ADVANCED ELECTRO-OPTICAL AND INFRARED: SENSING ACROSS THE SPECTRUM

In applications from smart phones to industrial machine vision, the past decade has seen an exponentially increasing demand for larger format cameras with greater functionality. This is also true of the electro-optic, infrared (EOIR) cameras used in military and aerospace systems, where in addition to format size, there is need for increased sensitivity, higher dynamic range, faster frame rates, and improved image processing across the entire optical spectrum. Raytheon is meeting these demands with sensors that detect throughout the ultraviolet to infrared wavebands, for applications on platforms ranging from the individual warfighter to ground vehicles to airborne and space assets.

The focal plane arrays (FPAs) that Raytheon develops and produces consist of a detector array integrated to a readout integrated circuit (ROIC). The detector array converts incoming optical energy of a particular waveband into electrical signals. The ROIC captures and formats the signals for each pixel and transmits the data to electronics for image and/or information processing.

Most FPAs output digital data with format sizes up to 64 megapixels. Dynamic ranges can exceed 20 bits, while sensitivities meet tactical and strategic requirements. Frame rates range from 30 Hz to greater than 1 kHz. With the advent of efficient 3D integration, smart sensors are feasible, from basic on-chip image processing to implementing neuromorphic algorithms. 

Advanced EOIR Sensors

Raytheon customers need EOIR systems for a wide breadth of applications in ground, maritime, airborne and space systems which can detect ultraviolet (UV), visible, short wave infrared (SWIR), mid-wave infrared (MWIR), long wave infrared (LWIR), and very long wave infrared (VLWIR) bands of the electromagnetic spectrum. Some applications require combinations of bands to be imaged in the same aperture, such as visible through SWIR, MWIR and LWIR and multiple LWIR wavelengths.

Figure 1: EOIR sensors for different parts of the optical spectrum are being developed and produced for a wide breadth of applications.

Detector Material

Raytheon works with and uses a variety of materials to optimize detection in a particular waveband to provide sensor capabilities across the optical spectrum (Figure 1). Silicon P-intrinsic-N (PiN) detectors are used for the visible band. Indium Gallium Arsenide (InGaAs) detectors are used for the SWIR band up to 1.7 micrometers (μm). Mercury Cadmium Telluride (HgCdTe) is a versatile material for all the infrared wavebands as its cutoff wavelength is tunable based on its stoichiometric composition. It has been the primary, high performance infrared (IR) detector for many years for both single and dual band FPAs. Over the past several years, Raytheon has developed an alternative solution in indium arsenide/gallium antimonide (InAs/GaSb) strained-layer-superlattice (SLS) bandgap engineered barrier device (nBn) structures. These III-V devices have high producibility and can similarly be architected with different wavelength cutoffs. With performance approaching that of HgCdTe detectors, they are candidates for next generation sensors, especially those used in tactical MWIR applications. The last in the list of primary detector materials is silicon microbolometers with vanadium oxide (VOx). In contrast to the materials previously discussed that are designed as photovoltaic devices, VOx microbolometers are microelectromechanical system (MEMS) devices whose resistance changes when thermal energy is absorbed. 

The common detector materials and the wavebands they address are depicted in Figure 2.

Figure 2: Material sets commonly used to sense in different parts of the optical spectrum.

Advanced ROICs Provide Greater Functionality

Readout integrated circuits (ROICs) provide the first level of signal conditioning, processing and capabilities such as gain and offset non-uniformity correction, bi-directionality for scanners, and windowing for regions of interest. Each ROIC has an array of unit cells that interconnect one-to-one with the detector array. The unit cells capture and preprocess the signals from the detectors. Peripheral circuitry in the ROIC controls timing and output of the data.

Figure 3: Readout Integrated Circuits (ROICs), shown above in relative scale, have been continually increasing in format size.

Advanced ROICs feature even greater capability and functionality for the FPA. Fabricated in deep submicron CMOS processes, these devices have enabled large format arrays, from 128 x 128 to 8000 x 8000 (8K x 8K) pixel elements with pixel pitches from 40 μm down to 8 μm. The chronological advancement of ROICs in relative format size is shown in Figure 3.

Figure 4: Digital visible scanners are replacing CCDs. IR scanners are available as well.

For imaging applications in both the visible and IR domains, ROICs are designed as either scanners or starers. Scanners produce an image line by line and starers produce an image simultaneously among the two dimensional array. Figure 4 shows a visible scanning array that provides time-delayed integration at a fast scan rate and ultra-low power with digital outputs. It can replace traditional charge coupled device (CCD) imagers. In Figure 5 is an example of a staring array, which acquires an image in one frame. Frame rate capabilities of various staring arrays range from 30 hertz (Hz) to 20 kilohertz (kHz). 

Figure 5: Staring arrays are produced for visible and IR application in small to large formats.

