequency signifies the point at which the FBAR exhibits maximum sensi-
tivity to the input signal, resulting in optimal performance. On the other hand, the anti-
resonant frequency, de-noted as fa, is measured to be 1.82 GHz, correlating with a maxi-
mum impedance of 219.32 dB. The anti-resonant frequency represents the point at which
the FBAR demonstrates minimum sensitivity to the input signal.
Figure 3. Displacement profile of the cavity-based single-port FBAR, at resonant frequency.
Figure 4. Impedance vs. frequency response of the single port cavity based FBAR for different active
area.
Figure 3. Displacement profile of the cavity-based single-port FBAR, at resonant frequency.
Sensors 2023, 23, x FOR PEER REVIEW 6 of 18
in the active region of the FBAR. Conversely, the remaining section of the FBAR had a
minimum displacement of 0 nm.
The displacement profile demonstrates the effective confinement of the acoustic sig-
nal within the piezoelectric (PZE) layer, with minimal dissipation of acoustic energy into
the silicon (Si) substrate. This confinement is crucial for achieving efficient resonator per-
formance and minimizing signal losses. Examining Figures 4 and 5, the variation of im-
pedance and phase with frequency is illustrated for different active area. It confirms the
confinement of acoustic signal by utilizing the active area optimizing techniques. The fre-
quency response from the results also demonstrates that the optimisation technique is
suitable to suppress higher harmonic and spurious modes. The resonant frequency, de-
noted as fr, is determined to be 1.762 GHz, accompanied by a least impedance of 0.11 dB.
This resonant frequency signifies the point at which the FBAR exhibits maximum sensi-
tivity to the input signal, resulting in optimal performance. On the other hand, the anti-
resonant frequency, de-noted as fa, is measured to be 1.82 GHz, correlating with a maxi-
mum impedance of 219.32 dB. The anti-resonant frequency represents the point at which
the FBAR demonstrates minimum sensitivity to the input signal.
Figure 3. Displacement profile of the cavity-based single-port FBAR, at resonant frequency.
Figure 4. Impedance vs. frequency response of the single port cavity based FBAR for different active
area.
Figure 4.
Impedance vs. frequency response of the single port cavity based FBAR for different
active area.
The effective electro-mechanical coupling coefficient
k2
e f f
has been calculated 7.86%.
The quality factors at resonant frequency, Qr, and anti-resonant frequency, Qa, are 906.6
and 612.1 with corresponding Figure of Merit’s (FoM1) of 71.26 and 48.11, respectively.
The significant difference in impedance values between the resonant and anti-resonant
frequencies highlights the distinctive electrical response of the FBAR device. This disparity
plays a crucial role in various applications such as signal filtering and frequency modula-
tion. The observed impedance characteristics and the distinct resonant and anti-resonant
frequencies validate the effective operation of the FBAR device. The impedance values
obtained provide important information for the optimization of the device performance
and the design of appropriate matching circuits.

Sensors 2023,23, 8920 7 of 17
Sensors 2023, 23, x FOR PEER REVIEW 7 of 18
Figure 5. Phase vs. frequency response of the single port cavity based FBAR for different active area.
The effective electro-mechanical coupling coefficient 𝑘
 has been calculated
7.86%. The quality factors at resonant frequency, Qr, and anti-resonant frequency, Qa, are
906.6 and 612.1 with corresponding Figure of Merit’s (FoM1) of 71.26 and 48.11, respec-
tively.
The significant difference in impedance values between the resonant and anti-reso-
nant frequencies highlights the distinctive electrical response of the FBAR device. This
disparity plays a crucial role in various applications such as signal filtering and frequency
modulation. The observed impedance characteristics and the distinct resonant and anti-
resonant frequencies validate the effective operation of the FBAR device. The impedance
values obtained provide important information for the optimization of the device perfor-
mance and the design of appropriate matching circuits.
3. Device Fabrication
The 2″ double side polished Si (100) wafer and four photo masks with the required
specifications were used for the fabrication of optimized single-port FBAR. The essential
steps in the device fabrication process flow are presented in Figure 6.
Figure 5.
Phase vs. frequency response of the single port cavity based FBAR for different active area.
