Annealing studies of detector materials for uncooled thermal imaging
Uncooled thermal imaging is a key technology for military applications. The sensors used are thermal detectors which respond to changes in temperature resulting from absorption of infrared radiation, generally in the 8-14μm waveband. In the UK these have been principally based on the pyroelectric effect in ferroelectric materials which have been incorporated into hybrid detectors where the detector elements are machined from bulk ceramic, either lead zirconate titanate (PZT) or lead scandium tantalate (PST), and then flip-chip bonded to a readout IC (ROIC). Since this is equipment and labour intensive, detectors are expensive to produce and, in addition, the detector structures used limit the achievable noise equivalent temperature difference (NETD) to ~100mK in the scene. In addition to the work on PST, excimer laser annealing of low-temperature deposited PZT has been investigated for use in a fully integrated detector fabrication process. Modelling has shown that the PZT could be heated preferentially to induce perovskite formation whilst the ROIC could be kept within its thermal budget, but that the short laser pulse resulted in extreme surface heating of and poor heat distribution in the PZT. Using a bespoke temporal pulse-extender the laser pulse was lengthened from 25ns to 375ns, lowering surface temperatures and improving heat distribution in the PZT. Pulse-extended excimer laser annealing of 0.5 μm thick PZT was found to be capable of transforming over half the film. The transformable depth was limited by a problem associated with the melting of the lead oxide component of the as deposited PZT, and was less than that required for the detector structures under consideration in the UK programme.