Application as a liquid sensor can be achieved with a sensing film or by direct contact of the liquid onto the surface of the BAW device.
For the sensing film case, attachment of the chemical or biological stimulus results in mechanical perturbations causing a corresponding change in the resonant frequency.
- Mass loading as a concentration of chemical adsorbs onto the surface of the sensing film or when a bio-molecule attaches to some selective surface chemistry. Mass loading results in a decrease in resonant frequency.
- Visco-elastic changes of the sensing film as a concentration of chemical diffuses into the bulk of the sensing film. Elastic stiffening of a sensing film will result in an increase in the resonant frequency, while elastic softening or swelling of the sensing film will result in a decrease in the resonant frequency.
- Viscous drag caused by the attachment of large biological molecules on the surface of the biologically specific sensing film.
For the direct contacting case, the mechanical properties of the fluid causes perturbations on the surface of the BAW device resulting in resonant frequency changes.
Viscous loading of the BAW: The density and viscosity of the fluid will strongly affect the BAW equivalent circuit parameters and resonant frequency.
When a thickness shear mode (TSM) resonator is placed in contact with a liquid, the resonant frequency and series resistance is dependent on the density and viscosity of the contacting liquid.
The illustration above shows the cross-sectional displacement profile for a TSM AT-cut resonator contacted by a viscous liquid. As shown, the oscillating surface generates plane-parallel laminar flow in the contacting liquid. The “viscously-entrained” liquid undergoes a phase lag that increases from the distance from the surface of the TSM device.
Plotting the series resonant frequency as a function of the percent weight of glycerol generates the plot above.
And finally, plotting the shift in frequency as a function of the square root of the density-viscosity product for the various concentrations of glycerol creates the plot shown above. You can see that the shift in frequency is directly proportional to the square root of the density-viscosity product.
Electrical Properties: Note that for a sensing film placed on top of a metal electrode, the TSM device will not detect any electrical properties because the metal film will short out the electric field that is coupled to the propagating acoustic wave.
A TSM device can be used to measure the conductivity of non-viscous solutions. For a TSM device with equivalent electrodes (shown right), the interaction with the conductive solution is not electro-acoustic in nature, but is mainly a function of the fringing electric fields.
Because the top electrode is metal, it will short out any electric field that is coupled to the propagating bulk acoustic wave. For this reason, the acoustic wave device does not interact with the solution via the propagating BAW. Instead, the fringing fields interacting with the conductive solution directly influences the C0 of the resonator causing the anti-resonant frequency (parallel frequency) of the device to be dependent on conductivity of the solution.
For a modified electrode structure(lower right), the mode shape plot (shown right) illustrates the strength of the acoustic mode as a function of position across the surface of the resonator. As can be seen, the acoustic activity in the center of the resonator is not very strong, but is still somewhat present. The acoustic wave in the center (no metal) does have an electric field that is coupled to the propagating acoustic wave that can interact with a conductive solution or a sensing film placed on the surface. Additionally, the liquid or sensing film in the center non-electrode region will behave as a “lossy electrode” coupling more acoustic energy into the TSM device as a function of conductivity of the solution or sensing film on its surface. So as the conductivity increases, the series resonant frequency, series resistance, and overall Q of the resonator will dramatically change and can be easily measured using a standard oscillator circuit.