Off-the-shelf readout ICs standardize detector interface

When manufacturing an infrared detector or system, careful consideration must be given to the readout integrated circuit (ROIC) early in the design process. The detector manufacturer must determine the feasibility of designing and building the ROIC in-house--with the accompanying design risk, time commitment, and expense--or seek a suitable alternative. A standard off-the-shelf advanced ROIC offers the potential of speeding the design process while reducing cost.

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Off-the-shelf readout ICs standardize detector interface

Infrared imaging devices can share a common electronic interface, which

lowers cost and increases

performance.

Jeff D. Frank

When manufacturing an infrared detector or system, careful consideration must be given to the readout integrated circuit (ROIC) early in the design process. The detector manufacturer must determine the feasibility of designing and building the ROIC in-house--with the accompanying design risk, time commitment, and expense--or seek a suitable alternative. A standard off-the-shelf advanced ROIC offers the potential of speeding the design process while reducing cost.

A mixed-signal, very-large-scale integrated circuit designated ISC9705 by Indigo Systems (Santa Barbara, CA) is an example of such an ROIC. With its 320 ¥ 256-pixel design, this submicron complementary metal-oxide silicon (CMOS) readout multiplexer incorporates integrated on-chip functionality for high performance and ease of use. It is compatible with p-on-n detectors fabricated from indium antimonide, mercury cadmium telluride, quantum-well infrared photodiodes (QWIPs), and indium gallium arsenide technologies that are indium bump-bonded to the readout device. Standard ROIC features include snapshot mode integration, flexible integration mode control, dynamic image transposition, dynamic windowing, multiple output configurations, variable gain, skimming, and high-voltage QWIP bias.

Advantages of standardized units

An off-the-shelf ROIC offers numerous advantages. First, design engineers avoid the lengthy and risky ROIC development procedure and work with a thoroughly tested device covering a broad range of applications. The development procedure is complex, requiring expertise in low-noise, mixed-signal IC design. In addition, when building ROICs, working within a silicon foundry`s exacting design parameters can be problematic; the failure rate of ROIC designs is historically high. A standardized ROIC benefits from a unique design process and close working relationships with various commercial silicon foundries, allowing a high first-pass success rate.

Using an off-the-shelf ROIC leads to significant cost savings as well. Development costs for an ROIC typically run between $300,000 and $800,000, whereas an off-the-shelf ROIC, sold in wafer form containing on average 20 yielded 11 ¥ 11-mm dies, will cost much less. At the same time, a detector manufacturer also is buying a developmental and production support environment that includes testing services and equipment and software packages for customers desiring their own in-house capabilities.

For end-use applications, Indigo`s standard ROIC is supported by fully configured interface field-programmable gate arrays, greatly simplifying the design task of integrating new focal-plane devices into embedded systems. In addition, manufacturers can use complete imaging electronics packages as foundations for infrared imaging demonstrators and products. Using this array of extended products, services, and capabilities leads to reduced risks, costs, and product-development times.

Design features

Designed for operation from 60 K to 310 K, the ISC9705 features 30 ¥ 30-µm pixels and a 20-million-electron-device storage capacity. Dynamic range exceeds 72 dB, and readout noise is less than 500 electrons. This ROIC is fabricated using a state-of-the-art 0.6-µm lithographic process incorporating high-speed CMOS transistors.

The ROIC operates in two modes: the simplified hands-off default mode directly supports single-output National Television System Committee (NTSC) or Programmable Array Logic (PAL) operation, with the mode defaulting to single-output, full-window, normal-scan order and no reference output (see Fig. 1). In control, or programmable, mode, the ROIC supports advanced features including dynamic image transposition, dynamic windowing, multiple output configurations, and signal skimming (see Fig. 2). Both default and control modes support integrate-while-read and integrate-then-read operations, variable gain, and high-voltage QWIP bias operations.

Other ROIC features include snapshot mode integration, which allows the simultaneous image acquisition of all pixels in an array. This eliminates temporal and spatial artifacting. Dynamic image transposition provides on-focal-plane-array inversion of an image in either or both axes. This feature eliminates the need for expensive and power-hungry external frame buffers and adapts a system to variable optics that change image orientation on an array.

In addition, multiple output configurations make the device suitable for hand-held imagers and high-speed data-acquisition systems. It can operate in single-, dual-, and four-output modes, with each output capable of an output bandwidth of 10 million pixels/s. This flexibility allows full-frame rates of up to 110 frames/s in single-output mode and 346 frames/s in four-output mode. Finally, dynamic windowing permits the readout of a two-dimensional subarray. Rates up to 15,000 frames/s are possible when implementing small windows.

After a silicon foundry ships raw wafers to Indigo, the designs are characterized (at cryogenic temperatures if necessary). The wafers then are passed through an extensive production probe testing process. The fully tested wafers are shipped to customers with a user`s manual plus a CD-ROM containing a wafer map showing the specific locations of specification-compliant, process-development, and nonfunctional dies and complete test data on all usable dies.

Application trends

The company plans to introduce other off-the-shelf ROICs, including small-format (128 ¥ 128-pixel) and large-format (640 ¥ 480-pixel) devices for cooled IR detectors, with at least one 640 ¥ 480-pixel version optimized specifically for QWIPs. Various linear devices will fulfill spectrometer and telecommunications applications such as wavelength division multiplexing. Use in devices such as hand-held imagers and covert security systems will be made possible by supporting electronics chips that allow construction of low-power, very-lightweight low-cost cameras.

Off-the-shelf ROICs and accompanying devices promise to simplify detector design and manufacturing. With minimal expense, time, and risk involved in ROIC design and product-development cycles greatly reduced, detector manufacturers will be able to run more-efficient, less-costly operations, and users will be secure in the knowledge that they are working with a thoroughly tested design. o

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Readout integrated-circuit design (320 ¥ 256 pixels) offers standardized interfaces for different infrared detectors.

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FIGURE 1. Default mode for the ISC9705 readout integrated circuit (ROIC) unit is single-output operation with full-window, normal scan order and no reference output.

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FIGURE 2. In full-function programmable mode, the ROIC unit supports dynamic image transposition, dynamic windowing, multiple output configurations, and signal skimming.

JEFF D. FRANK is vice president of business development at Indigo Systems Corp., 5385 Hollister Ave., Ste. 103, Santa Barbara, CA 93111.

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