3 - Structures

Bulk acoustic wave (BAW, otherwise also called thickness-shear mode, or TSM) sensors have been applied to a wide variety of mass, chemical, and biochemical measurement applications. For BAW sensors, small quartz crystal disks of 10-15mm diameter and 0.1-0.2mm thickness are used. The resonance frequencies are between 6 and 20 MHz. The historic development of BAW quartz sensors originates from the use of quartz resonators as time bases for frequency control. The high reliability and stability of quartz oscillators rely on the stability of a mechanical resonance in a structure composed of single-crystal quartz. It has been well known that the deposition of a small mass of material on the surface of a quartz microbalance (QMB) lowers its resonance frequency. QMB sensors are commonly used for thin-film thickness monitoring in vacuum evaporation equipment. Detection limit is 0.1ng/mm².

In surface acoustic wave (SAW) devices, the acoustic wave energy is constrained to the surface; the wave propagates along the surface of a solid medium. Waves can be generated quite easily in piezoelectric substrates using an interdigital transducer (IDT) electrode. The first interdigital transducer excites an SAW whose frequency is mainly determined by the elastic constants of the piezoelectric material and the geometrical sizes of the generator IDT. The receiver IDT will receive this wave after traveling along the surface of the substrate. The propagation path is the sensitive area. All changes in the boundary conditions for SAW propagation lead to a variation of the SAW received by the second IDT. The SAW resonator is a delay line with an amplified feedback. Standing waves are formed by reflection from gratings. Any change in the environment leads to a change in the resonance frequency. Detection limit is 0.05 pg/mm².

In Lamb-wave (Flextural Plate Wave) device, waves propagate in the bulk of plates whose thickness is small compared with the ultrasonic wavelength: the thickness/wavelength ratio is less than one. Particle motions of Lamb waves are similar to those of Rayleigh waves; however, in a thin plate, the waves give rise to a series of symmetric and antisymmetric plate modes. The lowest order antisymmetric modes have a unique flexural character, hence, the name "flexural plate wave," or FPW. The core of the device is an ultrasonic delay line consisting of a composite plate of a low stress insulation, metallization, and piezo-electric film. IDTs on the piezoelectric layer launch and receive the waves and, together with the amplifier, form a feedback oscillator whose output frequency depends on the mass per unit area of the membrane, including the chemically sensitive film. The advantages are in the low-MHz operation frequency range, and the possibility of operation while immersed in a liquid.

Shear horizontal acoustic plate mode (SH-APM) sensor. Typical plates are a few acoustic wavelengths thick. The waves are generated and detected by IDTs on the lower surface and reflect between the upper and lower surface of the substrate. This beam interacts with the receptor polymer film and viscously coupled fluid sample when it reflects from the upper surface. Changes in the film due to mass loading or viscoelastic stiffening will alter the phase of the reflected wave and results in a phase shift in the electrical signal. Similar to SAW sensors, a dual delay line configu-ration can be used to compensate nonspecific responses and undesired environmental effects. The advantage of the APM sensor is that electrical connections can be made on the surface of the device, which is not immersed in solution. Typical APM device frequencies are 25-200 MHz.

Love wave sensor. Mass loading of this so called surface skimming bulk wave (SSBW) device results in a composite acoustic device, known as Love plate, with an SH wave guided at the interface of the deposited polymer over-layer and the SSBW substrate. SSBWs are acoustic waves with only a shear displacement component propagating just below the surface of a piezoelectric substrate. SSBW suffers from a considerable acoustic loss. A thin overlayer of a dielectric material on the surface may convert the SSBW into a guided SH or Love wave, increasing the coupling coefficient of the wave and reducing the losses. Generally, devices using thin films to help guide the waves are called Love wave devices. The substrate is generally piezoelectric quartz crystal; the overlayer is silica or polymethyl-methacrylate, which exhibit much lower shear acoustic velocities than quartz, and so fulfill the necessary requirement for the guidance of a Love wave.