Contact Behavior among Vertically Aligned Carbon Nanotube Bumps under Compression for Flexible Multilayer Substrates

Abstract

Vertically aligned carbon nanotubes (VACNTs) are promising candidates as vertical interconnects in multilayer substrates. Therefore, VACNTs were applied to the via structure in a multilayer flexible substrate as flexible bumps by surface activated bonding indeed. During the manufacturing process, VACNT bumps were compressed and deformed by the bonding pressure. In this research, VANCT deformation and structural mechanics were evaluated and analyzed under compression by an indenter. VACNT bumps were compressed to about 20% of their original height as multiple VACNT bundles folded under pressure more than 0.40 MPa. The VACNT compression height depended on the bonding pressure, which was within the range 0.04 to 0.20 MPa. Moreover, it was concluded that friction force between neighboring VACNTs caused plastic deformation of the VACNT bundles. © The Author Flexible assembly technology is one of the key factors for the development of modern electronics devices. This technology incorporates weight reduction, roll-to-roll technology for decreased production cost, reduction of mechanical stress caused by plastic deformation for die attachment or flip chip interconnects, and the application for wearable devices. High-density multilayer integration for flexible substrates provides miniaturization, high performance, and multi functionalization of devices. To achieve multilayer assembly of flexible substrates, interlayer connections are required for electrical contact between each layer and mechanical robustness. Metal nanoporous structures like adhesive silver paste 1 or structures produced by de-alloying and extracting one component from an alloy [2][3][4][5][6][7] possess the properties of electrical contact and robustness against mechanical deformation.On the other hand, carbon nanotubes (CNTs) are a popular solution for flexible interconnects. CNTs are expected to be a substitutional wiring material for copper because of their high maximum allowable current density of 10 9 A/cm 2 estimated from ion beam irradiation. 8CNTs can be constructed in vertically aligned structures by controlling the catalyst density and synthesis condition. 9 Vertically aligned carbon nanotubes (VACNTs) can serve as via structures by metal deposition on top of the VACNTs.10 VACNT bundles consist of entwined and nominally vertical carbon nanotubes which have the attractive material properties of compressibility and flexibility.11,12 Horizontal CNT wirings can also be obtained through use of horizontally grown CNTs 13 that are subsequently connected with metal electrodes.14 CNTs are typically synthesized by the thermal chemical vapor deposition (CVD) method at temperatures exceeding 500• C. In order to avoid this high temperature process, VACNTs are transferred after synthesis to other substrates by using thermocompression, 11,15 an adhesive conducting polymer, 16 a solder alloy, 17 or an adhesive conducting metal.18 For the thermocompression bonding method, the top surface of the VACNTs is coated by a metal and bonded to a metallic substrate at...

Bonding and transferring of carbon nanotube bumps using magnetron sputtering

“…At the bonded interface, VACNTs were bonded almost vertically to the substrate; otherwise, VANCTs above the interface were buckled and folded by the bonding load, as the authors previously reported. 36)…”