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A flexible, gigahertz, and free-standing thin film piezoelectric MEMS resonator with high figure of merit

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In this paper, a 2.6 GHz air-gap type thin film piezoelectric MEMS resonator was fabricated on a flexible polyethylene terephthalate film. A fabrication process combining transfer printing and hot-embossing was adopted to form a free-standing structure. The flexible radio frequency MEMS resonator possesses a quality factor of 946 and an effective coupling coefficient of 5.10%, and retains its high performance at a substrate bending radius of 1 cm. The achieved performance is comparable to that of conventional resonators on rigid silicon wafers. Our demonstration provides a viable approach to realizing universal MEMS devices on flexible polymer substrates, which is of great significance for building future fully integrated and multi-functional wireless flexible electronic systems.
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A flexible, gigahertz, and free-standing thin film piezoelectric MEMS resonator with
high figure of merit
Yuan Jiang, Menglun Zhang, Xuexin Duan, Hao Zhang, and Wei Pang
Citation: Appl. Phys. Lett. 111, 023505 (2017); doi: 10.1063/1.4993901
View online: http://dx.doi.org/10.1063/1.4993901
View Table of Contents: http://aip.scitation.org/toc/apl/111/2
Published by the American Institute of Physics
A flexible, gigahertz, and free-standing thin film piezoelectric MEMS
resonator with high figure of merit
Yuan Jiang, Menglun Zhang,
a)
Xuexin Duan, Hao Zhang, and Wei Pang
b)
State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072,
China
(Received 17 April 2017; accepted 1 July 2017; published online 13 July 2017)
In this paper, a 2.6 GHz air-gap type thin film piezoelectric MEMS resonator was fabricated on a
flexible polyethylene terephthalate film. A fabrication process combining transfer printing and
hot-embossing was adopted to form a free-standing structure. The flexible radio frequency MEMS
resonator possesses a quality factor of 946 and an effective coupling coefficient of 5.10%, and
retains its high performance at a substrate bending radius of 1 cm. The achieved performance is
comparable to that of conventional resonators on rigid silicon wafers. Our demonstration provides
a viable approach to realizing universal MEMS devices on flexible polymer substrates, which is of
great significance for building future fully integrated and multi-functional wireless flexible
electronic systems. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4993901]
The rapid development of flexible electronic systems
such as flexible displays,
1
wearable devices,
2
and e-skins
3
is
enabled by significant advancements in basic flexible elec-
tronic components, e.g., flexible transistors,
46
light-emitting
diodes (LEDs), passive components,
7,8
and batteries.
9
Among different realization technologies, film bulk acoustic
resonators (FBARs), traditionally used as basic building
blocks of modern RF filters,
10
are natural candidates for
wireless flexible electronics. Moreover, as MEMS devices
that bridge the mechanical and electrical domains, FBARs
demonstrated significant sensing
1113
and actuating
1416
potentials. Transplanting this technology into the flexible
electronic realm will enrich the functionalities of the future
flexible electronic systems.
Flexible FBARs in previously reported works were real-
ized by either chemical thinning of the silicon wafer
17
or fab-
ricating FBARs directly on polymer substrates, such as
polyimide (PI), that can withstand high processing tempera-
tures.
1821
By thinning the silicon wafer, semi-flexible
FBARs with high performance can be obtained. However,
this process is time-consuming, and further thinning of the
silicon wafer would result in very fragile devices. Using the
latter method, thin and flexible FBARs can be fabricated, but
the device performance is limited. The performance of a
FBAR is typically indicated by its figure of merit (FOM),
defined as the product of the quality factor (Q) and the effec-
tive coupling coefficient (k2
teff ). The difficulty in depositing a
high quality piezoelectric thin film on a polymer substrate
22
and lack of an effective acoustic reflection structure (air cav-
ity or Bragg reflector) result in a FOM below 10, compared
to a FOM of 39 in the previous method.
In this work, high-performance FBARs are achieved
using a process combining transfer printing
23
and hot-
embossing: the composite membranes are first fabricated on
a silicon wafer and then transfer printed on a flexible
polyethylene terephthalate (PET) substrate with hot-
embossed air cavities, forming suspended FBARs. The flexi-
ble resonator has a resonant frequency of 2.656 GHz, a Q
factor of 948, and a k2
teff of 5.10%. The device FOM is
around 48, which is comparable to conventional FBARs fab-
ricated on rigid silicon wafers.
