CNTs Attractive Alternative for Interconnects

Carbon nanotubes (CNTs) could play an important role in how on-chip interconnects are fabricated in the future. CNTs have unique properties, including the ability to sustain a high current density, exceeding 10⁹ A/cm², which is about three orders of magnitude more than that of conventional wiring. CNTs also have ultrahigh thermal conductivity, enable ballistic transport along the tube, have extremely high mechanical strength, and are resistant to electromigration (EM).

It's not yet clear when CNTs might be required or if they will actually prove to be “production-worthy,” but it is clear that there are at least two major challenges moving forward with the copper technology used today. First, copper resistivity rises dramatically from its bulk value as the width of the wire is reduced, caused by increased surface and grain boundary electron scattering. Second, smaller lines create higher current density, which in turn create higher temperatures and a greater potential for EM problems.

Alternative options for future nanoscale interconnect, according to Gael Close and H.S. Philip Wong of Stanford University (Stanford, Calif.), include conducting polymers, metallized DNA, metallic nanowires and metallic CNTs.¹ Of these, computer simulations show that nanotubes have the most promise, and could outperform copper in terms of power, delay, cross talk and reliability.

At this year's International Interconnect Technology Conference (IITC), to be held June 4-6 at the Hyatt Regency Hotel at the San Francisco airport, research efforts into the use of CNTs for interconnects will be reported. A MIRAI-Selete (Yokohama, Japan) team will discuss how they grew a forest of CNTs not just in the vias, but also all over the substrate's surface.

Another paper on the topic of nanotubes will explain how to make the CNT bundles denser in order to reduce their resistance. To date, it has proven impossible to grow closely packed CNT bundles. A team from Rensselaer Polytechnic Institute (RPI, Troy, N.Y.) decided instead to focus on densifying the bundles once they were grown. CNTs are grown using chemical vapor deposition (CVD) methods in quartz-tube furnaces using two different recipes. With Recipe 1, silicon substrates with 500 nm thermally grown SiO₂; are used for CNT bundle growth. At first, the SiO₂; surface is selectively masked with Ti/Au thin films. The substrate is then exposed to a mixture of ferrocene and xylene vapor at 800°C. Xylene acts as the source of carbon atoms, while iron from ferrocene serves as the catalyst for CNT growth. Vertically aligned CNTs grow on SiO² but not on Au/Ti, forming CNT bundles with desired shapes defined by Au/Ti patterns.

With Recipe 2, the patterned catalyst Fe/Al (1.5 nm/10 nm) is deposited on the SiO₂;/Si substrate using a shadow mask. Vertically aligned CNTs are grown on the catalyst at 775°C with ethylene as the carbon source. As-grown CNT bundles are immersed in an organic solvent isopropyl alcohol in a Petri dish. The IPA solvent is then dried out by evaporation in atmosphere at room temperature. IPA evaporation lowers the liquid level, which is equivalent to the withdrawal of CNT bundles from the liquid bath. CNTs are expected to be pulled toward each other by the capillary forces, and then stick together because of van de Waals forces.

After densification, the bundle shape changes from the initial cylinder shape to the dog-bone shape. If CNTs are perfectly aligned, the densified bundle is expected to have a nail shape. The dog-bone shape is believed to be caused by messy CNT networks at the bundle ends (Fig. 1a). They function as anchors similar to the fixed CNT roots. Consequently, only the middle part of a CNT bundle can be compacted (Fig. 1b ).

1. Side view of a CNT bundle end before (left) and after (right) densification process.

Figure 2 shows the densification of CNT bundles grown with Recipe 2. These CNTs have a smaller diameter (~8 nm), higher initial site density (~50 tubes/µm²) and better alignment. Unlike CNT bundles grown with Recipe 1, the bundle end was also significantly densified.

2. Recipe 2 CNT bundles before (left) and after (right) densification.