Thin Film Piezoelectric Transducer Research Articles

Lateral Extensional Mode Piezoelectric ZnO-on-Nickel RF MEMS Resonators for Back-End-of-Line Integration.

High motional resistance and incompatibility with post-CMOS fabrication due to thermal budget constraints are imperative issues associated with the back-end-of-line integration of lateral extensional vibrating micromechanical resonators. This paper presents piezoelectric ZnO-on-nickel resonators as a viable means for mitigating both of the issues. Lateral extensional mode resonators equipped with thin-film piezoelectric transducers can exhibit much lower motional impedances than their capacitive counterparts due to piezo-transducers' higher electromechanical coupling coefficients. Meanwhile, the employment of electroplated nickel as the structural material allows the process temperature to be kept lower than 300 °C, which is low enough for the post-CMOS resonator fabrication. In this work, various geometrical rectangular and square plates resonators are investigated. Moreover, parallel combination of several resonators into a mechanically coupled array was explored as a systematic approach to lower motional resistance from ~1 kΩs to 0.562 kΩs. Higher order modes were investigated for achieving higher resonance frequencies up to 1.57 GHz. Local annealing by Joule heating was also exploited for quality factor improvement after device fabrication by ~2× enhancement and breaking the record of MEMS electroplated nickel resonators in lowering insertion loss to ~10 dB.

A method for the calibration of wideband ultrasonic sensors for optoacoustics.

A method for calibration of ultrasonic sensors for optoacoustics that provides both frequency response and sensitivity is presented. In order to obtain the bandwidth and the frequency response of an uncalibrated sensor, a point source with broadband spectra generated by a laser-induced bubble on a copper wire submerged in water is employed. On the other hand, the sensitivity measurement relies on the spatial symmetry of the pressure pulse and on a calibrated transducer. Therefore, two sensors are employed to detect the pressure pulse at the same distance from the source. The symmetry of the acoustic field that arrives at both transducers is adjusted and verified by means of an optical interferometer that provides a null signal when the copper wire is placed at the right position. The method is tested on the characterization of a thin-film polymeric piezoelectric transducer with a cylindrical focused shape.

RETRACTED: Design of a piezoelectric energy conversion based wind generator

This manuscript presents a new experimental wind generator based on piezoelectric energy conversion for low power applications. The aim is to demonstrate an alternative renewable energy generation method for low power applications. The generator has four blades of a propeller equipped with a total of twenty-four (24) thin film piezoelectric transducers (TFPTs). The output voltage is generated using a newly developed circuit topology. The generator was tested at three wind speeds 10 m/s, 14 m/s and 18 m/s, with a maximum output voltage of 10.2 V being produced at a wind speed of 18 m/s. Results show that this generator has promise to be suitable for low power batteryless applications, for example wireless sensor nodes (WSN).

Flexible piezoelectric AlN transducers buckled through package-induced preloading for mechanical energy harvesting

There is a high demand for novel flexible micro-devices for energy harvesting from low-frequency and random mechanical sources. The research of new functional designs is required to strategically enhance the performances and to increase the control on mechanical flexibility. In this work we report the fabrication and characterization of bi-stable and statically balanced thin-film piezoelectric transducers based on Aluminum Nitride (AlN). The device consists of a piezoelectric layer sandwiched between two thin Molybdenum electrodes that were deposited on a Kapton substrate by reactive sputtering and patterned by UV lithography. In order to improve the out-of-plane flexibility, the mechanical design is distinguished by a post-buckled flexure that introduces a negative stiffness to compensate the otherwise positive stiffness of the system. The buckling was introduced by a new method, called Package-Induced Preloading (PIP) where the mechanisms are laminated over a package with a geometry extending out-of-plane. The induced buckling resulted in bi-stable and statically balanced mechanisms which demonstrated an enhanced voltage output during a triggered snapping step. A preliminary study shows potential for the statically balanced designs and the PIP method for wind energy harvesting, revealing prospective applications and future improvements for the development of energy harvesters.

