by Laura Peters, contributing editor

October 20, 2010 – Researchers will soon report they are close to achieving the density of (CNTs) needed to manufacture carbon nanotube interconnect vias for production applications.

At the upcoming International Electron Devices Meeting (IEDM, 12/6-8 in San Francisco, CA), a group from Grenoble, France-based CEA LITEN and CEA LETI, École Polytechnique Fédérale Lausanne in Switzerland, and the UK’s Cambridge University will present its methods used to achieve vias with a density of 2.5 × 10¹² tubes/cm² — equivalent to 8 × 10¹² walls/cm², nearing the value of 3 × 10¹³ walls/cm² required for interconnect vias. This density is an order of magnitude beyond the previous state of the art.

Carbon nanotubes are ideal as interconnect vias because they can carry currents of over 10⁸ A/cm². However, there are two key challenges preventing CNT incorporation in interconnect vias: reaching the necessary density and having a viable integration scheme. These researchers grew CNTs on metal alloy (99.5% aluminum, 0.5% copper) or polysilicon substrates using an iron catalyst. The AlCu alloy was chosen due to its low resistance at very small linewidths. Using a root-growth method at 580°C, 200mm wafers, and the process flow shown in Figure 1, the result was double- and triple-walled CNTs with via geometries from 250nm to 1μm (Figure 2).

Figure 1. Process flow for CNT fabrication: wet etch is followed by catalyst deposition, CNT growth, encapsulation with Al2O3 by ALD, CMP, then top contact. (Source: CEA/École Polytechnique/Cambridge U.)

To measure the density achieved, the researchers dipped the CNTs in alcohol, yielding a filling factor as high as 64%. The group was also able to measure the density as a function of via diameter, which varied from 5 × 10¹² to 8 × 10¹² walls/cm².

Cross-section of a 250nm CNT via on AlCu after CMP. (Source: CEA LITEN)	High-density CNT growth in 500nm vias on AlCu line. (Source: CEA LITEN)