Acoustic wave technologies and devices have been in commercial use for telecommunications, pressure sensing for automotive, non-destructive testing and healthcare for more than 60 years; their application also extends to industry and academia, where dozens of physical, chemical, and biological sensors can be found. Most acoustic wave devices can be used as sensors because they are sensitive to mechanical, chemical, or electrical perturbations on their surfaces, being able to monitor not only mass/density changes, but also changes in elastic modulus, viscosity, dielectric and conductivity properties. Examples of sensors based on acoustic waves include: temperature sensors, moisture, strain, pressure, shock, acceleration, flow, viscosity, ionic contaminants, pH levels, electric, magnetic and radiation fields, gas and explosives. _x000D_
The Quartz Crystal Microbalance (QCM), a resonator made of a piezoelectric material sandwiched between two metallic electrodes, embodies the typical operating mechanism of most acoustic wave devices: By applying an alternate voltage to its electrodes, the structure experiences deformations at a determined resonant frequency, which may change depending on disturbances on its surface such as addition or removal of mass or changes on their environment such as temperature drift. Despite the extensive use of QCM in various disciplines of science and technology in past decades, current thin film acoustic wave devices fabricated with MEMS technology are smaller, thinner, can reach higher frequencies for improved resolution and can be monolithically integrated with CMOS circuitry, thus offering low-cost and mass production of acoustic wave microsensors ready to be integrated in Lab-on-a-Chip systems. The Thin Film Bulk Acoustic Resonators (FBAR) are the latest development thin film acoustic wave devices; they have a structure similar to QCM devices but with the advantage of being several orders of magnitude smaller and capable of work in the GHz range, which provides them with superior sensitivity. Unfortunately, membrane-type FBARs have low yield at fabrication due to residual stress and when operating as sensors they tend to be brittle when submerged in liquid environments, whereas FBARs using Bragg acoustic mirrors require multiple precisely controlled deposition steps for the acoustic mirrors, raising complexity of the device and causing its performance to be dependent on manufacturing defects. The output signal of most acoustic wave sensors is frequency shift of their resonance modes, which are correlated with quantitative measurement of physical changes on the detection surface or chemical or biological molecules bound to the chemically modified surface in the case of biosensors. _x000D_
The latter represents one of the main drawbacks of current acoustic wave devices; most of them require elaborate mechanisms for signal control and relatively complex read-out circuitry capable of detecting frequency shift in the impedance curve caused by changes of the acoustic load coupled to the resonator. To overcome that problem, this work describes the design, simulation, characterization and tests of a BAW sensor operating in longitudinal mode capable of studying the compressional properties of fluids by interrogating the liquid samples with short pulses of ultrasound. This would allow applications such as identification and classification different liquids at microscale or studying concentration changes of mixtures simply by measuring difference in output voltage level._x000D_
This contribution overcomes current restrictions of film acoustic resonators such as fragility of operation in liquid environments, high manufacturing cost or limitations regarding narrow microchannels; offering an alternative to applications that demand ultra-low consumption, miniaturization, versatility and ease of readout.