New Horizons for Uncooled Ir Sensors

The performance requirements for space-based IR imaging sensors are generally so severe that uncooled detector technologies have historically been ignored – and rightly so. The reasons for this have often been inaccurately represented on the basis of theoretical analysis with unnecessary assumptions. This paper shows that the performance of uncooled or minimallycooled IR detectors can approach or even exceed that of cooled photon detectors. It also describes the practical barriers to achieving such performance. A detailed analysis of signal and noise shows that the backgroundlimited performance of a well-designed thermal detector is not much different from that of a photon detector. The analysis also shows that these distinct detector types are optimally suited for different types of applications. The barriers to achieving backgroundlimited sensitivity are quite different for thermal detectors. In this paper we quantify the barriers, and discuss their implications. INTRODUCTION Uncooled IR imaging is at last, after some 25 years of development, commonplace. Many companies working with several different technologies are producing a diverse variety of products in quantity. Some products are lower-cost versions of products that have been around for a long time, while other products were difficult to imagine prior to the advent of uncooled IR focal plane arrays (FPAs). The initial measure of viability of these devices was a sensitivity comparable to the poorest of cooled IR sensors. We have seen various technologies mature to the point that FPAs with 320×240 pixels and with pixel sizes on the order of 50 μm perform significantly better than that standard. Larger arrays with smaller pixels are further blurring the boundaries between uncooled and cooled technologies. The words “radiation-limited performance” are now being commonly spoken in reference to development activities for uncooled IR FPAs. What is meant by that is usually the oft-quoted 1.8 ×10 cm·Hz/W “ultimate” D* limit for thermal detectors. Although barriers to achieving that level of performance certainly remain, it by no means represents the ultimate limit for these detectors. The assumptions in the analysis that leads to this magical D* are nonessential, and are in fact not true of modern micromachined devices. This, of course, has serious implications for the future of space-based IR imaging sensors. REVIEW OF FUNDAMENTALS What we commonly call “uncooled” detectors are better characterized as “thermal” detectors, as opposed to “photon” detectors. A thermal detector is characterized by the fact that its response is proportional to the amount of energy in the absorbed photon stream. The response of a photon detector is proportional to the number of absorbed photons. In a photon detector, a photon is absorbed directly by the IR-sensitive material, and the charge carriers generated by that absorption are sensed either directly or by a concomitant change in some property of the material. A thermal detector, by contrast, comprises three distinct parts: An IR absorber, a thermal isolation means, and a temperature sensor or transducer, as illustrated in Figure 1. The purpose of the IR absorber is to convert IR electromagnetic energy into heat energy. The thermal isolation, modulated by the thermal mass (heat capacity), converts the heat energy to a temperature change. The transducer converts the temperature change to a change in a measurable (usually electrical) parameter. The three parts of a thermal detector are functionally independent, although the device may be designed in such a manner that they are interdependent.