ACOUSTIC WAVE DEVICES FOR CHEMICAL SENSING
This appendix describes several types of chemical sensors that are based on acoustic wave formats. There are many other formats being developed for chemical sensing, as discussed in Chapter 6, "Chemical Sensing."
SURFACE ACOUSTIC WAVE DEVICES
Surface acoustic wave (SAW) devices are simple, rugged sensors that consist of a piezoelectric substrate and two patterned electrodes to launch and detect acoustic waves. The surface is covered with a coating that sorbs selected chemicals present in the environment. As an acoustic wave travels from one electrode to the other, the presence of molecules sorbed on the coating changes the velocity and attenuation of the SAW. Measurements of these changes can be used to indicate the identity and concentration of a specific chemical species in the environment.
By using coatings with selective absorption properties, investigators are developing sensors that can detect specific chemical species for both gas-phase and liquid-phase environments (Kepley et al., 1987). Typically, durable oxide-based coatings that are chemically modified to provide the required sorption characteristics are used. An important area of materials research is the development of coatings for particular chemicals. Another area of current materials research is the development of coatings that selectively sorb ionic species from solution for use in applications such as monitoring electroplating processes or waste streams for toxic species such as chromium, cadmium, or lead.
Polymer coatings that absorb a wide variety of chemicals have been found to be ideally suited for monitoring the highly regulated ozone-depleting chlorinated hydrocarbons. Simultaneous measurement of the wave velocity and attenuation can be used to identify a chemical species and its concentration.
SAW devices are fabricated from piezoelectric materials, typically quartz. SAWs rely on two interdigitated transducers—one to launch and the other to detect a wave that travels from one end of the device to the other. Each transducer is composed of many pairs of photolithographically defined fingers, and each finger is only a few micrometers wide. SAW devices operate in the 100-MHz-frequency regime.
The SAW is extremely sensitive to tiny mass changes and is capable of detecting 100 picogram/cm2, which corresponds to a sensitivity to less than 0.01 monolayer of carbon. The velocity and the attenuation of acoustic waves result from changes in surface mass in SAW devices. Measuring both these properties simultaneously helps determine the nature and cause of the sensor response. However, the motion of the SAW is subject to extreme damping in liquids. As a result, applications are confined to the gas phase.
Typical applications of SAW devices are monitoring of volatile organic compounds in effluents on-line for environmentally conscious manufacturing
and in waste-monitoring and remediation. The sensitivity and versatility of SAW sensors permit this technology to be engineered into many industrial processes, improving productivity and reducing environmental emissions.
A recent application of SAW sensors is the selective detection of organophosphates, which are a common class of chemical warfare agent. The detection of these chemicals was made possible by the recent development of thin films of self-assembled monolayers as the active surface of SAW devices. The sensitivity of this film/SAW device combination endows the sensor with immunity to interference from water vapor and common organic solvents while providing sensitivity in the part-per-billion concentration of organophosphates. As a result, arrays of such sensors with appropriate coatings can be used to detect the production of chemical weapons.
ACOUSTIC PLATE MODE SENSORS
The material of choice for acoustic plate mode sensors is quartz because of its low coefficient of thermal expansion. These sensors are typically made from ST-cut quartz, with mode propagation along the x-direction of the crystal. The plates are lapped to the desired thickness, and both sides of the plates are optically polished. Input and output transducers are defined on the face of the crystal to which the sensitive coating has been applied. The transducers are formed using evaporated metal films and standard photolithography techniques. Standard metallization, typically gold on chromium, is used. Dimensions of the quartz plate are not critical; typical dimensions are 23 mm × 7.6 mm × 0.5 mm.
Different transducer geometries are used to excite different modes (Martin et al., 1989). In all cases, the transducer finger dimensions scale with periodicity d: finger and width separation are d/4, while finger length is 50d, where d is selected based on the desired mode to be excited. Typical mode frequencies are 100 or 150 MHz for d = 50 and 32 µm, respectively. There are advantages to operating at higher frequencies; the sensitivity increases as the square of the operating frequency. This in creased performance is available until the increased complexity of high-frequency operation or the increased fabrication complexity due to decreased dimensions of the photolithography outweigh the advantages.
