Carbon nanotubes (CNTs) have unique electrical properties. One of them is that CNTs can sustain a current density 2-3 orders higher than that of Cu. This property makes them a good candidate for interconnect materials of LSIs in the 32-nm technology node and beyond. In fact, we try to replace Cu vias (vertical wiring) with bundles of multi-walled carbon nanotubes (MWNTs)[1].
There are many issues to address to realize CNT vias. One is to grow high-density CNTs in a tiny via hole, since the electrical resistance is inversely proportional to the CNT density. To tackle this issue, we developed a method to form size-controlled catalyst particles and deposit them densely at the bottom of via holes. With this approach, we grew high-density MWNTs (1E11-1E12 /cm2) at 510 °C[1].
Another key issue is to grow CNTs at a low temperature, ideally at 400 °C or lower, since various materials used in LSIs are vulnerable to heat. In this paper, we describe the growth of MWNTs at temperatures down to 400 °C, and the electrical properties of CNT vias made at 450 °C.
To make CNT vias, a substrate composed of Cu wiring (100 nm thick), a Ta barrier layer (5 nm), a TiN layer (5 nm), and a SiO2 layer (350 nm) was first prepared. Via holes were made by etching the SiO2 layer. Co particles of 4 nm were then deposited at the via-hole bottom. The particles were size-classified with an impactor, and deposited by a system using differential pumping[1]. MWNTs were grown by thermal chemical vapor deposition at 450 °C using C2H2 diluted by Ar at 1 kPa. A Ti contact layer (50 nm) and Cu wire (300 nm) were then deposited at the top of MWNTs.
The resistance of CNT vias of 1.9 μm in diameter was as low as 1.2 Ω, which was one fifth of that of CNT vias from film catalyst. The difference can be explained by the difference in the CNT density. The resistances of twelve 1.9-μm vias ranged 1-2 Ω. The temperature dependence of the resistance suggests that carrier transport may be ballistic.
In the presentation, we will also explain high-quality MWNTs obtained at 400 °C and planarization of CNT vias by chemical mechanical polishing. This work was partly supported by NEDO.
[1] Sato. S. et al., Proc. IEEE IITC2006, 230 (2006) |