Toward 40 GHz excitation of diamond-based HBAR

We have obtained an excitation of longitudinal bulk acoustic waves in a diamond-based High overtone Bulk Acoustic Resonator (HBAR) at microwave and enhanced frequency bands as EHF up to 40 GHz. As an effective piezoelectric transducer, an aluminum-scandium nitride film is employed. The frequency response of acoustic overtones excited in the HBARs with a different aperture in the 1.0 up to 40 GHz range is measured. Some peculiarities of HBAR's frequency response concerned with the thin film piezoelectric transducer are studied. It is established that the maximal magnitude of the quality factor varies near the value of about 10 000 within all the frequency bands investigated. The approximately frequency-independent Q-factor behavior can explain in terms the Landau–Rumer approximation describing the acoustic attenuation in a diamond. An estimation of the HBAR's quality parameter as Q × f ≈ 4 × 1014 Hz at 40 GHz is obtained. In our opinion, that value exceeds all known data among the given devices. The results obtained will be helpful in the development of microwave and EHF acoustic resonators as well as highly sensitive acoustic sensors. The development of the EHF band opens up the possibility of obtaining acoustic waves of submicrometer and nanometer scales (in various media) and implementing acoustic studies of micro- and nanoobjects.

Impedance Spectroscopy of Lead-Free Ferroelectric Coatings

This paper presents impedance measurements of ferroelectric structures involving lead-free oxide and polymer-oxide composite coatings for sensing and energy harvesting applications. Three different ferroelectric materials grown by conventional microfabrication technologies on solid or flexible substrates are investigated for their basic resonant characteristics. Equivalent electrical circuit models are applied to all cases to explain the electrical behavior of the structures, according to the materials type and thickness. The analytical results show good agreement with the experiments carried out on a basic types of excited thin-film piezoelectric transducers. Additionally, temperature and frequency dependences of the dielectric permittivity and losses are measured for the polymer-oxide composite device in relation with the surface morphology before and after introduction of the polymer to the functional film.

Peculiarities of energy trapping of the UHF elastic waves in diamond-based piezoelectric layered structure. II. Lateral energy flow

Features of an acoustic energy trapping and outflow in a High-overtone Bulk Acoustic Resonator (HBAR) were considered. On the example of a specified resonator based on a piezoelectric layered structure (PLS) as “Al/AlN/Mo/(100) diamond”, the simulation of the propagation of microwave acoustic waves in the range of 10–6000 MHz by Finite Element Method (FEM) in COMSOL Multiphysics has been carried out. It was found that the magnitude of acoustic energy retained in the substrate under the bottom electrode depends substantially on the relation between the HBAR’s antiresonance frequency fa,n and the cut-off frequencies fs,n-k and fs,n-k+1 of the substrate. The term “cut-off frequency” has the meaning of the frequency point when integer value of half-waves fits along the thickness of substrate. Parameter gk,n estimating the degree of the HBAR’s energy trapping has been proposed. If 0.5 < gk,n < 1, then a retention of acoustic energy in the HBAR’s center as a maximal ET-effect at the overtone frequency fa,n should be fulfilled. Increasing the width of the top electrode as well as the HBAR’s aperture should expand the frequency interval in which the retained elastic energy in the vicinity of the HBAR’s center will be greater than the similar outside. It was shown that the area-averaged Umov-Poynting’s power energy flow vector from the HBAR’s center in the lateral directions increases when approaching the Thin Film Piezoelectric Transducer half-wave resonance frequency from below, and then decreases sharply in the point of complete energy trapping. This can be explained by the outflow of acoustic wave energy from the center to the periphery as a consequence of the transformation of an operational longitudinal acoustic mode into waves of Lamb type mainly. Results on the peculiarities of an energy trapping effect will be useful developing the HBARs and HBAR-based acoustic sensors operating at microwave frequencies.

Thin film piezoelectric acoustic transducer for fully implantable cochlear implants

