Uncooled infrared focal plane arrays

Abstract

After a long incubation period, commercialization of uncooled infrared focal plane arrays (IRFPAs) is progressing rapidly. In the high-end field, pixel pitch reduction is approaching the diffraction limit of optics, and full high-definition IRFPAs have been reported. On the other hand, low-cost infrared array sensors (IRASs), which are small-format IRFPAs for non-imaging applications, are finding new applications in areas such as home appliances. This paper reviews the basics of and advances in uncooled IRFPAs, including IRASs. Some emerging trends in business and applications are also discussed.

A CMOS-Based Thermopile Array Fabricated on a Single SiO2 Membrane

“…Low-cost IR image sensors are increasingly used for a variety of applications, ranging from home appliances and security to automotive and IR cameras for smartphones [1]. These sensors typically consist of an array of thermopiles or bolometers, with the former being inherently insensitive to changes in ambient temperature [1] and being easier to manufacture using low-cost Complementary Metal-Oxide-Semiconductor (CMOS) fabrication processes [2,3], which makes the technology more attractive for commercialization. Thermopile sensors detect a temperature difference ∆T across each thermopile element and are composed of several thermocouples with their hot junctions thermally isolated, typically by a thin dielectric membrane [4,5].…”

“…Micromachining of the silicon substrate can be used to form a membrane with a high thermal resistance to enhance IR heating and thus responsivity [2,3]. A typical array consists of thermopile pixel elements, each fabricated on an isolated membrane, formed by bulk etching of the substrate [1,4]. However, this fabrication method requires critical process control to minimize substrate etching tolerances and undercut.…”

Low-Cost Microbolometer Type Infrared Detectors

“…With diodes in series connection, the sensitivity is given by [ 14 ]: where n is the number of the diodes in the series. The typical value of the sensitivity for a single diode at 300 K is ~2 mV/K under a bias voltage of 0.6 V [ 59 ], which is equivalent to a temperature coefficient of only ~0.33%/K. However, as the number of the diodes in the series increases, the temperature coefficient could become comparable to the TCR of VO x .…”

“…Although such requirement could be achieved via one-by-one pumping through a fine-bore tube, the cost becomes a bottleneck in lowering the cost of uncooled IRFPAs. Figure 9 shows the concept of the wafer-level packaging (WLP) technology for IRFPA, which is a popular option for cost reduction [ 59 , 101 , 102 ]. In this technology an IR transparent cap wafer is bonded to the IRFPA wafer under vacuum and then the hermetical sealing is achieved using solders.…”

“…Today, one of the most attractive thermal IR detectors for imaging purpose is the microbolometer IRFPA. Comparing to other thermal IR detectors like thermopile detector [ 47 , 48 , 49 , 50 ], pyroelectric detector [ 51 , 52 , 53 , 54 ], and superconducting transition edge sensor (TES) bolometer detector [ 55 , 56 , 57 , 58 ], it is promising for the commercial imaging applications because of its respectable performance, small pixel size, and ease to make [ 59 ]. Attributing to the continuous efforts and the technological advances, the pixel size of the microbolometer detector fabricated via the low-cost manufacture technology based on silicon LSI (large scale integration) circuit process has been reduced to beyond 17 μm [ 18 , 19 , 20 ].…”