3. Device Fabrication
The 2
00
double side polished Si (100) wafer and four photo masks with the required
specifications were used for the fabrication of optimized single-port FBAR. The essential
steps in the device fabrication process flow are presented in Figure 6.
Sensors 2023, 23, x FOR PEER REVIEW 8 of 18
Figure 6. The essential steps in the device fabrication process flow.
The fabrication process of the device began with a series of cleaning steps to ensure
the removal of organic residues and contaminants. Degreasing was carried out initially,
followed by a standard piranha cleaning process (H2SO4:H2O2 = 3:1) to eliminate any re-
maining organic residue. Subsequently, the wafer was immersed in a 5% HF solution to
remove the native oxide layer and thorough rinsing with deionized (DI) water was per-
formed to ensure a clean surface. To create an insulating layer, a thermal oxidation process
was employed. The cleaned Si wafer was placed in a thermal chamber and subjected to a
dry-wet-dry thermal oxidation process at a temperature of 1075 °C. This process resulted
in the growth of a SiO2 layer with a thickness of 0.9 µm. After the oxidation process, the
wafer was dried using N2 gas.
The deposition of the boom electrode was carried out using the e-beam evaporation
technique. Aluminum (Al) was chosen as the material for the boom electrode, and a layer
with a thickness of 150 nm was deposited onto the SiO2 layer. The paerning of the boom
Al electrode was achieved using standard bright field photolithography, combined with
the application of aluminum type-F chemical etchant. Figure 7 provides a microscopic
view of the boom electrode after the paerning process on the SiO2 layer. The image
demonstrates the successful and precise paerning of the boom Al electrode. The com-
patibility of the boom SiO2 layer with the aluminum type-F chemical etchant is evident
from the results, further confirming the effectiveness of the fabrication process.
These fabrication steps are crucial in establishing the foundation for the subsequent
layers and components of the device. The careful cleaning and oxide growth processes
ensure a clean and well-prepared surface, while the precise deposition and paerning of
the boom electrode contribute to the overall functionality and performance of the device.
The RF reactive magnetron spuering system is used to grow the bulk zinc oxide
layer on the boom Al electrode at room temperature. This bulk ZnO layer was paerned
with bright field photolithography and then etched using a wet chemical etchant (1%
Figure 6. The essential steps in the device fabrication process flow.
The fabrication process of the device began with a series of cleaning steps to ensure
the removal of organic residues and contaminants. Degreasing was carried out initially,
followed by a standard piranha cleaning process (H
2
SO
4
:H
2
O
2
= 3:1) to eliminate any

Sensors 2023,23, 8920 8 of 17
remaining organic residue. Subsequently, the wafer was immersed in a 5% HF solution
to remove the native oxide layer and thorough rinsing with deionized (DI) water was
performed to ensure a clean surface. To create an insulating layer, a thermal oxidation
process was employed. The cleaned Si wafer was placed in a thermal chamber and subjected
to a dry-wet-dry thermal oxidation process at a temperature of 1075
◦
C. This process
resulted in the growth of a SiO
2
layer with a thickness of 0.9
µ
m. After the oxidation
process, the wafer was dried using N2gas.
The deposition of the bottom electrode was carried out using the e-beam evaporation
technique. Aluminum (Al) was chosen as the material for the bottom electrode, and a layer
with a thickness of 150 nm was deposited onto the SiO
2
layer. The patterning of the bottom
Al electrode was achieved using standard bright field photolithography, combined with the
application of aluminum type-F chemical etchant. Figure 7provides a microscopic view of
the bottom electrode after the patterning process on the SiO
2
layer. The image demonstrates
the successful and precise patterning of the bottom Al electrode. The compatibility of the
bottom SiO
2
layer with the aluminum type-F chemical etchant is evident from the results,
further confirming the effectiveness of the fabrication process.
Sensors 2023, 23, x FOR PEER REVIEW 9 of 18
hydrochloric acid) solution. DC spuering was used to deposit 20 nm Cr and a 130 nm
thin Au layer as the top electrode of the resonator and paerned using a liftoff process.
Figure 7. Microscopic view of the boom electrode after the paerning.
