Silicon will be joined in the years between 2013 and 2019 by more exotic compounds. One is the carbon nanotube, which is a sheet of carbon atoms rolled up into a cylinder. This is a uniquely flexible substance: depending on how it’s rolled up, it can behave like a metal or a semiconductor. The tube can be between 1nm and 25nm across, and can be used to form part of a transistor in conjunction with more established silicon structures. Similarly, transistors made from a cocktail of other elements -- indium, aluminium, tin, gallium and so on -- exhibit promising abilities to let electrons move easily in smaller areas than silicon supports.
Another problem area for ultra-small devices is the wires used to connect them together. The industry has already moved from aluminium to copper, which is expected to work well for the 32nm architectures due in 2009. It may be acceptable for 22nm in 2011, but after that the reductions in size constrain the passage of electrons to the point where resistance becomes a serious problem. Once again, carbon nanotubes – this time metallically configured – are interesting: they can carry more current than copper tracks and are effective at smaller widths. They are, however, difficult to add to current manufacturing technologies.
To that end, Intel is experimenting with self-assembling nanotubes. One promising effect is the way they align themselves in an electric field, and it may be possible to put a disorganised mass of nanotubes onto a silicon matrix and have them correctly place themselves in response to a series of signals.
There will come a point where silicon is no longer able to underlie ever smaller transistors. At 1.6nm, there is just enough room for the essential parts of a transistor to be built out of a single atom apiece, and at that point -- expected well past 2020 -- there will be nowhere further for the standard CMOS silicon transistor to go. Intel is investigating a wide variety of exotica -- optical, quantum, biological, plastic, spintronics -- that can go further in one or more areas such as cost, size, speed and energy efficiency. None has more potential than standard silicon technology in all respects, and the company says that chips will most probably start to use two or more heterogeneous ideas as appropriate to cope with different functions.
Intel is in no mood to abandon Moore’s Law, and there’s little doubt that there are plenty of avenues for continuing its advance. Whether these are commercially interesting, and what problems await in production is a different matter -- high-K dielectrics have proved more difficult to implement than expected, and like all chip companies Intel will never talk about yield. That’s the number of saleable chips that a particular production process can make, compared to the number of failures -- new technologies can have horrendously low yields while the bugs are worked out of the system, and until the last ones are banished no technology can be counted a success.
However, worldwide transistor production is still increasing at over 50 percent per year -- a rate substantially unchanged since 1956, leading to a current production rate of 1019 transistors per year. That’s only a hundred times fewer than the number of stars in the universe, a gap that should close in the next four years. As long as designers can work out what to do with them, engineers can keep them cool and fed with data, and marketers can sell the results -- it’s steady as she goes.
Talkback
Thanks to Rupert Goodwins for an excellently clear and instructive article. I'm a physics graduate, but one from three decades ago who has thus lost contact with the advances that have been made in chip design and manufacture. I now consider myself much better educated and am fascinated by the ways that science can keep Moore's Law valid.