Monday, September 28, 2009

We Did All We Could

As you read this on your computer screen, it's easy to take for granted the billions of transistors--driving the screen, running the programs, storing the data, and bringing it to you over the internet--that make it all possible.

This embarrassment of transistors is affordable because they're made in a parallel process that produces vast numbers of similar devices at once, combined into integrated circuits (ICs). Making sure that they each behave the way they're supposed to demands extraordinarily clean and reproducible manufacturing processes. In fact, after inventing the transistor, Bell Labs was late to the IC party because they didn't think anybody could get them all to work at once.

Later, Bell Labs' parent, AT&T, did get good at ICs. Towards the end of my time in semiconductor device research at Bell Labs I worked with the excellent developers of the upcoming integrated circuit generations for what was then AT&T Microelectronics, who had moved to Orlando, Florida.

One benefit of visiting Orlando and learning about their challenges was that I managed to design some test structures that they included on the photomasks they used to develop their process. It took some convincing for them give up even a tiny piece (about 0.002 square centimeters!) of their very precious real estate. They also need to be sure that my devices wouldn't flake off and mess up other structures that they needed to do their real work.

Months later, it was a real rush to get the first silicon wafers with my devices on them.

First, the structures looked exactly like what I designed. Instead of looking at multicolored rectangles in a CAD program on a computer screen, though, I was looking at multicolored rectangles in a microscope: real semiconductor devices.

Second, there were lots of them. Even though the entire array of test structures was over a square centimeter in area, there were dozens of repetitions on each eight-inch-diameter silicon wafer.

Third, they were all the same. They didn't just look the same: on the unfortunate occasions when I blew one up with too much voltage, I learned that its repeated version would have very much the same electrical behavior.

I also made friends with people who did testing, robotically stepping across the wafer to measure each repetition. So for simple measurements, after a lot of up-front planning, I could sit back and let the data roll in. Whenever development ran a lot of 25 wafers through the several hundred steps it took to get finished ICs, they also made me hundreds of test structures, and measured them, too.

Compared to what I was used to in the physics labs, where you might work weeks to get a sample or two, this was heaven.

With lots of people helping out, we also did something more challenging, which was to explore new ways to process the wafers. For example, my research colleague Joze Bevk devised a scheme to improve the addition of electrical dopants into the narrow poly-crystalline-silicon ribs that formed the gates of the transistors. Our development colleagues helped track the wafers through step after step of the modified process.

One day, when Joze and I were visiting Orlando, our colleague Steve Kuehne approached us. In the matter of a surgeon telling waiting relatives "I'm sorry. We did all we could," Steve gave us the bad news: "The gates are falling off." Joze and I were very disappointed at this failure, since from Steve's grave expression it was clear that the result was a disaster.

Over the next hour or so, as we discussed what sort of stresses in the materials might cause these terrible problems, an interesting fact emerged. Out of many millions of gates on the test wafer, perhaps 20 had fallen off! Only the high-throughput measurement tools in the development line, which scan the entire wafer looking for anomalies, could even detect them. This is what Steve meant when he said the gates were falling off. For him, a process with even that many broken devices was a non-starter.

I don't doubt that the developers could have devised modifications of the process that reduced the problem, it if had seemed worthwhile--or if they had invented it themselves. Nonetheless, it was a powerful reminder of the degree of reproducibility that IC manufacturing demands.

When I see a news story about some new technique that's going to change the way ICs are made (like this one or this one--not to pick on IBM), I remember how few failures are deadly. If you can see variation in a handful of devices, then someone is going to have to do an awful lot of work before they can be made by the billions.

Now go back to taking them for granted.

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