Flip-chip hybridization, also known as bump bonding, is a packaging technique for microelectronic devices that directly connects an active element or detector to a substrate readout face-to-face, eliminating the need for wire bonding. In order to make conductive links between the two parts, a solder material is used between the bond pads on each side. Solder bumps, composed of indium metal, are typically deposited by thermal evaporation onto the active regions of the device and substrate. While indium bump technology has been a part of the electronic interconnect process field for many years and has been extensively employed in the infrared imager industry, obtaining a reliable, high-yield process for high-density patterns of bumps can be quite difficult.
Under the right conditions, a moderate hydrogen plasma exposure can raise the temperature of the indium bump to the point where it can flow. This flow can result in a desirable shape where indium will efficiently wet the metal contact pad to provide good electrical contact to the underlying readout or imager circuit. However, it is extremely important to carefully control this process as the intensity of the hydrogen plasma treatment dramatically affects the indium bump morphology.
To ensure the fine-tuning of this reflow process, it is necessary to have real-time feedback on the status of the bumps. With an appropriately placed viewport in a plasma chamber, one can image a small field (a square of approximately 5 millimeters on each side) of the bumps (10-20 microns in size) during the hydrogen plasma reflow process. By monitoring the shape of the bumps in real time using a video camera mounted to a telescoping 12× magnifying zoom lens and associated optical elements, an engineer can precisely determine when the reflow of the bumps has occurred, and can shut off the plasma before evaporation or dewetting takes place.
This reflow process has been demonstrated to yield streak-free imagers, and repair misaligned or otherwise damaged indium bumps. It has also been demonstrated to yield non-resistive indium bump contacts in hybridized imagers having large arrays of information processing contacts. Without the reflow process, some 15 percent of the indium contacts were affected by unwanted resistance from oxidized or otherwise damaged indium bumps.
This technology has broad applications to all types of hybridized sensors and is not limited to space applications. Bump-bonding technology, in general, is useful in applications where a reduction in the packaging size of a completed device is advantageous. Because wire bonds are unnecessary in bump-bonded devices, the flip chips can sit directly on their corresponding circuit boards, resulting in a reduction of carrier area and height.
This work was done by Todd J. Jones, Shouleh Nikzad, Thomas J. Cunningham, Edward Blazejewski, Matthew R. Dickie, Michael E. Hoenk, and Harold F. Greer 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:
Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Pasadena, CA 91109-8099 E-mail:
Refer to NPO-47422, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Optimization of Indium Bump Morphology for Improved Flip Chip Devices
(reference NPO-47422) is currently available for download from the TSP library.
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Overview
The document titled "Optimization of Indium Bump Morphology for Improved Flip Chip Devices" is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL). It focuses on enhancing the performance of flip chip devices through the optimization of indium bump morphology, which is crucial for reliable electronic connections in aerospace applications.
The document outlines the significance of reflow processes in the manufacturing of flip chip devices. Reflow is a critical step where solder bumps are melted to create electrical connections between the chip and the substrate. The document discusses two types of reflow conditions: short reflow times, referred to as "tent," and long reflow times, referred to as "ball." The "tent" method, characterized by a height of 6-7 microns, is noted for its superior ability to wet the pad, potentially leading to better morphology and improved device performance.
Real-time monitoring of the reflow process is emphasized as a key factor in achieving optimal results. The document mentions the use of commercially available zoom lenses that enable real-time feedback, allowing for adjustments to be made during the reflow process. This capability is essential for maintaining the best reflow conditions, which are described as having a narrow time window of approximately 20 seconds. Deviations from these conditions can lead to poor reflow outcomes, negatively impacting the reliability of the flip chip devices.
The document also includes images that illustrate the reflow process before and after, highlighting the changes in morphology that occur during reflow. The use of a color camera for imaging is recommended to ensure sufficient contrast for effective monitoring.
Overall, the Technical Support Package serves as a resource for understanding the advancements in indium bump morphology optimization and the importance of real-time monitoring in the reflow process. It aims to provide insights that can be applied not only in aerospace technology but also in broader technological, scientific, and commercial applications. The document is part of NASA's efforts to disseminate aerospace-related developments that have potential wider applications, reflecting the agency's commitment to innovation and collaboration in technology development.
