Extreme printing

Next-generation ultraviolet lithography technique could make it possible to create nano-scale integrated circuits with double the processing speed. Siobhan Wagner reports

A new way of printing circuit boards using advanced lithography could produce semiconductors with double the amount of processing speed.

Extreme ultraviolet lithography (EUVL) would create circuit patterns on nano-scale chips using a beam of light with a wavelength 14 times shorter than conventional techniques. Current lithography uses a light beam with a 193nm wavelength, but EUVL would use a 13.5nm wavelength beam.

The aim is to produce semiconductor components that are more than half to possibly even a quarter of the size of current 65nm-sized circuitry. ‘You’ll be able to put in more transistors so you get faster speed,’ said Samir Ellwi, vice-president of strategic technology at Powerlase, a Crawley-based laser manufacturer that has been working with EUVL research groups around the world for the past 10 years.

The technique presents some unique technical requirements for semiconductor manufacturers. While current lithography processes take place in the open, EUVL needs to be carried out in a vacuum because all matter absorbs extreme UV radiation. Also, there are no materials that can be used to make refractive lenses that operate at the 13.5nm wavelength, so the light must be focused using only specially-shaped reflective mirrors.

One major challenge facing the industry has been deciding on the most appropriate source for a UV light beam with such a short wavelength. Powerlase is exploring two potential sources: laser produced plasma (LPP) and discharge produced plasma (DPP). The company is developing the LPP approach with the University of Central Florida (UCF).

The LPP process begins by heating a droplet of liquid tin suspended in a vacuum with an intense laser pulse. A diode-pumped laser from Powerlase is aimed at the target. As the droplet is heated it creates an environment known as the plasma medium, which emits light at 13.5nm.

DPP, on the other hand, works on a different principle. The process creates an electric field between two conductive electrodes, a cathode and an anode. The field between the two electrodes will pinch the EUV target, which can be a tin droplet, a vapour of tin or ablated tin, creating a high temperature plasma medium that emits the 13.5nm wavelength light. A special mirror collects this light and focuses it for use.

Powerlase is supplying lasers to Japan’s Extreme Ultraviolet Lithography System Development Association (EUVA), which is working to perfect the use of DPP as a light source.

Ellwi said that LPP appears to have the greatest potential. ‘With DPP you can be limited with the collection of the light,’ he said. ‘You are restricted because you have to take the light out of the interaction chamber between the two electrodes.’

A second challenge to the industry is ensuring potential throughput is high enough. Ellwi said that current EUVL laboratory demonstrations only produce 10 wafers/hr. ‘One critical point that relates directly to the throughput is having an EUV source with sufficient output power,’ he said.

The 13.5nm EUV light accounts for just two and a half per cent of total light generated. For successful EUV lithography to be possible, the power at the EUV target needs to reach 100W. This will ensure sufficient light is captured by the stepper (lithography machine) for successful semiconductor printing.

Should the EUVL work continue to progress, Ellwi is confident 32nm scale components can be achieved. ‘I believe by 2012 we will have full production machines worldwide,’ he said.