This paper reports the development of a single cantilever thin film PLD-PZT transducer prototype. The device was experimentally characterized by attaching it on an acoustically vibrating membrane resembling the behavior of the eardrum. Acceleration characteristic of the sensor attached on the membrane was obtained by using a Laser Doppler Vibrometer (LDV) as the output voltage was measured by an oscilloscope. A voltage output of 114 mV was obtained, when the device was excited at 110 dB Sound Pressure Level (SPL) at 1325 Hz. This is the highest value for a thin film piezoelectric transducer in the literature to our knowledge. Using the results of a finite element analysis for this single-channel prototype, which are within 92% agreement with the experimental results, we performed an optimization study to propose a multi-frequency acoustic sensor to be placed on the eardrum for fully-implantable cochlear implant (FICI) applications. The proposed multi-channel transducer consists of eight cantilever beams. Each of these beams resonates at a specific frequency within the daily acoustic band (250–5000 Hz), senses the eardrum vibration and generates the required voltage output for the stimulation circuitry. The total volume and mass of the transducer are 5 × 5×0.2 mm3 and 12.2 mg, respectively. High sensitivity of the transducer (391.9 mV/Pa @900 Hz) enables transmission of strong signals to be the readout circuit, which can easily be processed. Expected to satisfy all the requirements (volume, mass, and stimulation signal at the hearing band) of FICI applications for the first time in literature, the proposed concept has a groundbreaking nature and it can be referred to as the next generation of FICIs since it revolutionizes the operational principle of conventional CIs.

Rapid Wave Velocity Measurement by Brillouin Scattering Using Coherent Phonons Induced by ScAlN Piezoelectric Thin-Film Transducer

It is difficult to perform 2-D imaging of elastic properties using the Brillouin scattering technique because the weak thermal phonon signal in the sample leads to low measurement accuracy and long measurement times. To improve the phonon signal, we artificially induced acoustic phonons using a ScAlN thin-film piezoelectric transducer, which has a giant piezoelectricity. The film was grown using RF magnetron sputtering of a ScAl alloy target on a silica glass bar sample. Using a microwave probe, the electric power applied to the film was 1 mW at 875 MHz. We obtained the enhancement of the Brillouin scattering signal in the silica glass bar sample due to the induced phonons. Compared with and without the induced phonons from the ScAlN film transducer, the peak intensity improved by nearly 3 orders of magnitude. This technique can significantly shorten the time required for the Brillouin scattering measurements.

High-speed real-time heterodyne interferometry using software-defined radio.

This paper describes the design and performance of a phase demodulation scheme based on software-defined radio (SDR), applied in heterodyne interferometry. The phase retrieval is performed in real time by means of a low-cost SDR with a wideband optoelectronic front-end. Compared to other demodulation schemes, the system is quite simpler, versatile, and of lower cost. The performance of the demodulator is demonstrated by measuring the displacement per volt of a thin-film polymeric piezoelectric transducer based on polyvinylidene fluoride for ultrasonic applications. We measured displacements between 3.5pm and 122pm with 7% relative uncertainty, in the frequency range from 20kHz to 1MHz.

Peculiarities of energy trapping of the UHF elastic waves in diamond-based piezoelectric layered structure. I. Waveguide criterion

Finite Element Modeling of the peculiarities of the trapping energy phenomenon in application to the piezoelectric layered structure (PLS) “Al/(0 0 1) AlN/Mo/(1 0 0) diamond” has been fulfilled. The resonant properties of longitudinal bulk acoustic waves (BAW) as well as frequency dependence of impedance within the 1 – 6 GHz band have been studied. The investigation of distribution of elastic energy flow and elastic displacements in a PLS cross-section allowed us to obtain an important information on energy trapping (ET) in PLS.Experimentally and as a result of modeling, it has been found that Q minimums are observed in PLS at quarter-wave resonance in the thin-film piezoelectric transducer (TFPT). Maximal Q value was observed at half-wave resonance in TFPT.It has been established that the ET-effect depends considerably on the mutual location of the n-th overtone’s antiresonant frequency fa,n and cut-off frequencies of substrate fs,n−k−1 and fs,n−k where fs,n−k−1<fa,n<fs,n−k, and k=0, 1,2,… is the number of half-length waves to be stowed on the thickness of TFPT. The total violation of the ET-effect will be arisen at the condition f >fs,n−k, when the BAW energy excites the symmetrical or antisymmetrical Lamb waves.

