Rex Watton

Ferroelectric materials and IR bolometer arrays: From hybrid arrays towards integration

Abstract Large arrays of bolometer elements offer uncooled and simple operation, and a thermal imaging performance which challenges the cooled semiconductor (photon) detectors. A hybrid array technology, exploiting the pyroelectric property of ferroelectric ceramic materials in the bolometer elements, is the basis of a successful range of linear and 2-D arrays. However, other technologies will compete for cost effectiveness in large area devices. Direct integration of ferroelectric thin films onto suitable thermal microstructures on the silicon readout IC is attractive if deposition processes which are compatible with the silicon IC are established. Deposited lead based perovskite films show considerable potential. Thin film resistance bolometers will also compete, and have some advantages of compatibility in fabrication.

Pyroelectric Materials and Devices

The pyroelectric effect is the phenomenon whereby a change in temperature in a polar dielectric engenders a change in its electrical dipole moment. It can be used to create a flow of current in an external circuit connected to a piece of the material. The effect has been known for many years but it is only since about 1960 that its technological applications have been seriously considered. These have been almost entirely in the field of the detection of electromagnetic radiation, especially in the two “atmospheric window” infrared (IR) bands of 3-5 pm and 8-14 pm. The ambient temperature operation of pyroelectric detectors, leading to low power consumption, low cost, compactness and lack of any requirement for logistical support in the form of cooling fluids or high-pressure gas, gives them a number of important advantages over photon detectors used in these wavebands, such as MCT or InSb. These must be cooled to cryogenic temperatures to obtain optimum performance. Hence, pyroelectric detectors have found a huge range of applications in products ranging from fire alarms to intruder detectors, in instrumentation such as gas analysis and laser beam characterization and in military/paramilitary applications such as thermal imaging. Detectors have been demonstrated which work from the visible [1] through the infrared [2] to sub-millimetre [3,4] and millimetre [5] wavelengths. They have also been used at radiation modulation frequencies from a few Hz [2] to many GHz [6].

Ferroelectrics in uncooled thermal imaging

Uncooled Thermal Imaging (TI) in the UK involves large arrays of ferroelectric bolometer elements, at a pitch of from 100 micrometer down to 40 micrometer. A Hybrid Array Technology, exploiting the pyroelectric property of ferroelectric ceramic materials for the bolometer elements, has produced a range of successful solder bump bonded 2-D arrays. However, in innovative technologies under research, direct deposition of the ferroelectric material as a thin film onto suitable thermal microstructures on the silicon readout IC will provide substantial reductions in costs as well as improved performance. A route has been defined for this Integrated Array Technology, leading to performance enhancements by a factor of three over the Hybrids. In achieving the performance, the optimized ferroelectric signal readout, signal conditioning and processing architectures perfected for the Hybrid Arrays will be retained. Microscan mechanisms, readily incorporated in the IR chopped format used with ferroelectric imaging, have been demonstrated for the Hybrids, and will be even more closely matched to the improved thermal diffusion MTF of the Integrated devices. The ferroelectric capacitative detector filters the high frequencies, limiting noise bandwidths for very large arrays, and with microscan technology added, the ferroelectric arrays retain their potential to provide high quality IR imaging at very large equivalent array sizes.

Ferroelectric IR bolometers - from ceramic hybrid arrays to direct thin film integration

Abstract Bolometer elements in large arrays, at a pitch from 100 down to 40μm, offer uncooled and simple operation in thermal imaging applications. 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 for the ceramic has been coupled to fabrication technologies including solder bump bonding of the elements to the multiplexer IC. However, other designs will compete for low costs in the large arrays required for civil applications. 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. The successful technology will form the basis for a rapid growth of commercial IR imaging and...

PyX3: Ir bolometers and thermal imaging: The role of ferroelectric materials

Abstract 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 pyroelectric materials, is the basis of a successful range of linear and 2-D arrays. Ceramic uniformity for large area arrays is good but wafer processing is demanding. Other technologies will compete for cost-effectiveness in large area devices. Ferroelectric thin films have recently shown marked improvement in the merit figures and, if compatible with the silicon IC, may allow a more direct array fabrication. Thin film resistance bolometers have advantages in ease of fabrication, but there are also some inherent disadvantages, when compared with the ferroelectric operation.

Epitaxy and interfacial phase in thin films of lead scandium tantalate deposited by reactive sputtering on a platinum interlayer

Abstract Lead scandium tantalate (PST) films with a lead-rich composition were deposited at 500°C by RF sputtering on sapphire substrates coated with Pt and were investigated by cross-sectional transmission electron microscopy. The films consisted almost entirely of the perovskite phase. The grains of perovskite PST had a crystallographic orientation almost parallel to that of the underlying Pt. Some elongated voids were observed. The presence of an interfacial phase, with interplanar spacings consistent with a pyrochlore phase, was demonstrated by electron diffraction and dark-field imaging. The crystallographic orientation of this phase is also nearly parallel to that of the Pt. Energy-dispersive X-ray microanalysis showed that the interfacial phase is lead-deficient. The interfacial phase is irregular in thickness. A reduction in the Pb content of the film leads to a thicker interfacial layer. The implications for the mechanism of growth of PST on Pt and for improvement of the film quality are discussed.

IR bolometer arrays: the route to uncooled, affordable thermal imaging

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.

Laser-assisted etching of ferroelectric ceramics for the reticulation of IR detector arrays

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.

Uncooled IR imaging: hybrid and integrated bolometer arrays

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.

Ferroelectric arrays: the route to low-cost uncooled infrared imaging

Large arrays of ferroelectric elements have considerable potential for thermal imaging applications, offering operation and performance which challenges the cooled semiconductor detectors. A hybrid array technology, exploiting ferroelectric ceramic materials is the basis of a successful range of two dimensional arrays. The success is based on hot pressed ceramic (operated in both pyroelectric and dielectric bolometer modes), and the technologies for solder bump bonding and element reticulation. Arrays have been made with up to 105 elements at pitches down to 40 micrometer, thus providing high resolution in compact systems. Ferroelectrics have now been produced as thin films with high figures of merit and, once deposition techniques compatible with silicon integrated circuits have been demonstrated, these will allow a more direct array fabrication. These integrated array technologies have potential for high yield and low cost arrays of very large numbers of elements. Thermal imaging systems research has led to a signal conditioning architecture optimized for pyroelectric and dielectric bolometer arrays and exploiting the ac coupling of the ferroelectric response to the radiation. The resulting compact sensors give high image quality with low fixed pattern nose in reliable, low power formats.