Large format detector arrays are responsive uniformly over spectral 1-5μm wavelength
range and are available with RVS' high quality HgCdTe detector epitaxial layers on large
area 15 cm diameter wafers. Large wafers enable both low cost High Definition (HD)
staring FPAs, as well as the ability to approach giga-pixel format detector arrays with a
seamless 10cm ×10cm continuous image plane size possible. With a 15 cm diameter
detector substrate it is a straightforward growth path to a 5k×5k μm pitch 25 Mega-pixel
infrared focal plane array (FPA) with smaller pitches allowing even greater format along
the 10cm die length. This paper describes arrays 1.5 to 4 Mega-pixel infrared HgCdTe
developed by RVS for demanding higher performance applications. Performance data
for both the detector and ROIC for typical SWIR and MWIR FPAs operating at 85K will
be presented. This paper will provide FPA performance capability for small pitch large
format HgCdTe/Si detector arrays fabricated at RVS and manufacturing readiness low
cost Mega-pixel infrared FPAs for current and future wide FOV high-resolution systems.
High-performance large-format detector arrays responsive to the 1-5μm wavelength range of the infrared spectrum
fabricated using large area HgCdTe layers grown on 6-inch diameter (211) silicon substrates are available for advanced
imaging applications. This paper reviews performance and capabilities of Raytheon Vision Systems (RVS) HgCdTe/Si
Focal Plane Arrays (FPA) and shows 2k x 2k format MWIR HgCdTe/Si FPA performance with NEdT operabilities
better than 99.9%. SWIR and MWIR detector performance for HgCdTe/Si is comparable to established performance of
HgCdTe/CdZnTe wafers. HgCdTe devices fabricated on both types of substrates have demonstrated very low dark
current, high quantum efficiency and full spectral band fill factor characteristic of HgCdTe. HgCdTe has the advantage
of being able to precisely tune the detector cutoff via adjustment of the Cd composition in the MBE growth. The
HgCdTe/Si detectors described in this paper are p-on-n mesa delineated architecture and fabricated using the same
mature etch, passivation, and metallization processes as our HgCdTe/CdZnTe line. Uniform device quality HgCdTe
epitaxial layers and application of detector fabrication processes across the full area of 6-inch wafers routinely produces
high performing detector pixels from edge to edge of the photolithographic limits across the wafer, offering 5 times the
printable area as costly 6×6cm CdZnTe substrates. This 6-inch HgCdTe detector wafer technology can provide
applications demanding very wide FOV high resolution coverage the capability to produce a very large single piece
infrared detector array, up to a continuous image plane 10×10 cm in size. Alternatively, significant detector cost
reduction through allowing more die of a given size to be printed on each wafer is possible, with further cost reduction
achieved through transition towards automated detector fabrication and photolithographic processes for both increased
yields and reduced touch labor costs. RVS continues to improve its FPA manufacturing line towards achieving low cost
infrared FPAs with the format, size, affordability, and performance required for current and future infrared applications.
Raytheon Vision Systems (RVS) is producing large format, high definition HgCdTe-based MWIR and SWIR focal plane
arrays (FPAs) with pitches of 15 μm and smaller for various applications. Infrared sensors fabricated from HgCdTe
have several advantages when compared to those fabricated from other materials -- such as a highly tunable bandgap,
high quantum efficiencies, and R0A approaching theoretical limits. It is desirable to operate infrared sensors at elevated
operating temperatures in order to increase the cooler life and reduce the required system power. However, the
sensitivity of many infrared sensors, including those made from HgCdTe, declines significantly above a certain
temperature due to the noise resulting from increasing detector dark current.
In this paper we provide performance data on a MWIR and a SWIR focal plane array operating at temperatures up to
160K and 170K, respectively. The FPAs used in the study were grown by molecular beam epitaxy (MBE) on silicon
substrates, processed into a 1536x1024 format with a 15 μm pixel pitch, and hybridized to a silicon readout integrated
circuit (ROIC) via indium bumps to form a sensor chip assembly (SCA).
This data shows that the noise equivalent delta temperature (NEDT) is background limited at f/3.4 in the SWIR SCA
(cutoff wavelength of 3.7 μm at 130K) up to 140K and in the MWIR SCA (cutoff wavelength of 4.8 μm at 115K) up to
115K.
