Cool growth of nanotubes

The holy grail has been found for the silicone microchip industry, allow the technology wall to be pushed back

Low temperature carbon nanotubes allow growth on glass substrates

RESEARCHERS from the University of Surrey's Advanced Technology Institute (ATI) and leading plasma tool manufacturer, Surrey NanoSystems, have launched the world's first commercial tool for low-temperature growth of carbon nanotubes. A joint venture, Surrey NanoSystems is financed by a leading UK technology venture company, IP Group PLC.

The new machine, NanoGrowth, represents a platform for both low-temperature and high temperature growth of carbon nanotubes which can provide high-quality, high-speed connections to the next generation of silicon chips and other leading technologies in the display and medical fields. The low temperatures used permit the use of substrate materials previously unable to withstand the high growth temperatures that can be found in more conventional high temperature growth processes.

This new tool will revolutionise nanotube fabrication, and will have an impact on the nanotechnology industry generally, helping to take it into a new commercial dimension. Professor SRP Silva, Director of the Advanced Technology Institute (ATI) of Surrey University, and non-executive Director of Surrey NanoSystems, comments: “The high degree of thermal control and automation offered by this tool allows more precise control of growth parameters, for accurate and repeatable processing.”

Capability for low temperature growth has been identified as the holy grail of nanotechnology by many leading scientists, since this is the key requirement for integration of carbon nanotechnology into the conventional silicon microchip industry. Detrimental carrier diffusion processes may occur in silicon microchips at the elevated temperatures required for conventional carbon nanotube growth, and so a lower temperature growth process is needed, explains Dr Jose V Anguita, Research Fellow at ATI. The NanoGrowth breakthrough is its ability to grow at much lower temperatures, allowing for compatibility with silicon technology.

Carbon nanotubes have been identified as the ideal interconnection material in modern silicon microchips, due to the high stability of the carbon nanotube structures which provide them with robustness to electron migration induced defects. Electron migration is a problem of increasing importance as the dimensions of current copper or aluminium interconnections are reduced. The increasing current densities impose a technological barrier imposed by the physical properties of these metals. This has been identified as a possible bottleneck by IC manufacturers in road mapping of silicon technology, and can only be overcome by switching to materials with much higher stable atomic lattice structures, such as those found in carbon nanotubes. This will push back the technology wall.

The exploitation of the extraordinary mechanical and electrical properties of carbon nanotubes in precision applications - such as integrated circuits and flat panel displays - has previously been hindered by current growth techniques which require the substrate temperature to be elevated to 1000°C or more, resulting in damage or material compatibility issues. In contrast, the NanoGrowth tool is specifically designed to deliver nanomaterial growth uniformly across wafers up to 3-inches in diameter, and employs a patented, unique thermal processing technology to maintain the substrate at temperatures that are compatible with complementary metal-oxide semiconductor (CMOS) processing. Ben Jensen, Technical Director of Surrey NanoSystems, predicts that the process will be running on 6-inch and 12-inch substrates within the next 3 years.

The integrated thermal control system maintains the substrate at the desired temperature during growth, allowing carbon nanotube materials to be grown with precision - even on highly heat sensitive materials such as plastic. The system may also be used to grow related nanomaterials, including doped silicon and metal oxide nanowires, on suitable substrates. Conventional high temperature PECVD and CVD processing is also supported. In early 2007 new modules will be introduced to enable developers to use ICP material growth and Sputter Etch techniques on the same Platform. The NanoGrowth Catalyst is also completing trials, and for the first time gives the user the ability to deposit the growth catalyst in an integrated single platform

NanoGrowth represents a platform for both low-temperature and high-temperature growth of carbon nanotubes

solution without breaking vacuum. Catalyst growth is available in PVD Sputter and Nanocluster processes.
The machine's processing capability is fully programmable, providing great flexibility for research or development use. Its design permits and encourages scientists who have limited knowledge of plasma based processing or CVD equipment. User-friendly software - NanoSoft, a field-proven SCADA (supervisory control and data acquisition) package - allows accurate control and high repeatability over all stages of processing via graphical MIMIC-style displays.