Excitation of hypersonic acoustic waves in diamond-based piezoelectric layered structure on the microwave frequencies up to 20 GHz

First ultrahigh frequency (UHF) investigation of quality factor Q for the piezoelectric layered structure «Al/(001)AlN/Mo/(100) diamond» has been executed in a broad frequency band from 1 up to 20GHz. The record-breaking Q·f quality parameter up to 2.7·1014Hz has been obtained close to 20GHz. Frequency dependence of the form factor m correlated with quality factor has been analyzed by means of computer simulation, and non-monotonic frequency dependence can be explained by proper features of thin-film piezoelectric transducer (TFPT). Excluding the minimal Q magnitudes measured at the frequency points associated with minimal TFPT effectiveness, one can prove a rule of Qf∼f observed for diamond on the frequencies above 1GHz and defined by Landau-Rumer’s acoustic attenuation mechanism. Synthetic IIa-type diamond single crystal as a substrate material for High-overtone Bulk Acoustic Resonator (HBAR) possesses some excellent acoustic properties in a wide microwave band and can be successfully applied for design of acoustoelectronic devices, especially the ones operating at a far UHF band.

Study of high-overtone bulk acoustic resonators based on the Me1/AlN/Me2/(100) diamond piezoelectric layered structure

The Me1/AlN/Me2/(100) diamond structure has been theoretically analyzed and experimentally investigated in the range 0.5–10 GHz using high-overtone bulk acoustic resonators with different electrodes topologies based on the Al/AlN/Mo/(100) diamond structure. The maximum quality parameter Q × f ≈ 1014 Hz was obtained at f = 9.5 GHz. The layered structure has been analyzed using the developed HBAR software v. 2.3. It is demonstrated that the features in the frequency dependences of the parameters of such resonators are related to the behavior of a loaded thin-film piezoelectric transducer. The calculation results are in good agreement with the experiment. The frequency dependences of the equivalent parameters of the resonators have been calculated. It is shown that the synthetic type IIa diamond single crystal in combination with aluminum nitride is promising for implementation of high-Q acoustoelectronic microwave devices.

Electrical control of a single magnetoelastic domain structure on a clamped piezoelectric thin film—analysis

This paper presents an analytical model coupling Landau-Lifshitz-Gilbert micromagnetics with elastodynamics and electrostatics to model the response of a single domain magnetoelastic nano-element attached to a piezoelectric thin film (500 nm). The thin film piezoelectric is mounted on a Si substrate, globally clamping the film from in-plane extension or contraction. Local strain transfer to the magnetoelastic element is achieved using patterned electrodes. The system of equations is reduced to eight coupled partial differential equations as a function of voltage (V), magnetic potential ϕ, magnetic moments (m), and displacements (u), i.e., fully coupled material. The weak forms of the partial differential equations are solved using a finite element formulation. The problem of a Ni single domain structure (i.e., 150 nm × 120 nm × 10 nm) on a thin film (500 nm) piezoelectric transducer (PZT)-5H attached to an infinite substrate is studied. Discretization in the single domain structure is on the order of the exchange length (8.5 nm), providing spatial and temporal information on the local mechanical and magnetic fields. A −0.5 V potential is applied to a pair of surface electrodes, producing out-of-plane deformation and in turn straining the magnetoelastic single domain nanostructure in-plane. This strain is sufficient to reorient a single domain structure representative of an idealized memory element.

Microbioreactor designed for integration with piezoelectric transducers for cellular diagnostics

This article reports a perfusion-based microbioreactor that can be integrated with a thin-film piezoelectric transducer array for rapid disease diagnosis, such as identifications of cancer cells and infection induced cell abnormality. By using the gap between suspended transducers as the high-aspect ratio barriers to establish high flow resistance into the culture chamber, we have verified that diffusion dominated the main transport mechanisms in the culture chamber. The fluid flow was successfully suppressed under 0.03, 0.3, and 3.0 μl/min volume flow rates, and a very low shear flow region was achieved on the transducer surfaces. This design offers minimal influences of mechanical forces on cellular detection and cells cultured on the surface of transducers. Finite element simulation showed that the shear stress on transducer surface could be maintained lower than milli-Pascal range. Detailed design, simulation results and experimental verifications of the microbioreactor are discussed.

