Developments show that carbon nanotubes - rolled-up sheets of graphite about 1 nm in diameter ( Small-diameter nanotubes can become superconducting; Can carbon nanotubes handle high-energy particles?) - could provide the technology of tomorrow.

The ever-shrinking world of electronics has become even smaller following the first demonstration of digital logic circuits made from carbon nanotubes. A research team in the Netherlands used different combinations of "nanotube transistors" to create several devices, including a voltage inverter and a NOR logic gate. As conventional silicon microelectronics approach a fundamental size limit, these devices, operating at room temperature, are an important step towards nanoelectronics.

Carbon nanotubes have been used to make a variety of electronic components, including diodes and field-effect transistors (FETs). A FET can be made from a nanotube by attaching gate, source and drain electrodes.

Now several of these FETs have been placed together on a single silicon chip to form different circuits. Their simple inverter device consists of a nanotube FET and a large bias resistance. It converts a high-input voltage to a low one - that is, "one" to "zero" - and vice versa. By adding an extra FET in parallel, the researchers made a NOR gate, needing two "zero" inputs to give a "one" output or two "ones" to give a "zero".

Any of the standard logic gates can be created using different arrangements of these FETs. The team also created a "static random access memory" and an oscillator. The devices have gains of a factor of 10.

Electron-beam lithography was used to deposit aluminium gate electrodes on a silicon wafer. The nanotubes were placed on top, and gold electrodes were added using an evaporation technique. There are still challenges to overcome, such as the difficulty in positioning the nanotubes accurately on the wafer, but emerging techniques, including a way to grow nanotubes directly on the chip, may solve this problem.

This could lead to electron-beam lithography equipment for chip making that could produce parallel beams as well as electron microscopes, based on a first report of field emission measurements from carbon nanotubes with an integrated gate electrode from a French-UK team.

The nanotubes are used as field-emission sources in the device. The integrated gate electrode enables the device to achieve emissions at low field voltages. The team used a self-aligning technique, similar to the technique used to pattern CMOS gates, to create the gate, the insulator and the base for growing the nanotubes with one lithography mask.

With the gate aperture formed, the nanotubes are grown inside using plasma-enhanced chemical vapour deposition. This ensures that the nanotubes are always centred in the gate apertures, which are just 2 µm in diameter.

The major achievement of this work is the self-alignment of the multiwalled 30 nm diameter carbon nanotubes. The next step is to grow one carbon nanotube per nanohole and to have it perfectly centred in the gate aperture. Preliminary results show that it is possible. A single nanotube per gate would boost the efficiency of the applied field by avoiding the electric field screening when nanotubes are close together. The nanocathode is intended for parallel electron-beam lithography and microscopy. Other applications could be in instrumentation and microwave vacuum amplifiers.

Further reading

A Bachtold et al. 4 October 2001 Science