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tElectro-acoustic sensors proving an adequate operation in liquid media are appropriate candidates forbiosensing or liquid monitoring applications. In this context, electro-acoustic devices based on Lambwaves have been widely used for such purpose during the last years. More particularly, S0mode Lambwave resonators (S0-LWRs) have shown promising in-liquid performance. However, a theoretical back-ground describing their in-liquid sensing mechanisms has only been published recently. In this workwe present the experimental verification of the previously developed theoretical background based on afinite element model. We discuss about similarities and discrepancies between model and experiments,stating a final model correctly describing the in-liquid sensitivity features of S0-LWRs. These devicesshow appreciable different sensitivities of the resonant frequency to liquid viscosity and density, beingmore sensitive to the latter. Additionally, when they are not electrically isolated, the influence of theliquid electrical properties is superimposed to the mechanical ones and can be correctly extracted.

Sensors based on thin film electroacoustic (TEA) devices have emerged as a promising alternative to quartz crystal microbalance and surface acoustic wave devices, in view of sensibility, miniaturization and easy integration. TEA devices include quasi-shear film bulk acoustic resonators (QS-FBAR) and S-0 mode Lamb wave resonators (S-0-LWR) based on AlN films. Despite the work done on the application of TEA devices as in-liquid biological and chemical sensors, a theoretical framework for S-0-LWRs properly describing their sensing mechanisms is still needed. Here we validate a finite element analysis model on QS-FBARs and study the sensing mechanisms of S-0-LWRs in liquid media. We show that S-0-LWRs can sense changes in the dielectric permittivity of the liquid and demonstrate different sensitivities to viscosity and density. A complementary assessment of the S-0-LWRs losses, dependent in a non-specific manner on the square root of the density viscosity product, provides the ability to discriminate density from viscosity changes on the entire device surface. Finally, with an S-0-LWR optimization study we show that resolution improves with the decrease of the membrane thickness; however, a trade-off between sensitivity, quality factor and membrane fragility has to be considered.

In a narrowband 2-port resonator on 37 degrees-cut quartz with Al electrodes (h(Alu) = 100 nm) exploiting surface transverse waves (STW), we have observed parasitic ripples which have been attributed to transverse modes unusually situated on the left side of the main resonance. To suppress these modes, we have used metallization coefficient weighting across the aperture, with more metal in the middle and reduced metallization close to the busbars. The parasitic modes indeed disappeared, but at significantly higher frequency, we have found an additional strong response which does not exist in a resonator with uniform electrode metallization. 3-D simulations showed that the structure has another very different mode, with the wave propagating mainly along the edge of the busbars, but excited with the interdigital electrode system.

Sensors based on thin film electroacoustic (TEA) devices have emerged as a promising alternative to quartz crystal microbalance and surface acoustic wave devices, in view of sensibility, miniaturization and easy integration. TEA devices include quasi-shear film bulk acoustic resonators (QS-FBAR) and S0 mode Lamb wave resonators (S0-LWR) based on AlN films. Despite the work done on the application of TEA devices as in-liquid biological and chemical sensors, a theoretical framework for S0-LWRs properly describing their sensing mechanisms is still needed. Here we validate a finite element analysis model on QS-FBARs and study the sensing mechanisms of S0-LWRs in liquid media. We show that S0-LWRs can sense changes in the dielectric permittivity of the liquid and demonstrate different sensitivities to viscosity and density. A complementary assessment of the S0-LWRs losses, dependent in a non-specific manner on the square root of the density viscosity product, provides the ability to discriminate density from viscosity changes on the entire device surface. Finally, with an S0-LWR optimization study we show that resolution improves with the decrease of the membrane thickness; however, a trade-off between sensitivity, quality factor and membrane fragility has to be considered.

A concept is demonstrated for the development of a phononic surface acoustic wave interdigital transducer (IDT) able to excite acoustic waves in a transverse manner with respect to the applied electric field. The basic principles of the design and fabrication of the transversely coupled phononic transducer are presented. More specifically, the IDT consists of a regular interdigital electrode configuration on which a 2D array of low profile masses is superimposed. The transversely coupled mode of excitation is explained in terms of the specific localization of the elastic energy along with the operation within the complete frequency bandgap of the composite transducer grating. The experimental results provide solid evidence for the suggested mode of operation.

We present a novel low-cost and low-power MEMS gas sensor concept based on an ultrasonic resonance cavity. The sensor consists of a capacitive micromachined ultrasonic transducer (CMUT) embedded to an acoustic resonance cavity. The sensor operation was demonstrated with carbon dioxide CO2 and methane CH4, the lowest resolvable concentrations are about 10 - 20 ppm - a competitive result with the existing commercially available CO2 sensors. In addition, the sensor is able to measure gas concentration and humidity independently, and thus can be used as a combo sensor for gas concentrations and humidity.

