Acoustic wave technology sensors - Microwave …

1 ieee transactions ON Microwave theory AND TECHNIQUES, VOL. 49, NO. 4, APRIL 2001795 Acoustic Wave technology SensorsBill DraftsAbstract A brief overview of Acoustic wave sensor physics, ma-terials, sensor types, and applications is presented in this is placed on the different types of Acoustic wave sensors ,their respective advantages, and their specific applications in Terms BAW sensors , SAW sensors , sensor INTRODUCTIONACOUSTIC wave devices have been in commercial usefor over 60 years. The telecommunications industry is thelargest user of these devices, consuming approximately threebillion Acoustic wave filters annually, primarily for mobile cellphones and base stations. These devices are typically surfaceacoustic wave (SAW) devices, and act as bandpass filters inboth the RF and IF sections of the transceiver are several new emerging applications for Acoustic wavedevices as sensors that may eventually equal the demand ofthe telecommunications market.

2 These include automotiveapplications (torque and tire pressure sensors ), medical applica-tions (biosensors), and industrial and commercial applications(vapor, humidity, temperature, and mass sensors ). Acousticwave sensors are competitively priced, inherently rugged, verysensitive, and intrinsically reliable. Some are also capable ofbeing passively and wirelessly interrogated (no sensor powersource required).II. ACOUSTICWAV ETECHNOLOGYOVERVIEWA coustic wave sensors are so named because they utilize amechanical, or Acoustic , wave as the sensing mechanism. As theacoustic wave propagates through or on the surface of the ma-terial, any changes to the characteristics of the propagation pathaffect the velocity and/or amplitude of the wave. Changes in ve-locity can be monitored by measuring the frequency or phasecharacteristics of the sensor and can then be correlated to thecorresponding physical quantity that is being all Acoustic wave devices and sensors use a piezo-electric material to generate the Acoustic wave.

3 Piezoelectricitywas discovered by the brothers Curie in 1880, received itsname in 1881 from Hankel, and remained largely a curiosityuntil 1921, when Cady discovered the quartz resonator forstabilizing electronic oscillators [1]. Piezoelectricity refersto the production of electrical charges by the imposition ofmechanical stress. The phenomenon is reciprocal. Applying anappropriate electrical field to a piezoelectric material createsManuscript received August 21, 2000; revised January 7, author is with Microsensor Systems Inc., Apopka, FL 32703 Item Identifier S 0018-9480(01) mechanical stress. Conversely, by applying an appropriatemechanical stress, an electric field will be created. Piezoelec-tric Acoustic wave sensors apply an oscillating electric fieldto create a mechanical wave, which propagates through thesubstrate and is then converted back to an electric field PIEZOELECTRICSUBSTRATEMATERIALS FORACOUSTICWAV ESENSORST here are several piezoelectic substrate materials that may beused for Acoustic wave sensors and devices.

4 The most commonare quartz (SiO) and lithium tantalate (LiTaO), and to a lesserdegree, lithium niobate (LiNbO). Each material has specificadvantages and disadvantages, which include cost, temperaturedependence, attenuation, and propagation velocity. Table I listssome relevant specifications for each material, including themost popular cuts and orientations [2]. An interesting propertyof quartz is that it is possible to select the temperature depen-dence of the material by the cut angle and the wave propagationdirection. With proper selection, the first-order temperature ef-fect can be minimized. An Acoustic wave temperature sensormay be designed by maximizing this effect. This is not true ofLiNbOor LiTaO, were a linear temperature dependence al-ways exists for all material cuts and propagation materials that have commercial potential include gal-lium arsenide (GaAs), silicon carbide (SiC), langasite (LGS),zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium ti-tanate (PZT), and polyvinylidene flouride (PVDF).

5 IV. FABRICATION OFACOUSTICWAV EDEVICESThe sensors are made by photolithography, using a process asdetailed in Fig. manufacturing process begins by carefully polishingand cleaning the piezoelectric substrate. As shown in Fig. 1(a),metal, usually aluminum, is then deposited uniformly onto thesubstrate. The device is then coated with a photo-resist, whichis spun on and then baked to harden it. The coated device isthen exposed to UV light through a mask [see Fig. 1(b)]. Themask contains opaque areas, which correspond to the areas tobe metallized on the final device. The exposed areas undergoa chemical change, allowing them to be removed using adeveloping solution [see Fig. 1(c)]. This exposes areas of metal,which are chemically etched away. The remaining photo-resistis then removed, leaving the final device, as shown in Fig. 1(d).The pattern of metal that remains on the device is called aninterdigital transducer (IDT). By changing the length, width,position, and thickness of the IDT, the performance of the sensorcan be 9480/01$ 2001 IEEE796 ieee transactions ON Microwave theory AND TECHNIQUES, VOL.

