Employing an optical readout architecture expands the capabilities offered by uncooled thermal imagers, such as extremely fast frame rates, dual-band imaging, and multi-megapixel resolution. It also affords the ability to incorporate multiple pixel designs on the same infrared sensor chip, which we have taken advantage of to fabricate an optical readout photomechanical imager with 12 distinct pixel designs in the sensor chip layout. Using this methodology, we were able to quickly sort the designs in terms of performance and suitability for manufacturing, and thus, in an expedient and highly cost-effective manner, determine which pixel designs have merited future consideration for full-scale prototyping. A fast frame rate MWIR photomechanical imager based on one of the best pixel designs was built and tested for high-speed imaging of small arms fire.
In an optical-readout photomechanical imager, the infrared sensor array is physically separated from the ROIC. The modularity of the optical readout architecture allows for extra design freedom that is not possible in bolometers, negating fundamental trade-offs, such as NETD versus thermal time constant. For successful commercialization, the photomechanical imager must meet application-specific performance and functional targets, and to this end, Agiltron has advanced the photomechanical imaging platform over several technology generations. Improvements have been made to both the optical readout system and the photomechanical sensor chip, which enabled reductions in size, weight, and power (SWAP) and NETD over successive generations. The current-generation photomechanical imager has the size equivalent to a digital camera and an /1-equivalent NETD and MDTD of less than 100 mK.
Multispectral Imaging has recently made considerable improvements to the sensitivity, uniformity and dynamic range of infrared FPAs based on capacitively read, bimorph microcantilever sensor technology. The company is presently prototyping 160x120 imaging arrays with 50 μm pitch pixels and is actively pursuing the development of next generation 25 μm pitch pixel arrays. Measured peak NETD values for recently fabricated 50μm pitch focal plane arrays are in the 40-50mK range, with individual pixels in the 10-15mK range. The modeled and measured tradeoffs discussed in this paper lead to a possible 2-3 times further improvement in average NETD. A number of factors influence the performance of these devices which includes the optimization of sometimes competing design requirements. For example, the tuning and optimization of the infrared optical resonant cavity structure while maximizing the change in sensor capacitance during IR irradiance. Similarly there are tradeoffs between structural rigidity, which increases the structure resonant frequency improving noise immunity, and thermal response times. These tradeoffs are discussed with reference to real world sensor structures. Results from detailed thermo-electromechanical-optical modeling of the operation of the 25 μm pitch pixels will be discussed in reference to the design and fabrication of 25 μm pitch test pixels. The most recent infrared sensitivity and other performance measurements from the development of the company's first commercial 160 x 120 pixel imaging array product will also be presented.
This paper reports on the development of small pixel pitch infrared FPAs based on the capacitively read bimorph microcantilever sensor technology. The heat sensing bimorph microcantilever structures are fabricated directly onto the CMOS control and amplification electronics to produce a high performance, low cost imager that is compatible with standard silicon IC foundry processing and materials. Positional responsivities of greater than 0.3 mu m/K have been modeled and measured for 50 mu m pitch pixels, corresponding to a temperature coefficient of capacitance, Delta C/C, (equivalent to TCR for microbolometers) above 30%/K. This responsivity, along with noise capacitances in the sub-attofarad range and nominal sensor capacitances of 15 fF, give modeled NEDT < 20 mK for these devices. Very preliminary infrared imagery has been obtained with a recently fabricated imaging array and assembled camera and control system, and indicates that NEDTs in 1K range arc! presently being measured with this system. Considerable electronic noise is evident in the output response which, when eliminated, will lead to much reduced NEDT numbers in the near future. The measured thermal time constants for these arrays are in the 5 - 10 msec range.At smaller pixel pitches, the positional responsivity decreases rapidly with feature size resulting in increased system NEDTs. Modeling the performance of microcantilever based IR sensors with innovative sensor structures and pixel pitches down to 17 pm indicates NEDTs < 20 mK and thermal time constants in the 5 msec range, are feasible with this technology. Results from detailed thermo-electro-opto-mechanical modeling of the operation of the 25 mu m pitch pixels are presented.