Most ROICs designed at Raytheon have on-chip analog-to-digital converters (ADCs). Massively parallel architectures with ADCs at the columns are used for high sensitivity applications and provide 14 to 16 bits of dynamic range. For those sensors needing dynamic ranges greater than 18 bits, Raytheon has developed a family of digital-in-pixel architectures, where signal digitization occurs within the pixel. Digital ROICs have been designed for both single color and dual band FPAs, with some designs capable of well capacities greater than one billion electrons. These high well capacities allow high scene contrast imaging without pixels saturating.

Figure 6: Compact Dewar and cooler example.

Small Form Factor Cameras

FPAs operate at a variety of temperatures. HgCdTe or III-V nBn/SLS detectors are cryogenically cooled for high performance. InGaAs-based SWIR detectors are generally thermoelectrically cooled (TEC) to approximately 280 degrees Kelvin (K) and microbolometer arrays operate at ambient room temperature. All FPAs are packaged in some form of evacuated environment to enable cryogenic cooling, provide temperature stabilization, or prevent condensation on the detectors in various operating conditions. These environments are designed to be as compact as possible (Figure 6) and the form factor of the associated electronics is also small (Figure 7). The electronics provide clocks and biases to the FPA, calculate gain and offset coefficients, and perform digital formatting, all while accommodating the increasing data rate from the FPAs, which can exceed 50 gigabits per second for large arrays.

Figure 7: Compact camera electronics example.

As discussed in this edition’s article, “Raytheon’s Three Dimensional Heterogeneous Integration (3DHI) Electronics Technology,” 3D Integration is becoming more pervasive to address the needs for compact sensors. A method of vertically bonding electronic components, this wafer-to-wafer integration is used to hybridize Si PiN detector arrays to the ROIC. Figure 8 is a cross section of a detector and ROIC directly bonded together. An example of multiple wafers bonded together is shown in Figure 9. This technology promises to enable compact intelligent sensors through the integration of FPAs with electronics. 

Figure 8: Cross section of wafer bonded ROIC and detector.

Imagery across the Spectrum

In addition to the visible scanner, visible starers have been produced in formats ranging from 512 x 540 to 8K x 8K. 

In the SWIR domain up to 1.7 μm cutoff wavelength, low noise tactical cameras in a small pitch, 1920 x 1200 format have been developed using Raytheon’s InGaAs detectors. HgCdTe is used for cutoffs at longer wavelengths in the SWIR band. Figure 10 shows one of the largest SWIR FPAs for a ground telescope made with HgCdTe detectors.

Figure 9: Multiple stacked wafer example.

FPAs for Active SWIR applications are made with avalanche photo diode (APD) arrays. APDs operate near the breakdown region of a diode to provide gain at the front end of the signal chain. A LADAR receiver fabricated with HgCdTe detectors was used for autonomous docking in a Space Shuttle mission. Current development is focused on dual mode (Linear and Geiger mode) APDs using III-V materials.

Figure 10: The VISTA sensor is a precision aligned FPA composed of 1k x 1k subarrays. Credit: ESO/J. Emerson/VISTA and Digitized Sky Survey 2. Acknowledgment: Davide De Martin.
Figure 11: High operating temperature (HOT) Mid-wave IR (MWIR) sensors have been made with Mercury Cadmium Telluride (HgCdTe) and III-V bandgap engineered barrier device (nBn)/strained-layer-superlattice (SLS) detectors.
Figure 12: A 720p dual band camera images the Mid-wave IR (MWIR) and Long Wave IR (LWIR) bands with the same Focal Plane Array (FPA).
Figure 13: Long Wave IR (LWIR) example of a microbolometer FPA. These sensors operate uncooled.

Figure 11 shows MWIR capability for tactical applications. The focus has been on high performance HgCdTe and III-V nBn/SLS detectors that operate at higher temperatures (e.g., 120 K). Dual band capability is shown for the MWIR and LWIR bands in Figure 12. Because each pixel has back-to-back diode detectors that can sense in both wavebands, the MWIR and LWIR imagery is perfectly co-registered. 

In figure 13 is an example of LWIR capability using a 3 megapixel microbolometer-based FPA that can operate athermally at room temperature. Cameras with a large 1920 x 1200 format have also been produced.

Conclusion

Raytheon is developing next generation EOIR sensors that span the entire optical spectrum. Increasingly higher yield processes and decreasing form factors continue to reduce product size, weight, power, and cost (SWAP-C). And with the advent of higher functional ROICs and 3D wafer integration, a new era of intelligent sensors is emerging, where actionable information is provided rather than just raw signal data alone. On the ground, in the air and in the far reaches of space, Raytheon’s sensors continue to provide the capabilities required for customer mission critical applications. ‭ 

— Leonard Chen