The dark field photolithography and buffered oxide etchant (BOE) were applied to
paern the backside of the SiO2 layer. Then, a TMAH solution was used for the etching of
bulk Si to form the backside cavity. Figure 8a,b represent the SEM top view of the single-
port cavity-based FBAR and a slanted view of the backside cavity, respectively. The active
layer stack was successfully paerned because the surface exhibited a clean profile free of
inhomogeneities.
Figure 8. SEM diagram of the (a) top view of single-port cavity-based FBAR, and (b) slanted view
of the backside cavity.
4. Result and Discussion
4.1. Study of ZnO Morphology
The crystalline phase of the deposited ZnO thin layer was determined using the PAN-
alytical X’pert Powder system, equipped with a CuKα1 X-ray source with a line focus and
a radiation wavelength of 1.54059 Å at 1.6 KW. Figure 9 displays the X-ray diffraction
Figure 7. Microscopic view of the bottom electrode after the patterning.
These fabrication steps are crucial in establishing the foundation for the subsequent
layers and components of the device. The careful cleaning and oxide growth processes
ensure a clean and well-prepared surface, while the precise deposition and patterning of
the bottom electrode contribute to the overall functionality and performance of the device.
The RF reactive magnetron sputtering system is used to grow the bulk zinc oxide layer
on the bottom Al electrode at room temperature. This bulk ZnO layer was patterned with
bright field photolithography and then etched using a wet chemical etchant (1% hydrochlo-
ric acid) solution. DC sputtering was used to deposit 20 nm Cr and a 130 nm thin Au layer
as the top electrode of the resonator and patterned using a liftoff process.
The dark field photolithography and buffered oxide etchant (BOE) were applied to
pattern the backside of the SiO
2
layer. Then, a TMAH solution was used for the etching
of bulk Si to form the backside cavity. Figure 8a,b represent the SEM top view of the
single-port cavity-based FBAR and a slanted view of the backside cavity, respectively. The
active layer stack was successfully patterned because the surface exhibited a clean profile
free of inhomogeneities.

Sensors 2023,23, 8920 9 of 17
Figure 8.
SEM diagram of the (
a
) top view of single-port cavity-based FBAR, and (
b
) slanted view of
the backside cavity.
4. Result and Discussion
4.1. Study of ZnO Morphology
The crystalline phase of the deposited ZnO thin layer was determined using the
PANalytical X’pert Powder system, equipped with a CuK
α
1 X-ray source with a line
focus and a radiation wavelength of 1.54059 Å at 1.6 KW. Figure 9displays the X-ray
diffraction (XRD) pattern of the deposited piezoelectric ZnO film, scanned within the 2
θ
range of 20
◦
to 60
◦
. The XRD pattern reveals the presence of a highly crystalline film with
a prominent (002) phase at 2
θ
= 34.58
◦
, closely matching the reference peak indicated by
JCPDS card no. 36-2828 [
46
]. Notably, the film exhibited negligible strain. The XRD analysis
confirmed the excellent crystallinity and structural quality of the deposited ZnO thin film.
The dominant (002) phase peak indicates the preferred orientation and alignment of the
crystal lattice along the ZnO film growth direction. The observation of a single crystalline
nature signifies the absence of significant defects or grain boundaries that could adversely
affect the film performance.
Sensors 2023, 23, x FOR PEER REVIEW 10 of 18
(XRD) paern of the deposited piezoelectric ZnO film, scanned within the 2θ range of 20°
to 60°. The XRD paern reveals the presence of a highly crystalline film with a prominent
(002) phase at 2θ = 34.58°, closely matching the reference peak indicated by JCPDS card
no. 36-2828 [46]. Notably, the film exhibited negligible strain. The XRD analysis confirmed
the excellent crystallinity and structural quality of the deposited ZnO thin film. The dom-
inant (002) phase peak indicates the preferred orientation and alignment of the crystal
laice along the ZnO film growth direction. The observation of a single crystalline nature
signifies the absence of significant defects or grain boundaries that could adversely affect
the film performance.
The agreement between the experimental results and the reference peak from the
JCPDS card further validates the accurate determination of the film’s crystallographic
phase. This conformity provides confidence in the quality and reliability of the deposited
ZnO thin film for its intended applications. The precise characterization of the crystalline
phase is crucial in piezoelectric materials, as it directly influences their electromechanical
properties. A well-defined and single crystalline structure ensures an improved piezoe-
lectric response, high coupling coefficients, and enhanced performance in devices utiliz-
ing the ZnO thin film.
