Uncooled pyroelectric detector arrays using ferroelectric ceramics and thin films

Pyroelectric infrared detectors have been of interest for many years because of their wide wavelength response, good sensitivity and lack of need for cooling. They have achieved a wide market acceptance for such applications as people sensing, IR spectrometry (especially for environmental protection) and flame/fire protection. Arrays of such detectors, comprising a pyroelectric material interfaced to an application specific integrated circuit for signal amplification and read out, provide an attractive solution to the problem of collecting spatial information on the IR distribution in a scene. Over the past 15 years, this has been particularly explored for uncooled thermal imaging. However, pyroelectric arrays have the property that they are only sensitive to changes in the IR flux. This means that they are particularly-well suited to the monitoring of movements of people in applications such as retail outlets and in safety and healthcare applications. The applications of low cost arrays with limited (few hundred elements) for people sensing and imaging radiometry will be illustrated. The performances and costs of uncooled pyroelectric arrays are ultimately driven by the materials used. For this reason, continuous improvements in materials technology are important. In the area of bulk ceramics, it is possible to obtain significant improvements in both production costs and performance though the use of tape-cast, functionally-gradient materials. The use of directly-deposited ferroelectric thin films on silicon ASIC´s is offering considerable potential for low cost high performance pyroelectric arrays. The challenges involved in developing such materials will be discussed, especially from the aspect of low temperature deposition and other fabrication issues, such as patterning. Sol gel deposition provides an excellent technique for thin film growth and will be illustrated with an example of a Mn-doped PZT film that can be grown at 560°C with a FoM FD exceeding that of many bulk materials. The ability to grow such high-performance thin films onto silicon brings with it the ability to fabricate novel MEMS-based pyroelectric device structures. A new concept that will be presented here is the use of arrays of thin film pyroelectric detector elements with i- ntegrated radiation collectors designed to greatly enhance the intensity of the radiation falling on the element. The collectors´ design is based on the Compound Parabolic Concentrator. These are non-imaging collectors, originally developed for solar cells technology in the 1960´s, that yield a concentration ratio near to the thermodynamic maximum. The CPC structure is etched in a silicon wafer, upon which has been defined a pyroelectric IR sensor with low thermal conductance (spiral leg structure) fabricated in a high sensitivity PZT thin film. To our knowledge, this is the first time that CPCs have been applied to microfabricated IR sensors. First experimental assessment of the performance of these structures will be presented.