Polarimetry sensor development has been in work for some time to determine the best use of polarimetry to differentiate
between manmade objects and objects made by nature. Both MWIR and LWIR Focal Plane Arrays (FPAs) have been
built at Raytheon Vision Systems each with exceedingly higher extinction ratios. This paper compares field imagery
between MWIR and LWIR micro-grid polarimetric sensors independently and during simultaneous image collects.
LWIR polarimetry has the largest polarimetric signal level and an emissive polarimetric signature which allows
detection at thermal crossover and is less dependent on sun angles. Polished angled glass and metal objects are easily
detected using LWIR polarimetry. While LWIR
polarimetry has many advantages its resolution is not as
good as MWIR.
MWIR polarimetry has higher resolution than LWIR. With
good sun angles plastic drums, and wet surfaces provide
good polarization signatures. With poor sun angles
detection can be challenging.
To gain acceptance polarimetric sensors must provide
intelligence signatures that are better than existing nonpolarimetric
Infrared sensors. This paper shows several
examples of images without polarimetric processing and
identical images with MWIR and/or LWIR polarimetric
fusion onto the non-polarized images to show the
improvement of detection using polarimetric sensors. It is
the author's belief that the fastest way to gain acceptance of
polarimetric remote sensing is through field demonstration
as shown in Figure 1.
This paper presents the infrared detector performance improvement accomplishments by Raytheon Vision Systems (RVS) and by AVYD Devices Inc (AVYD). The RVS-AVYD collaboration has resulted in the demonstration of very large imaging focal plane arrays with respectable operability and performance which could potentially be useful in a variety of promising new applications to advance performance capability for future near and short wave infrared imaging missions. This detector design concept potentially permits ultra-small pixel large format imaging capabilities for diffraction limited resolution down to 5μm pitch focal planes. In this paper, we report on the work performed at the RVS's advanced prototype engineering facility, to fabricate planar detector array wafers with a combination of RVS's Hg1-xCdxTe production material growth and detector fabrication processes and AVYD's p-type ion-implantation process. This paper will review the performance of a 20μm pitch 1,024 x 1,024 format SWIR focal plane array. The detector array was fabricated in Hg1-xCdxTe material responsive from near-infrared to 2.5μm cutoff wavelength. Imaging capability was achieved via interconnect bump bond connection of this detector array to an RVS astronomy grade readout chip. These focal plane arrays have exhibited outstanding quantum efficiency uniformity and magnitude over the entire spectral range and in addition, have also exhibited very low leakage current with median values of 0.25 electrons per second. Detector arrays were processed in engineering grade Hg1-xCdxTe epitaxial layers grown with a modified liquid phase epitaxy process on CdZnTe substrates followed by a combination of passivation/ion implantation/passivation steps. This paper will review the detector performance data in detail including the test structure current-voltage plots, spectral cutoff curves, FPA quantum efficiency, and leakage current.
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 has achieved significant improvements in VLWIR (very long wavelength infrared) detector materials. These small bandgap detector materials are susceptible to tunneling of carriers across the bandgap via either band-to-band or trap-assisted tunneling phenomena. RVS' new procedures reduce exposure of the highly sensitive p-n junction to possible contamination sources during growth and processing. This reduction in impurities reduces the tunneling component of the detector current and yields high quality detectors out to the VLWIR range. Evidence of a highly uniform detector fabrication process is detailed within spanning the LWIR to VLWIR wavelength range.
This paper details significant improvements in current-voltage (I-V) modeling capabilities using an automated iterative non-linear fitting program. The properties of a particular infrared (IR) detector's I-V curve are dependent upon the current limiting mechanisms in the device which depend upon the temperature, applied bias, and cutoff wavelength or detector bandgap. This model includes ideal diode diffusion, generation-recombination, band-to-band tunneling, trap-assisted tunneling, shunt resistance, and avalanche breakdown as potential current limiting mechanisms in an IR detector diode. The modeling presented herein allows one to easily distinguish, and more importantly to quantitatively compare, the amount of influence each current limiting mechanism has on various detector's I-V characteristics. Modeling of the trap-assisted-tunneling mechanism leads to an estimate of the density of occupied trap states at a given temperature. This model is now routinely applied to Raytheon Vision Systems’ test structures to better understand detector current limitations.
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.
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