24
Bending test shows that the
device performance is sustainable for a substrate bending
radius of 1 cm. Our proposed method could be easily adopted
for the fabrication of other piezoelectric MEMS devices
2527
on polymer substrates as well.
Figure 1illustrates the main steps used to fabricate the
flexible FBAR devices and the fabricated flexible devices.
The receiver flexible substrate and the device donor wafer
were prepared separately. A piece of commercially available
dicing tape (UDT-1025c, Denka), which contains a 100 lm
PET layer and a 25 lm UV-curable adhesive layer, was cho-
sen as the receiver substrate. A silicon mold with pentagon-
shaped protrusions (10 lm in height) was fabricated by
photolithography and reactive ion etch (RIE). Then, the
dicing tape was laminated to the silicon mold, and together
they were put between two heated and pressurized plates
(0.2 MPa and 95 C) for 5 min. After cooling down to room
temperature, the dicing tape was peeled off from the silicon
mold, and thus, pentagon-shaped air cavities were formed in
the adhesive layer of the dicing tape, as shown in Fig. 2(a).
The fabrication of the donor device began with the
deposition of 300 nm silicon oxide as a sacrificial layer on
the silicon wafer. The sacrificial layer was then patterned by
a buffered oxide etchant (7:1 BOE), exposing parts of the
silicon surface underneath. The bottom electrode was formed
by deposition and patterning of 300 nm molybdenum (Mo).
Part of the bottom electrode was formed on the silicon sur-
face, thus serving as anchors during the release step. 600 nm
aluminum nitride (AlN) was deposited and patterned as the
piezoelectric layer. Another layer of 300 nm Mo was depos-
ited and patterned as the top electrode, forming the sand-
wiched thin film structure. Removal of the sacrificial layer in
a 1:10 solution of HF (hydrofluoric acid, 49%) for 3 h
a)
Electronic mail: zml@tju.edu.cn
b)
Electronic mail: weipang@tju.edu.cn
0003-6951/2017/111(2)/023505/4/$30.00 Published by AIP Publishing.111, 023505-1
APPLIED PHYSICS LETTERS 111, 023505 (2017)
released the FBAR membranes from the substrate, while
anchors kept the membranes on the substrate, as shown in
Fig. 2(b).
Finally, a soft elastomer stamp made from polydime-
thylsiloxane (PDMS) was used to pick up the device from
the donor wafer. In Fig. 2(c), the anchor left on the wafer
shows a clean fracture along the designed mechanical weak
edges. It is worth mentioning that the silicon wafer can be
reused after removing the left anchors. The stamp carrying
the FBAR membrane was carefully aligned with the air cav-
ity and then brought into contact with the adhesive surface
using a microscope and a XYZ translation stage. After
retracting the stamp, the device membrane eventually
remained on the adhesive surface because of a larger adhe-
sion force at the FBAR–adhesive interface rather than the
FBAR–PDMS interface. In the last step, UV exposure from
the back side of the tape fully cured the adhesive. The adhe-
sive turned harder and became less adhesive after curing,
rendering the device more mechanically stable and less sus-
ceptible to particulates or contaminants, and made the whole
device stable. An intentional half-cut device is shown in Fig.
2(d) in order to demonstrate the suspended structure.
The transfer process is carried out at room temperature.
The applied pressure during the pick and place steps was
controlled by monitoring deformations of the device and the
stamp in the microscope, as an intimate contact must be
ensured without inducing too much stress to the device. To
optimize the transfer yield, sharp corners and narrow inter-
sections should be avoided in the geometry design to mini-
mize stress concentration during the transfer process. Round
holes at the connection edge [as shown in Fig. 2(b)] help
create stress concentration at the anchor sites, making sure
that the anchors break easier than other parts of the device.
Hole densities of 0.5 and 0.75 were finally chosen from
balancing between the easiness of retrieval and the stability
of attaching devices on the substrate during the washing and
cleaning process. Discussion about the design principle of
the device geometry and information of the design evalua-
tion experiment can be found in the supplementary material.
Our proposed fabrication method is quite robust and gen-
eral, as ten FBARs were transferred in succession with a yield
of 100% (Figs. S1 and S2, supplementary material). On the
other hand, since the receiver substrate experiences no harsh
conditions such as high temperature and corrosive etchant,
there are a vast number of choices for other flexible substrates
such as PDMS and PI. The air cavity can be formed by
alternative methods like molding and plasma etching (Fig.
