Focal Plane Arrays

The basic concept of a modern thermal-imaging system is to form a real image of the infrared (IR) scene, detect any variation in the imaged radiation, and, by suitable electronic processing, create a visible representation of this variation analogous to that achieved by conventional television cameras. IR camera construction is similar to that of a digital video camera. Detectors are only a part of usable sensor systems. Instead of charge-coupled device (CCD)/complementary metal-oxide semiconductor (CMOS) image arrays that video and digital still cameras use, the IR camera detector (Fig. 4.1) is a focal plane array (FPA) of micrometer-sized pixels made of various material sensitive to IR wavelengths. Once a detector is selected, optics (lens) material and filters can be selected to somewhat alter the overall response characteristics of an IR camera system.

The term “focal plane array” (FPA) refers to an assemblage of individual detector picture elements (“pixels”) located at the focal plane of an imaging system. Although the definition could include ID (“linear”) arrays as well as 2D arrays, it is most frequently applied to the latter. Usually, the optics part of an optoelectronic image device is restricted to focusing of the image onto the detector’s array. These so-called “staring arrays” are scanned electronically, usually by circuits integrated with the arrays. The architecture of detector-readout assemblies has assumed a number of forms which have been described in detail [1-4]. The types of readout integrated circuits (ROICs) include the function of pixel deselecting, antiblooming on each pixel, subframe imaging, and output preamplifiers, and may include yet other functions.

FIGURE 4.1 Schematic representation of an imaging system, showing important sub-systems.

FPA technology has revolutionized many kinds of imaging, from у rays to the terahertz and even radio waves; the rate at which images can be acquired has increased by more than a factor of a billion in many cases. Figure 4.2 illustrates the trend in array size over the past 50 years. Imaging IR FPAs have been developing in-line with the ability of silicon integrated circuit (IC) technology to read and process the array signals, and also to display the resulting image. The progress in IR arrays has also been steady, mirroring the development of dense electronic structures, such as dynamic random-access memories (DRAMs). FPAs have had nominally the same development rate as DRAM ICs, which have followed Moore’s Law with a doubling-rate period of approximately 18 months, although with FPAs lagging DRAMs by about 5-10 years. The 18-month doubling time is evident from the slope of the graph presented in the Fig. 4.3, which shows the log of the number of pixels per array as a function of the first year of commercial availability. Array size will continue to increase but perhaps at a rate that falls below the Moore’s Law curve [6]. An increase in array size is already technically feasible [7]. Flowever, the market forces that have demanded larger arrays are not as strong now that the megapixel barrier has been broken. CCDs greater than three gigapixels offer the largest formats (Fig. 4.2).

Astronomers were the driving force towards the day when the optoelectronic arrays could match the size of photographic film. Since large arrays dramatically multiply the data output of a telescope system, the

FIGURE 4.2 Imaging array formats compared with the complexity of silicon microprocessor technology and dynamic random-access memory (DRAM), as indicated by transistor count and memory bit capacity (adapted after Ref. [5] with completions). The timeline design rule of MOS/CMOS features is shown at the bottom. Note the rapid rise of CMOS imagers which are challenging CCDs in the visible spectrum. Infrared arrays with size greater than 100 megapixels are now available for astronomy applications. Imaging formats of many detector types have progressed beyond that required for high-definition TV.

development of large-format mosaic sensors of high sensitivity for ground- based astronomy is the goal of many astronomic observatories around the world. Raytheon manufactured a 4x4 mosaic of 2 kx2 к HgCdTe sensor chip assemblies (SCAs) with 67 million pixels and assisted in assembling it to the final focal-plane configuration to survey the entire sky in the southern hemisphere at four IR wavelengths [6]. Also, Teledyne fabricated a large detector system for the Euclid Near-Infrared Spectrometer and Photometer (NISP) instrument which consists of a 4x4 mosaic focal plane of 16 H2RG (2 kx2 к pixels) SCAs and 16 SIDECAR ASIC Sensor Chip Electronics (SCE) modules. These SCAs were made with substrate- removed molecular beam epitaxy (MBE) HgCdTe material system with a 2.3-pm cutoff wavelength. The SCAs are mounted on a buttable molybdenum package that enables close-packing of the 16 flight SCAs in the NISP

FIGURE 4.3 The number of pixels on an infrared array has been growing exponentially, in accordance with Moore’s Law, for 40 years with a doubling time of approximately 18 months. The pixel density of infrared arrays will keep increasing with time but perhaps slower than that predicted by Moore’s curve.

FIGURE 4.4 The top and bottom sides of the Euclid H2RG SCA molybdenum flight package (after Ref. [8]).

focal plane (Fig. 4.4). The mass of a fully assembled H2RG SCA is less than 115 grams [8].

The trend of increasing pixel number is likely to continue in the area of large-format arrays. It is predicted that a focal plane of 100 megapixels and larger will be possible, constrained by only budgets but not by technology

[9]. This increasing trend will be continued using a close-butted mosaic of several SCAs. “Butting” refers to tiling closely together separate pieces of semiconductor to produce one large sensitive array operated as a single image sensor. In most cases, butting is used to make imagers that are larger than the largest imager a single wafer can hold.

A number of architectures are used in the development of IR FPAs [10- 14]. In general, they may be classified as hybrid or monolithic, but these distinctions are often not as important as proponents or critics state them to be. The central design questions involve performance advantages versus ultimate producibility. Each application may favor a different approach, depending on the technical requirements, projected costs, and schedule.