A variety of single color and multicolor Electro Optical/Infrared (EO/IR) Focal Plane Array technologies are being developed for Space System Applications. There has been a significant research and development effort to date to improve the technology in single and multicolor wavelength bands that include the SWIR, MWIR and LWIR spectral regions. This review paper will address the current technologies and new ones that will be useful for near term applications and also can provide further improvements for future systems applications.
LWIR and Multi-color LWIR Focal Planes are being developed for a variety of Space applications. A variety of Focal Plane array technologies, that includes HgCdTe, PbSnTe and other novel technologies are being developed. These detectors FPAs will require High quality two-dimensional, small unit cell, Silicon CMOS based ROICs for efficient read-out circuits. This Paper will discuss some of the salient features of various approaches being developed for the IR focal plane applications. We will also present Trade-off analysis for design of a trans-impedance input amplifier with reset and compact signal average for low noise performance. We will also discuss the various approaches for design of an A/D converter with high linearity and speed. We will discuss approaches to achieve ROIC with low noise floor, high dynamic range and high frame rate.
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 Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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 Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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 Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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.
High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation.
In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures.
We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si<sub>3</sub>N<sub>4</sub> and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.
Optical sensors aboard space vehicles designated to perform seeker functions need to generate multispectral images in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral regions in order to investigate and classify man-made space objects, and to distinguish them relative to the interfering scene clutter. The spectral imager part of the sensor collects spectral signatures of the observed objects in order to extract information on surface emissivity and target temperature, both important parameters for object-discrimination algorithms. The Adaptive Spectral Imager described in this paper fulfills two functions simultaneously: one output produces instantaneous two-dimensional polychromatic imagery for object acquisition and tracking, while the other output produces multispectral images for object discrimination and classification. The spectral and temporal resolution of the data produced by the spectral imager are adjustable in real time, making it possible to achieve optimum tradeoff between different sensing functions to match dynamic monitoring requirements during a mission. The system has high optical collection efficiency, with output data rates limited only by the readout speed of the detector array. The instrument has no macro-scale moving parts, and can be built in a robust, small-volume and lightweight package, suitable for integration with space vehicles. The technology is also applicable to multispectral imaging applications in diverse areas such as surveillance, agriculture, process control, and biomedical imaging, and can be adapted for use in any spectral domain from the ultraviolet (UV) to the LWIR region.
Advances in basic infrared science and developments in pertinent technology applications have led to mature designs being incorporated in civil as well as military area defense systems. Military systems include both tactical and strategic, and civil area defense includes homeland security. Technical challenges arise in applying infrared sensor technology to detect and track targets for space and missile defense. Infrared sensors are valuable due to their passive capability, lower mass and power consumption, and their usefulness in all phases of missile defense engagements. Nanotechnology holds significant promise in the near future by offering unique material and physical properties to infrared components. This technology is rapidly developing. This presentation will review the current IR sensor technology, its applications, and future developments that will have an influence in military and civil defense applications.
Low Cost Multi-color infrared (IR) sensors/focal plane arrays are required for surveillance and other homeland security applications. These sensors require multi-color focal plane arrays (FPA) that will cover 3-5 (MWIR) and 8-14 (LWIR) micron bands. There has been a significant progress in developing HgCdTe on Silicon substrates [1,2]. Two-color IR FPA eliminate the complexity of multiple single color IR FPAs and provide a significant reduction of weight and power in a simpler, reliable and affordable systems.
Infrared focal plane arrays are a critical component in many of the military and civilian applications for advanced imaging systems. Advanced material growth and etching techniques have made possible the fabrication of infrared detectors in various configurations and in a broad range of wavelengths for a variety of applications. In the last decade, many researchers have explored advances in the processing and growth techniques, which have made it possible to realize complex detector concepts, array architectures and improvements in the producibility of these devices.
In this paper, infrared detector materials and structures will be reviewed with emphasis on applicability to designs of focal plane arrays for single and multi-wavebands. Key developments and status of the technology will be presented along with projections and challenges for the continued evolution of the technology.
The traditional approaches using multiple focal plane arrays (FPAs), filters, beam splitters, cooling circuits etc, complicate system design, reliability and create difficulty in spatial alignment and temporal registration of image at the pixel level. Multiple colors on a single full-resolution FPA will greatly improve spectral discrimination capability at longer ranges. The integrated multicolor infrared FPAs in which a single pixel location is sensitive to two (or three) separate IR spectral bands will be the future generation technology for its use in target acquisition and signature recognition in a wide variety of space and ground based applications.
