The performances of large focal-plane arrays of quantum-well infrared photodetectors (QWIPs) would be improved, according to a proposal, by incorporating microlenses. In comparison with a similar QWIP array that lacked microlenses, a QWIP array with microlenses could be made to exhibit less dark current [and thus a greater signal-to-noise ratio (SNR)] at a given temperature. Alternatively, the QWIP array with microlenses could be made to exhibit a given SNR at a higher temperature; this would be advantageous in that it would open up the possibility of operating QWIPs in infrared cameras at higher temperatures, reducing the cost of cooling the QWIPs to obtain adequate SNRs.
It would be advantageous to reduce dark current for two reasons:
- The noise current of a photodetector is proportional to the square root of its dark current.
- Along with signal current, dark current contributes to filling of the charge-storage wells of a readout multiplexer. Because of this and because the amount of charge that can be stored is finite, the available charge-integration time decreases with increasing dark current. As the charge-integration time decreases, the SNR decreases.
Assuming that the QWIPs in a given array were designed to operate with back-side illumination, the microlenses would be formed on the back side (the substrate side) of the array (see figure). There would be one microlens for each pixel. The basic function of the microlenses would be to concentrate incident infrared radiation (or preserve optical area) into a fraction of the area of each pixel. Concomitantly, the active device area in each pixel would be reduced to encompass only the reduced illuminated area plus (if desired) an appropriate margin. Inasmuch as the dark current of a QWIP is proportional to its area, the dark current would be reduced accordingly.
Suppose, for example, that a QWIP array had a 50-µm pixel pitch with active pixel areas of 45 by 45 µm. Microlenses could be used to concentrate light into 15-by-15-µm active pixel areas. If the active pixel areas were reduced to 15 by 15 µm, then the dark current at a given temperature would be reduced to 1/9 of its previous value, and therefore, the noise current at that temperature would be reduced to 1/3 of its previous value.
This work was done by Sarath Gunapala, Sumith Bandara, and Fred Pool of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240
Refer to NPO-20309
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Microlenses on focal-plane arrays of QWIPs
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Overview
The document discusses advancements in Quantum Well Infrared Photodetectors (QWIPs) through the integration of microlenses on focal-plane arrays. The primary focus is on how these microlenses can significantly enhance the performance of QWIP arrays by addressing the challenges posed by high dark current, which negatively impacts the signal-to-noise ratio (SNR) and overall sensitivity of infrared detection systems.
Microlensed QWIPs utilize a microlens array placed on the back surface of the QWIP focal plane array. Each microlens is designed to concentrate incident infrared radiation onto a smaller active area of the pixel, effectively reducing the pixel size from a larger area (e.g., 45 µm x 45 µm) to a smaller one (e.g., 15 µm x 15 µm). This reduction in active area leads to a corresponding decrease in dark current, which is proportional to the pixel area. Consequently, the noise current is also reduced, enhancing the SNR by a factor of three compared to QWIPs without microlenses.
The document outlines the mathematical relationships governing the performance improvements, including the calculation of peak detectivity (D*) and noise current (in). The use of microlenses allows for longer charge-integration times, which is crucial for maintaining high SNR in infrared systems. By reducing the dark current, the microlensed QWIPs can operate at higher temperatures, further improving their functionality.
The research was conducted by a team from Caltech for NASA's Jet Propulsion Laboratory, highlighting the innovative nature of this technology. The findings suggest that microlensed QWIPs could lead to significant advancements in infrared imaging systems, making them more sensitive and efficient.
Overall, the document emphasizes the potential of microlenses to transform QWIP technology, offering a solution to the limitations imposed by dark current and enabling the development of more effective infrared detection systems. This advancement could have wide-ranging applications in various fields, including aerospace, environmental monitoring, and security.
