Thin-Film Thickness-Mode Sensors
What is a Thin-Film Thickness-Mode Sensors?
These are BAW sensors based on thickness-mode waves that, as opposed to TSM quartz crystals, are of the longitudinal type, at least in the early implementations of the concept. They are made by electroded piezoelectric thin films and are therefore also termed film bulk acoustic resonator (FBAR) sensors. Films of piezoelectric materials, such as AlN or ZnO, are created in the form of diaphragms photolithographically defined and etched starting from a silicon substrate. In this way, a very low thickness can be obtained that causes a high resonant frequency, up to 1000 MHz and above. This, in turn, determines a high mass sensitivity in gravimetric applications.
As opposed to free-standing, or suspended, homogeneous resonators, composite resonators can also be used where the piezoelectric film is deposed on a nonpiezoelectric substrate, such as silicon, with intermediate layers with different acoustic impedances. Composite film resonators can display improved thermal stability due to the property matching that can be obtained among different layers. A significant case is when the layers have alternate high and low acoustic impedances, thereby forming a Bragg reflector which acts as an acoustic mirror that isolates the film from the substrate.
This configuration is often termed as solidly-mounted resonator (SMR). The structures of suspended and SMR FBARs. The SMR solution has the effect to decrease the effective thickness and is especially interesting for sensing applications, because it avoids the need of etching away the silicon to form the thin suspended diaphragm. This advantageously mitigates the problem of fragility.
Composite resonators can also be made by resonant piezo-layers (RPL) of lead-zirconate-titanate (PZT) films screen printed on alumina substrate. RPL sensors display a mass sensitivity comparable or slightly higher than TSM quartz sensors at the same frequency, though the thermal stability is worse. Most likely due to their porosity, thick-film RPL sensors with chemically functionalized surface apparently offer an improved sensitivity as sorption sensors in air.
Thickness-longitudinal-mode sensors have many analogies with TSM quartz sensors. One important difference is that, in the former ones, the vibrations normal to the sensor surface irradiate energy into a surrounding liquid, which makes thin-film thickness longitudinal mode sensors generally unsuitable for (bio)chemical applications in solutions.
For this reason, efforts have been aimed to the development of shearmode FBAR sensors. Recently reported devices have a configuration similar to thickness-longitudinal-mode sensors with the difference that they exploit the oriented growth of ZnO piezoelectric films to generate thickness-shear-mode vibrations. As an alternative, shear-wave generation using lateral field excitation has also been reported. Shear-mode FBARs are expected to have a high potential especially for highly integrated biochemical sensor arrays, though the very high operating frequencies (in the range 1-10 GHz) can pose significant challenges to the readout electronic circuits and instrumentation.