S3, supplementary material). Among those methods, hot-
embossing technique represents the most efficient patterning
method and can be easily implemented within a roll-to-roll
fabrication system. Since the transfer print process is also
scalable, a high-throughput production can be envisioned
by upgrading the current manual transfer process to an auto-
mated transfer print system equipped with robotic arms, par-
allel stamp arrays, and computer vision modules.
The resonance characteristics of flexible FBARs were
measured with an Agilent E8368b vector network analyzer
FIG. 1. (a) Illustration of the main fabrication process of a flexible FBAR
(drawn not to scale). (b) Photo of a plastic substrate carrying flexible resona-
tors and zoomed-in microscopy images of one resonator in an air cavity.
FIG. 2. Scanning electron microscopy (SEM) images of (a) the hot-
embossed air cavity on the adhesive, (b) the anchor structure that tethers the
FBAR at the silicon wafer, (c) the anchor left on the silicon wafer after the
device transfer, and (d) FBAR transferred on the adhesive (the composite
membrane was intentionally half-cut before transfer in order to show the
suspended structure).
023505-2 Jiang et al. Appl. Phys. Lett. 111, 023505 (2017)
using a 150 lm pitch GS probe (Cascade Microtech).
Analytical results were calculated using a one-dimensional
Mason model.
28
The measured and analytical reflection coef-
ficients (S11) are plotted on a Smith chart in Fig. 3(a). The
measured S11 circle is very close to the edge of the Smith
chart, indicating that the FBAR has a relatively high FOM.
10
Figure 3(b) presents the measured and analytical impedance
spectra. The two impedance peaks correspond to the series
resonant frequency (f
s
) at 2.656 GHz and the parallel reso-
nant frequency (f
p
) at 2.713 GHz, respectively. The effective
coupling coefficient (k2
teff ) is calculated to be 5.10%, indicat-
ing a good electrical–mechanical conversion efficiency of
the piezoelectric AlN. It can be seen from Figs. 3(a) and 3(b)
that the analytical data agree well with the measured data,
showing that the Mason model can be effectively utilized to
guide the design of the flexible air-gap type FBAR as well.
The device Q factor as a function of frequency can be
calculated
29
using
Qf
ðÞ
¼2pfsf
ðÞ
jS11j
1jS11j2;
where sis the group delay of the measured S11. Q factors at
the series resonant frequency (Q
s
) and the parallel resonant
frequency (Q
p
) were calculated to be 947 and 694, respec-
tively. Temperature characteristic of flexible FBAR was
tested by adhering the substrate to a heated wafer chuck, and
each dataset was measured at 5 min after the chuck reached
its set temperature. Figure 3(c) shows that the thermally
induced frequency shift of the flexible FBAR is quite linear
from 30 to 60 C. The corresponding coefficient of frequency
(TCF) is about 24 ppm/C, which is comparable with rigid
AlN FBARs on silicon. A further increase of the temperature
will soften the adhesive below the electrodes, rendering
probe tests difficult. Other polymers with a higher glass tran-
sition temperature, e.g., polyimide and polyimide adhesives,
can be used as substrates for a wider operating temperature
range.
The mechanical flexibility of FBARs was investigated
by attaching them on plastic columns of radii ranging from
3 cm to 0.5 cm, as shown in Fig. 4(a). The FBARs functioned
well at bending radii of 3 cm, 2 cm, and 1 cm, and there were
no obvious impedance spectral shifts at these bending levels.
A small resonant frequency shift (less than 100 kHz, about
38 ppm) was observed in the zoom-in view, which is
acceptable for most applications. This shift is much lower
than the expected value of 9 MHz in theory, as derived in the
supplementary material. The discrepancy may be caused by
alternative factors other than thickness change, including
parasitic capacitance and inductance of the electrodes, and
residual stresses left in the Mo and AlN films during deposi-
tion. The effect of device bending on strain distribution was
studied using a FEA solver. Figure 4(b) shows that the maxi-
mum strain resides in the center of the pentagon-shaped
membrane, due to the presence of air cavity. The maximum
strain increases exponentially with the decrease of bending
radii. Bending radii of 3 cm, 2 cm, and 1 cm correspond to
strain values of 0.25%, 0.35%, and 0.7%, respectively. The
maximum strain increased significantly to 1.3% at the radius
of 0.5 cm, causing the electrode to break. Further
FIG. 3. Measured and analytical results: (a) the Smith chart and (b) the impedance spectrum. (c) Resonant frequency drift versus temperature change.
