Fast work

A collaboration between researchers in Florida and a UK supplier of high-powered lasers could be the key to fabricating the next generation of high-speed microprocessors. The result of the research, a source of extreme ultraviolet (EUV) light, could be used to make processors capable of operating at 10GHz and above.

Computer chips are made by etching components such as transistors on to silicon wafers using photolithography. The first step of this process uses ultraviolet light to project an image of the circuit design on to the silicon surface in a device called a stepper.

The wafer is coated with a photo- sensitive material called a photoresist, and after exposure is developed in the same way as a photograph. The photo-resist which was not exposed to the UV dissolves away, and the exposed areas of silicon are then etched away in acid to create the circuit pattern.

To make processors faster, chip makers need to cram more components on to their surfaces; therefore, the components have to be much smaller. This means that the UV image projected on to the chip has to have a higher resolution, and this resolution depends on the wavelength of the UV light being used — the smaller the wavelength, the higher resolution is possible.

This is one of the biggest bottlenecks for producing fast processors. current UV sources are almost at their resolution limit. If chip makers are to keep pace with Moore's Law, the dictum which says that processing power will double every 18 months, they need to develop smaller-wavelength UV sources.

EUV has a wavelength of 13.5nm, compared with the current industry standard of 65nm. 'We must use a light source with a wavelength short enough to allow the minimum feature size on a chip to go down to possibly as low as 12nm,' said Martin Richardson, who leads the team at the University of Central Florida, Orlando.

Researchers have been trying to develop EUV since the late 1990s. The source of the UV is heated plasma; the hot ionised gas gives off ultraviolet radiation as its excited atoms lose energy. However, the plasma itself causes problems with the photolithograpy equipment. Samir Ellwi, vice-president for strategic innovation at Powerlase, which supplied the lasers for the Florida team, explained that currently the plasma is produced and heated electrostatically. 'This produces charged and neutral particles as well as ultraviolet, and these propagate on the optics and can damage the collection optics used in the projection system,' he said.

The Florida team, led by Martin Richardson, has developed a system which makes the plasma in a quite different way, using a high-powered, short-pulse Nd:YAG laser to hit tiny droplets of a 30 per cent solution of tin ions. The tin solution is forced through a nozzle by a vibrating piezo-electric crystal, creating droplets 35µm across and travelling at 200m/s.

When the laser hits the droplet, it vaporises. The laser energy ionises the tin, forming a hot plasma of tin ions in a high, unstable energy state. As the tin ions give up the energy, they radiate UV light in the crucial 13.5nm wavelength. 'The photons are very clean; they are still accompanied by charged particles,' said Ellwi. 'But they do not propagate on the collection optics and so do not damage the optics at all.'