QUARTZ CRYSTAL MICROBALANCE
The quartz crystal microbalance (QCM) is based on a resonant mode in a bulk crystal. The performance of these sensors relies on the changes in frequency as a result of mass loading on the surfaces. QCM sensors can also measure density-viscosity product in liquids as described below. The sensors are typically circular with about a 10:1 ratio of diameter to thickness; the precise dimension is determined by the desired operating frequency. Typical operating frequencies are about 5 MHz (Martin et al., 1991).
Electrodes are formed by evaporation of a chromium adhesion layer followed by evaporation of gold. A large electrode covering the center one-fourth of one side serves as ground, and a radio frequency electrode of one-half the diameter is deposited on the other side of the plate. The active area is determined by the area of the smaller electrode, since the radio frequency field is confined primarily to the quartz region beneath this electrode.
The resonance decreases in frequency and peak intensity and broadens as the QCM is loaded by the density-viscosity product of the liquid in which it is immersed. Because the admittance of the unloaded device can be measured precisely and because the behavior under loading can be calculated in detail, the QCM readily gives the product of the viscosity and density of the liquid. The QCM has a wide variety of applications that range from monitoring the viscosity-density product for oil or other liquids during refining or processing, measurement of degradation of jet fuels in high-temperature environments, to monitoring the life of motor oil in automobile engines.
CATALYTIC GATE SENSORS
Catalytic gate sensors refer to sensors based on silicon integrated circuit technology. The best known example is the hydrogen sensor that is fabricated from a metal-oxide—semiconductor field-effect
transistor. The sensor consists of a silicon base, a thin silicon dioxide insulating layer, and a metallic outer layer that catalytically decomposes H2 or other hydrogen-containing molecules. The initial demonstration used palladium metal (Hughes et al., 1992; Lundström 1981; Lundström and Söderberg, 1981; Keramati and Zemel, 1982; Ruths et al., 1981). When hydrogen-containing molecules adsorb on the palladium surface, a fraction of the molecules decompose to release hydrogen atoms that diffuse rapidly through the palladium electrode to the underlying oxide layer. The atoms are trapped at interface states between the metal and the oxide, changing the amount of charge trapped at the interface which in turn changes the bias voltage in the channel between the source and drain and thus the gain in the transistor.
This type of sensor is fabricated using standard silicon integrated-circuit processing technology, with the exception of the catalytic gate. These sensors are extremely sensitive and can accurately measure hydrogen concentrations down to parts per billion. Such sensitivity is important in development of technologies to measure and control impurities in integrated circuit fabrication. Although palladium was used in the earlier demonstrations of these devices, PdNi alloys have proven to be more robust. PdNi alloys are capable of reversible behavior after exposure in 100 percent H2 atmosphere, whereas pure palladium irreversibly forms a palladium hydride (Hughes and Schubert, 1992). Optimum alloy sensors for hydrogen detection, as well as development of alloy systems to detect other species, are an area of active materials research.
Recent extensions of this type of sensor technology have included adding resistors formed from hydrogen absorbing material, such as PdNi, to detect high concentrations of hydrogen. This capability is important if one wishes to use these sensors to detect explosive concentrations of H2. Combining the two sensors on the same device and adding temperature sensing and control permits detection of hydrogen over a wide range of concentrations in gases under adverse environmental conditions. Since the sensors are fabricated using standard microelectronics techniques, further addition of integrated circuits on the sensor chip provide ''smart" sensors; such sensors measure hydrogen concentrations and can also conduct signal processing, calibration, and output buffering, as well as initiate some action based on the measurement result.
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Hughes, R.C., and W.K. Schubert. 1992. Thin-films of PdNi alloys for detection of high hydrogen concentrations. Journal of Physical Chemistry 71(l):542-544.
Kepley, L., R.M. Crooks, and Aj. Ricco. 1987. Selective surface acoustic wave-based organophosphonate chemical sensor employing a self-assembled composite monolayer: A new paradigm for sensor design. Analytical Chemistry 64:3191-3193.
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