Thin film S0 mode Lamb wave resonators with reduced spurious transverse-mode responses are designed, fabricated and evaluated. The proposed solution consists of a higher velocity busbar topology which improves the waveguiding under the active part of the resonator. To suppress the spurious responses a narrow matching layer is introduced between the busbar and the active part of the resonator. 3D finite element analysis was initially used to verify the approach. Both eigen-frequency and frequency response analyses were used to reveal the physics of the waveguiding and the energy confinement effects in the proposed topology. Finally, measurements of S0 Lamb wave resonators have confirmed the efficient suppression of the spurious transverse modes. The proposed technique can successfully be employed also in S0 Lamb wave resonators with suspended edges (referred by some authors also as contour-mode resonators), while eliminating the need of tethers thus improving the heat dissipation.

2D composite grating building blocks with hexagonal symmetry are proposed and analyzed as efficient reflectors and transducers in the design of low loss surface acoustic wave (SAW) resonators. Eigen-frequency and frequency response finite element analyses (FEA) are used to study the propagation and excitation characteristics of SAWs in the 2D composite structures. The proposed structures have the advantage of being compatible with the planar SAW technology, while exhibiting a complete frequency bandgap. Three distinct types SAW phononic transducers are here invented and their operation confirmed both analytically and experimentally. The proposed research paves the way for subsequent implementation of the phononic bandgap technology in high performance microacoustic RF components.

Monitoring under harsh environments, particularly high temperatures (> 600 degrees C), is on high demand nowadays. Applications such as gas control in propulsion systems or fire detection in early stages are good examples. During the last decades, materials and devices have been extensively investigated for these applications. Few have proven thermal and chemical stability. Among the most used devices are surface acoustic wave (SAW) resonators using a langasite (LGS) substrate. Their main disadvantage is related to their transducer topology. Their long and narrow electrode strips are subjected to destructive agglomeration. In order to solve this problem, solidly mounted bulk resonators (SMR) are here proposed as an alternative. They provide rigidity, high performance and large electrodes. Here we investigate the performance of SMR devices after annealing under vacuum condition at 700 degrees C for a cumulative time of 24h. SMRs are composed of porous-SiO2/Mo Bragg mirrors and Ir or Mo electrodes. Their performance shows and initial overall improvement with subsequent stabilization. Q(p)xk(eff)(2) in the order of 72 are achieved. Further investigations on full dielectric Bragg mirrors will be performed. With this initial study we demonstrate that SMRs can be a good alternative to SAWs for high temperature applications.

Resonant surface acoustic wave (SAW) structures employing 2D phononic gratings with hexagonal symmetry are micro-fabricated and tested in a proof of principle study. The proposed structures have the advantage of being compatible with the planar SAW technology, while ensuring operation within the complete frequency bandgap of the phononic grating. More specifically, 2D phononic gratings exhibiting a complete frequency bandgap are employed in the design of three distinct types of resonant test structures. The proposed test structures are fabricated on to a 128 degrees YX LiNbO3 substrate by means of low resolution photolithography. Frequency response measurements of the SAW phononic resonators confirmed operation within the complete frequency bandgap of each grating structure. The experimental results obtained demonstrate that surface phononic gratings can be successfully used in the design of SAW resonators. The proposed research paves the way for subsequent implementation of the phononic bandgap technology in high performance micro-acoustic RF components.

A microwave bandpassfilter with a large ratio between the output andthe input impedance has been designed and fabricated. Consequently,it functions both as a voltage transformer and a bandpassfilter, or trans-filter for brevity. It represents a two-port micro-acoustic resonatoremploying Lamb waves in a thin piezoelectric AlNfilm grown ontoa Si carrier substrate with a centre frequency of around 887 MHz.The transfilter has a transformer ratio of 10 and a voltage efficiencyof over 80%. The component has a small size ( < 0.5 mm2) and isshown to sustain power levels of 250 mW. It can be used in avariety of cases where both voltage amplification and frequencyfilter-ing are required. Examples include: charge pumps in RFID tags,energy scavenging, remotely triggered switches, wake-up radios inwireless networks, stand-by units in home electronics etc., WiSENET and VR granted "Thin Film Guided Microacoustic Waves in Periodical Systems: Theory and Applications "

2D composite grating building blocks with hexagonal symmetry are proposed and analyzed asefficient reflectors and transducers in the design of low loss surface acoustic wave (SAW) resonators. Eigen-frequency and frequency response finite element analyses are used to study the propagation and excitation characteristics of SAWs in the 2D composite structures. The proposed structures have the advantage of being compatible with the planar SAW technology, while introducing design principles reminiscent to the semiconductor devices. More specifically, thefrequency bandgap characteristics of the various resonator building blocks can be adjusted so that losses related to the SAW diffraction, impedance mismatch, and parasitic SAW radiation from theelectrical contacts can be suppressed. Three distinct types SAW resonant topologies employing 2D gratings are deduced from the analytical results., VR granted "Thin Film Guided Microacoustic Waves in Periodical Systems: Theory and Applications"

