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

Monday, 6 October 2014: 11:10
Expo Center, 1st Floor, Universal 9 (Moon Palace Resort)
M. Fujino, H. Terasaka, and T. Suga (The University of Tokyo)
Carbon nanotubes (CNTs) are expected to be a substitutional wiring material instead of copper, because of the properties of high maximum allowable current density, and the low resistivity. Especially, bundled vertically aligned CNTs (VA-CNTs) have the flexible structure and it is considered that VA-CNTs are suitable for vertical interconnect among flexible substrates.
In this research, we fabricated VA-CNTs as bump shaped structures and located on flexible substrates for flexible multilayer substrates and their electrical property was investigated.
At first, VA-CNT bumps were synthesized on SOI substrate by the thermal chemical vapor deposition method using Fe as catalyst and acetylene gas as source gas. Secondly, Au was sputtered on the substrate of VA-CNT bumps by magnetron sputter. In this process, surface shells of top of VA-CNT were torn by plasma energy and sputtered Au particle was directly contacted to the edges of torn layer of VA-CNTs and inner layers of CNTs for low contact resistance. Subsequently, the substrate of VA-CNT bumps was bonded to Au electrodes on polyimide substrate by surface activated bonding method, that surfaces of specimens were cleaned and activated by Ar ion beam or Ar plasma in vacuum, then they were bonded each other.
After bonding, the SOI substrate was removed from VA-CNTs and polyimide substrate, then sputtering process and surface activated bonding to polyimide substrate process were performed again to opposite side. The bonded substrates have a flexible structure and its flexibility against the electrical property was evaluated.
As a result, the bonded structure was confirmed to be tolerant of bending 2 to 3 mm curvature. Furthermore, the resistivity of VA-CNT bumps was decreased from 106 to 7.8 Ω under more than 0.75 MPa compression.
Thereafter, the VA-CNT bump were required to be bonded more than 0.75 MPa for good conductivity, and the conductivity was stable under the compression more than 0.75 MPa.
It indicates that the flexibility of VA-CNT/polyimide substrates were effective on condition of preload to VA-CNT bumps.