Comment

28 July 2009

Molecular manufacturing a boost to Moore's Law

There is now a new method of attaching molecules to semiconducting silicon that may help manufacturers reach beyond the current limits of Moore’s Law as microprocessors get smaller and more powerful.

Moore’s Law, suggested by Intel co-founder Gordon Moore in 1965, said the number of transistors that can go on an integrated circuit doubles about every two years. But even Moore has said the law cannot go on forever.

A new method of attaching molecules to semiconducting silicon may help manufacturers reach beyond the current limits of Moore’s Law.

The challenge is to get past the limits of doping, a process that has been essential to creating the silicon substrate at the heart of all modern integrated circuits, said James Tour, Rice University’s Chao Professor of Chemistry and professor of mechanical engineering and materials science and of computer science. Scientists at Rice and North Carolina State University partnered in the research.

Doping introduces impurities into pure crystalline silicon as a way of tuning microscopic circuits to a particular need, and it is been effective so far even in concentrations as small as one atom of boron, arsenic, or phosphorus per 100 million of silicon.

But as manufacturers pack more transistors onto integrated circuits by making the circuits ever smaller, doping gets problematic.

“When silicon gets really small, down to the nanoscale, you get structures that essentially have very little volume,” Tour said. “You have to put dopant atoms in silicon for it to work as a semiconductor, but now, devices are so small you get inhomogeneities. You may have a few more dopant atoms in this device than in that one, so the irregularities between them become profound.”

Manufacturers who put billions of devices on a single chip need them all to work the same way, but that becomes more difficult with the size of a state-of-the-art circuit at 45 nanometers wide—a human hair is about 100,000 nanometers wide—and smaller ones on the way.

Monolayer molecular grafting—basically, attaching molecules to the surface of the silicon rather than mixing them in—essentially serves the same function as doping, but works better at the nanometer scale. “We call it silicon with afterburners,” Tour said. “We’re putting an even layer of molecules on the surface. These are not doping in the same way traditional dopants do, but they’re effectively doing the same thing.”

Tour said years of research into molecular computing with an eye toward replacing silicon has yielded little fruit. “It’s hard to compete with something that has trillions of dollars and millions of person-years invested into it. So we decided it would be good to complement silicon, rather than try to supplant it.” Tour and his team wrote a paper, “Controllable Molecular Modulation of Conductivity in Silicon-Based Devices,” and it is online at http://pubs.acs.org/doi/abs/10.1021/ja9002537.

Tour anticipates interest in the process, in which carbon molecules could bond with silicon either through a chemical bath or evaporation. “This is a nice entry point for molecules into the silicon industry. We can go to a manufacturer and say, ‘Let us make your fabrication line work for you longer. Let us complement what you have.’ ”

For related information, go to www.isa.org/manufacturing_automation.