Plasma lithography could improve chips

A lithography method based on thin plasma beams could make smaller, better performing computer chips.

The new class of nanolithography, under development at the Centre for Materials Under Extreme Environments at Purdue University in Illinois, US, has the potential to extend Moore’s law.

The unofficial rule states that the number of transistors on integrated circuits, or chips, doubles about every 18 months.

Computer chips are currently created with ultraviolet light through a process called photolithography. The process involves projecting the image of a mask onto a light-sensitive material, then chemically etching the resulting pattern.

The plasma-based lithography generates ultraviolet light that has a wavelength of 13.5nm, less than one-tenth the size of current lithography.

The researchers shape and control plasma into beams using magnetic fields. This is because plasma, which is a partially ionised, gaslike material, conducts electricity.

The method is adopted from fusion-energy research. In experimental fusion reactors, magnetic fields are used to keep plasma-based nuclear fuel from touching the metal walls of the containment vessel.

The remaining challenge for the researchers is to produce the plasma in a way that is less energy consuming. The current techniques for producing plasma include either a laser or an electric current.

The laser method creates plasma by heating xenon, tin or lithium. The plasma produces high-energy packets of light, called photons, of extreme ultraviolet light.

Lead researcher Ahmed Hassanein, head of Purdue’s School of Nuclear Engineering, said producing plasma with lasers or electric currents are equally inefficient.

‘In either case, only about one to two per cent of the energy spent is converted into plasma,’ he said.

Purdue’s nuclear engineers and scientists are working with US Department of Energy’s Argonne National Laboratory to improve this.

Hassanein’s team have developed a computer simulation method to visualise the entire process of the plasma evolution. The simulator, called HEIGHTS (high-energy interaction with general heterogeneous target systems), runs on supercomputers at Argonne’s laboratories.

The system simulates the laser interacting with the target, and the target evaporating, ionising and turning into a plasma. The simulation also shows what happens when the magnetic forces ‘pinch’ the plasma cloud into a smaller diameter spot needed to generate the photons.

Beyond the challenges involved with producing the plasma, the method for focusing the plasma beams has presented difficulties for the researchers. The Purdue team was not able to use traditional lenses in their design because they absorb the photons that make up light.

The researchers are instead using mirrors to focus the beams. However, the mirrors present other significant challenges. Research has shown that plasma condenses on the mirrors, reducing their reflectivity and limiting the efficiency of the process.

‘We are trying to help find innovative ways of producing these photons, optimising the production and mitigating the effects of the plasma on the mirrors,’ Hassanein said. ‘So we are trying to improve the entire system.’