Dual-Mode Solidly Mounted Resonator-Based Sensor for Temperature and Humidity Detection and Discrimination.

Sensors based on solidly mounted resonators (SMRs) exhibit a good set of properties, such as high sensitivity, fast response, low resolution limit and low production cost, which makes them an appealing technology for sensing applications. However, they can suffer from cross-sensitivity issues, as their response can be altered by undesirable ambient factors, such as temperature and humidity variations. In this work we propose a method to discriminate humidity variations from the general frequency response using an SMR specifically manufactured to operate in a dual-mode (displaying two close resonances). The two modes behave similarly towards humidity changes (-1.94 kHZ/(%RH)) for resonance one and -1.62 kHZ/(%RH) for resonance two), whereas their performance under temperature changes is significantly different, displaying 2.64 kHZ/°C for resonance one and 34.21 kHZ/°C for resonance two. This allows for the decoupling process to be carried out in a straightforward manner. Frequency response is tracked under different humidity conditions, in the -20 °C to room temperature region, proving that this behavior is reproducible in any given environment.

Homogeneity and Thermal Stability of Sputtered Al0.7Sc0.3N Thin Films.

This work presents a study on the homogeneity and thermal stability of Al0.7Sc0.3N films sputtered from Al-Sc segmented targets. The films are sputtered on Si substrates to assess their structural properties and on SiO2/Mo-based stacked acoustic mirrors to derive their piezoelectric activity from the frequency response of acoustic resonators. Post-deposition annealing at temperatures up to 700 °C in a vacuum are carried out to test the stability of the Al0.7Sc0.3N films and their suitability to operate at high temperatures. Despite the relatively constant radial composition of the films revealed from RBS measurements, a severe inhomogeneity in the piezoelectric activity is observed across the wafer, with significantly poorer activity in the central zone. RBS combined with NRA analysis shows that the zones of lower piezoelectric activity are likely to show higher surface oxygen adsorption, which is attributed to higher ion bombardment during the deposition process, leading to films with poorer crystalline structures. AFM analysis reveals that the worsening of the material properties in the central area is also accompanied by an increased roughness. XRD analysis also supports this hypothesis, even suggesting the possibility of a ScN non-piezoelectric phase coexisting with the AlScN piezoelectric phase. Thermal treatments do not result in great improvements in terms of piezoelectric activity and crystalline structure.

Direct growth of few-layer graphene on AlN-based resonators for high-sensitivity gravimetric biosensors.

We present the successful growth of few-layer graphene on top of AlN-based solidly mounted resonators (SMR) using a low-temperature chemical vapour deposition (CVD) process assisted by Ni catalysts, and its effective bio-functionalization with antibodies. The SMRs are manufactured on top of fully insulating AlN/SiO2 acoustic mirrors able to withstand the temperatures reached during the CVD growth of graphene (up to 650 °C). The active AlN films, purposely grown with the c-axis tilted, effectively excite shear modes displaying excellent in-liquid performance, with electromechanical coupling and quality factors of around 3% and 150, respectively, which barely vary after graphene integration. Raman spectra reveal that the as-grown graphene is composed of less than five weakly coupled layers with a low density of defects. Two functionalization protocols of the graphene are proposed. The first one, based on a covalent binding approach, starts with a low-damage O2 plasma treatment that introduces a controlled density of defects in graphene, including carboxylic groups. After that, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) chemistry is used to covalently bind streptavidin molecules to the surface of the sensors. The second functionalization protocol is based on the non-covalent bonding of streptavidin on hydrophobic graphene surfaces. The two protocols end with the effective bonding of biotinylated anti-IgG antibodies to the streptavidin, which leaves the surface of the devices ready for possible IgG detection.

Tungsten Oxide Layers of High Acoustic Impedance for Fully Insulating Acoustic Reflectors.

