Used in low-cost thermal imaging, 8-to-12μm infrared micro-bolometer hybrid detectors are very demanding in terms of offset skimming and technological fluctuation compensation: typical offset values are about 100 times larger than the signal, while the fixed-pattern noise (FPN) is about 10 times larger. State-ofthe-art image sensors feature noise-equivalent temperature difference (NETD), i.e. thermal resolutions, of about 50mK.To reach such an NETD, and considering the dependency of both offset and FPN to the temperature of the focal plane, a thermo-electrical cooler (TEC) is used to prevent the temperature of the focal plane from varying either spatially and temporally.This paper presents a 640×480 17μm-pitch TEC-less IR bolometer imager that features an NETD below 50mK over a 60°C range, while keeping a low 4V power supply. It also features a figure of merit of only 400mK/ms, which reflects its ability to deliver 50mK NETD lag-free images at 60fps. Getting rid of the TEC is a key issue to spread the use of microbolomoter imagers to cost-sensitive markets (e.g. automotive) or to extend the battery life of portable devices (e.g. gun sight). Few works to achieve TEC-less capabilities have been reported to date [1][2][3][4].This circuit is based on an enhanced differential pixel read-out ( Fig. 22.8.1), which differs from the classical bolometer read-out architectures [1,2]. As with all microbolometer imagers, this circuit is based on sensitive thermistor arrays (i.e. the bolometers), processed on top of a CMOS read-out circuit (ROIC). A bolometer's resistance varies according to its temperature, which is linked to biasing, self-heating, focal-plane temperature, and scene temperature. The differences between the sensitive bolometers and a "blind", i.e. shielded or a thermalized, reference bolometer gives an image of the scene temperature. This difference is less accurate when the characteristics of the reference bolometer differ from the sensitive bolometer (i.e. polarization, duty cycle, and physical design). The read-out circuit also impacts the accuracy of the difference and its sensitivity to temperature [5]. To sum up the TEC-less characteristic implies to keep output NETD over a large thermal range, without tuning either the supplies or polarizations of the circuit. So it depends on conversion gain, mismatch, and the offset level drift. This work is an improvement on [1], for which thermal drift of NMOS and PMOS differs, except for a specific polarization point. Finding such a polarization point for each imager design makes this architecture suffer from a lack of flexibility. The presented circuit is also an improvement over [2] and [3], for which the TEC-less ability is obtained by using a temperature probe and correction tables in order to modify polarization conditions. Such an approach implies costly calibration steps and power-consuming off-chip processing.In the presented circuit, the reference is provided by a shielded bolometer with the same characteristics as the sensitive bolometer. A shielded bolom...
