Cross-talk in integrated-circuit focal-plane arrays of quantum-well photodetectors (QWIPs) equipped with microlenses would be reduced, according to a proposal, by etching deep trenches into the substrates of these devices. The proposal applies, more specifically, to GaAs-based, back-side-illuminated QWIP arrays with microlenses - devices like those described in the preceding article.
The cross-talk problem in such a device without trenches would arise as follows: The microlenses would be formed by patterning the back side of the substrate, as described in the preceding article. The lenses would focus the incident infrared light into and through sub-pixel-size active device (QWIP) areas. Most of the focused light would not be absorbed by the QWIPs, due to lower quantum efficiency, and would, instead, be scattered from patterned reflective surfaces on the front side. A significant portion of the light scattered in each pixel would travel through the unthinned substrate to neighboring pixels, where some of it would be absorbed, thereby giving rise to cross-talk. The cross-talk-reduction problem would thus become one of preventing the scattered infrared light from traveling through the substrate to neighboring pixels.
The problem could not be solved by thinning the entire substrate to the membrane level because such thinning would make it impossible to achieve the required focal length of the microlenses. The proposal would afford the optical advantage of microlenses, without the optical disadvantage of thinning the entire substrate. Instead of thinning the entire substrate, one would etch the substrate only along the boundaries between neighboring pixels; in other words, one would etch deep trenches in the substrate between microlens/pixel units. Such trenches are shown in the figure of the preceding article. Because of the large difference between the indices of refraction of air and the GaAs substrate, the trenches would be highly effective as optically isolating cavities to reduce cross-talk.
This work was done by Sarath Gunapala, Sumith Bandara, and John Liu of Caltech m 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 OfficeJPLMail Stop 122-1164800 Oak Grove DrivePasadena, CA 91109(818) 354-2240
Refer to NPO-20311
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Trenches would reduce cross-talk among mirolensed QWIPs
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Overview
The document discusses a novel approach to reducing optical cross-talk in microlensed quantum well infrared photodetector (QWIP) focal plane arrays, developed by researchers at NASA's Jet Propulsion Laboratory. QWIPs are designed to detect infrared (IR) light, utilizing quantum wells that can be engineered to have discrete energy states. When a photon strikes the well, it excites an electron from the ground state to the first excited state, generating a photocurrent. The design of these quantum wells allows for the detection of specific wavelengths of light, particularly in the range of 6 to 25 micrometers, by adjusting the potential depth and width of the well.
The document highlights a significant challenge in QWIP arrays: pixel-to-pixel optical cross-talk, which occurs when scattered light from one pixel affects the readings of neighboring pixels. This issue is exacerbated in microlensed QWIP arrays, where microlenses focus incoming light onto sub-pixel-sized active areas. However, much of the focused light is not absorbed and instead scatters, potentially reaching adjacent pixels and causing erroneous signals.
To address this problem, the proposal suggests etching deep trenches around each microlens in the substrate. This method creates optically isolating cavities that prevent scattered light from traveling through the substrate to neighboring pixels. The trenches are designed to maintain the required focal length of the microlenses while effectively reducing cross-talk. This solution is particularly advantageous as it combines the benefits of microlenses—improved light collection—with a means to mitigate the optical disadvantages associated with traditional thinning of the substrate.
The document also emphasizes the importance of minimizing dark current, which is the current that flows through a biased detector in the absence of light. High dark current can hinder the performance of QWIPs, especially at elevated temperatures. The proposed trench isolation technique aims to enhance the overall performance of QWIPs, making them more suitable for high-temperature applications.
In summary, the document presents a promising innovation in the design of QWIP arrays, focusing on trench isolation as a solution to cross-talk issues, thereby improving the efficiency and accuracy of infrared light detection in various technological applications.