Gravimetric sensors based on solidly mounted resonators require fully insulating acoustic reflectors to avoid parasitics when operating in liquid media. In this work, we propose a new high-acoustic impedance material, tungsten oxide ([Formula: see text]), for acoustic reflectors. We have optimized the sputtering conditions of [Formula: see text] to obtain nonconductive layers with mass density around [Formula: see text] and acoustic velocities for the shear and the longitudinal modes up to 2700 and 4500 m/s, respectively. Compared to other conventionally used high impedance layers, [Formula: see text] films display several manufacture advantages, such as high deposition rates, great reproducibility, and good adhesion to underlying substrates. We have demonstrated the applicability of [Formula: see text] in practical shear mode bulk acoustic wave resonators that display good performance in liquid environments.

High-acoustic-impedance tantalum oxide layers for insulating acoustic reflectors.

This work describes the assessment of the acoustic properties of sputtered tantalum oxide films intended for use as high-impedance films of acoustic reflectors for solidly mounted resonators operating in the gigahertz frequency range. The films are grown by sputtering a metallic tantalum target under different oxygen and argon gas mixtures, total pressures, pulsed dc powers, and substrate biases. The structural properties of the films are assessed through infrared absorption spectroscopy and X-ray diffraction measurements. Their acoustic impedance is assessed by deriving the mass density from X-ray reflectometry measurements and the acoustic velocity from picosecond acoustic spectroscopy and the analysis of the frequency response of the test resonators.

Influence of crystal quality on the excitation and propagation of surface and bulk acoustic waves in polycrystalline AlN films.

We investigate the excitation and propagation of acoustic waves in polycrystalline aluminum nitride films along the directions parallel and normal to the c-axis. Longitudinal and transverse propagations are assessed through the frequency response of surface acoustic wave and bulk acoustic wave devices fabricated on films of different crystal qualities. The crystalline properties significantly affect the electromechanical coupling factors and acoustic properties of the piezoelectric layers. The presence of misoriented grains produces an overall decrease of the piezoelectric activity, degrading more severely the excitation and propagation of waves traveling transversally to the c-axis. It is suggested that the presence of such crystalline defects in c-axis-oriented films reduces the mechanical coherence between grains and hinders the transverse deformation of the film when the electric field is applied parallel to the surface.

DCS Tx filters using AlN resonators with iridium electrodes.

In this paper we present the design, fabrication technology, and characterization of BAW filters for the Digital Cellular System (DCS) Tx-band at 1.75 GHz. The filters are fabricated with AlN-based solidly mounted resonators (SMR) using iridium electrodes, in an attempt to increase the effective electromechanical coupling factor of the BAW devices and achieve the bandwidth requirements of DCS filters. The design and optimization of the filters is performed with a simulation tool that uses a circuit model to compute the filter frequency response. Tx filters with balanced inputs and outputs and different topologies are designed and fabricated. The experimental filter response is compared with the simulations to determine the suitability of each design. DCS bandwidth requirements are fulfilled by using Ir/AlN/Ir stacks.

Sputtered SiO2 as low acoustic impedance material for Bragg mirror fabrication in BAW resonators.

In this paper we describe the procedure to sputter low acoustic impedance SiO(2) films to be used as a low acoustic impedance layer in Bragg mirrors for BAW resonators. The composition and structure of the material are assessed through infrared absorption spectroscopy. The acoustic properties of the films (mass density and sound velocity) are assessed through X-ray reflectometry and picosecond acoustic spectroscopy. A second measurement of the sound velocity is achieved through the analysis of the longitudinal lambda/2 resonance that appears in these silicon oxide films when used as uppermost layer of an acoustic reflector placed under an AlN-based resonator.

Circuital model for the analysis of the piezoelectric response of A1N films using SAW filters.

In this paper we describe a method to assess the piezoelectric response of a piezoelectric thin film deposited on a conductive substrate. It is based on analyzing the frequency response of a surface acoustic wave (SAW) filter made on the piezoelectric thin film. For this analysis, we use a circuital model that takes into account the theoretical response of the ideal filter along with all the external and internal parasitic effects that deteriorate the response. Using this model, we can obtain the electromechanical coupling factor of the piezoelectric material (k2m) with good accuracy. If parasitic effects are not considered, k2m can be underestimated by a factor of up to 20. We have tested our model using SAW filters made on A1N thin films sputtered on substrates with different conductivities. A discussion on the relation between the different circuital elements and the physical properties of the filters also is provided.