Uncooled 17 μm ¼ VGA IRFPA development for compact and low power systems

ABSTRACT The high level of accumulated expertise by ULIS and CE A/LETI on uncooled microbolom eters made from amorphous silicon enables ULIS to develop ¼ VGA IRFPA formats with 17µm pixel-pitch to enable the development of small power, small weight (SWAP) and high performance IR systems. ROIC architecture will be described where innovations are widely on-chip implemented to enable an easier operation by the user. The de tector configuration (integration time, windowing, gain, scanning direction ), is driven by a standard I²C link. Like most of the visible arrays, the detector adopts the HSYNC/VSYNC free-run mode of operation driven with only one master clock (MC) supplied to the ROIC which feeds back pixel, line and frame synchronizations. On-chip PROM memory for customer operational condition storage is available for detector characteristics. Low power consumption has been taken into account and less than 60 mW is possible in analog mode at 60 Hz and < 175 mW in digital mode (14 bits). A wide electrical dynamic range (2.4V) is maintained despite the use of advanced CMOS node. The specific appeal of this unit lies in the high uniformity and easy operation it provides. The reduction of the pixel-pitch turns this TEC-less ¼ VGA array into a pro duct well adapted for high resolution and compact systems. NETD of 35 mK and thermal time constant of 10 ms have been measured leading to 350 mK.ms figure of merit. We insist on NETD trade-off with wide thermal dynamic range, as well as the high characteristics uniformity and pixel operability, achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This technology node associated with adva nced packaging technique, paves the way to compact low power system. Keywords: Uncooled microbolometer, LWIR, amorphous silicon technology, IRFPA. 1 INTRODUCTION For some years the infrared sensors market tends to follow the evolution observed in the past for visible imaging sensors. It is particularly true for cost decreasing, to address high volumes applications, while more and more performances are expected into the sensors. Indeed with smaller pixel pitch, higher resolution [1] and easier system integration, microbolometer infrared sensors intend to address new low-cost/high-volume applications such as automotive for night vision, pedestrian detections [2] [3], security / surveillance and thermography. A ¼ VGA sensor has been designed for large volume application taking into account 17µm pixel-size technology and high reliability packaging technique. We will describe first the interest of amorphous silicon for microbolometer application regarding material properties, simplified technological process and simplified sensor operation, and then discuss key vacuum package and ROIC technological breakthroughs prior to present benefits of LWIR thermal sensor improvement for commercial as well as military applications. 2 KEY TECHNOLOGIES FOR FIR IMAGING SENSOR 2.1 Amorphous Silicon microbolometer Amorphous silicon presents attractive properties for micro bolometer applications. The amorphous silicon bolometer resistance R variation versus temperature is described by Arrhenius law in which activation energy Ea depends on the sensitive material. R