Key elements for hybridization technology development of $2000\times 2000$ pixels Infrared Focal Plane Arrays
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Lynred develops infrared imaging sensors covering the entire infrared spectrum (SWIR, MWIR, LWIR, and VLWIR) through a variety of technologies including MCT (mercury cadmium telluride). Today with the increasing demand for better spatial resolution we observe the emergence of very large arrays detectors with ultra-fine pitch. The hybridization of such IR detector array onto a readout circuit represents one of the major limiting factors for the development of these detectors. Lynred has recently developed 2000×2000 pixels IR detectors down to 15 μm and 10 μm pitches, with successful and reproducible hybridization processes. In this paper we describe the main barriers and solutions put in place in order to realize these detectors, and the resulting prototypes are illustrated.Keywords:
Mercury cadmium telluride
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Infrared detector
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.
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In the case of a thermal imaging system to which a uncooled infrared detector is applied, the application range is expanding in various ways in the field of military and civilian use. In particular, in the case of a military thermal system, it is required to develop a high-resolution infrared sensor to upgrade the system, and in the case of a civil thermal system, it is required to miniaturize and lighten the thermal detector to apply to various applications. By reducing the pixel pitch of infrared sensors from 17um pitch to 8um pitch size, it is easy to develop miniaturized and high-resolution sensors, but overcoming the problem of decreasing infrared sensor performance is the key to technology development. The structural separation of an area absorbing incident infrared rays and an area for thermal isolation is required to maximize the performance of an infrared sensor with a reduced pixel pitch, and to implement this, it is required to develop a multi-layered microbolometer. In this paper, we develop an SXGA-class (1280*1024) uncooled infrared detector with an 8um pixel pitch and fabricate using multilayered FPA(focal plane array) design techniques and 0.18um 3D MEMS technology design techniques to improve absorption area and thermal isolation reduction as the pitch decreases. For miniaturization and weight reduction of infrared detectors, ceramic packages were applied to manufacture uncooled infrared detectors of NETD 50 mK or less. In addition, environmental and reliability test evaluation of uncooled infrared detectors was performed to verify system applicability and practicality.
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Mercury cadmium telluride
Photodiode
Dot pitch
Infrared detector
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Quantum Efficiency
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Mercury cadmium telluride
Infrared detector
Cadmium telluride photovoltaics
Focal Plane Arrays
Vapor phase
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Mercury cadmium telluride
Infrared detector
Mercury
Cadmium telluride photovoltaics
Solid-state physics
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The two-dimensional spatial response of a pixel in SCD's back-side illuminated InSb Focal Plane Array (FPA) is measured directly for arrays with a small pitch, namely 30, 20 and 15&mgr;m. The characterization method uses a spot-scan measurement and de-convolution algorithm to obtain the net spatial response of a pixel. Two independent methods are used to measure the detector spatial response: a) direct spot-scan of a pixel with a focused beam; b) uniform illumination upon back-side evaporated thin gold coating, in which sub-pixel apertures are distributed in precise positions across the array. The experimental results are compared to a 3D numerical simulation with excellent agreement for all pitch dimensions. The spatial response is used to calculate the crosstalk and the Modulation Transfer Function (MTF) of the pixel. We find that for all three pixel dimensions, the net spatial response width (FWHM) is equal to the pitch, and the MTF width is inversely proportional to the pitch. Thus, the spatial resolution of the detector improves with decreasing pixel size as expected. Moreover, for a given optics and smaller array pitch, the overall system spatial resolution is limited more by the optical diffraction than by the detector. We show actual improved spatial resolution in an imaging system with a detector of smaller array pitch.
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Mercury cadmium telluride
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Infrared detector
Cadmium telluride photovoltaics
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Mercury cadmium telluride
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More and more systems are requested to be more compact keeping constant system performances. One of the best approach is to reduce the pixel pitch of the IR detector while new technology improvements are carried out to improve the detector performance. The last developments at SOFRADIR / France for cooled IR detectors are following these trends. As a matter of fact, HgCdTe (Mercury Cadmium Telluride / MCT) staring arrays for infrared detection do show constant improvements regarding their compactness, by reducing the pixel pitch, and regarding performances. Among the new detectors, the family of 15 μm pixel pitch detectors is offering a mid-TV format (384 x 288), a TV format (640 x 512) and a HD-TV format (1280 x 1024). The latest development concerning the mid-TV format is performed according to very challenging specifications regarding compactness and low power consumption. Thanks to recent improvements, the MCT technology allows to operate detectors at higher temperature (HOT detectors), in order to save power consumption at system level. In parallel, the 15μm pitch permits to reach challenging density and spatial resolution. This Focal Plane Arrays (FPA) is proposed in different tactical dewars, corresponding to various systems solutions.
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