FIG. 4. (a) Resonance curves of a flexible FBAR at different bending radii,
along with a magnified view of the resonance peak. (b) Correlation of the
bending radius versus maximum strain in a device membrane solved using a
FEA program.
023505-3 Jiang et al. Appl. Phys. Lett. 111, 023505 (2017)
improvements to the flexibility of our device include using a
thinner substrate, optimization of the device shape to avoid
stress concentration, and adding a top encapsulation layer
according to the neutral plane theory.
30
In summary, a flexible piezoelectric MEMS resonator
has been demonstrated on a polymer substrate. Transfer print-
ing and hot-embossing were used to form the free-standing
structure, which is the key to retain high device performance.
The 2.6 GHz FBAR has a Q
s
of 947, a Q
p
of 694, a k2
teff of
5.10%, and a TCF of 24 ppm/C. The device functioned
well at bending radii of 3 cm, 2 cm, and 1cm. The fabrication
process we developed could be adopted for other MEMS
devices. It greatly enriches the library of available building
blocks for a multi-functional flexible electronic system.
See supplementary material for a detailed comparison
with previous works, experiments for yield estimation, the
process demonstration on alternative substrates, the design
principles of the device geometry, and the deduction of the
theoretical frequency shift of an FBAR under tensile strain.
This work was supported by Natural Science
Foundation of China (NSFC No. 51375341), the 111 Project
under Grant No. B07014, and the National High Technology
Research and Development Program of China (863 Program)
under Grant No. 2015AA042603.
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Today, the vast majority of microelectromechanical system (MEMS) sensors are mechanically rigid and therefore suffer from disadvantages when used in intimately wearable or bio-integrated applications. By applying new engineering strategies, mechanically bendable and stretchable MEMS devices have been successfully demonstrated. This article reviews recent progress in this area, focusing on high-performance flexible devices based on inorganic thin films. We start with the common design and fabrication strategies for flexibility and stretchability, summarize the recent application-oriented flexible devices, and conclude with criteria and opportunities for the future development of flexible MEMS sensors.
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This paper proposes a flexible lateral-field-excited (LFE) film bulk acoustic resonator (FBAR) using transferred single-crystalline LiTaO3 piezoelectric thin films. Coplanar electrodes are adopted to excite the acoustic wave and the flexible LFE FBAR exhibits three wave modes: thickness-shear mode (TSM), thickness-longitudinal mode (TLM), and the second harmonic of TSM. Because of the adoption of high-quality transferred single-crystal piezoelectric thin films, a record high quality factor (Q) of 1837 is obtained for the TSM with a resonance frequency of 2.510 GHz. The TLM (4.700 GHz) and the second harmonic of TSM (7.490 GHz) also exhibit a high Q of 1516 and 928, respectively. The Qs are among the highest reported values for flexible piezoelectric MEMS resonators, and comparable to those of the silicon-based FBARs. The temperature sensitivities of the three wave modes are in the range of −58 to −67 ppm/℃ over the temperature range of 20–90 ℃. The electrical performance remains almost unchanged under a minimum bending radius of 3 mm. The flexible LFE FBARs exhibit high mechanical flexibility and a remarkable figure of merit. The excellent performance would enable application scenarios including low-loss, high-selectivity filters for future flexible front-end modules in the Internet of Things (IoT), and flexible temperature sensors that can be integrated into wearable healthcare and biochemical systems.
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A solidly mounted resonator on flexible Polyimide (PI) substrate with high effective coupling coefficient (Kt2) of 14.06% is reported in this paper. This high Kt2 is resulting from the LiNbO3 (LN) single crystalline film and [SiO2/Mo]3 Bragg reflector. The quality of LN film fabricated by Crystal-ion-slicing (CIS) technique using Benzocyclobutene (BCB) bonding layer was close to the bulk crystalline LN. The interfaces of the Al/LN/Al/[SiO2/Mo]3 Bragg reflector/BCB/PI multilayer are sharp and the thickness of each layer is consistent with its design value. The resonant frequency and the Kt2 keep stable when it is bended at different radii. These results demonstrate a feasible approach to realizing RF filters on flexible polymer substrates, which is an indispensable device for building integrated and multi-functional wireless flexible electronic systems.
