Improving Contact Resistance at the Nanotube−Cu Electrode Interface Using Molecular Anchors

Abstract

It is anticipated that future nanoelectronic devices will utilize carbon nanotubes (CNT) and/or single graphene sheets (SGS) as the low-level on-chip interconnects or functional elements. Here we address the contact resistance of Cu for higher level on-chip interconnects with CNT or SGS elements. We use first-principles quantum mechanical (QM) density functional and matrix Green's function methods to show that perfect Cu-SGS contact has a contact resistance of 16.3 MΩ for a one square nanometer contact. Then we analyzed possible improvements in contact resistance through incorporation of simple functional groups such as aryl (-C 6 H 4 -), acetylene (-CC-), carboxyl (-COO-), and amide (-CONH-), on CNT. We find that all four anchors enhance the interfacial mechanical stabilities and electrical conductivity. The best scenario is -COOH functionalized CNT which reduces the contact resistance to the Cu by a factor of 275 and increases the mechanical stability by 26 times.

Contact Resistance for “End-Contacted” Metal−Graphene and Metal−Nanotube Interfaces from Quantum Mechanics

“…24 There we used ab initio quantum mechanical (QM) studies to show that Ti leads to the lowest contact resistance of 24.2 kΩ/nm 2 followed by Pd (221 kΩ/nm 2 ), Pt (881 kΩ/nm 2 ), Cu (16.3 MΩ/nm 2 ), and Au (32.6 MΩ/nm 2 ) for the "sidecontacted" metal electrode (Figure 1b) to CNT or graphene. 25 Although the Cu-graphene interface has a contact resistance 672 times higher for Ti, we found that incorporation of bifunctional groups (anchors) can reduce the Cu-graphene contact resistance by a factor of 275, making Cu better than Pd by 3.7 times. 25 In this paper, we use QM to determine the electrical properties (e.g., contact resistance) for "end-contacted" (or vertical) metal-graphene and metal-CNT electrodes (Figure 1a).…”

“…25 Although the Cu-graphene interface has a contact resistance 672 times higher for Ti, we found that incorporation of bifunctional groups (anchors) can reduce the Cu-graphene contact resistance by a factor of 275, making Cu better than Pd by 3.7 times. 25 In this paper, we use QM to determine the electrical properties (e.g., contact resistance) for "end-contacted" (or vertical) metal-graphene and metal-CNT electrodes (Figure 1a). We find that this "end-contacted" metal electrode improves the contact resistance by up to a factor of 6751 while simultaneously increasing mechanical stabilities dramatically.…”

“…We previously reported contact resistances for the “side-contacted” metal electrode (Figure b) to CNT or graphene . There we used ab initio quantum mechanical (QM) studies to show that Ti leads to the lowest contact resistance of 24.2 kΩ/nm 2 followed by Pd (221 kΩ/nm 2 ), Pt (881 kΩ/nm 2 ), Cu (16.3 MΩ/nm 2 ), and Au (32.6 MΩ/nm 2 ) for the “side-contacted” metal electrode (Figure b) to CNT or graphene . Although the Cu−graphene interface has a contact resistance 672 times higher for Ti, we found that incorporation of bifunctional groups (anchors) can reduce the Cu−graphene contact resistance by a factor of 275, making Cu better than Pd by 3.7 times …”

Creating covalent bonds between Cu and C at the interface of metal/open-ended carbon nanotubes

“…14 According to DFT simulations, proper CNT functionalization 29 or connecting CNTs to the metal surface via a short linker molecule is more conducive to achieving low-resistance Cu-CNT interfacial interactions than direct CNT-metal bonding. 30 Furthermore, linker molecules have been used for bonding CNT tips to Al and Au. [31][32][33][34] The bond between the metal and the linker molecule is critical for utilizing linkers as intermediates for CNT-metal bonding.…”

Sidewall Chemical Bonding of Aligned Carbon Nanotubes with Metals: A Promising Route for Hybrid Materials Development

“…Linker molecules as bridging agents may also aid in overcoming the energy mismatch between CNTs and metals, thus providing a low-cost method for forming chemical bonds between CNTs and metals under mild conditions. Theoretical studies have shown that CNT–linker–metal structures provide better interface conductivity than mere physical metal–CNT interactions. , The formation of a self-assembled monolayer (SAM) on Au surfaces is commonly used to decorate the metal with organic linkers that have been used to immobilize CNTs on Au metal. − However, SAMs formation is metal-specific and results in weak linker–substrate interactions . Covalent bonding between diazonium species and substrates, which was introduced by Pinson et al, has been widely used to chemically bond organic groups to various surfaces, including C, , Fe, stainless steel (SS), , indium tin oxide (ITO), , Cu, − Al, and Au. − Computational and spectroscopic analysis have confirmed covalent bond formation between metals such as Au, Iron substrates and short organic aryl groups via the diazonium salt–substrate grafting reaction. , Linker molecules containing aminophenyl groups, previously used to attach nanostructures to substrates, have an amino group that can react with carboxyl groups on CNTs …”