Ferroelectric and piezoelectric thin films and their applications for integrated capacitors, piezoelectric ultrasound transducers and piezoelectric switches

Ferroelectric and piezoelectric thin films are gaining more and more importance for the integration of high performance devices in small modules. High-K 'Integrated Discretes' devices have been developed, which are based on thin film ferroelectric capacitors integrated together with resistors and ESD protection diodes in a small Si-based chip-scale package. Making use of ferroelectric thin films with relative permittivity of 950–1600 and stacking processes of capacitors, extremely high capacitance densities of 20–520 nF/mm2, high breakdown voltages up to 140 V and lifetimes of more than 10 years at operating voltages of 5 V and 85°C are achieved. Thin film high-density capacitors play also an important role as tunable capacitors for applications such as tuneable matching circuits for RF sections of mobile phones. The performance of thin film tuneable capacitors at frequencies between 1 MHz and 1 GHz is investigated. Finally thin film piezoelectric ultrasound transducers, processed in Si– related processes, are attractive for medical imaging, since they enable large bandwidth (>100%), high frequency operation and have the potential to integrate electronics. With these piezoelectric thin film ultrasound transducers real time ultrasound images have been realized. Finally, piezoelectric thin films are used to manufacture galvanic MEMS switches. A model for the quasi-static mechanical behaviour is presented and compared with measurements.

A microfabricated piezoelectric transducer platform for mechanical characterization ofcellular events

During the last decade, it was discovered that the mechanical properties and interactions ofcells and their surrounding extra-cellular matrix play important roles in cellular activities.Substantial efforts have been made to develop various methodologies and tools to study cellmechanics. In this paper, we report an ongoing study on integrating the concept ofa smart structure with a microfabricated thin film piezoelectric transducer forcharacterizing the various changes in mechanical properties associated with cellularevents. A microbridge sensor integrated with a thin film piezoelectric transducerwas created from silicon dioxide and zinc oxide sandwiched between two goldelectrodes. The cells to be tested were cultured on the microbridge surface. Thesurface tractions exerted by the cells on the microbridge directly modulated theselected resonant behaviors, which were detected with the custom designed effectivesurface electrode. Our theory and simulation results showed, for the first time,that the application and changes in these surface tractions resulted in resonantand anti-resonant frequency shifts in the impedance response of the piezoelectrictransducer. Both spatial and temporal information of dynamic cellular activitiescould be inferred from the changes in the impedance spectra. The design, theory,finite-element simulation, microfabrication techniques, and preliminary test results arediscussed.

Electrical control of parametric processes in silicon waveguides

We demonstrate electrical tuning of phase mismatch in silicon wavelength converters. Active control of birefringence induced by a thin-film piezoelectric transducer integrated on top of the waveguides is used for dispersion engineering. The technology provides a solution for compensating the phase mismatch caused by fabrication errors in integrated waveguides. It also offers a mean to dynamically control the relative dispersion between interacting waves and hence, to introduce electronic control of optical parametric processes.

Thin Film Piezoelectric MEMs Devices

Thin film piezoelectric devices, processed in Si‐related processes, are attractive for ultrasound transducers and piezoelectric switches. Thin film ultrasound transducer enable large bandwidth (> 100%), high frequency operation. In piezoelectric micromachined ultrasonic transducers (PMUTs) the ultrasonic waves are generated by flexural motion of the membrane, which is coupled to strain in the piezoelectric film. We have investigated the piezoelectric properties of thin films for ultrasound transducers and piezoelectric switches. Thin film piezoelectric ultrasound transducers as well as piezoelectric switches have been designed, processed and characterized. Thin film piezoelectric devices with excellent quality and reliability have been realized.

Design study of piezoelectric micro-machined mechanically coupled cantilever filters using a combined finite element and microwave circuit analysis

A new mechanical filter structure is presented which comprises two silicon cantilevers mechanically coupled by a silicon linkage with thin film piezoelectric transducers providing electrical input and output signals. The resonance behaviour of such a structure results in a band-pass filter response, having a band-width determined by the frequency separation between the closely spaced in-phase and out-of-phase vibrational modes of the two coupled cantilevers. A detailed configuration design analysis, filter simulation and optimisation of performance is undertaken using a new modelling approach combining microwave circuit theory and finite element analysis to evaluate the generalised ( A, B, C and D) and scattering ( S) circuit parameters of the filter. Two significant features of the filters have emerged from the derived analyses and simulations: (1) with correct design filter Q-values can reach several thousand which is much higher than the Q-values (∼80) of uncoupled cantilevers, (2) the Q-value is determined by the configuration of the silicon linkage and so is under the designer's control. The position and length of the linkage that give optimum Q and minimum insertion loss are determined.