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AlNLamb waveresonators enjoy advanced and attractive properties for enabling the next-generation single-chip radio frequency front-end, but their moderate effective electromechanical coupling coefficient (k²eff) poses a limit to their application in filters and multiplexers. Despite the fact that the reported k²eff enhancement techniques of doped AlN thin films which are expensive and trade off the quality factor (Q), the transducer topology itself extensively impacts the k²eff value. Although an AlN cross-sectional Lame mode resonator exhibiting a k²eff of 6.34% has been demonstrated without the need for changing the piezoelectric material, a detailed study of transducer design for AlNLamb waveresonators has not been conducted. In this work, we investigate the impact of (i) transducer configurations, (ii) electrode materials, (iii) electrode thicknesses, and (iv) interdigital transducer duty factors on the k²eff dispersive characteristics of one-port AlNLamb waveresonators by using the finite element analysis approach. By properly designing and optimizing the transducers, the k²eff of one-port AlNLamb waveresonators can be boosted to as high as 7.7%, showing great potential for applications of cellular frequency selection.
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In this paper, we have modeled and analyzed affinities and kinetics of volatile organic compounds (VOCs) adsorption (and desorption) on various surface chemical groups using multiple self-assembled monolayers (SAMs) functionalized film bulk acoustic resonator (FBAR) array. The high-frequency and micro-scale resonator provides improved sensitivity in the detections of VOCs at trace levels. With the study of affinities and kinetics, three concentration-independent intrinsic parameters (monolayer adsorption capacity, adsorption energy constant and desorption rate) of gas-surface interactions are obtained to contribute to a multi-parameter fingerprint library of VOC analytes. Effects of functional group's properties on gas-surface interactions are also discussed. The proposed sensor array with concentration-independent fingerprint library shows potential as a portable electronic nose (e-nose) system for VOCs discrimination and gas-sensitive materials selections.
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We show that AlN-based piezocapacitors with relatively high piezoelectric coefficient (d33) values (3–4 pC/N) can be fabricated on polyimide (PI) substrates at 160 °C or even at room temperature by sputtering processes. With respect to PI, a reduction of the piezoelectric performances was observed on polyethylene naphthalate (PEN). With the same approach, a d33 value as high as 8 pC/N was achieved on rigid substrates (SiO2/Si). In all cases, a thin Al buffer layer was deposited, immediately before AlN, without breaking the vacuum in the deposition chamber, in order to preserve the interface from contaminations that would obstruct the optimal atomic stratification with the desired [0001] growth axis. The piezoelectric behavior was thus correlated to the degree of texturing of the AlN layer through the evaluation of the XRD texturing coefficients and to the morphology by means of AFM analyses. We show that a high level of roughness introduced by the PEN substrate, coupled with the effect of the substrate flexibility on the piezoelectric coefficient, reduces the impact of the AlN texturing on the d33 values
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This paper presents high fill-factor piezoelectric micromachined ultrasonic transducer (PMUT) arrays fabricated via a novel process using cavity SOI wafers. The simple three-mask fabrication process enables smaller diameter PMUTs (25 m) and finer pitch than previous processes requiring through-wafer etching. PMUTs were fabricated with diameters from 25 to 50 m, resulting in center frequencies from 13 to 55 MHz in air. Two types of devices, having different piezoelectric layers, lead zirconium titanate (PZT), and aluminum nitride (AlN), were fabricated and characterized. Comparing 50- m diameter devices, the PZT PMUTs show large dynamic displacement sensitivity of 316 nm/V at 11 MHz in air, which is higher than that of the AlN PMUTs. Electrical impedance measurements of the PZT PMUTs show high electromechanical coupling % and 50- electrical impedance that is well-matched to typical interface circuits. Immersion tests were conducted on PZT PMUT arrays. The fluid-immersed acoustic pressure generated by an unfocused array of 40- m diameter, 10-MHz PZT PMUTs, measured with a needle hydrophone 1.2 mm away from the array, was 58 kPa with a 25 input. Beam forming based on electronic phase control produced a narrow, 150- m diameter, focused beam over a depth of focus >0.2 mm and increased the pressure to 450 kPa with 18 input. [2014-0324]
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The film bulk acoustic resonator (FBAR) is a widely-used MEMS device which can be used as a filter, or as a gravimetric sensor for biochemical or physical sensing. Current device architectures require the use of an acoustic mirror or a freestanding membrane and are fabricated as discrete components. A new architecture is demonstrated which permits fabrication and integration of FBARs on arbitrary substrates. Wave confinement is achieved by fabricating the resonator on a polyimide support layer. Results show when the polymer thickness is greater than a critical value, d, the FBARs have similar performance to devices using alternative architectures. For ZnO FBARs operating at 1.3-2.2 GHz, d is ~9 μm, and the devices have a Q-factor of 470, comparable to 493 for the membrane architecture devices. The polymer support makes the resonators insensitive to the underlying substrate. Yields over 95% have been achieved on roughened silicon, copper and glass.
