1. Field of the Invention
This invention pertains generally to electrical equipment for measuring and testing, and more specifically to the design of piezoelectric resonators and systems to utilize the acoustoelectric effect to measure electrical properties of a medium.
2. Description of the Related Art
Piezoelectric materials are materials which generate electricity when subjected to mechanical stress and, conversely, generate mechanical stress when a voltage is applied. There are many materials which are piezoelectric. These piezoelectric materials have found application in many diverse technologies, ranging from mechanical actuators and gas igniters to very precise timekeeping.
The uses for piezoelectric devices derive from the conversion of electricity to motion or vibration and, often, the reconversion of that motion back into electricity. For example, a precision clock oscillator will utilize a quartz crystal of very precise dimension and mass. Electrodes are formed on the surface of the crystal, and an electric field is applied. This stimulates a mechanical stress in the quartz.
If the applied voltage changes at or near the resonant frequency of the crystal, a sustained vibration may be generated in the quartz. At the resonant frequency of the quartz, which may be determined by cut angle, thickness, length, width and mass, an electrically alternating current may pass through the crystal with very little loss. Outside of this frequency, larger losses will occur in the alternating current as it is passed through the crystal.
The Q of a crystal is a measure of how narrow a band of frequencies is passed by the crystal with minimum attenuation relative to the resonant frequency of the crystal. Often the Q of a piezoelectric material will determine the useful application. For example, very low Q materials are capable of converting wide frequency bands to and from mechanical energy. These materials are often used as sonic transducers in applications such as in microphones or speakers. The low Q allows for many tones to be produced.
Other applications demand a great deal of precision, such as timekeeping. For these applications, a material with a very high Q is preferred, since only a very narrow band of frequencies may then be passed through the piezoelectric material. In these precision applications, the piezoelectric material is usually associated with an electronic oscillator circuit, where the oscillator circuit will be caused to oscillate at the resonant frequency of the piezoelectric material.
With modern manufacturing methods, precision crystals of quartz or similar very high Q material may be made to oscillate at a frequency which is accurate to within a few parts per million. As noted above, this frequency is dependent upon the type of material, mass and dimensions of the crystal resonator. During the production of the quartz resonators, layers of conductive electrode material are typically deposited to a precision of only a few atomic layers, since the resonators will be sensitive to changes in mass as small as this.
The characteristic sensitivity of high Q piezoelectric materials to changes in mass has led industry to a number of diverse applications. For example, a quartz resonator may be coated with an absorbent which is selective to a particular compound. The amount or concentration of that compound may be determined just by monitoring the change in resonant frequency of the quartz as the compound is absorbed. As more of the compound is absorbed, the mass of the vibrating structure is increased.
Similarly, amounts of material deposited in a vacuum deposition chamber may be very accurately monitored by including a quartz resonator in the deposition area. As more material is deposited upon a surface of the quartz resonator, the frequency of the quartz will also change, thereby indicating with great precision the thickness of the deposited layer.
Many other similar applications for precision resonators have been devised. For the purposes of this disclosure, these applications will be referred to herein as crystal microbalances. That is, the addition or subtraction of mass in the region of vibration of the piezoelectric material results in a change in the resonant frequency of vibration. Common applications for crystal microbalances include gas sensing, mass detection for very small masses, film thickness monitoring, microbe and similar biological sensing, and frequency control. Other more recent applications include viscosity and density detectors.
The use of mass or viscosity sensing limits the applicability of the very sensitive quartz resonator to those situations where a change of mass, density or viscosity may be expected or generated. The present inventors have sought to overcome the limitations of the prior art through the use of a new type of sensor which utilizes the acoustoelectric effect that is characteristic of piezoelectric materials. For the purposes of this disclosure, the acoustoelectric effect will be defined as an electrical interaction between a medium and a vibrating piezoelectric material, wherein the medium acts to electrically load the piezoelectric material proportionate to one or more electrical characteristics of the medium.
The acoustoelectric effect as defined herein was introduced by the present inventors and others in a paper entitled "Theory and Applications of Quartz Resonators as Sensors for Viscous Conductive Liquids," incorporated herein by reference. The sensor which formed the topic of that paper was neither reproducible nor sufficiently sensitive to form the resonator structure for a commercial sensor. Furthermore, the theory in that paper ignored the existence of the metal electrode in deriving the acoustoelectric effect.
Additional structures were attempted which addressed the issues of sensitivity and reproducibility. These sensors utilized a lithium niobate piezoelectric material of high piezoelectric coupling in an acoustic plate mode arrangement. The acoustic plate mode device propagates the wave from a first electrode, commonly referred to as an interdigital transducer or IDT, to a second IDT. The amount of acoustoelectric interaction present at the surface of the acoustic plate mode device affects the propagating properties of the wave through the device. By monitoring the time delay or phase change and attenuation, it is possible to determine through the acoustoelectric effect various electrical properties of the medium. For example, liquid properties such as conductivity, ion concentration and dielectric constant can be monitored with high sensitivity.
However, lithium niobate is a relatively expensive material which restricts the applicability of the device to those applications which will tolerate the increased cost. Moreover, lithium niobate is extremely sensitive to changes in ambient temperature. Although the high electromechanical coupling coefficient of lithium niobate allows sufficient sensitivity, the acoustic plate mode device utilizes the surface acoustic wave device structure which requires a more elaborate oscillator circuit. Acoustic plate mode quartz devices constructed similarly were found not to have sufficient sensitivity.
The present invention seeks to overcome the limitations of the prior art both in terms of cost and reproducibility. The present invention thereby provides a sensor which may be affordably mass produced while at the same time providing great sensitivity and reproducibility.