Figure 9. Grown ZnO layer, XRD paern.
The morphology of the deposited ZnO film was inspected using atomic force micros-
copy (AFM, Brukar multimode 8). Figure 10a shows AFM micrographs which display that
the ZnO film is uniformly distributed on the entire Si substrate and has a very low rough-
ness of around 6.71 nm (calculated for 1 µm × 1 µm area). It is evident that the deposited
film has a very smooth surface and a small grain size. The 3D micrograph in Figure 10b
also depicts the uniform distribution of the grains in the ZnO film. The two-dimensional
AFM micrograph represents the distribution of particles as well as the horizontal size of
particles, something which cannot be achieved by a 3D AFM diagram.
Figure 9. Grown ZnO layer, XRD pattern.
The agreement between the experimental results and the reference peak from the
JCPDS card further validates the accurate determination of the film’s crystallographic phase.
This conformity provides confidence in the quality and reliability of the deposited ZnO thin
film for its intended applications. The precise characterization of the crystalline phase is
crucial in piezoelectric materials, as it directly influences their electromechanical properties.

Sensors 2023,23, 8920 10 of 17
A well-defined and single crystalline structure ensures an improved piezoelectric response,
high coupling coefficients, and enhanced performance in devices utilizing the ZnO thin film.
The morphology of the deposited ZnO film was inspected using atomic force mi-
croscopy (AFM, Brukar multimode 8). Figure 10a shows AFM micrographs which display
that the ZnO film is uniformly distributed on the entire Si substrate and has a very low
roughness of around 6.71 nm (calculated for 1
µ
m
×
1
µ
m area). It is evident that the
deposited film has a very smooth surface and a small grain size. The 3D micrograph
in Figure 10b also depicts the uniform distribution of the grains in the ZnO film. The
two-dimensional AFM micrograph represents the distribution of particles as well as the
horizontal size of particles, something which cannot be achieved by a 3D AFM diagram.
Sensors 2023, 23, x FOR PEER REVIEW 11 of 18
(a)
(b)
Figure 10. (a) Two-dimensional atomic force microscopy plot; (b) 3D atomic force microscopy im-
age.
The morphologies of the as-prepared samples were analyzed by (JEM2100F) trans-
mission electron microscope (TEM) and high-resolution transmission electron microscope
(HRTEM). TEM images in Figure 11 show an overview of the ZnO thin film showing na-
norod-like morphology. The inset in Figure 11a shows the TEM micrograph, which shows
that the film has grown uniformly throughout the substrate; there are no cracks in the
film. Figure 11b,c show the rod morphology, with a rod diameter of 15 nm to 20 nm. Fig-
ure 11d and the inset in Figure 11b show the HRTEM image of ZnO nanorods aligned in
002 plane, a phenomenon which was also confirmed by XRD spectroscopy.
Figure 10.
(
a
) Two-dimensional atomic force microscopy plot; (
b
) 3D atomic force microscopy image.

Sensors 2023,23, 8920 11 of 17
The morphologies of the as-prepared samples were analyzed by (JEM2100F) trans-
mission electron microscope (TEM) and high-resolution transmission electron microscope
(HRTEM). TEM images in Figure 11 show an overview of the ZnO thin film showing
nanorod-like morphology. The inset in Figure 11a shows the TEM micrograph, which
shows that the film has grown uniformly throughout the substrate; there are no cracks in
the film. Figure 11b,c show the rod morphology, with a rod diameter of 15 nm to 20 nm.
Figure 11d and the inset in Figure 11b show the HRTEM image of ZnO nanorods aligned in
002 plane, a phenomenon which was also confirmed by XRD spectroscopy.
Sensors 2023, 23, x FOR PEER REVIEW 12 of 18
Figure 11. TEM micrograph.
