Large format corrugated quantum well infrared photodetector (C-QWIP) focal plane arrays (FPAs) have been
developed over the past two years. The results of this development have demonstrated the potential for this technology
to satisfy requirements for very large format high performance long-wave infrared (LWIR) imaging systems. One
particular C-QWIP design has focused on developing an FPA that operates in the 8 to 10 &mgr;m spectrum with integration
times in the millisecond regime when used against warm backgrounds. This FPA is very suitable for many LWIR
applications and has been integrated into a camera system. The specifications of that camera are described in this paper.
The characterization of this camera system includes standard electro-optical tests and compares the results of those tests
to theoretical models for the FPA. This paper concludes by describing the ongoing effort to tailor the system
specifically for the C-QWIP. This includes design features of the read-out integrated circuit (ROIC), dewar-cooler
design and interfacing electronics, and video processing. This thorough characterization of the camera has
demonstrated the utility of the C-QWIP FPA for LWIR imaging and has established a path forward to further improve
the performance of imaging systems implementing this technology.
Current generation QWIP detectors, although very cost effective, have relatively narrow spectral range and low quantum efficiencies. Tactical operation is generally limited to a single spectral band. These limitations arise from the design approach and restrict
applications to those that can tolerate these performance limitations.
Using recent device design improvements, a novel material, and special processing approaches, High Quantum Efficiency Dual Band C-QWIP detectors are currently being developed. These are expected to overcome traditional limitations in the QWIP design approach and deliver extremely high performance.
In the first phase of the program, single color LWIR and VLWIR C-QWIP FPAs in large (1024x1024) format will be demonstrated with targeted peak quantum efficiency of 35%, and correspondingly high BLIP operating temperatures. In the next phase of the program, the team will continue to improve QE towards 50% with conversion efficiency of 75%, and demonstrate dual band MW/LW FPAs. The detector gain will be optimized for operation in either low background or high background applications. These goals will
be accomplished using highly producible/low cost materials and processes. System considerations include ROIC well capacity, noise performance, as optics configuration and other concerns will be addressed. A robust design for high performance in a variety
of applications will be shown.
This work is being performed by the Army Research Laboratory (ARL) and L-3 Cincinnati Electronics (CE), with funding provided by the Missile Defense Agency.
Recent requirements for modern low noise thermal imaging systems demand higher performance and more detailed characterization of the system. The statistical uncertainty inherent to the test system can often provide misleading information about system performance. An example would be a test that eliminates pixels based on certain performance parameters such as noise or responsivity. If the test uncertainty exceeds the true variance of the parameter, the test will yield results indicative of the test system rather than the parameter. This results in good pixels being eliminated that potentially impacts operability goals. A sign that test uncertainty dominates the test results is when operability remains nearly uniform between multiple tests while the pixels marked bad by the test changes between tests. In order to minimize the uncertainty in a test, one must consider all aspects of the test system that can affect test results. Those aspects include the physical construction of the test station as well as the underlying statistics associated with the measurement. This paper will show ongoing efforts at L-3 Cincinnati Electronics to lower test uncertainty, increase test repeatability, and qualify test systems for both focal plane array and system level electro-optics testing of thermal imagers.
The evolution of InSb Focal Plane Arrays (FPAs) at L-3 Communications Cincinnati Electronics (L-3 CE) has resulted in large format, high reliability, and high yields for 256x256, 640x512, 1Kx1K and even 2Kx2K formats using our patented front-side illuminated, reticulated pixel design. Baseline processes matured at 30um pitch and gradually were made producible at 25um pitch. Recent progress in process technology, specifically dry etch plasma processes and photolithography tools, has created a new set of processes/design capabilities which enable 15um pixel pitch FPAs, thus allowing us to develop a 15um pitch FPA with 4 times as many pixels, in the same foot print as the previous 30um pitch designs. We have developed a new 15um pitch, reticulated pixel design, implemented on a 512x512 format, which can then be sized into larger arrays, similar to the evolution that occurred on 30um pitch FPAs. As unit cell dimensions shrink by a factor of two, both the feature size and the alignment tolerances begin to limit optical fill factor. Addition of a novel micro-optic design, which optimizes signal collection to near 100% efficiency while maintaining near theoretical pixel MTF, will be presented.
Cincinnati Electronics has developed a high resolution, high sensitivity, commercial infrared camera, the IRC-160ST. This camera incorporates a 160 X 120 element Indium Antimonide (InSb) multiplexed focal plane array (FPA) integrated with a miniature closed cycle Stirling cooler. The resulting product is small enough for handheld applications and can be operated from an AC power source or battery. Highlights of this particular design include a high reliability integrated dewar/cooler assembly, compact electronic viewfinder, image correction electronics, display controls, and low power consumption. Size and power reductions resulted from incorporating an integrated dewar/cooler assembly (IDCA). Thermal performance issues relevant to the packaging of the IDCA are presented. Performance characteristics of the FPA/electronics such as uniformity, NEDT, and stability of corrections also are presented.
The study presents recent performance results for a 2D InSb hybrid focal plane array. The short wavelength (1 to 3 microns) response at liquid helium temperatures was improved, making the device more useful for astronomy and other scientific applications. The detectors exhibit high short wavelength quantum efficiency and good uniformity.
The performance of a recently developed indium antimonide (InSb) two-dimensional multiplexed medium wavelength infrared (MWIR) hybrid focal plane array (FPA) is presented. The CMOS FET switch array multiplexer individually buffers each detector in a 64 x 64 element array through a source follower amplifier. This multiplexer was designed for demanding low-background, high sensitivity requirements. The detector array consists of InSb photodiodes spaced on 100 micron centers, bump bonded through indium columns to the silicon multiplexer, and is thinned for backside illumination. The array is responsive to radiation in the 1 to 5.5 micron region. The FPA has been demonstrated to have a near theoretical read-out noise performance of less than 500 electrons. The, charge storage capacity is approximately 4 million electrons giving a dynamic range of 78 dB. The device is linear to better than 99.95%over the lower 30% of its dynamic range, and greater than 90% over its total range, reflecting the capacitive discharge nature of the charge integration. Dark currents of less than 10 pA are obtained at 77K with reverse biases as great as 0.5V, and less than 0.2 fA at 25K. Quantum efficiency greater than seventy-five percent has been achieved at the peak wavelength. Functional element yields of 99% have been obtained.
A recently developed indium antimonide high-sensitivity two-dimensional multiplexed medium wavelength infrared (MWIR) hybrid focal plane array (FPA) is described with special attention given to its multiplexer circuit and the detector array. The device parameters and the performance characteristics of the MWIR FPA are presented in tabular form, demonstrating excellent performance. The multiplexer schematic is included together with graphs of quantum efficiency at various wavelengths and mean dark current at various initial biases and temperatures.
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