TR: Your ambition is to use this blueprint to build an entirely new type of computer, one fabricated using chemistry rather than lithography, isn’t it?
WILLIAMS: Our goal is to manufacture circuits in simple chemical fume hoods using beakers and normal chemical procedures. Instead of making incredibly complex and perfect devices that require very expensive factories, we would make devices that are actually very simple and prone to manufacturing error. They would be extraordinarily inexpensive to make, and most of the economic value would come in their programming.
TR: It seems slightly counterintuitive that the way to make microelectronics even smaller and more powerful is to allow them to be defective.
WILLIAMS: A year ago we published a paper in Science in which we talk about what is going to be required to make a computer using chemical assembly. The answer was that you need to have a computing architecture that would allow the systems to have a lot of manufacturing defects, a lot of mistakes. We call that architecture defect-tolerant. We discussed an example of a computer that has been built here at Hewlett-Packard called Teramac. This is our computer archetype; we think that in the future things that are based on molecular-scale or nanometer-scale objects are going to have to have as part of their organizing principles these defect-tolerant designs because it’s going to be impossible to make such small things perfectly.
TR: Tell us a little about the origins of your interest in Teramac.
WILLIAMS: James Heath, a UCLA chemistry professor, and I spent at least a year and a half studying it before we were ready to build anything. We were having a series of discussions with a computer architect at HP, Philip Kuekes, about defect tolerance, and Phil started talking to us about this computer that he had helped build. They had decided to build it from imperfect or defective silicon components, because those would be much cheaper, and just deal with whatever problems that came up by using clever software.
TR: In other words, you pay for a material’s perfection.
WILLIAMS: Absolutely. Perfection costs a lot of money. And as you get more and more complex, the cost of perfection gets higher and higher. That’s the main reason why the cost of fabs is increasing exponentially. What we’re saying is that if we can make things that are imperfect but still work perfectly then we can build them a lot more cheaply.
TR: How do you make something that is imperfect work perfectly?
WILLIAMS: Teramac has an architecture that relies on very regular structures called crossbars, which allows you to connect any input with any output. If any particular switch or wire in the system is defective, you can route around it. You can avoid the problems. It turned out that Teramac had a huge bonus. Not only is it capable of compensating for manufacturing mistakes, but Teramac could also be programmed very rapidly and it executed those programs with blinding speed because it had this huge communications bandwidth.
TR: As constructed, Teramac uses silicon chips, albeit defective ones. But your interest is in using this architecture to build a computer using chemical processes. Why is it so promising for that application?
WILLIAMS: Teramac was built as a tool to demonstrate the utility of defect tolerance for building complex systems more cheaply. Even though it was a success, a desktop Teramac is not yet economically viable. It may be that Teramac-like architectures will help to extend silicon integrated circuits a generation or so by making fabs cheaper to build, but we see the huge potential for this architecture in chemical manufacture of integrated circuits. Assembling devices and ordering them by chemical means will be an inherently error-prone process. However, we now have proof that a highly defective system can operate perfectly.
TR: This actual architecture could provide an actual way to do computing?
WILLIAMS: It’s real. The hardware was built, tested and programmed. The concepts are very well understood and very robust. Now the second stage of all this is to see if we can use the ideas coming out of basic research in nanotechnology-the ideas of self-assembly, constructing little regular units using chemical procedures-to actually make something that would be useful. Our Science paper this July is, we believe, the first major step in that direction in that we demonstrate that molecular electronic switching is possible.
TR: What’s next?
WILLIAMS: Within two years, we hope to assemble chemically an operational 16-bit memory that fits in a square 100 nanometers on a side. Today, one bit in a silicon memory is much larger than a square micrometer. So, we’re looking for a scaleup of at least three orders of magnitude in memory density. Our longer-term goal, frankly, is to build an entire computer using nothing but chemical processes. That particular goal is 10 years from now if everything goes well, and even then we’ll be making fairly simple circuits. But it’s got to start someplace.