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

July 2017
Applied Physics Letters 111(2):023505

DOI:10.1063/1.4993901

Authors:

Menglun Zhang

Tianjin University

Xuexin Duan

Tianjin University

Show all 5 authorsHide

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.

Measured and analytical results: (a) the Smith chart and (b) the impedance spectrum. (c) Resonant frequency drift versus temperature change.

…

(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.

…

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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,

Xuexin Duan, Hao Zhang, and Wei Pang

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,

wearable devices,

and e-skins

enabled by signiﬁcant advancements in basic ﬂexible elec-

tronic components, e.g., ﬂexible transistors,

4–6

light-emitting

diodes (LEDs), passive components,

7,8

and batteries.

Among different realization technologies, ﬁlm bulk acoustic

resonators (FBARs), traditionally used as basic building

blocks of modern RF ﬁlters,

are natural candidates for

wireless ﬂexible electronics. Moreover, as MEMS devices

that bridge the mechanical and electrical domains, FBARs

demonstrated signiﬁcant sensing

11–13

and actuating

14–16

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

or fab-

ricating FBARs directly on polymer substrates, such as

polyimide (PI), that can withstand high processing tempera-

tures.

18–21

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

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

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.

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

25–27

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

Electronic mail: zml@tju.edu.cn

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.

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.

Figure 3(b) presents the measured and analytical impedance

spectra. The two impedance peaks correspond to the series

resonant frequency (f

) at 2.656 GHz and the parallel reso-

nant frequency (f

) 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

using

ðÞ

¼2pfsf

ðÞ

jS11j

1jS11j2;

where sis the group delay of the measured S11. Q factors at

the series resonant frequency (Q

) and the parallel resonant

frequency (Q

) 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.

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

of 947, a Q

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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Oct 2022
SENSOR ACTUAT A-PHYS

Recently, ultrahigh-frequency (UHF) ultrasonic transducers (> 100 MHz) have been widely applied in research fields such as medical diagnostics, nondestructive testing, energy harvesting, and so on, due to their capability of creating high-resolution ultrasonic images and detecting micro-scale defects. To accomplish this, in this study, ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (i.e., PIN-PMN-PT) polycrystalline single-phase thin film was fabricated for UHF ultrasonic transducers due to their extraordinary piezoelectric properties and electromechanical coupling coefficients. A tri-layered structure (of 1) aluminum substrate, 2) the PIN-PMN-PT thin film, and 3) a silver electrode) was used to create a PIN-PMN-PT thin film compatible with intense thermal treatment and having low cost. To analyze the performance of the newly developed PIN-PMN-PT thin film, data were collected on its crystallographic and piezoelectric characteristics, fabrication process, analysis of piezoelectric constants, and ultrasonic pulse-echo response waveforms according to different design conditions (various front matching layers, backing materials, and media). These data were provided by X-ray diffraction, X-ray fluorescence, X-ray reflectometry, field emission scanning electron microscopy, and polarization-electric field hysteresis curves. Moreover, pulse-echo simulation was conducted using PiezoCAD software based on the Krimholtz-Leedom-Matthaei model. The results showed that the proposed PIN-PMN-PT thin film exhibited ultrahigh-resonance frequencies of ~ 500 MHz. Ultimately, it was concluded that the new high-frequency PIN-PMN-PT thin film could be applied in a variety of fields because its performance has been verified.

The Potential of Acoustic Wave Devices for Gas Sensing Applications

Article

Mar 2022
SENSOR ACTUAT A-PHYS

Here we present a critical review related to the important acoustic device technologies being used for the gas sensing applications. Acoustic devices find wide applications in chemical and biological sensing due to miniaturized size, high sensitivity, integration with on-field electronics, ruggedness and wireless sensing ability. An overview on the acoustic devices such as quartz crystal microbalance (QCM), Film bulk acoustic wave resonators (FBAR), and surface acoustic wave resonators (SAW) is discussed with their working principle, selection of piezoelectric, electrode material for fabrication, resonance frequencies, operation of the respective acoustic device-based gas sensors, temperature dependency and sensing layers for different gases. Recent advancements made in acoustic devices based on gas sensing technology, their structures and working mechanism along with their fabrication method, as well as the recent advances made in the design are reviewed and discussed.

A four-state programmable mid-infrared band-stop filter exploiting a Ge 2 Sb 2 Te 5 film and VO 2 nanoparticles

Article

Oct 2021
APPL PHYS LETT

We designed and demonstrated a four-state programmable mid-infrared band-stop absorber that exploits two different phase-change materials. This programmability is possible by exploiting Fabry–Pérot resonances in a Ge2Sb2Te5 film and vanadium dioxide nanoparticles' (VO2 NPs) dual layer. The reflectivity trough can be tuned to four different infrared (IR) wavelengths from 1906 to 2960 nm by heating the structure to different temperatures. The near-perfect absorber is reconfigurable, lithography-free, industrially scalable, polarization-insensitive, and omnidirectional. Our strategy opens a path for programmable infrared photonics.

Transducer design for AlN Lamb wave resonators

Article

Full-text available

Apr 2017

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.

Detection of Volatile Organic Compounds by Self-assembled Monolayer Coated Sensor Array with Concentration-independent Fingerprints

Article

Full-text available

Apr 2016

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.

AlN texturing and piezoelectricity on flexible substrates for sensor applications

Article

Full-text available

Jun 2015

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

Modeling, Fabrication, and Characterization of Piezoelectric Micromachined Ultrasonic Transducer Arrays Based on Cavity SOI Wafers

Article

Full-text available

Jan 2015

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]

Film bulk acoustic resonators integrated on arbitrary substrates using a polymer support layer

Article

Full-text available

Mar 2015

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.

Stretchable silicon nanoribbon electronics for skin prosthesis

Article

Full-text available

Dec 2014

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.

Theoretical study of thermally stable SiO2/AlN/SiO2 Lamb wave resonators at high temperatures

Article

Full-text available

Feb 2014

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.

On-chip integrated multiple microelectromechanical resonators to enable the local heating, mixing and viscosity sensing for chemical reactions in a droplet

Article

Sep 2017
SENSOR ACTUAT B-CHEM

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.

Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare

Article

Feb 2016
ADV MATER

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.

Flexible structured high-frequency film bulk acoustic resonator for flexible wireless electronics

Article

May 2015

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.