Carbon nanotubes grown from a surface typically have poor bonding strength at the interface. A process has been developed for adding a metal coat to the surface of carbon nanotubes (CNTs) through a “wicking” process, which could lead to an enhanced bonding strength at the interface. This process involves merging CNTs with indium as a bump-bonding enhancement.
Classical capillary theory would not normally allow materials that do not “wet” carbon or graphite to be drawn into the spacings by capillary action because the contact angle is greater than 90°. However, capillary action can be induced through JPL’s ability to fabricate oriented CNT bundles to desired spacings, and through the use of deposition techniques and temperature to control the size and mobility of the liquid metal streams and associated reservoirs. A reflow and plasma cleaning process has also been developed and demonstrated to remove indium oxide, and to obtain smooth coatings on the CNT bundles.
This work was done by James L. Lamb, Matthew R. Dickie, Robert S. Kowalczyk, and Anna Liao of Caltech; and Michael J. Bronikowski of Atomate Corporation for NASA’s Jet Propulsion Laboratory. NPO-46594
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Carbon Nanotube Bonding Strength Enhancement Using Metal “Wicking” Process
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Overview
The document outlines a research initiative conducted by NASA's Jet Propulsion Laboratory (JPL) focused on enhancing bonding strength in electronic packaging through the use of carbon nanotubes (CNTs) and Indium in a "wicking process." The primary objective of the project is to develop a proof of concept for utilizing oriented CNTs and CNT bundles to significantly increase the pitch (density) of connections in bump bonding hybridization, aiming for an order of magnitude improvement over current technologies.
The research identifies several secondary objectives, including enhancing electrical conductivity, bonding strength at the CNT interface, and thermal management. Single Wall CNTs can exhibit electric current densities up to 1,000 times greater than traditional metals like silver or copper, which could lead to reduced contact area requirements and minimized I²R heating issues. The bonding strength enhancement is particularly relevant for CNT field emitters, where the CNT/In hybrid may provide a solution to existing challenges. Additionally, the higher aspect ratio and strength of the CNT/In hybrid could improve thermal management by offering a greater surface-to-volume ratio and facilitating gas cooling between chips.
The document emphasizes the importance of increasing bump density as chip applications continue to shrink, particularly for Focal Plane Arrays and optical interfaces in analog electronic neural networks. The hybridization of Indium bumps and CNTs is expected to yield benefits such as improved thermal management and increased current densities, while also enhancing interface bonding strength.
Two New Technology Reports were filed as a result of this research: NTR 46592, which discusses bump bonding enhancement through the use of oriented CNTs with a metal in a wicking process, and NTR 46594, which focuses on bonding strength enhancement via the same method.
The document acknowledges contributions from key individuals, including Michael J. Bronikowski and Risaku Toda, who provided valuable technical advice. It also notes that no publications have been produced from this research yet, but the findings are considered relevant to the strategic focus area of remote-sensing systems, particularly in the development of advanced instruments, sensors, and detectors.
Overall, this research represents a significant step toward improving hybrid packaged devices, addressing alignment and planarity issues, and enhancing the performance of electronic components in various applications.
