An earlier paper  discussed the merits of adaptive coded apertures for use as lensless imaging systems in the thermal
infrared and visible. It was shown how diffractive (rather than the more conventional geometric) coding could be used,
and that 2D intensity measurements from multiple mask patterns could be combined and decoded to yield enhanced
imagery. Initial experimental results in the visible band were presented. Unfortunately, radiosity calculations, also
presented in that paper, indicated that the signal to noise performance of systems using this approach was likely to be
compromised, especially in the infrared.
This paper will discuss how such limitations can be overcome, and some of the tradeoffs involved. Experimental results
showing tracking and imaging performance of these modified, diffractive, adaptive coded aperture systems in the visible
and infrared will be presented. The subpixel imaging and tracking performance is compared to that of conventional
imaging systems and shown to be superior. System size, weight and cost calculations indicate that the coded aperture
approach, employing novel photonic MOEMS micro-shutter architectures, has significant merits for a given level of
performance in the MWIR when compared to more conventional imaging approaches.
This paper discusses the potential thermal imaging performance achievable from thermal detector arrays and concludes that the current generation of thin-film ferroelectric and resistance bolometer based detector arrays are limited by the detector materials used. It is proposed that the next generation of large uncooled focal plane arrays will need to look towards higher performance detector materials - particularly if they aim to approach the fundamental performance limits and compete with cooled photon detector arrays. Two examples of bolometer thin-film materials are described that achieve high performance from operating around phase transitions. The material Lead Scandium Tantalate (PST) has a paraelectric-to-ferroelectric phase transition around room temperature and is used with an applied field in the dielectric bolometer mode for thermal imaging. PST films grown by sputtering and liquid-source CVD have shown merit figures for thermal imaging a factor of 2 to 3 times higher than PZT-based pyroelectric thin films. The material Lanthanum Calcium Manganite (LCMO) has a paramagnetic to ferromagnetic phase transition around -20oC. This paper describes recent measurements of TCR and 1/f noise in pulsed laser-deposited LCMO films on Neodymium Gallate substrates. These results show that LCMO not only has high TCR's - up to 30%/K - but also low 1/f excess noise, with bolometer merit figures at least an order of magnitude higher than Vanadium Oxide, making it ideal for the next generation of microbolometer arrays. These high performance properties come at the expense of processing complexities and novel device designs will need to be introduced to realize the potential of these materials in the next generation of thermal detectors.
Recently there has been great interest in doped manganite thin films exhibiting colossal magnetoresistance (CMR), in particular for magnetic sensing applications. These perovskite oxides exhibit a metal-insulator transition at the Curie temperature which is accompanied by a large change of resistance as the material loses its ferromagnetic properties. Since this resistance change occurs over a relatively narrow temperature range it is accompanied by very large temperature coefficients of resistance in the region of the phase transition, making these materials ideal candidates for infrared detectors utilizing resistance bolometers. This paper reports measured physical and electrical properties, the latter including 1/f noise, of doped manganite thin film CMR material deposited by pulsed- laser deposition. The potential performance of CMR based resistance bolometer devices is reported.
This paper reviews the use of thin film ferroelectric materials for application in microbolometer infrared detector arrays. A key issue is the need for high temperature processing to achieve the required ferroelectric crystal phase. Results of thermal trials on silicon readout circuits are presented which indicate failure due to disruption of the AlCuSi metallisation. Higher temperatures can be used if oxygen is excluded. A low temperature lead zirconate titanate (PZT) sol-gel deposition is reported which has been used to fabricate fully integrated detector arrays directly on readout silicon wafers. Higher performance is obtained from dielectric bolometer materials, and materials merit figures nearly 4 times that of PZT are reported for sputtered lead scandium tantalate (PST) films. These require post-deposition annealing to temperatures above that allowable for silicon readout survival. Results on excimer laser annealing are presented which demonstrate crystallisation of a ferroelectric film without heating the underlying substrate. A new composite thermal detector array design is introduced, based on an indirect fabrication method. This uses a high density interconnect wafer as a high temperature substrate for ferroelectric film growth. After fabrication of the detector pixels, individual arrays are flip-chip bonded to readout silicon die.
Uncooled, compact and low power IR imaging is offered by large arrays of bolometer elements, at a pitch of 40 micrometers to 100 micrometers. A hybrid array technology, exploiting the pyroelectric property of ferroelectric ceramic materials in the bolometer elements, has produced a range of successful linear and 2-D arrays. High merit figures for the ferroelectric ceramic have been coupled to fabrication technologies including reticulation and solder bump bonding of the elements to the readout multiplexer IC. However, other designs will compete for cost-effectiveness in the large arrays now required for security and other civil applications such as night driving aids. In particular, direct deposition of thin film ferroelectric material onto suitable thermal microstructures on the silicon readout IC could provide substantial reductions in costs and improved performance. This integration will require processes compatible with the silicon IC. Already, lead-based perovskite films are showing considerable potential but other bolometer types are also candidates, such as thin film resistance bolometers. Thermal imaging systems research has resulted in signal conditioning and processing architectures which are optimized for the bolometer arrays. The ac coupling of the ferroelectric response to the IR radiation has been exploited through the use of radiation chopping and image difference processors (IDP), which remove fixed pattern noise and limit low frequency noise. The image detail observable with the pixellated aery has been enhanced by microscan modes. The successful technology will form the basis for a rapid growth of commercial IR imaging and monitoring into the next century.
Large arrays of bolometer elements have considerable potential for thermal imaging applications, offering uncooled operation, and a performance which challenges the cooled semiconductor detectors. A hybrid array technology, exploiting ferroelectric materials, is the basis of a successful range of linear and 2-D arrays. The success is based on hot-pressed ceramics (operating in both pyroelectric and dielectric modes), and the technologies for solder bump bonding and element reticulation. Arrays are increasing in size, from 104 elements up to 105 elements while the pitch is reducing, thus providing high resolution in compact systems. Ferroelectric thin films have recently shown marked improvement in the merit figures and, if compatible with the silicon IC, will allow a more direct array fabrication. These integrated array technologies have potential for high yield and low cost in very large area devices.
Focal plane arrays using ferroelectric ceramics have shown considerable success for ambient
temperature IR imaging. To realise their full potential, however, the arrays must be reticulated, ie.
the individual detector elements must be physically separated. In the research reported here, the use
of focused argon-ion laser radiation to etch lead-compound ferroelectric ceramics in aqueous KOH is
demonstrated. The experimental conditions required to laser-etch the desired structures for detector
elements are discussed. Finally, the use of a reflective metal layer as an etch stop is assessed and
laser-reticulated elements at 1 00 microns pitch are shown.