Excitation Principles of BAW Sensors
Other Excitation Principles of BAW Sensors
The most known quartz crystal microbalance may reveal some limitations when applied as chemical or biochemical sensor. Sensitivity to the mass of molecular species is a very unique advantage of acoustic sensors. However, acoustic sensors are inherently nonspecific. The core of chemical analysis involving surfaces is therefore a method for immobilization of the target molecule on the surface of the transducer, hence mainly a question of surface chemistry and application to complex (bio)molecular systems. From that point of view, the necessity of metal electrodes at the surface interacting with the medium to be investigated is a limitation of applicable surface chemistry. In addition, a simple replacement method for the sensor element, which does not require a skilled operator, is an issue of practical interest. Electrical connection to electrodes on the sensor element can therefore become a critical design factor.
Two other principles can overcome these limitations, lateral field excitation (LFE) and direct magnetic generation. The classical LFE design is characterized by two electrodes covering completely the left and the right side of a quartz disc just leaving a small straight gap between them. A lateral electrical field is confined in the gap and excites acoustic vibration, thenceforth the name. Magnetic excitation has been utilized for nondestructive material testing, for example in automotive industry. In a static magnetic field acoustic waves are generated and detected in the material by radio frequency (RF) coils placed next to the test sample. The device has therefore been called electromagnetic acoustic transducer (EMAT).
Just recently both principles have been modified for microacoustic resonator sensors. LFE sensors utilize the same piezoelectric crystal that is used in QCM, namely AT-cut quartz. The electrodes are located only in the bottom surface leaving the top sensing surface blank.
The bare surface gives now access to the large variety of silicon based surface chemistry. On the other hand one loses the shielding effect of the top electrode. The aspect ratio between electrode gap distance and crystal thickness is about 3-6. The electric field is not completely confined between the electrodes. Consequently, the electric field penetrates partly into the medium adjacent to the sensing surface of the crystal. This feature can provide access to additional relevant physical material properties of the material under investigation, namely the electrical parameters permittivity and conductivity. The sensor response to electrical properties can be much larger than that to density-viscosity.
Lateral Field Excited (LFE) Sensor
For the understanding of the extraordinary sensor response to electrical properties of a liquid analyte, one must consider the change in the (electrical) boundary conditions at the sensing surface. As a result of liquid application the electrical field distribution changes depending on conductivity and permittivity of the liquid and experimental conditions (grounding). As long as the sensor faces a medium which features a relative permittivity, ε, lower than that of quartz the electrical field is distributed mainly in lateral direction. For a medium featuring a dielectric permittivity higher than that of quartz the internal lateral electric field component decreases in strength and components of the traditional thickness field excitation (TFE) will be amplified. As a consequence, the wave propagation properties of the acoustic wave change, hereby modifying the resonance frequency of the sensor. In other words, the sensitivity to electrical properties of the adjacent liquid does not directly appear in the sensor response, they become effective via changes in the acoustic wave generation scheme and acoustic properties of the crystal. Distinction of the contributions to the sensor response from liquid density and viscosity on the one hand and permittivity and conductivity on the other requires advanced analysis.
By combining magnetic direct generation with an acoustic resonator it is possible to excite a mechanical resonance in the element. The coil is driven with a stationary RF current or around mechanical resonance. When coinciding with the electrical resonance of the coil which can be adjusted by bridging the coil with a parallel capacitance, the result is a detectable signal response that is improved by the quality factors Q of both resonances. This combination of utilizing a single planar spiral coil was termed magnetic acoustic resonator sensor (MARS).
The advantage of such an acoustic sensor is the ability to utilize a large variety of different materials and material combinations which have been exempt before, i.e., there is no need for piezoelectric materials. Furthermore, a variety of different modes of vibration can be excited. The planar coil setup for magnetic direct generation can also be used to remotely excite piezoelectric transducers. A static magnetic field is not necessary here, since the excitation mechanisms are fundamentally different. This magneto-piezoelectric coupling has been successfully employed to bare, electrode-free quartz crystals. Due to the absence of a large parallel capacitance an additional feature of this excitation principle is the possibility to generate evanescent waves over the megahertz to gigahertz frequency range with the unique ability to focus the acoustic wave down onto the chemical recognition layer.
Magnetic direct generation with spiral coil. For non-piezoelectric resonators a permanent magnet below the spiral coil and a conductive lower surface is required
The description of magnetic direct generation of acoustic waves in nonpiezoelectric plates requires Maxwell’s equations and involves two mechanisms. For the first mechanism, according to Lenz’s law, in a conductive layer placed in parallel above the coil eddy currents are generated that will flow in the opposite clockwise direction as the primary current in the coil. The second mechanism involved consists of the interaction between the eddy currents and a magnetic field. Superposing the induced movement of a charge with the magnetic field will result in the Lorentz force that is capable of exciting acoustic waves in the plate.
A first option to provide such a magnetic field is to generate it externally in the form of a static field. The Coulomb force due to the electric component of the electromagnetic field created by the primary current is negligible when using an additional strong static magnetic field. For a spiral coil, and therefore circular flowing eddy currents, the direction of the Lorentz forces will be radial. Due to these forces alternating with the primary current frequency, the crystal lattice of the material will start to vibrate and an acoustic wave is generated in the sensor element. A standing acoustic wave then appears if the frequency corresponds to one of the eigenmodes of the element and if the force distribution is compatible with the mode shape of the resonance. At resonance, the vibration of the crystal lattice achieves significantly increased displacements resulting in a second perpendicular induction current component, which is superposed with the eddy currents. Both induced currents affect the mutual inductance between primary coil and the resonator element, whereas the second part only takes a measurable effect in mechanical resonance, which can thus be detected by an RF analyzer circuit monitoring the coil parameters.
A second option to provide the required magnetic field is to exploit the same RF field that is generated by the coil and used to produce the eddy currents. The magnetic field is assumed to be sinusoidal at frequency f. As a consequence, the interaction between the eddy currents and the magnetic field itself causes Lorentz forces at frequency 2f that can set the conductive structure into resonance if 2f coincides with the frequency of a proper vibration mode. This frequency doubling action is a distinctive consequence of the nonlinearity in the force generation mechanism.
As a further alternative to classical solutions with quartz crystal sensors, a configuration and method has been developed for contactless readout of the resonance response of a TSM resonator array. The configuration uses a crystal with a large common electrode on the front face, and a number of small equal electrodes on the back face, as shown in. This leads to localized sensing regions via the confined energy trapping under the small back electrodes. Each back electrode is capacitively coupled to a tip electrode separated by a stand-off distance. The tip consists of a small disc and a guard ring, which confine the electric field to the electrode area and make the measurement unaffected by the stray parallel capacitances. A localized mass load added on the front electrode can be consistently detected and measured by scanning the correspondent back electrode, irrespective of the tip-to crystal stand-off distance.
The proposed method may be attractive for the perspective development of monolithic TSM sensor arrays with contactless scanning, because it avoids the problems of routing connections to multiple electrodes, at the same time minimizing the influence of stray contributions external to the crystal.