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Sensory receptors in human skin transmit a wealth of tactile and thermal signals from external environments to the brain. Despite advances in our understanding of mechano- and thermosensation, replication of these unique sensory characteristics in artificial skin and prosthetics remains challenging. Recent efforts to develop smart prosthetics, which exploit rigid and/or semi-flexible pressure, strain and temperature sensors, provide promising routes for sensor-laden bionic systems, but with limited stretchability, detection range and spatio-temporal resolution. Here we demonstrate smart prosthetic skin instrumented with ultrathin, single crystalline silicon nanoribbon strain, pressure and temperature sensor arrays as well as associated humidity sensors, electroresistive heaters and stretchable multi-electrode arrays for nerve stimulation. This collection of stretchable sensors and actuators facilitate highly localized mechanical and thermal skin-like perception in response to external stimuli, thus providing unique opportunities for emerging classes of prostheses and peripheral nervous system interface technologies.
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Aluminum nitride (AlN) and silicon dioxide (SiO2) bilayer structure has been widely utilized in temperature-compensated micromechanical resonators as SiO2 has unique positive temperature coefficients of elasticity. However, the thermal expansion mismatch would cause large bending deformation and stress distribution in the resonant plate. In this study, a symmetrical SiO2/AlN/SiO2 sandwiched structure is proposed to reduce the temperature-induced deformation in the asymmetrical AlN/SiO2 bilayer plate. The thermal compensation at high temperatures for the Lamb wave resonators utilizing the lowest-order symmetric (S0) mode in the SiO2/AlN/SiO2 sandwiched structure is theoretically investigated herein. While operation temperature rises from room temperature to 600 °C, the temperature-induced bending deformation in the symmetrical SiO2/AlN/SiO2 composite plate is much less than that in the AlN/SiO2 composite plate conventionally used for temperature compensation. Furthermore, the different material properties of the AlN and SiO2 layers make the displacements of the S0 mode not purely symmetric with respect to the neutral axis, whereas the symmetrical SiO2/AlN/SiO2 sandwiched membrane still can enable a pure S0 mode which shows higher phase velocity and larger electromechanical coupling coefficient than the lowest-order quasi-symmetric (QS0) mode traveling in the AlN/SiO2 bilayer membrane. With proper thickness selection of AlN and SiO2, the S0 mode in the symmetrical SiO2/AlN/SiO2 sandwiched membrane can simultaneously offer excellent thermal compensation, high phase velocity, large electromechanical coupling coefficient, and small thermally induced deformation at high temperatures.
Article
On-chip integrating several functional components for developing integrated lab-on-a-chip microsystem remains as a challenge. In this work, by employing multiple microelectromechanical resonators both as actuators and sensors, on-chip heating, mixing and chemical reaction monitoring are successfully demonstrated. Mechanism studies using COMSOL simulations indicate that the local heating and mixing are induced by the acoustic wave attenuation during its transmission in liquid. On-line chemical reaction monitoring is realized by viscosity sensing using the same resonator through impedance analysis. Classic Diels-Alder reaction in a single droplet was performed to verify the feasibility of using such microsystem for mixing, heating and online reaction monitoring at microscale.
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Flexible and stretchable physical sensors that can measure and quantify electrical signals generated by human activities are attracting a great deal of attention as they have unique characteristics, such as ultrathinness, low modulus, light weight, high flexibility, and stretchability. These flexible and stretchable physical sensors conformally attached on the surface of organs or skin can provide a new opportunity for human-activity monitoring and personal healthcare. Consequently, in recent years there has been considerable research effort devoted to the development of flexible and stretchable physical sensors to fulfill the requirements of future technology, and much progress has been achieved. Here, the most recent developments of flexible and stretchable physical sensors are described, including temperature, pressure, and strain sensors, and flexible and stretchable sensor-integrated platforms. The latest successful examples of flexible and stretchable physical sensors for the detection of temperature, pressure, and strain, as well as their novel structures, technological innovations, and challenges, are reviewed first. In the next section, recent progress regarding sensor-integrated wearable platforms is overviewed in detail. Some of the latest achievements regarding self-powered sensor-integrated wearable platform technologies are also reviewed. Further research direction and challenges are also proposed to develop a fully sensor-integrated wearable platform for monitoring human activity and personal healthcare in the near future.
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Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.