Chips in a flash

Researchers at Princeton have found a fast method for printing ultra small patterns in silicon wafers, a discovery that could greatly reduce the size and cost of computer chips.

Researchers in the lab of electrical engineer Stephen Chou used the new technique to make patterns with features measuring 10 nanometers. The method involves pressing a mould against a piece of silicon and applying a laser pulse for just 20 billionths of a second. The surface of the silicon briefly melts and resolidifies around the mould.

The method is said to eliminate the costly and time-consuming step of etching (photolithography), which had been the only way to make such small patterns in silicon. While the etching process takes 10 or 20 minutes to make a single chip, Chou’s imprint method accomplishes it in a quarter of a millionth of a second.

Electrical engineer Fabian Pease of Stanford University said the new method could allow electronics manufacturers to continue the rapid pace of miniaturisation that has continued for three decades, but appeared to be running up against fundamental physical limits.

Chou has made a career of breaking what had appeared to be physical limits of miniaturisation. In 1996, he developed a method for imprinting nanometer-scale patterns into plastic polymers. That breakthrough is said to have simplified the process of making moulds, but costly etching was still required to transfer these patterns into silicon.

Chou believed that imprinting would work directly in silicon and could be made to happen much faster.

‘People’s intuition is that mechanical processes are very slow, so imprinting cannot be fast,’ said Chou. ‘But I knew there is no scientific proof of that. So how do you design an experiment to explore the speed limit of the imprint process?’

The key turned out to be an excimer laser, a tool that is commonly used in laser surgeries because it can heat just the thinnest surface layer of a material without causing damage underneath. Using conventional etching, Chou made a template of the pattern he wanted out of quartz, which is transparent to the laser beam, and pressed it against the silicon. A brief laser pulse melted the silicon surface around the mould. The silicon does not stick to the quartz.

Revealed by electron microscopes, the patterns the researchers produced look like long, squared-off channels. Each ridge measures 140 nanometers across and is topped by a much smaller ridge just 10 nanometers wide.

Chou dubbed the method Laser-Assisted Direct Imprint, or LADI and the University has initiated the process of filing for a patent. He believes the LADI process will mesh well with another of his earlier breakthroughs, his creation in 1996 of the world’s smallest transistor, which requires only a single electron of current. Making common use of such small transistor has been inhibited by lack of a convenient manufacturing process, he said.

Another benefit of LADI, said Chou, is that it eliminates the chemicals used in conventional lithography and is thus more environmentally friendly.

In addition to its commercial applications, the discovery opens an interesting avenue of scientific research, said Chou. Understanding the physics behind melting and solidifying on such small scales will require input from many fields, including materials science, mechanics and microfluidics.

‘Scientifically, people are still trying to understand how it works, because it is amazing that it works at all,’ said Chou.