In partnership with the ATI, Surrey NanoSystems have created generic nanotube and nanowire recipes which are provided with the machine offering ready-to-use growth processes from the moment of installation. Users will also have full manual control and will be able to adapt these recipes, or create their own custom processes, using a menu-driven user interface to control all process parameters required for repeatable growth of the structure or material.

Sophisticated data-logging, trending and batch tracking facilities will help users to develop and document their own processes, and to optimise recipes for commercial production, remote monitoring is also possible. Automation allows users to spend minimal time optimising growth hardware and allows researchers to concentrate their time on engineering the devices and applications. A practical approach to engineering the electrical system of NanoGrowth has been adopted, where faults are easy to locate, parts can be replaced with minimal machine downtime and by personnel with little prior experience of electrical systems.

This revolutionary low temperature carbon nanotube growth process will be of considerable use in both academic and commercial laboratories for the development of practical nanomaterial production techniques for high technology applications. According to Dr Guan Yow Chen, Research Fellow at ATI, the NanoGrowth tool platform will also be extended to demonstrate other forms of materials such as silicon and metal oxides.

The high current carrying capability make nanotubes ideally suited for power electronics and nano-electronic applications, both of which are rapidly expanding areas. Other likely applications include low-resistance nanowires and three terminal devices in integrated circuits, micro-miniature heat-sinks, ultra-tough polymer composites, gas sensors and electron sources for flat panel displays - which because of the lower processing temperatures can now be fabricated on low-cost substrates.

Low temperature carbon nanotubes allow growth on glass substrates enabling new research avenues that include solar cells and solid state lighting. The low cost of the source material employed in carbon nanotube growth gives much promise to low cost alternatives. This technology could bring the manufacturing costs to affordable levels.

A further application of nanotube growth on glass is to enhance the performance of electrically-conducting yet optically transparent substrate materials such as indium-tin oxide (ITO). ITO glass is currently used as an electrical contact to pixel devices found in flat panel screens, solar cells and even in low-technology conventional kitchenware such as oven windows, where it is used as an infrared reflector. Applications such as medical imaging, implantable sensor technology, micro-delivery and lab-on-a-chip components are also in the horizon.

NanoGrowth technology also has the potential to add functionality to ‘plastic electronics’, thereby complementing, rather than competing with it. The much larger surface to volume ratios in the nanotubes coupled with the significantly higher carrier mobility will allow for additional functionality and scope in the devices developed for plastic electronics. More efficient lighting solid state sources as well as solar cells could be envisaged.

ATI will be working together with Surrey NanoSystems on initially providing a whole host of recipes to fully realise the potential of the NanoGrowth system for growth of functional nanomaterials. These will be extended to integrated micro-fluidics with nanotube sensors, solar cells and light emitting devices. The ability to grow precise nanomaterials at pre-determined locations will open up a whole host of applications in electronics, biomedical, structural and engineering applications. The tool will contribute to the establishment of a new platform technology in the nanoscale, and allow for an affordable route to exploit the nanoscale properties of materials for engineering applications.

Work is scheduled to investigate the growth of next generation three-dimensional nanomaterials, which have high surface areas. These structures are expected to have interesting potentials in applications such as highly efficient and environmentally friendly fuel cells, batteries and sensors. The NanoGrowth system is particularly well suited as a starting point for this, aiming at facilitating the research performed as well as providing a common platform for the generation of these 3D materials. One of the goals of this future work is to investigate routes and techniques to achieve control over the growth and formation. There are already a number of universities and institutions who are already looking into the properties and applications of novel 3D material.
The researchers at Surrey University believe that the standardisation of the growth technology will play a key role in aiding the natural transitions of new products from first idea, to development, through to mass production. This transition has up to now been made more difficult by the fact that carbon nanotube growth systems have been custom user-made, therefore the processes and observations found by a research group has not been transferable to other research groups using a somewhat different configuration. The promotion of platform standardisation, together with industrial level control system would accelerate the research and commercialisation of nanomaterials.

The research project was initially funded by the South East of England Development Agency (SEEDA), which provided assistance in the early development work with the aim of realising the potential of nanotechnology in the South East by enabling mass production of nanomaterials as an affordable platform technology.

By Maria Anguita. Maria is a freelance science and health writer. She has previously edited several science and health magazines. She holds a degree in biochemistry with medical biochemistry from Bristol University and an MA in international journalism from City University, London.