This paper reviews the historical progress of HgCdTe material and device development at Raytheon Vision Systems
starting with the initial work in 1965 at what was then the Santa Barbara Research Center, a subsidiary of the Hughes
Aircraft Company and progressing up to the present time. Because of the long history, all the details cannot be presented
in a single paper; instead, we focus only on a few major accomplishments. In HgCdTe material preparation these
include: the early bulk single crystal growth methods; the advent of liquid phase epitaxial growth from Hg melts; and,
the most recent molecular beam epitaxial methods. For IR photodetector devices, we started with just single element
detectors operating either in photoconductive or photovoltaic mode, then progressed to multi-element linear arrays, then
to 2-D arrays on Si read-out circuits and, finally to the very large focal plane (>2k × 2k), dual-band, and APD arrays of
today. Some applications of these devices in IR systems will be presented. Technical issues will be discussed only to
the extent necessary to support the historical narrative. Some interesting anecdotes will be included.
Raytheon Visions Systems (RVS) is furthering its capability to deliver state-of-the-art high performance large
format HgCdTe focal plane arrays (FPAs) for dual-band long-wavelength infrared (LWIR) detection. Missile
seekers are designed to acquire targets of interest at long ranges and discriminate targets from clutter. The use of
dual-band long wavelength infrared detector technology provides the ability for these seekers to combine these
operations into the same package with enhanced performance. Increasing the format size of dual-band longwavelength
FPAs and tailoring the detector design for specific long-wavelength bands enables seekers to be
designed for increased field-of-view, longer target acquisition ranges, and improved accuracy. This paper will
review in further detail the aspects of detector design, MBE wafer growth, wafer fabrication, and detector
characterization that are contributing to development and demonstration of high performance large format dual-band
LWIR FPAs at RVS.
Raytheon has developed a 3rd-Generation FLIR Sensor Engine (3GFSE) for advanced U.S. Army systems. The sensor
engine is based around a compact, productized detector-dewar assembly incorporating a 640 x 480 staring dual-band
(MW/LWIR) focal plane array (FPA) and a dual-aperture coldshield mechanism. The capability to switch the
coldshield aperture and operate at either of two widely-varying f/#s will enable future multi-mode tactical systems to
more fully exploit the many operational advantages offered by dual-band FPAs. RVS has previously demonstrated high-performance
dual-band MW/LWIR FPAs in 640 x 480 and 1280 x 720 formats with 20 μm pitch. The 3GFSE includes
compact electronics that operate the dual-band FPA and variable-aperture mechanism, and perform 14-bit analog-to-digital
conversion of the FPA output video. Digital signal processing electronics perform "fixed" two-point non-uniformity
correction (NUC) of the video from both bands and optional dynamic scene-based NUC; advanced
enhancement processing of the output video is also supported. The dewar-electronics assembly measures approximately
4.75 x 2.25 x 1.75 inches. A compact, high-performance linear cooler and cooler electronics module provide the
necessary FPA cooling over a military environmental temperature range. 3GFSE units are currently being assembled
and integrated at RVS, with the first units planned for delivery to the US Army.
Raytheon Vision Systems (RVS) has developed and demonstrated the first-ever 1280 x 720 pixel dual-band MW/LWIR
focal plane arrays (FPA) to support 3rd-Generation tactical IR systems under the U.S. Army's Dual-Band FPA
Manufacturing (DBFM) program. The MW/LWIR detector arrays are fabricated from MBE-grown HgCdTe triple-layer
heterojunction (TLHJ) wafers. The RVS dual-band FPA architecture provides highly simultaneous temporal detection in
the MWIR and LWIR bands using time-division multiplexed integration (TDMI) incorporated into the readout integrated
circuit (ROIC). The TDMI ROIC incorporates a high degree of integration and output flexibility, and supports both
dual-band and single-band full-frame operating modes, as well as high-speed LWIR "window" operation at 480 Hz
frame rate. The ROIC is hybridized to a two-color detector array using a single indium interconnect per pixel, which
makes it highly producible for 20 μm unit cells and exploits mature fabrication processes currently used to produce
single-color FPAs. High-quality 1280 x 720 MW/LWIR FPAs have been fabricated and excellent dual-band imagery
produced at 60 Hz frame rate. The 1280 x 720 detector arrays for these FPAs have LWIR cutoff wavelengths ≥10.5 μm
at 78K. These FPAs have demonstrated high-sensitivity at 78K with MW NETD values < 20 mK and LW NETD values
<30 mK with f/3.5 apertures. Pixel operability greater than 99.9% has been achieved in the MW band and greater than
98% in the LW band.
Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm2 to 50cm2, long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (<1000cm-2) and excellent uniformity (composition<0.1%, cut-off wavelength Δcenter-edge<0.1μm). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5μm and 10.5μm, respectively, exhibit significant improvement in 78K LW performance with >70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1×103 Ωcm2. Two-color 20μm unit-cell 1280×720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.
Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Army's Third Generation FLIR systems. RVS has produced 640 x 480 two-color FPAs with a 20 micron pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheon's approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 μm unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 micron at 78K. These 20 micron pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously.
HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm2 in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048 x 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6 in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 x 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations.
The Navy faces an ever evolving threat scenario, ranging from sub-sonic sea skimming cruise missiles to newer, unconventional threats such as that experienced by the USS Cole. Next generation naval technology development programs are developing “stealthy” ships by reducing a ships radar cross section and controlling electromagnetic emissions. To meet these threat challenges in an evolving platform environment, ONR has initiated the “Wide Aspect MWIR Array” program. In support of this program, Raytheon Vision Systems (RVS) is developing a 2560 X 512 element focal plane array, utilizing Molecular Beam Epitaxially grown HgCdTe on silicon detector technology. RVS will package this array in a sealed Dewar with a long-life cryogenic cooler, electronics, on-gimbal power conditioning and a thermal reference source. The resulting sub system will be a component in a multi camera distributed aperture situation awareness sensor, which will provide continuous surveillance of the horizon. We will report on the utilization of MWIR Molecular Beam Epitaxial HgCdTe on Silicon material for fabrication of the detector arrays. Detector arrays fabricated on HgCdTe/Si have no thermal expansion mismatch relative to the readout integrated circuits. Therefore large-area focal plane arrays (FPAs) can be developed without concern for thermal cycle reliability. In addition these devices do not require thinning or reticulation like InSb FPAs to yield the high levels of Modulation Transfer Function (MTF) required by a missile warning sensor. HgCdTe/Si wafers can be scaled up to much larger sizes than the HgCdTe/CdZnTe wafers. Four-inch-diameter HgCdTe/Si wafers are currently being produced and are significantly larger than the standard 1.7 inch x 2.6 inch HgCdTe/CdTe wafers. The use of Si substrates also enables the use of automated semiconductor fabrication equipment.
Raytheon Vision Systems (RVS) in collaboration with HRL Laboratories is contributing to the maturation and manufacturing readiness of third-generation two-color HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256x256 30μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral regions. FPAs configured for MWIR/MWIR, MWIR/LWIR and LWIR/LWIR detection are used for target identification, signature recognition and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer-heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all two-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single mesa, single indium bump, and sequential mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.
Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.
Raytheon Infrared Operations (RIO) has achieved a significant technical breakthrough in uncooled FPAs by reducing the pixel size by a factor of two while maintaining state-of-the-art sensitivity. Raytheon has produced high-quality 320 X 240 microbolometer FPAs with 25 μm pitch pixels. The 320 X 240 FPAs have a sensitivity that is comparable to microbolometer FPAs with 50 micrometers pixels. The average NETD value for these FPAs is about 35 mK with an f/1 aperture and operating at 30 Hz frame rates. Good pixel operability and excellent image quality have been demonstrated. Pixel operability is greater than 99% on some FPAs, and uncorrected responsivity nonuniformity is less than 4% (sigma/mean). The microbolometer detectors also have a relatively fast thermal time constant of approximately 10 msec. This state-of-the-art performance has been achieved as a result of an advanced micromachining fabrication process. The process allows maximization of both the thermal isolation and the optical fill-factor. The reduction in pixel size offers several potential benefits for IR systems. For a given system resolution (IFOV) requirement, the 25 μm pixels allow a factor of two reduction in both the focal length and aperture size of the sensor optics. The pixel size reduction facilitates a significant FPA cost reduction since the number of die printed on a wafer can be increased. The pixel size reduction has enabled the development of a large-format 640 X 512 FPA array applicable to wide-field-of-view, long range surveillance and targeting missions, and a 160 X 128 array where applications for miniaturization and temperature invariance are required as well as low cost and low power.
The objectives of the Integrated Imaging Sensors (I2S) Program are rtwofold. The first is to develop and deliver a rifle sight containing a single aperture and optical path for receiving, combining, and viewing radiation from the separate infrared (IR) and visible bands in a single image simultaneously. The second is to develop a sensor array sensitive in the radiation band spanning approximately from 0.4 μm to 1.7 μm by "fusing" indium-gallium-arsenic material onto silicon charge coupled devices. The ability to coincidentally and simultaneously form images from these two separate radiation bands is expected to significantly improve the detection and identification of objects from the case where only one radiation band is employed. Additionally, extending the cutoff of the visible band from 0.9 μm to 1.7 μm is expected to enhance viewing in this band as there is more available light, and further lessons the exacting requirement of desigining nearly noise free detectors.
