Carbon Nanotubes Grown at Predefined Sites on Silicon Wafers
Brian Dance, Contributing Editor -- Semiconductor International, 9/1/2002
Scientists at Infineon Technologies AG (Munich, Germany) claim to have made a crucial breakthrough in carbon nanotube research. They modified an established microelectronics process to grow carbon nanotubes at predefined sites on 150 mm wafers. These nanotubes have properties that make them one of the most attractive materials for future semiconductor technology.
The extremely high electrical conductivity of carbon nanotubes is of great interest. The electrical length is almost independent of tube length, as ballistic transport effectively bypasses Ohm's law. Quantum mechanical effects result in a resistance of 6.5 kΩ/tube. This resistance can be further reduced by the operation of a number of tubes in parallel. This remarkable property enables current densities in the tubes to be up to 1010A/cm2, whereas copper starts to melt at current densities on the order of 10 MA/cm2. It has been predicted that chip wiring will be required to handle current densities of 3.3 MA/cm2 10 years from now — a value that is almost impossible to attain with conventional semiconductors without extreme heating.
The thermal conductivity of the carbon nanotubes has been calculated as almost twice that of diamond, whose thermal conductivity of 3000 W/(Km) exceeds that of all other conventional materials. It is expected that this extremely high value, together with the high electrical conductivity, will greatly simplify the design of devices such as fast, high-power processors. Manufacturing techniques to produce carbon nanotubes, such as laser ablation and arc discharge, are difficult to combine with semiconductor production technology. However, the Infineon group grew carbon nanotubes by modifying a deposition process that is widely used in microelectronics. "Many process parameters, such as the temperature and materials, are completely compatible with the standard processes used in semiconductor manufacturing," said Franz Kreupl, one of the researchers.
"The present results are completely reproducible and the structures grow at the predefined locations with sufficient homogeneity over the whole wafer," said Soenke Mehrgardt, chief technology officer at Infineon. "The growth process lasts only a few minutes. These are optimum prerequisites for integration in semiconductor production line processes."
The first possible application of the carbon nanotubes is in vias that are the contact bridges between two metal layers in ICs. Owing to the high current densities and associated heating, conventional vias tend to distort and impair the operational ability of the chips. This problem should be rectified by using nanotubes that can handle much larger current densities and have a much greater mechanical stability. "Within this discovery, we can consider replacing all of the metal conductors in the chips with carbon nanotubes," Kreupl said.
Another important characteristic of the carbon nanotubes is that they can be made semiconducting and can be doped, enabling production of new types of active switching elements such as field-effect transistors. Varying the tube diameter can control the energy bandgap of the semiconducting tubes. Infineon says that the energy bandgap typically corresponds to 1 eV for a diameter of 1 nm, which is comparable to the relationship for silicon-based transistors. Infineon researchers are working on the growth of semiconducting carbon nanotubes on wafers by the same catalytic deposition technique.
"The whole topic has a very promising future. It is very possible that this technology could completely replace silicon-based semiconductor technology," said Wolfgang Hoenlein, senior director in the field of nano-process research, who is leading the Infineon team. In this case the relatively expensive silicon could be replaced by glass, for example. Infineon's experts are already trying scenarios in which carbon nanotubes are used to extend the current planar microelectronics into a proper 3-D technology.
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