Carbon Nanotubes (CNT)
Carbon Nanotubes for Interconnect and Sensor Applications
Since the discovery of Carbon Nanotubes (CNT) in the year 1991 big efforts were made to profit of their outstanding structural, electronic and mechanical properties in various applications. For electronic applications the capability of carrying extreme current densities, the insensitivity to electro-migration, and the excellent thermal conductance protrude. This makes CNTs an ideal candidate as an via-material in future interconnect systems of IC-devices which is our major interest. Beyond that we are working on the implementation of CNTs as basic elements in NEMS and as sensors with high sensitivity and selectivity. The aim of our work is to achieve selective growth of high quality and dense CNT films at temperatures lower than 450°C. Furthermore good electrical contacts at the CNT-metal interface and the alignment of MWNTs as well as SWNTs are essential for our purposes. For the integration of CNTs we follow two approaches:
(1) Chemical vapour deposition (CVD) is applied for direct growth of CNTs on substrates. For this we use a special high temperature horizontal CVD reactor (Fig. 1) for processing samples up to 4” in diameter. Optionally we can apply an electric field during the process to align CNT-growth. For growth of CNTs we use single metallic catalysts (e.g. Ni) as well as bimetallic catalysts and ethylene or acethylene diluted in N2/H2 as reaction gases. The predominant aim is to grow vertical and densely packed CNT-arrays at low temperatures with acceptable structural integrity.
(2) Dieelectrophoresis (DEP) technique is used as the second approach to selectively deposit and align CNTs. The protruding challenge of this technique is the deposition of high quality single walled CNTs and multi walled CNTs at low temperatures which is key factor for integration of CNTs in temperature sensitive systems. Our aim is the horizontal deposition of CNTs from a mild dispersion with a high alignment degree and with only few residuals after deposition.
In the CVD process we are investigating the influence of process parameters like substrate, catalyst, temperature, and gas composition. We are able to grow multi walled carbon nanotubes (MWCNTs) in dense films at temperatures ranging from 450°C to 700°C. REM and TEM observations revealed MWCNTs with controllable diameters in the range from 10 to 20 nm. Raman measurements were applied to study the effect of process conditions on the quality of the CNTs and revealed that MWCNTs with a relatively small defect density can be obtained. Furthermore with the preparation of an single damascene architectures with a Cu metallization and a TiN barrier, selective growth of CNTs was achieved (Fig. 2).
In the DEP-experiments we are trying to get debundled CNTs placed between electrodes. For this we have tried CNT-dispersions with different CNT-material and different dispersing agents. In this way we have achieved dense alignment of SWNTs on electrode structures (Fig. 3).