Detector May Keep Chips From Biting Dust
Just a speck of dust, too small to be seen by the human eye, can gum up a silicon chip.
So manufacturers of computer chips go to extraordinary measures to be sure that the vacuum chambers in which the chips are etched and fabricated are exceedingly clean. Chips are manufactured in a “batch” on top of a flat wafer. To test for cleanliness, manufacturers have to interrupt the production process, put in a blank wafer to serve as a monitor, run a test, and then restart the process if the chamber is still clean.
That takes up precious time, and it doesn’t provide continuous real-time monitoring of the process. Manufacturers run the test every now and then, so occasionally wafers containing tens of thousands of microchips and worth up to $100,000 are ruined before anyone knows the chamber is contaminated.
Now a chemist at Bell Labs has developed an instrument that detects particles 100,000 times smaller than the head of a pin, and it could provide continuous monitoring of the manufacturing process.
“Right now this is a research instrument, and we are collecting data to determine how good it is,” said William Reents, inventor of the instrument. It is being tested at Bell Labs, the research arm of Lucent Technologies, in Murray Hill, N.J.
“We have preliminary data saying it should work, but we don’t have a complete set of data showing how well,” Reents said. However, the early results are encouraging enough for the lab to begin seeking out companies to license to build the instrument.
If it works as well as Reents thinks it will, it could further streamline the semiconductor manufacturing industry and save millions of dollars each year in faulty chips.
Chips are fabricated in “clean rooms” that are thousands of times cleaner than a hospital operating room, but “it doesn’t take much of a dust particle to wipe a chip out,” said a spokesman for National Semiconductor in Santa Clara, Calif. Some chips are lost, no matter how carefully the area is controlled, so the Bell project “looks like something we would want to take a close look at,” he added.
Once the chips are completed, they are coated with silicon dioxide, sealing them from the outside world.
Microchips are most vulnerable during the manufacturing stage. They are produced in vacuum chambers that gradually become contaminated as various gases are used to deposit and etch material, thus forming the chips.
“What you would like to have is a real-time monitor, something that’s sitting there the whole time, looking for particles,” Reents said. “That way, when it [contamination] does happen you will know immediately, and you will lose only the wafer that’s in there and nothing else.”
Currently, if a manufacturer runs a wafer monitor test once for every 100 wafers, and something goes wrong after two wafers are run, “then you run 98 wafers under a high particle condition,” Reents said. “You have no idea there’s a problem” until 98 wafers have been wasted.
Reents’ instrument sucks a small amount of gas out of the vacuum chamber during the production process. A high-intensity pulse laser strikes the particles, breaking them into charged atoms and molecules. These then accelerate at various velocities because of differences in their weights, and they strike a detector.
The weights and the charges are recorded, and that tells Reents instantly each particle’s size and composition.
As tested in the lab, the system has been able to detect particles down to one-thousandth of a micron. An average dust particle is 50 microns, so we are talking really small here. That might seem like overkill, but as chips get smaller and smaller, they are more vulnerable to ever smaller bits of contamination.
“This is really benign,” Reents said of his invention, because it has no effect on the manufacturing process. “They don’t have to change anything in order to detect particles.”
He said he does not know yet what a commercial version of the research instrument will cost, and he’s not even absolutely certain it will work. As we talked, a team of engineers in another part of the lab was gearing up for the first in a long series of tests to see if the instrument works as well in a production setting as Reents expects.
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Lee Dye can be reached via e-mail at leedye@compuserve.com.
