Uncooled IR imaging: hybrid and integrated bolometer arrays

Abstract

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.