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.

2 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. 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.

3 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.

4 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.

5 , 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.

6 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.

7 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).IV. FABRICATION OFACOUSTICWAV EDEVICESThe sensors are made by photolithography, using a process asdetailed in Fig.

8 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.]

9 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. 49, NO. 4, APRIL 2001 TABLE IPHYSICALPARAMETERS OFMORECOMMONLYUSEDPIEZOELECTRICMATERIALS Fig.

10 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.