This paper reviews key developments and status of this important technology. Projections and challenges for the continued evolution of this technology are also discussed.
Multi-color infrared (IR) focal planes are required for high performance sensor applications. These sensors will require multi-color focal plane arrays (FPA) that will cover various wavelengths of interest in MWIR/LWIR and LWIR/VLWIR bands. There has been a significant progress in HgCdTe detector technology for multi-color MWIR/LWIR and LWIR/VLWIR focal plane arrays [1,2,3]. Two-color IR FPA eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in a simpler, reliable and affordable systems.
The complexity of multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally material characteristics are only relatively controllable and depend on the process repeatability. In this context the ability of performing simulation experiments where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications.
Complex multi-color detector pixels cannot be designed and optimized by using a conventional 1D models. Several additional physical phenomena need to be taken into account. In designing a conventional photovoltaic IR detector array, a trade off exists on the choice of the pixel pitch, the pixel area and its height. The main goal of the device optimization is to reduce the pixel cross talk while keeping a high filling factor and detection efficiency. If the pixel height is made comparable to the lateral pixel dimension the contribution of the lateral photocurrent and lateral generation-recombination current becomes relevant and a full 2D simulation needs to be performed. It also important to point out that the few attempts to perform 2D simulations have reached the conclusion that for advanced IR arrays a full 3D approach should be used. The most challenging aspect of the array design and simulation is the pixel cross-talk effects. Since this is caused by the interaction with the four nearest neighboring pixels, even a description based on a 2D simulation model in most cases is not adequate. It is consequently important to include results from 3D simulation models as a guide to build lower dimensionality models.
The advanced planar ion-implantation-isolated heterojunction process, which utilizes the benefits of both the boron implantation and the heterojunction epitaxy techniques, has been developed and used to produce longwave and very longwave HgCdTe focal plane arrays in the 320v256 format. The wavelength of these arrays ranges from 10.0-17.0μm. The operability of the longwave HgCdTe arrays is typically over 97%. Without anti-reflection coating and with a 60° FOV cold shield, the D* of the 10.0μm array is 9.4x10<sup>10</sup>cm x (Hz)<sup>1/2</sup> x W<sup>-1</sup> at 77K. The 14.7μm and 17.0μm very longwave HgCdTe array diodes have excellent reverse characteristics. The detailed characteristics of these arrays are presented.
SWIR HgCdTe photodiode test chips and 256x256 Focal Plane arrays with a 2.1 micron cutoff wavelength have been fabricated and tested.
The base material was n-type HgCdTe. P-type junctions were created by ion implantation. Test chip arrays with 60-micron pixels exhibited an average R<sub>o</sub>A of 509 ohm-cm<sup>2</sup> and internal quantum efficiency (QE) of 98% at 295 K; R<sub>o</sub>A and QE were uniform. Average R<sub>o</sub>A increased to 2.22x10<sup>4</sup> at 250 K and internal QE remained high at 93%. The mini-array of 30-micron pixels had lower R<sub>o</sub>A values, 152 and 6.24x10<sup>3</sup> ohm-cm2 at 295 and 250 K, but 100% internal quantum efficiency at both temperatures. There was no bias dependence of quantum efficiency, demonstrating that our junction formation process does not give rise to valence band barriers.
FPA test data have demonstrated NEI operability greater than 98% at 220 K and greater than 97% at 250 K along with QE operability in excess of 99.9% at 220 K and in excess of 99.8% at 250 K.
Boron implantation and heterojunction epitaxy have been the standard techniques for the production of HgCdTe focal plane arrays for a variety of applications. Each of these techniques has its special advantageous features. In this paper, we will describe an advanced HgCdTe junction formation technique, the planar ion-implantation-isolated heterojunction process, which utilizes the benefits of both the boron implantation and the heterojunction epitaxy
techniques. HgCdTe arrays in the format of 320x256 and 640x512 have been produced by this method. The characteristics of these arrays are reported.
Mechanisms of incorporation of native defect and dopants in HgCdTe alloys are reviewed. Origin of the native defect related deep centers in limiting the minority carrier lifetime is explored. Primary and secondary mechanisms operative in the activation of n type and p type dopants in HgCdTe are discussed along with implications for fabrication of high performance detectors.