The elemental mapping of the as-prepared samples was analyzed by TEM. The spec-
tra obtained from the EDS, shown in Figure 12c, confirm the qualitative elemental purity
of the material. Quantitative EDS analysis confirmed the uniform distribution of Zn and
O elements. The peak at 1.037 eV corresponds to the Lβ1 of Zn, 8.63886 corresponds to the
Kα1 of Zn, 9.5720 corresponds to the Kβ1 of Zn, and 0.525 corresponds to the Kα1 of O.
The elemental homogeneity of the samples was checked using an EDS mapping tech-
nique. The elemental maps micrographs of Zn and O show a uniform density distribution,
confirming the uniform distribution of Zn and O throughout the material. The obtained
EDS maps are presented in Figure 12. Figure 12b,d show the elemental mapping of Zn
and O, respectively, for the red rectangle visible in Figure 12a.
Figure 12. The elemental mapping of the TEM image and EDS spectra.
4.2. Radio Frequency Characteristic of the Resonator
The Radio Frequency characteristic of the single-port cavity-based FBAR was char-
acterized with respect to impedance on an RF coplanar probe station using GSG-250
probes. A vector network analyzer (Model MS2028C, Anritsu, Hongkong) was used to
2 nm
a) b)
c) d)
d002 =0.26
nm
700
nm
Element Weight % Atomic %
O K 25.07 57.75
Zn K 74.93 42.25
Totals 100.00
O ka1
Zn ka1
b)
d)
a)
c)
Figure 11. TEM micrograph.
The elemental mapping of the as-prepared samples was analyzed by TEM. The spectra
obtained from the EDS, shown in Figure 12c, confirm the qualitative elemental purity of
the material. Quantitative EDS analysis confirmed the uniform distribution of Zn and
O elements. The peak at 1.037 eV corresponds to the L
β
1 of Zn, 8.63886 corresponds to the
Kα1 of Zn, 9.5720 corresponds to the Kβ1 of Zn, and 0.525 corresponds to the Kα1 of O.
Sensors 2023, 23, x FOR PEER REVIEW 12 of 18
Figure 11. TEM micrograph.
The elemental mapping of the as-prepared samples was analyzed by TEM. The spec-
tra obtained from the EDS, shown in Figure 12c, confirm the qualitative elemental purity
of the material. Quantitative EDS analysis confirmed the uniform distribution of Zn and
O elements. The peak at 1.037 eV corresponds to the Lβ1 of Zn, 8.63886 corresponds to the
Kα1 of Zn, 9.5720 corresponds to the Kβ1 of Zn, and 0.525 corresponds to the Kα1 of O.
The elemental homogeneity of the samples was checked using an EDS mapping tech-
nique. The elemental maps micrographs of Zn and O show a uniform density distribution,
confirming the uniform distribution of Zn and O throughout the material. The obtained
EDS maps are presented in Figure 12. Figure 12b,d show the elemental mapping of Zn
and O, respectively, for the red rectangle visible in Figure 12a.
Figure 12. The elemental mapping of the TEM image and EDS spectra.
4.2. Radio Frequency Characteristic of the Resonator
The Radio Frequency characteristic of the single-port cavity-based FBAR was char-
acterized with respect to impedance on an RF coplanar probe station using GSG-250
probes. A vector network analyzer (Model MS2028C, Anritsu, Hongkong) was used to
2 nm
a) b)
c) d)
d002 =0.26
nm
700
nm
Element Weight % Atomic %
O K 25.07 57.75
Zn K 74.93 42.25
Totals 100.00
O ka1
Zn ka1
b)
d)
a)
c)
Figure 12. The elemental mapping of the TEM image and EDS spectra.

Sensors 2023,23, 8920 12 of 17
The elemental homogeneity of the samples was checked using an EDS mapping tech-
nique. The elemental maps micrographs of Zn and O show a uniform density distribution,
confirming the uniform distribution of Zn and O throughout the material. The obtained
EDS maps are presented in Figure 12. Figure 12b,d show the elemental mapping of Zn and
O, respectively, for the red rectangle visible in Figure 12a.
4.2. Radio Frequency Characteristic of the Resonator
The Radio Frequency characteristic of the single-port cavity-based FBAR was charac-
terized with respect to impedance on an RF coplanar probe station using GSG-250 probes.
