Yap et al. Nanoscale Research Letters 2012, 7:105 http://www.nanoscalereslett.com/content/7/1/105 N A N O E X P R E S S Open Access Carbon nanotube bumps for the flip chip packaging system Chin Chong Yap1,2, Christophe Brun1,3, Dunlin Tan1,2, Hong Li2, Edwin Hang Tong Teo1,2,4*, Dominique Baillargeat1 and Beng Kang Tay1,2 Abstract Carbon nanotube [CNT] interconnection bump joining methodology has been successfully demonstrated using flip chip test structures with bump pitches smaller than 150 μm. In this study, plasma-enhanced chemical vapor deposition approach is used to grow the CNT bumps onto the Au metallization lines. The CNT bumps on the die substrate are then ‘inserted’ into the CNT bumps on the carrier substrate to form the electrical connections (interconnection bumps) between each other. The mechanical strength and the concept of reworkable capabilities of the CNT interconnection bumps are investigated. Preliminary electrical characteristics show a linear relationship between current and voltage, suggesting that ohmic contacts are attained. Keywords: CNT bumps, interconnects, flip chip, packaging Introduction Flip chip technology is one of the off-chip interconnect methodologies used in electronic packaging. According to the International Technology Roadmap for Semicon- ductor, the forecasted requirement for flip chip bump pitches will be shrinking them beyond 150 μm [1]. However, traditional solder bumps had difficulties downscaling beyond the 100-μm pitch size due to the high diffusive and softening nature of the solder [2]. Carbon nanotubes [CNTs] show excellent electrical, thermal, and mechanical properties and have been viewed as one of the emerging choices for future flip chip interconnect [3]. As compared with metal, CNTs possess a higher current carrying capacity (109 A/cm2), and theoretical studies had also shown that CNTs have a negligible skin depth effect and are free from the high-frequency current crowding issue due to their large kinetic inductance and negligible magnetic inductance [3]. These advantages motivate the researchers to evalu- ate the performance of the CNT bump for interconnect usage in both direct current [DC] and high frequency applications [3,4]. * Correspondence: HTTEO@ntu.edu.sg 1CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore, 637553, Singapore Full list of author information is available at the end of the article CNT bumps had been demonstrated by several groups as potential off-chip interconnects [4-6]. Soga et al. have shown the bumps’ good mechanical flexibility and low bundle resistance of 2.3 Ω (for a 100-μm diameter bump) [5]. Hermann et al. demonstrated a reliable electrical flip chip interconnect using CNT bumps over 2,000 tempera- ture cycles [4]. CNT bumps for practical applications such as high-power amplifier application had also been demonstrated [6]. In all the mentioned works, the CNT bumps were grown using the chemical vapor deposition [CVD] approach. The mechanism for vertical alignment during the CVD approach is achieved by the van der Waals forces between the walls of CNTs, resulting in tubes that are not exactly ‘aligned’ [7]. The poor ‘align- ment’ forms bends, reduces the mean free path [m.f.p.], and increases the resistance of CNTs [8]. Plasma- enhanced CVD [PECVD] is able to resolve this issue by the introduction of electric field to achieve alignment as well as lower growth temperature [9]. In the flip chip scenario, the as-grown CNT bumps are usually pressed onto pre-patterned conductive adhesive [5] or solder materials [10] to form the connections. This approach requires heating up the structure to a mini- mum of 200°C in order to reflow the materials to obtain good contacts. Yung et al. demonstrated a large-scale assembly process using the CNT interconnection bump showing that CNTs adhere well to each other by van der © 2012 Yap et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.