The objectives of the Low Cost Microsensors (LCMS) Program are twofold. The first is to develop and deliver a long-range infrared (IR) sensor built upon an uncooled vanadium oxide (VOx) 640 X 512 format focal plane array (FPA) engine. The second is to develop an expendable microsensor built upon a VOx 160 X 128 format FPA engine. The 640 X 480 sensor is applicable to long-range surveillance and targeting missions and is a reusable asset. The 160 X 120 sensor is designed for applications where miniaturization is required as well as low cost and low power. The 160 X 120 is also intended for expendable military applications. The intent of this DUS&T effort is to further reduce the cost, weight, and power of uncooled IR sensors, and to increase the capability of these sensors, thereby expanding their applicability to military and commercial markets never before addressed by thermal imaging.
The objectives of the Low Cost Microsensors Program are twofold. The first is to develop and deliver a long-range infrared (IR) sensor built upon an uncooled vanadium oxide (VOx) 640 X 480 format focal plane array (FPA) engine. The second is to develop an expendable microsensor built upon a VOx 160 X 120 format FPA engine. The 640 X 480 sensor is applicable to long-range surveillance and targeting missions and is a reusable asset. The 160 X 120 sensor is designed for applications where miniaturization is required as well as low cost and low power. The 160 X 120 is also intended for expendable military applications. The intent of this DUS&T effort is to further reduce the cost, weight, and power of uncooled IR sensors, and to increase the capability of these sensors, thereby expanding their applicability to military and commercial markets never before addressed by thermal imaging.
Raytheon IRCOE has developed a family of uncooled, microbolometer FPAs. These FPAs have been designed to address commercial and high-performance military applications. The SB-151 is a high-sensitivity 320 X 240 FPA with 50 micrometers pixels. The SB-151 FPA has been fabricated with several microbolometer pixel designs that allow optimization of either sensitivity or response time. Noise equivalent temperature difference (NETD) values as low as 8.6 mK have been measured for the SB-151 FPAs with f/1 optics. NETD values less than 25 mK have been measured for FPAs with thermal time constants of approximately 18 msec.
Raytheon Systems Company has developed a prototype infrared imaging rifle-sight using an uncooled, microbolometer FPA. The high-sensitivity FPA (SBRC-151) used in the Long-wavelength Staring Sensor (LWSS) was developed by Raytheon Infrared Center of Excellence (IR COE). The NETD (noise equivalent temperature difference) sensitivity of the camera has been measured at 14 mK with f/1 optics and at 74 mK with an f/2.1 aperture stop. Excellent imagery has been demonstrated with the f/2.1 aperture. The 320 X 240 FPA utilizes a high-yield CMOS readout integrated circuit (ROIC) that achieves high sensitivity, low output nonuniformity, and large scene dynamic range. The ROIC provides multi-level, on-chip nonuniformity correction and on-chip temperature compensation. The FPA has 50 micrometer X 50 micrometer pixels and operates at frame rates up to 60 Hz with a single output. The LWSS was characterized by the U.S. Army's NVESD in 1997 using an earlier version of the SBRC-151 FPA. The NVESD measurements validated the Raytheon NETD data. The NVESD evaluation also demonstrated outstanding MRT and spatial noise characteristics. The VOx microbolometer detectors are produced at the Raytheon IR COE facility in Santa Barbara, CA using an advanced dry-etch fabrication process. In addition to the LWSS project, the IR COE has initiated production of the microbolometer FPAs (AE-189) for commercial applications. Over 600 FPAs have been produced on this project, and data is presented for the first 250 FPAs that have been packaged and tested. The pixel operability of the production radiometer FPAs (AE-189) is typically greater than 99.9%.
