IEDM: Carbon nanotube vias approach production densities
Researchers report they are close to achieving the density of (CNTs) needed to manufacture carbon nanotube interconnect vias for production applications.
At IEDM, a group from Grenoble, France-based CEA LITEN and CEA LETI, École Polytechnique Fédérale Lausanne in Switzerland, and the UK's Cambridge University presented 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 x 1013 walls/cm2 required for interconnect vias. This density is an order-of-magnitude beyond the previous state-of-the-art.
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.)
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).
(a) Cross-section of a 250nm CNT via on AlCu after CMP. (b) High-density CNT growth in 500nm vias on AlCu line. (Source: CEA LITEN)
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. — Laura Peters, Contributing Editor