A vector network analyzer (Model MS2028C, Anritsu, Hongkong) was used to measure
the radio frequency response of the resonators from 1 GHz to 2.5 GHz. The fabricated
device has an active area of 310
µ
m
×
310
µ
m and a backside cavity whose dimensions
are 300
µ
m
×
300
µ
m. The resonator impedance, phase, effective electromechanical coeffi-
cient of coupling, and Q-factor parameters were plotted. Figure 13 shows the measured
impedance (Z) and the phase (
φ
z) characteristic of the fabricated single-port resonator
without the cavity as a function of frequency.
Sensors 2023, 23, x FOR PEER REVIEW 13 of 18
measure the radio frequency response of the resonators from 1 GHz to 2.5 GHz. The fab-
ricated device has an active area of 310 µm × 310 µm and a backside cavity whose dimen-
sions are 300 µm × 300 µm. The resonator impedance, phase, effective electromechanical
coefficient of coupling, and Q-factor parameters were ploed. Figure 13 shows the meas-
ured impedance (Z) and the phase (𝝓z) characteristic of the fabricated single-port resona-
tor without the cavity as a function of frequency.
Figure 13. Measured impedance characteristic of a resonator without cavity.
The impedance (Z) and phase (𝝓z) characteristic of the fabricated single-port resona-
tor with cavity are presented in Figure 14a and Figure 14b, respectively. As it can be ob-
served from Figure 14a, the resonant frequency, fr, had a value of 1.765 GHz and the anti-
resonant frequency, fa, had a value of 1.844 GHz at a minimum impedance of 17.88 dB
and at a maximum impedance of 33.71 dB, respectively, with 𝑘
 equivalent to 10.57%.
As it can be seen, there was a perfect match between the resonant frequencies for the FBAR
FEM simulation, with a minimal error of 5 MHz. The resonator’s quality factors Qr and
Qa were estimated at 94.3 and 214, respectively, with corresponding Figure of Merit
(FoM1) values of 9.97 and 22.6, respectively.
(a)
Figure 13. Measured impedance characteristic of a resonator without cavity.
The impedance (Z) and phase (
φ
z) characteristic of the fabricated single-port resonator
with cavity are presented in Figure 14a,b, respectively. As it can be observed from Figure 14a,
the resonant frequency, fr, had a value of 1.765 GHz and the anti-resonant frequency, fa, had
a value of 1.844 GHz at a minimum impedance of 17.88 dB and at a maximum impedance of
33.71 dB, respectively, with
k2
e f f
equivalent to 10.57%. As it can be seen, there was a perfect
match between the resonant frequencies for the FBAR FEM simulation, with a minimal
error of 5 MHz. The resonator’s quality factors Qr and Qa were estimated at 94.3 and 214,
respectively, with corresponding Figure of Merit (FoM1) values of 9.97 and 22.6, respectively.
The finite-element-based simulation results were confirmed through practical RF
measurements of the fabricated single-port cavity-based acoustic resonator. Table 1shows
the comparison of simulated and measured RF performance parameters of fabricated single
port cavity based acoustic resonator. It is confirmed from the Table 1that the confinement
of acoustic signal has been improved by active area optimization technique. The measured
performance parameter of the FBAR depends on various factors: the quality of ZnO, SiO
2
,
BE, TE layer and bulk etching of Si using the TMAH process. This illustrates the reason
for the gap between the simulation and fabrication results in Table 1. There is difference in
the quality factor and coupling coefficient. This difference is due to the tradeoff between
the coupling coefficient and quality factor. The value of coupling coefficient is increasing
at the cost of reducing the quality factor [
47
,
48
]. By the selection of active area, the effects

Sensors 2023,23, 8920 13 of 17
of higher harmonic modes, undesired coupling and spurious modes from the frequency
response can be well suppressed.
Sensors 2023, 23, x FOR PEER REVIEW 13 of 18
measure the radio frequency response of the resonators from 1 GHz to 2.5 GHz. The fab-
ricated device has an active area of 310 µm × 310 µm and a backside cavity whose dimen-
sions are 300 µm × 300 µm. The resonator impedance, phase, effective electromechanical
coefficient of coupling, and Q-factor parameters were ploed. Figure 13 shows the meas-
ured impedance (Z) and the phase (𝝓z) characteristic of the fabricated single-port resona-
tor without the cavity as a function of frequency.