Raytheon Sensors and Communications Systems has developed a prototype infrared imaging rifle-sight using an uncooled, microbolometer FPA. The Longwavelength Staring Sensor (LWSS) has been characterized by NVESD, where it demonstrated NETD and MRT values that are unsurpassed for uncooled FPA technology. The NVESD-measured NETD values were 24 mK with f/0.7 optics and 42 mK with an f/1/0 aperture. When used with the f/0.7 optics, NVESD measured MRT values less than 60 mK at the nyquist spatial frequency. Similar measurements at f/1.0 produced MRT values less than 110 mK. Further optimization of the microbolometers is expected to produce FPAs with NETD values less than 20 mK for f/1.0 apertures. The high- performance uncooled microbolometer FPA (SBRC-151) used in the LWSS was developed by Raytheon Santa Barbara Research Center. The 320 X 240 FPA utilizes a high-yield CMOS readout integrated circuit (ROIC) that achieves high sensitivity, low output nonuniformity, and large scene dynamic range. The ROIC provides multi-level, on-chip nonuniformity correction and on- chip temperature compensation. The FPA has 50 micrometer X 50 micrometer pixels and operates at frame rates up to 60 Hz with a single output. The VOX microbolometer detectors are produced at SBRC using an advanced dry-etch fabrication process. In addition to the LWSS project, SBRC has initiated low-rate production of the microbolometer FPAs. This work is being performed in support of Raytheon-Amber for commercial radiometer cameras. The pixel operability of the production radiometer FPAs (AE-189) are greater than 99.9%.
SBRC has developed a high-quality 320 X 240 room- temperature infrared FPA that operates in the 8 - 14 micrometers spectral band. The FPA is based upon the silicon microbolometer technology that has been licensed from Honeywell. This monolithic uncooled FPA utilizes a novel BiCMOS readout circuit that provides high sensitivity and excellent output uniformity. The 320 X 240 FPA operates at frame rates up to 60 Hz with a single output. The microbolometers were fabricated monolithically on the silicon readout circuits at SBRC using VOx as the bolometer material. As advanced microbridge structure design was used that achieves an optical fill-factor greater than 65% in the 48 micrometers X 48 micrometers pixels. The structure also provides excellent thermal isolation for high responsivity and sensitivity. Initial measurements indicate the FPAs are operating with an NETD sensitivity of about 100 mK for an f/1 aperture. This FPA is ultimately expected to operate at sensitivities of less than 20 mK. The FPA also demonstrates peak-to-peak output nonuniformities of less than 100 mV. The FPAs have been mounted in permanently-sealed vacuum packages with single-stage thermoelectric temperature stabilizers. These vacuum packages have been integrated into a camera system that has produced high-quality infrared imagery.
SBRC has developed a high-quality 320 X 240 room-temperature infrared FPA that operates in the 8 - 14 micrometers spectral band. The FPA is based upon the silicon microbolometer technology that has been licensed from Honeywell. This monolithic uncooled FPA utilizes a novel BiCMOS readout circuit that provides high sensitivity and excellent output uniformity. The 320 X 240 FPA operates at frame rates up to 60 Hz with a single output. The microbolometers were fabricated monolithically on the silicon readout circuits at SBRC using VOx as the bolometer material. An advanced microbridge structure design was used that achieves an optical fill-factor greater than 65% in the 48 micrometers X 48 micrometers pixels. The structure also provides excellent thermal isolation for high responsivity and sensitivity. Initial measurements indicate the FPAs are operating with an NETD sensitivity of about 100 mK for an f/1 aperture. This FPA is ultimately expected to operate at sensitivities of less than 20 mK. The FPA also demonstrate peak-to-peak output nonuniformities of less than 100 mV. The FPAs have been mounted in permanently-sealed vacuum packages with single- stage thermoelectric temperature stabilizers. These vacuum packages have been integrated into a camera system that has produced high-quality infrared imagery.
HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.
Integrated two-color detector arrays offer significant system advantages (over separate arrays for each color) where two-color information is required. Using a single array with co-located spectral band sensitivities guarantees perfect pixel registration between the two different spectral band images. These two-color IR detectors can be made in HgCdTe using a pair of back-to-back-diodes incorporated in a triple-layer heterojunction (TLHJ). Use of HgCdTe allows any combination of bands between SWIR and LWIR. TLHJs can be operated in either a sequential or simultaneous mode by leaving the layer common to the two diodes floating or by contacting it. The effect of the choice of spectral bands on the meaning of sequential and simultaneous operation is discussed. State-of-the-art trend line performance for each spectral band of a TLHJ has been demonstrated using an all-LPE HgCdTe technology at SBRC. Mean MWIR RrA of 2 X 107 (Omega) -cm2 and LWIR of 1.6 X 103 (Omega) -cm2 have been shown. Quantum efficiencies are typical of trend line PV HgCdTe. Very high quality imaging has been demonstrated using 64 X 64 sensor chip assemblies in a sequential mode incorporating the above TLHJs. Simultaneous detectors have been made in miniarrays and test structures of various size unit cells. 128 X 128 simultaneous arrays are under study. Imaging and test results (performance and uniformity) for each band are comparable to state-of-the-art single-color HgCdTe arrays.