Pyroelectric thin films

The pyroelectric response of ferroelectrics may be exploited to detect tem-peratnre changes with extremely high sensitivity. The most common devices are nncooled infrared (IR) detectors, which may be used for spectroscopic analysis as well as imaging apphcations. Pyroelectric thin films based on perovskite-type complex oxides, including Pb(Sc,Ta)03 have been deposited by CSD for intruder alarms, gas sensors, and IR cameras. It is anticipated that these thin-film devices will be substantially less expensive to manufacture than existing bulk polycrystaUine devices, which require labor-intensive manufacturing procedures. 

Muralt (2001) Micromachined infrared detectors based on pyroelectric thin films by P. Muralt, Rep. Prog. Phys. 64,1339. This is a convenient review article. Abstract accessed February 2014 at http //iopscience.iop.org/0034-4885/ 64/10/203/... 

The results obtained may contribute to the development of more efficient pyroelectric devices on the basis of thin-film techniques. 

Pyroelectric Ceramics and Thin Films tion, Properties and Selection... 

The following discussion separates pyroelectric materials into 3 groups intrinsic pyroelectrics which are operated well below Tc, dielectric bolometer materials which are operated close to Tc, but with an electrical bias applied and ferroelectric thin films. 

There are many different types of pyroelectric, including single crystals, polymers, ceramics and thin films and several reviews [2,3,28,29] have considered the properties of many pyroelectric materials in detail, so the discussion here will be confined to a brief review of pyroelectric ceramics and thin films. 

Table 11.2 Pyroelectric properties of several thin film ferroelectrics, c = 2.7 X 106 Jm 3K 1...

In conclusion, it has been demonstrated that the pyroelectric properties of polar materials can be compared relatively simply through the measurement of a few key physical parameters (pyroelectric,dielectric and thermal coefficients) and the judicious use of appropriate figures-of-merit. It is essential that the dielectric properties are measured in the frequency range appropriate for device use, and this is typically in the range of a few to 100 Hz. The properties of many pyroelectric ceramics and thin films have been compared and it has been shown that good pyroelectric properties can be obtained from this films manufactured at relatively low temperatures, a fact that bodes well for their future applications in fully-integrated arrays. 

To reduce expense, efforts are made to exploit integrated thin film technologies. For example, arrays have been produced via thin film deposition of the pyroelectric onto a sacrificial layer, e.g. a suitable metal or polysilicon, which is then selectively etched away. Thermal isolation of the pyroelectric element is achieved through engineering a gap between it and the ROIC silicon wafer. Yias in the supporting layer permit electrical connections to be made between the detector and the wafer via solder bonds. Imaging arrays have been produced in this way incorporating sputtered PST and sol-gel formed PZT films. 

The ideal route would be one in which the pyroelectric detector material is laid down in thin film form by a route compatible with the production of the silicon ROIC. There are obvious parallels with the development of FeRAMS (see Section 5.7.5) and the substantial effort now devoted to their development will have a positive impact on the manufacture of pyroelectric arrays. Challenges he in the requirement to process the deposited films at temperatures not too high for the underlying integrated circuit, and the need to engineer the temperature diffusion characteristics within the element and its surroundings so as to optimise image definition. 

The pyroelectric coefiicient (p) is a useful parameter with which to compare different materials. If the thin film acts as a dielectric in a capacitor, and an external resistance is connected between the electrodes, then a pyroelectric current (I) flows in the circuit. This situation can be expressed as... 

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