BAW Technology

It uses a piezoelectric sensing element (crystal) which is excited by a high-frequency oscillator and operates in the thickness shear mode (TSM) of vibration. In this mode, shear displacement occurs on the crystal faces in the plane of the crystal plate. The figure right illustrates that the displacement profile occurs throughout the thickness of the sensing element and is at its maximum at the surfaces

Thus, the acoustic wave resonator supports a standing wave through its thickness. The wave pattern interacts with electrodes on the lower surface (hermetically sealed from the liquid) and with the fluid on the upper surface (top electrode). When placing a TSM BAW device into a liquid, a layer of fluid couples to the cyclical shear displacement of the vibrating surface. This displacement is atomic scale and at a frequency of 5.25 million cycles per second. Increases in viscosity and loading increase the damping of the TSM BAW and therefore, reduces the BAW sensor's frequency. The bulk of the liquid is unaff ected by the acoustic signal as only a thin layer (in the order of microns) is moved by the vibrating surface. The penetration depth into the fluid is ideal for measuring the viscosity of homogeneous fluids like lubricants and inks. The viscosity measurement will not be susceptible to large particles or debris because the small penetration depth makes them virtually unnoticeable. The viscosity measurement is achieved by correlating the measured BAW electrical parameters to the acoustic viscosity (AV) of the fluid. The general relationship between acoustic viscosity and kinematic viscosity (measured in centi Stokes (cSt)) is: Acoustic Viscosity (AV) = Kinematic Viscosity x Density² (cSt x (g/cm³)²) However, all viscosity measurements are shear rate and material dependent. Variations in material properties and homogeneity can influence the acoustic viscosity measurement. Therefore, using above mentioned formula in isolation may result in unsatisfying results.

SAW Technology: Rayleigh Surface Acoustic Wave Delay Line

Starting with the Rayleigh surface acoustic wave (SAW) delay line, we can see that propagating wave is confined to the top surface of the substrate. For a particle on the surface of the substrate, the propagation of the Rayleigh wave will cause the particle to experience a vertically aligned elliptical motion. Because of this, the SAW is a very sensitive probe for measuring mechanical and electrical properties on its surface. We also note that since there is a vertically polarized displacement, the Rayleigh SAW can only be used for gas sensing or physical sensing applications. Putting the SAW in an aqueous environment will result in the SAW being completely damped out due to energy loss into the liquid.

The Rayleigh SAW is sensitive to mechanical and electrical properties occurring on its surface. For mechanical properties, they are sensitive to mass loading and visco–elastic changes like stiffening and softening. For electrical properties, the devices can be sensitive to any property that interacts with the electrical field that is coupled to the propagating acoustic wave. This effect has been given the term electro–acoustic interactions. The Rayleigh SAW is also sensitive to stress or strain coupled into the SAW substrate through the packaging. Because of this, Rayleigh SAW devices make great platforms for torque and pressure sensing applications. Rayleigh SAW devices can also be tailored with special cuts of piezoelectric substrate to create a very linear SAW frequency versus temperature dependence. The result is a very high resolution temperature sensor.