Figure 13. Measured impedance characteristic of a resonator without cavity.
The impedance (Z) and phase (𝝓z) characteristic of the fabricated single-port resona-
tor with cavity are presented in Figure 14a and Figure 14b, respectively. As it can be ob-
served from Figure 14a, the resonant frequency, fr, had a value of 1.765 GHz and the anti-
resonant frequency, fa, had a value of 1.844 GHz at a minimum impedance of 17.88 dB
and at a maximum impedance of 33.71 dB, respectively, with 𝑘
 equivalent to 10.57%.
As it can be seen, there was a perfect match between the resonant frequencies for the FBAR
FEM simulation, with a minimal error of 5 MHz. The resonator’s quality factors Qr and
Qa were estimated at 94.3 and 214, respectively, with corresponding Figure of Merit
(FoM1) values of 9.97 and 22.6, respectively.
(a)
Sensors 2023, 23, x FOR PEER REVIEW 14 of 18
(b)
Figure 14. (a) Measured impedance characteristic of a cavity-based resonator; (b) measured phase
characteristic for a cavity-based resonator.
The finite-element-based simulation results were confirmed through practical RF
measurements of the fabricated single-port cavity-based acoustic resonator. Table 1 shows
the comparison of simulated and measured RF performance parameters of fabricated sin-
gle port cavity based acoustic resonator. It is confirmed from the Table 1 that the confine-
ment of acoustic signal has been improved by active area optimization technique. The
measured performance parameter of the FBAR depends on various factors: the quality of
ZnO, SiO2, BE, TE layer and bulk etching of Si using the TMAH process. This illustrates
the reason for the gap between the simulation and fabrication results in Table 1. There is
difference in the quality factor and coupling coefficient. This difference is due to the
tradeoff between the coupling coefficient and quality factor. The value of coupling coeffi-
cient is increasing at the cost of reducing the quality factor [47,48]. By the selection of ac-
tive area, the effects of higher harmonic modes, undesired coupling and spurious modes
from the frequency response can be well suppressed.
Table 1. Comparisons of simulated and measured RF performance parameters of proposed single-
port cavity-based acoustic resonator.
S. No. Parameter Active Area of 225
× 225 µm2
Active Area of
310 × 310 µm2 Measured
1. fs [GHz] 1.762 1.762 1.765
2. fp [GHz] 1.802 1.82 1.844
3. Qs 1046 906.6 94.3
4. Qp 992 612.1 214
5. k

 [%] 5.47 7.86 10.56
The silicon bulk micromachining process simplifies fabrication of freely suspended
membrane-based structures using wet chemical etching. However, the complete bulk
etching of silicon substrate below the active area through wet chemical etching is a signif-
icant process and residuals of Si can cause the resonant and anti-resonant modes of the
resonator to vanish. The effect of such residuals of Si substrate on resonant mode was
observed. The study illustrates the generation of a resonant mode in bulk Si substrate with
280 µm thickness and completely etched Si substrate below the active area, shown in Fig-
ures 13 and 14a.
Figure 14.
(
a
) Measured impedance characteristic of a cavity-based resonator; (
b
) measured phase
characteristic for a cavity-based resonator.
Table 1.
Comparisons of simulated and measured RF performance parameters of proposed single-
port cavity-based acoustic resonator.
S. No. Parameter Active Area of
225 ×225 µm2
Active Area of
310 ×310 µm2Measured
1. fs[GHz] 1.762 1.762 1.765
2. fp[GHz] 1.802 1.82 1.844
3. Qs1046 906.6 94.3
4. Qp992 612.1 214
5. k2
eff [%] 5.47 7.86 10.56
The silicon bulk micromachining process simplifies fabrication of freely suspended
membrane-based structures using wet chemical etching. However, the complete bulk etch-

Sensors 2023,23, 8920 14 of 17
ing of silicon substrate below the active area through wet chemical etching is a significant
process and residuals of Si can cause the resonant and anti-resonant modes of the resonator
to vanish. The effect of such residuals of Si substrate on resonant mode was observed. The
study illustrates the generation of a resonant mode in bulk Si substrate with 280
µ
m thickness
and completely etched Si substrate below the active area, shown in Figures 13 and 14a.