This work makes an overview of the progress made during the last decade with regard to a novel class of piezoelectric microwave devices employing acoustic Lamb waves in micromachined thin film membranes. This class of devices is referred to as either thin film Lamb wave resonators or piezoelectric contour-mode resonators both employing thin film aluminum nitride membranes. These devices are of interest for applications in both frequency control and sensing. High quality factor Lamb wave resonators exhibiting low noise, low loss and thermally stable performance are demonstrated and their application in high resolution gravimetric and pressure sensors further discussed. A specific emphasis is put on the ability of these devices to operate in contact with liquids. Future research directions are further outlined., TOPICAL REVIEW, VR granted "Thin Film Guided Microacoustic Waves in Periodical Systems: Theory and Applications "

The characteristics of one-port aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S0) mode with electrically open, grounded, and floating bottom electrode configurations are theoretically and experimentally investigated. The finite element analysis (FEA) is performed to take an insight into the static capacitance characteristics of the AlN Lamb wave resonators with various bottom surface conditions. The theoretical results predict that the floating bottom electrode efficiently reduces the static capacitance in the AlN thin plate and then promotes an efficient improvement in the effective coupling coefficient. Experimentally the AlN Lamb wave resonator without a bottom electrode exhibits a loaded quality factor (Q) as high as 3033 at its series resonance frequency, 948.1 MHz, but a low effective coupling coefficient of 0.18%. On the contrary, the Lamb wave resonator with an electrically floating bottom electrode shows an effective coupling coefficient up to 1.05% but a low loaded Q of 850 at its series resonance frequency, 850.3 MHz. In contrast to the floating bottom electrode, the Lamb wave resonator with an electrically grounded bottom electrode shows a smaller effective coupling coefficient of 0.78% and a similar loaded Q of 800 at the series resonance frequency, 850.5 MHz., VR funded " "Thin Film Guided Microacoustic Waves in Periodical Systems: Theory and Applications ""

Different « layered structures » were proposed in the past combining the advantageous features of substrates (e.g. low SAW attenuation in sapphire) and high quality of piezoelectric (e.g. AlN) films for the development of acoustic wave guides. Unfortunately, the achievable electromechanical coupling constant K2 is in most cases very small. Meanwhile in FBARs AlN films excited in thickness mode demonstrated K2 > 6%. We propose here a transducer structure comprising: 1) a substrate suitable for propagation of surface acoustic waves in the X-direction with low loss, 2) a metal layer deposited on said substrate and 3) micro-acoustic transducers (resonators) created in the form of piezoelectric (AlN, ZnO, etc.) bars or ridges with cross-section dimension in X-direction smaller than a half wavelength of any acoustic wave in the substrate material at operation frequencies and having top conducting electrodes. The thickness of the piezoelectric layer is close to λp /4 or λp /2 of the bulk wave excited in said transducers/resonators. Said transducers being placed periodically and separated (centre-to-centre) by a distance in the X-direction smaller than 1/2 wavelength of any acoustic wave in the substrate material are connected to the balanced source of voltage +/- V/2. We have simulated both cases. The principal results are that the resonator grating always provides larger coupling factors than the solid layer one, and that a mode can occur exhibiting a coupling of at least 4.2% depending on the layer thickness. In this mode the resonances in narrow FBARs are coupled through the substrate. Actually a new type of FBAR is described, wherein the isolation from the substrate is provided by destructive interference of the radiated bulk waves because of the resonators periodicity < λBulk /2. The coupling can be easily adjusted. The mode we see may be treated as not a SAW, but as a vibration in a system of coupled FBARs

The Surface Transverse Waves (STW) cut of langasite (LGS), Euler angles (0°, 22°, 90°), provides temperature stable delay of STW with coupling coefficient a few times larger than that of 36° rotated Y-cut of quartz which also supports STW. It was expected that, as in the case of quartz, one can get excellent Q-factor resonators based on low loss shear surface acoustic waves (STW). Such resonators are in great demand for high temperature sensors and for this application Platinum (Pt) electrodes are necessary. However, recent attempts to develop such resonators resulted in mediocre Q factors of the order of 1000 rather than the expected Q>10000 in the 434 MHz ISM frequency range. In this paper we show that for a thickness of Pt (h/λ close to 2%) on this cut of LGS there is no wave-guiding, which means that the STW energy is radiated into the busbars. This effect can be explained by the concave shape of the slowness curve for SAW in the bus-bar area due to a large increase in the wave velocity with deviation of the propagation direction from 0Z axis. The use of thicker Pt layers, (yielding a relative thickness h/λ in excess of 3%) makes the phase velocity of STW in the periodic grating at resonance drop below the wave velocity of all types of waves propagating under the bus-bars due to the high reflectivity of the electrodes, establishing the usual wave-guiding conditions., VR granted "Thin Film Guided Microacoustic Waves in Periodical Systems: Theory and Applications"

Aluminum nitride thin film bulk wave resonant pressure sensors employing c- and tilted c-axis texture, have been fabricated and tested for their pressure sensitivities. The c-axis tilted FBAR pressure sensors demonstrate substantially higher pressure sensitivity compared to its c-axis oriented counterpart. More specifically the thickness plate quasi-shear resonance has demonstrated the highest pressure sensitivity while further being able to preserve its performance in liquid environment.