NEC achieves low-temperature print fabrication of CNT transistors on plastic films

Mumbai: Japan's NEC Corporation says it has successful fabrication of carbon nanotube (CNT) transistors using advanced printing processes. It said fundamental operations of the new transistors, completely manufactured through innovative new printing processes on plastic film, have also been verified.

Researchers say CNT transistors have better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constantas the gate insulator, ensuring efficient charge injection into transistor channels and reducing direct-tunnelling leakage currents.

NEC's succesful fabrication indicates that future printing processes now hold the promise of producing complete CNT transistors, which will lower production costs, ease environmental impact and expand the potential markets for printed electronics. These processes can also be applied to production techniques using plastic films, which require special considerations for limited exposure to heat.

Main features of the new CNT transistors include:

• Each component of the CNT transistors, including electrodes, insulating layers, and CNT channels, are completely printed. The composition of materials, solvent concentration, and conditions for insulator ink and metal ink are each optimized in order to eliminate interference between stacked layers, and to maintain printing conditions.
• Due to the dispersive qualities and low-temperature volatility of CNT, the development of specialized materials and preparation methods that are compatible with CNT are required. In this regard, low-temperature production (below 200oC) enabled the use of plastic substrates which are difficult to utilize under high temperatures.

Printed electronics technologies are unique in that they allow transistors to be formed by printing directly onto substrates. This means that manufacturing processes can be dramatically simplified in comparison to conventional semiconductor fabrication. Consequently, the waste materials from manufacturing processes can be minimised, and CO2 emissions can be reduced by more than 90 per cent.

Fabricating electronic circuit components by the roll-to-roll (fabrication process for making electronic circuits on rolled substrates) printing processes, for example, allows manufacturers to produce large-area, low-cost electronic circuits that are environmentally friendly. Furthermore, new applications, such as bendable circuits, can be produced when components are printed on plastic substrates.

NEC researchers used CNTs as channel (the part of a transistor that connects electrodes. The faster the electrons move in the channel, the faster the electronic device can operate.) materials to investigate printed transistors, and the company expects CNTs to have 100 times greater mobility than organic semiconductors, in addition to being chemically stable.

In order to fabricate each transistor component through printing processes, various stacked layers that contain materials such as metals, insulators and semiconductors are required. In order to maintain optimum printing conditions, the process also requires equal ink coverage throughout underlying layers, and robustness between layers to prevent leaking.

Furthermore, due to low thermal tolerance, the range of available inks is limited when using plastic substrates. Inks for making CNT channels are particularly challenging due to the need for stable dispersion and low temperature volatility.

To overcome these issues, careful consideration was made for the components, solvent concentration, and process parameters of insulator inks and metal inks. Furthermore, inks that are compatible with the dispersive qualities and low-temperature volatility of CNTs were also used to successfully create functional CNT transistors through printing processes. The fabricated transistors demonstrate p-type conduction and an on/off ratio of 1,000.

NEC views new CNT transistor technology as a key to the development of electronic devices that are friendly to the environment, and will continue to proactively conduct research in this area.

NEC Corporation is one of the world's leading providers of Internet, broadband network and enterprise business solutions dedicated to meeting the specialized needs of a diversified global base of customers. NEC delivers tailored solutions in the key fields of computer, networking and electron devices, by integrating its technical strengths in IT and Networks, and by providing advanced semiconductor solutions through NEC Electronics Corporation.