Surface acoustic device

Zinc oxide (ZnO, wurtzite structure) eliminates oxygen on heating to form nonstoichio-metric colored phases, Zni+xO with x < 70 ppm. ZnO is almost transparent and is used as white pigment, polymer stabilizer, emollient in zinc ointments, creams and lotions, as well as in the production of Zu2Si04 for TV screens. A major application is in the rubber industry to lower the temperatures and to raise the rate of vulcanization. Furthermore, it is an n-type semiconductor (band gap 3.37 eV) and shows piezoelectric properties, making zinc oxide useful for microsensor devices and micromachined actuators. Other applications include gas sensors , solar cell windows and surface acoustic devices. ZnO has also been considered for spintronic application because of theoretical predictions of room-temperature ferromagnetism . 

Zinc oxide, ZnO, is a p-type semiconductor and shows piezoelectric properties which make this material useful for microsensor devices and micromachined actuators [69, 70]. The Al-doped material is used as a transparent electrode [71]. Other applications of ZnO include gas sensors [72], solar cell windows [73] and surface acoustic devices [74]. The organometallic compounds diethylzinc, ZnEt2 [75-77], and dimethylzinc, ZnMe2 [78], are frequently used as precursors for the deposition of zinc oxide. However, these reagents are highly pyrophoric and tend to react prematurely in the presence of water or oxygen. If ZnEt2 is combined with an alcohol in the reaction chamber, stable intermediates, presumably zinc alkoxides and/or alcohol adducts, are formed in the vapor-phase [79]. These compounds are more stable than dialkyl zinc rea-... 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

H. Wohljen (1984) Mechanism of operation and design considerations for surface acoustic device vapor sensors , Sensors and Actuators B, 5,307-25. 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

A new chemical sensor based on surface transverse device has been developed (99) (see Sensors). It resembles a surface acoustic wave sensor with the addition of a metal grating between the tranducer and a different crystal orientation. This sensor operates at 250 mH2 and is ideally suited to measurements of surface-attached mass under fluid immersion. By immohi1i2ing atra2ine to the surface of the sensor device, the detection of atra2ine in the range of 0.06 ppb to 10 ppm was demonstrated. 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

A wide variety of solid-state sensors based on hydrogen-specific palladium, metal oxide semiconductor (MOS), CB, electrochemical, and surface acoustic wave (SAW) technology are used in the industry for several years. Microelectromechanical systems (MEMS), and nanotechnology-based devices for the measurement of hydrogen are the recent developments. These developments are mainly driven by the demands of the fuel cell industry. Solid-state approaches are gaining rapid popularity within the industry due to their low cost, low maintenance, replacements, and flexibility of multiple installations with minimal labor. 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Atalar, A. and Koymen, H. (1987). Use of a conical axicon as a surface acoustic wave focusing device. IEEE Trans. UFFC 34, 53-63. [56,273]... 

Barrier layers for Cu metallization in surface acoustic wave (SAW) devices, which are increasingly used in the information technique and telecommunications industry, have been investigated by SIMS depth profiling in comparison to AES (Auger Electron Spectrometry).125 Development trends in SAW devices focus on smaller structures, higher input power or higher frequency. Two Cu metallization systems (of 150 nm thickness) on a LiNb03 substrate with different barrier layers ... 

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

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