Monday, December 14, 2009

Rules to Design By

Once something gets really too complicated, it's almost certain to fail. So how can computer chips, with their billions of components, work at all?

We know lots of other complicated systems, like the world economy or our own bodies. And we know those systems fail often dramatically or tragically.

Of course, computers fail, too, as you know if you've seen a "blue screen of death" recently. But although it won't make you feel any better, those crashes almost always arise from problems with the software, not the hardware it runs on.

So how do engineers ensure that integrated circuits, diced by the score from semiconductor wafers, have a very good chance of working?

Design rules.

Simply put, design rules are a contract between the engineers designing the process for making chips and the engineers designing circuits to put on them. The process engineers guarantee that, if the circuit designers follow these rules in the geometry of their circuits, the chips will work (most of the time).

You may have heard "minimum design rule" used as a shorthand to describe a particular "generation" of computer chips, such as the "32nm" technology recently introduced by Intel. But that is shorthand is somewhat misleading.

For one thing, the true gate length of the transistor--which is critical to their speed and power--is generally about half the generation name. In addition, the "coded" gate length--the length in computer-aided-design files--is not usually the smallest design rule. And this is just one of hundreds of rules that are required to define a technology.

Rather than dive into the details of transistor geometry, consider a simpler design rule: the minimum width of a metal wire connecting the transistors. Together with the spacing between the wires, this dimension determines how tightly the wiring can be packed, which for some circuits determines how many transistors can be used in a parcel of semiconductor real estate.

The minimum safe design width of a wire depends on how fine it can be made and still assure that it will conduct electricity. This has to be guaranteed even under variations over time of the process used to make it, as well as the variation in that process across a, say, 12-inch diameter wafer.

To test what number is safe, the process engineers will make a whole series of test patterns, each consisting of very long wires with various design widths. After measuring hundreds of these test structures, they have a good idea what they can reliably make.

In developing the process technology, they have hundreds of test structures, each aimed at testing one or more design rules. The structures are automatically measured on different positions on different wafers made in different processing runs. Only then will the engineers have the confidence to guarantee that any circuit that follows those rules will work.

After a long process, a set of design rules will be given to designers to use for their circuit layouts. None of this would work without computers to check whether a particular chip layout meets the rules, since the job is beyond human capacity. Therefore a key feature of the design rules is that they can be embodied in an efficient algorithm.

The design-rule paradigm has been extraordinarily successful. But its success depends on a characteristic of the failures it is intended to prevent: they are all dependent on the local properties of the circuit. Some of the more complex rules involve quantities like the area of the metal "antenna" that is connected to a particular device at some point during processing. And frequently the engineers will play it safe by crafting the rules to cover the worst possible situation. But if the rules are chosen and followed properly, there is no chance for a combination of small choices that satisfy the rules to join together to cause a problem in the larger circuit. That's what makes a chip with a billion transistors possible.


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