Fabrication pyroelectrics

In the most common LB films with the Y-type structure, the center of inversion exists, and hence they are not suitable for pyroelectric usages. On the other hand, since LB films with X- or Z-type structure have no center of symmetry, it is possible to construct the polar pyroelectric film with permanent dipoles pointing toward one direction. Similar structures can also be formed in hetero LB films with two different amphiphiles stacked altematingly. The first report on the pyroelectric LB film with X-or Z-type structure appeared in 1982 by Blinov et al. [12], It was followed by those of the alternate LB films by Smith et al. [13] and Christie et al. [14]. The polarized structure of the fabricated LB film can be checked by the surface potential measurements using the Kelvin probe [15], the Stark effect measurements [12], or the sign inversion of the induced current between heating and cooling processes. 

An alternative to the resistive elements is the pyroelectric anemometer based on LiTa03 [34]. These devices provide wide rangeability and high reproducibility but, because they are fabricated separately, can only be used as hybrid elements [35]. While this is a drawback in some applications, the results of Yu et al established that it is possible to use even relatively large pyroelectric anemometers (active regions 3 mm x 3 mm) to monitor flows as low as 10 mm min , values that silicon-based devices generally cannot match. 

These two factors are significant in determining the signal to noise ratio of the pyroelectric capacitor - multiplexor couple. Among the different materials only the copolymer is directly compatible with the semiconductor fabrication process. The cr olymer also shows a low thermal diffusion and the best merit factor. 

A 200 pm thick, 30 mm diameter, Z-cut, single-crystal LiTaOs wafer, unpolished on both sides, was used to fabricate the pyroelectric flow sensor. The wafer was first degreased by immersing in trichloroethane, acetone, and isopropyl alcohol for 10 min each, and then it was rinsed in deionized (DI) water and blown dry with nitrogen. After cleaning, nichrome (NiCr) and gold films were sequentially deposited on the front surface of the wafer. This was done in a cryopumped electron-beam evaporator at a pressure of about 10 Pa without breaking vacuum between the NiCr and Au evaporations. The last step minimized contamination of the devices. 

During recent years, the study of micro- and nanoscale fluids has shown significant opportunities for high detectivity of elementary devices with small size. For pyroelectric flow sensors, it is highly desirable to develop theoretical models, experimental methods for pyroelectric element preparation and sensor fabrication, and higher sensitivity with excellent mechanical properties. 

Research to obtain further improvements in the performance of pyroelectric imaging systems is proceeding in several directions. Some further improvement in the spatial resolution obtainable with the vidicon will be obtainable with the reticulated targets which are now being fabricated [8.64]. Improper ferroelectrics offer improved materials [8.64a]. However, another approach is to develop smaller and more rugged tubes with lower power consumption aimed at providing cheaper and more convenient tubes for those applications for which the present performance is adequate [8.65, 66]. 

Fig. 1.15 Schematic diagrams of (a) pyroelectric and (b) micro-thermoelectric gas sensors fabricated on Si substrates with micromachined membrane [(a) Reprinted with permission from Schreiter et al. (2006) and (b) from Shin et al. (2006). Copyright 2006 Elsevier]...

It is important that all indicated devices can function at room temperatures. This means that polymer-based sensors have low power consumption (of the order of microwatts) because no heater element is required for their operation. Properties of polymers that influence the operating parameters of sensors can be physicochemical, chemical, optical (photo- and electroluminescence, optoelectronic), redox, hydrophobic/ hydrophilic, piezoelectric/pyroelectric, and electrical (conductivity, resistivity). Moreover, the polymer itself can be modified to bind biomolecules to a biosensor (Mulchandani and Wang 1996). It is mentioned above that polymers have considerable potential for fabrication of multisensing arrays required for e-nose fabrication (Janata and Huber 1985). 

The ferroelectricity, combined with excellent mechanical properties and the ease of fabrication, have rendered PVF2 a versatile material for piezoelectric and pyroelectric applications (transducer, stems, ultrasonics, loudspeakers, microphones, finger-press switches, ultrared detectors). Furthermore, PVF2 can be found in semiconductor applications and in the electrical-electronic market (plenum cables, aircraft wiring, computer... 

The main noise source of a pyroelectric detector with a carefully designed input amplifier comes from the tan ft of tte element material itself. It may have (hvergeni values depending on the fabricating methods, especially in the case of thin films. 

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