Table 2, finally, compares the fabricated single-port cavity-based acoustic resonator
results with those reported in the literature. The measured results are in agreement with
those observed from the finite element analysis. This shows that the fabricated single-port
cavity-based acoustic resonator gives better RF performance, thereby making it a promising
device for sensing applications in future.
Table 2. RF performance parameters of the proposed single-port cavity-based acoustic resonator.
References Device Name PZE Material fr(GHz) fa(GHz) k2
eff QsFoM1
[49] FBAR ZnO 1.546 1.590 6.83 350 23.9
[50] FBAR ZnO 1.498 1.513 2.39 224 5.35
[51] FBAR ZnO 1.43 1.441 1.88 433 8.14
[52] FBAR ZnO 2.11 2.127 2.03 253.2 5.14
This work FBAR ZnO 1.765 1.844 10.57 214 22.6
5. Conclusions
In conclusion, this manuscript has demonstrated the successful fabrication and opti-
mization of a single port cavity based acoustic resonator utilizing zinc oxide as a piezo-
electric layer. Through a comprehensive approach, the study harnessed finite element
simulation, software-driven design, and advanced fabrication techniques to enhance the
radiofrequency (RF) performance of the resonator. The design process incorporated finite
element simulation within the COMSOL Multiphysics v4.4 software suite, enabling the
refinement of the single port FBAR’s performance characteristics. Leveraging an opti-mized
design parameter, the resonator’s mask was meticulously crafted using L-edit software,
ensuring precise control over its structural aspects. The fabrication process itself underwent
careful optimization, particularly in the deposition of the bulk ZnO layer on the bottom
electrode. The emphasis on the (002) plane and the quality of the bulk ZnO layer was
pivotal, as these factors significantly influence the confinement of the acoustic signal within
the resonator’s active area and backside cavity. X-ray diffraction (XRD) and atomic force
microscopy (AFM) analyses confirmed the exception-al single crystalline nature of the
deposited zinc oxide film, marked by its dominant (002) plane orientation, small grain size,
and remarkably smooth surface. Notably, the utilization of the TMAH jig for silicon bulk
etching played a crucial role in safeguarding the integrity of the frontside active devices
while optimizing the etch rate. The RF measurement phase involved the use of a Vector
Network Analyzer (VNA), which offered insights into the impedance and phase spectra of
the FBAR. The impedance response underscored the remarkable capabilities of the single
port FBAR, notably in its effective suppression of spurious modes and higher harmonic
modes, resulting in pristine confinement of the acoustic signal. The resonator’s key pa-
rameters were ascertained through comprehensive analysis. Its resonant frequency was
found to be 1.765 GHz with a quality factor (Qr) of 94.3, while the anti-resonant frequency
was measured at 1.844 GHz with a quality factor (Qa) of 214. The corresponding Figures
of Merit (FOMs) for the resonant and anti-resonant frequencies were calculated to be 9.97
and 22.6, respectively. In essence, this research not only advanced the understanding of
single port cavity based acoustic resonators but also showcased their immense potential
as a platform for future gas-sensing applications. The successful integration of advanced
simulation techniques, software-driven design, and meticulous fabrication processes has
paved the way for enhanced RF performance and precise signal confinement. This work

Sensors 2023,23, 8920 15 of 17
serves as the development of innovative and efficient gas sensing technologies, contributing
to the further evolution of sensor-based applications.
Author Contributions: Conceptualization, R.P.; Methodology, R.P.; Validation, S.K.T.; Formal analysis,
R.P. and S.S.; Investigation, S.S.; Data curation, M.S.A.; Writing—original draft, R.P.; Writing—review
& editing, M.S.A. and S.S.; Visualization, S.K.T. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not Applicable.
Informed Consent Statement: Not Applicable.
Data Availability Statement: Not Applicable.
Acknowledgments:
The authors would like to acknowledge the generous support of the CSIR-
CEERI, Pilani, and the support of the TEQIP-III (a Government of India project assisted by the
World Bank), for providing a research grant under the TEQIP-III Collaborative Research Scheme
(grant no. 1-5744664011).
Conflicts of Interest: The authors declare no conflict of interest.
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