6 49, NO. 4, APRIL 2001 TABLE IPHYSICALPARAMETERS OFMORECOMMONLYUSEDPIEZOELECTRICMATERIALS Fig. 1. Acoustic wave devices are manufactured using the samephotolithography process that integrated circuits use. The only difference isthat no junction exists in Acoustic wave 2. Typical Acoustic wave device consists of two sets of IDTs. Onetransducer converts electric-field energy into mechanical wave energy, whilethe other transducer converts the mechanical energy back to an electric ACOUSTICWAV EPROPAGATIONMODESA coustic wave devices are described by the mode of wavepropagation through or on a piezoelectric substrate. Acousticwaves are distinguished primarily by their velocities anddisplacement directions; many combinations are possible,depending on the material and boundary conditions. The IDTof each sensor provides the electric field necessary to displacethe substrate to form an Acoustic wave.

7 The wave propagatesthrough the substrate, where it is converted back to an electricfield at the other IDT. Fig. 2 shows the configuration of a typicalacoustic wave device. Transverse, or shear, waves have particledisplacements that are normal to the direction of wave propaga-tion and can be polarized so that the particle displacements areparallel to or normal to the sensing surface. Shear horizontalwave motion indicates transverse displacements polarizedparallel to the sensing surface, whereas shear vertical motionindicates transverse displacements normal to the the wave propagates through the substrate, the wave iscalled a bulk wave. The most commonly used bulk acousticwave (BAW) devices are the thickness shear mode (TSM) res-onator and the shear-horizontal Acoustic plate mode (SH-APM) sensor . If the wave propagates on the surface of the substrate, itis known as a surface wave. The most commonly used surfacewave devices are the SAW sensor and the shear-horizontal sur-face Acoustic wave (SH-SAW) sensor , also know as the surfacetransverse wave (STW) sensor .

8 The mode of propagation dra-matically affects the sensor s performance and how the sensoris Acoustic wave devices are sensors in that they are sen-sitive to perturbations of many different physical change in the characteristics of the path over which theacoustic wave propagates will result in a change in output. Allthe sensors will function in gaseous or vacuum environments,but only a subset of them will operate efficiently when theyare in contact with liquids. The TSM, SH-APM, and SH-SAWall generate waves that propagate primarily in the shear hori-zontal motion. The shear horizontal wave does not radiate ap-preciable energy into liquids, allowing liquid operation withoutexcessive damping. Conversely, the SAW sensor has a substan-tial surface-normal displacement, which radiates compressionwaves into the liquid, causing excessive damping. An exceptionto this rule occurs for devices utilizing waves that propagate at avelocity lower than the sound velocity in the liquid.

9 Regardlessof the displacement components, such modes do not radiate co-herently and are, thus, relatively undamped by Acoustic waves that are promising for sensors includethe flexural plate wave (FPW), Love wave, surface skimmingbulk wave (SSBW) and the Lamb wave. Before reviewing ap-plication examples, it is helpful to briefly review each BULKWAV ESENSORS TSM RESONATORThe TSM, also widely referred to as a quartz crystal microbal-ance (QCM), is the best-known, oldest, and simplest acousticwave device. As Fig. 3 depicts, the TSM typically consists of athin disk of AT-cut quartz with parallel circular electrodes pat-terned on both sides. The application of a voltage between theseelectrodes results in a shear deformation of the device is known as a resonator because the crystal res-onates as electromechanical standing waves are created. Thedisplacement is maximized at the crystal faces, making the de-vice sensitive to surface interactions. The TSM resonator wasoriginally used as a deposition sensor to measure metal deposi-tion rates in vacuum systems [3, p.]

10 39]. The sensor is typicallyused in an oscillator circuit, where the oscillation frequencyDRAFTS: Acoustic WAVE technology SENSORS797 Fig. 3. Although it is the oldest Acoustic wave device, the TSM resonator isstill used today for measuring metal deposition 4. In the SH-APM sensor , the waves travel between the top and bottomsurfaces of the plate, allowing sensing on either the crystal resonance and indicates mass accumulationon the device surface. In the late 1960 s, the TSM resonator wasshown to operate as a vapor TSM features simplicity of manufacture, ability towithstand harsh environments, temperature stability, and goodsensitivity to additional mass deposited on the crystal surface[4]. As a result of its shear wave propagation component, theTSM resonator is also capable of detecting and measuring liq-uids, making it a good candidate for a biosensor. Unfortunately,these devices have the lowest mass sensitivity of the sensorsexamined.