Focused ion beams – a new instrument for nano fabrication

Brussels, Jun 2003

Working at the scale of a few molecules demands very precise instruments. The NANO-FIB project is finding that focused ion beams can manipulate material at a smaller scale than ever before.

Nanotechnology aims to carry out new processes at a scale of just a few atomic particles – a nanometre is a mere thousand millionth of a metre. The European Union has highlighted this technology because of its revolutionary potential, but new tools and skills will have to be developed to exploit it to the full.

"The start of NANO-FIB was a crazy idea we had," explains project director Jacques Gierak of the Laboratory of Photonics and Nanostructure at the French National Centre for Scientific Research (CNRS). "We knew that ion beams had been used for years for nano-work, but we thought that if we could focus them into a very sharp pencil beam, they would become more precise." The rather limited and specialist applications could then be greatly widened.

Working with ion beams

The most familiar use of fabrication on a very small scale is probably in the microelectronics industry to manufacture microprocessors and other integrated circuits. Typically, a silicon crystal is etched and doped with other elements from the periodic table groups III to V in a precise pattern that lays down electronic pathways required by the application.

Current light beam patterning methods are reaching their limit, although extreme ultraviolet technology will extend the possibilities of optical imaging. Future possibilities include electron beam lithography which can print a pattern in a single operation, but electron scattering limits its definition to about 10 nanometres (nm).

Ion beams are less scattered than electrons and focused ion beams are already used in etching and doping applications, usually to remove sputtered material, but they require very high ion doses that can only be produced by huge amounts of energy, of the order of tens of keV. This technology is based largely in the USA. The 'crazy idea' of the NANO-FIB team was that they could develop a new focused ion beam (FIB) instrument operating at a higher resolution level that would therefore find a new market.

Liquid metal ion source

The FIB is based on a liquid metal ion source which produces an optically bright beam of ions that can be sharply focused. The active electrode is a fine tungsten needle coated with the metal gallium in liquid form and placed in a vacuum. A potential of several kilovolts distorts the liquid into a cone and, at a critical voltage, its apex becomes a jet. This jet is focused with an electrostatic device, rather than the magnetic lens used to focus electrons, because the gallium ions are much heavier than electrons.

A fully dedicated ion beam column has been built into a new machine, called NANO-FIB after the project, with a current focusing capability of about 8 nm. When the focused beam hits a surface it can be used to make an image of it, to etch it or to inject a defect.

Because of its lower energy and finer focus, NANO-FIB has a number of potential applications. An arrangement of gold nano-particles on a graphite surface has already been produced. The graphite, which has an atomically flat surface, can readily be given a pattern of defects by the machine. Preformed gold clusters are then deposited on to it and act as artificial atoms. This long-term research is hoping to develop nano-electronic devices.

Other potential target materials are also highly sensitive to ion irradiation. These include optoelectronic crystals in the III to V range, such as gallium arsenide, that could be used in low-dimensionality systems or as laser components. Thin magnetic crystalline films could be nano-patterned for use in ultra high-density magnetic data storage machines. Unlike the high-energy FIB machines, NANO-FIB is a single-pass machine that should allow fast reproducible results. A typical target application is selective epitaxial growth of chip materials, creating a pattern of variable chemical components but uniform crystalline structure.

Overview of the NANO-FIB consortium organisation and tasks

Universities and SMEs

"We put together a consortium of the main university experts in Europe in charged particle optics and physics, together with industrial partners and some very innovative SMEs," explains Gierak. "Partners originally came from France, Germany, the Netherlands and the UK, and we have been joined more recently by the Institute of Scientific Instruments, from the Czech Republic. The three-year project would never have been possible without European support – although it runs until the end of 2003, it has already produced the new NANO-FIB instrument and several patents."

The SMEs are Raith of Dortmund, a manufacturer of beam lithography and nano-navigation products, and FuG Electronik, Rosenheim, which makes specialised power supplies. At first they will help to exploit and market the prototype, but a new company may be set up later as wider possibilities emerge.

DG Research
http://europa.eu.int/comm/dgs/research/i ndex_en.html
http://europa.eu.int/comm/research/indus trial_technologies/30-06-03_pro-nanofib_ en.html