Silicon Valley loves to see sparks of genius, but not during the production of sensitive integrated circuits. The microprocessor chips go through hundreds of processing steps on a wafer-fabrication line—a fab. Many of those steps involve subjecting the wafers to high-energy plasmas that deposit minute quantities of materials to form components. During the processing of expensive wafers, the last thing process technicians want are sparks, or arcs, caused by the plasma in the vapor-deposition equipment. Even a small arc can seriously damage a wafer that has a value of $10K to $30K. An undetected arc near the beginning of a fab line may mean damaged wafers go through the remaining steps—a waste of time, energy, and materials.
The semiconductor industry can't predict when arcs will occur, but it can now detect when they occur so technicians can weed out damaged wafers. That substitution improves processing yields—and profits. The detection process owes a lot to Paul Buda and the design team at Schneider Automation (Raleigh, NC).
When someone at Schneider says, "We could do that if only..." Buda and his small group get the call—as when a semiconductor company needed help to detect arcs in vapor-phase sputtering systems used to deposit materials on wafers.
During on-site investigations, Buda says, "We found the client's equipment had arc detectors, but they only detected arcs that could drive a hole in the side of the processing chamber and not the small arcs that can still cause considerable damage." Buda devised a solution on the trip home.
The arc-detection system centers on power-supply monitoring units, a sophisticated data-acquisition system, and tight integration with existing process-monitoring tools, as well as eliminating interference from other equipment. The final system had to fit within existing process equipment, and it had to use industry-standard protocols to communicate with other devices, including a PLC.
"We look for a 'voltage collapse' at the switching power supplies that power the plasma. When the system sees one, it tells an operator, 'Look at this batch.' Then the operator can decide what to do," says Buda. In some cases, little or no damage appears, so wafers can continue on. But at other times, seriously damaged wafers need to come off the line. Schneider Automation's arc-detection systems now operate in about 50 vapor-phase sputtering systems.
Producing the plasma takes a lot of energy, mostly contained in the radio frequency (RF) spectrum at around 13 MHz. "Protection from radio-frequency interference took a lot of careful investigation and design work," says Buda. "In one case, we had to mount the arc-detection equipment in an enclosure that also contained a 2 kW RF generator. Shielding, grounding, and installation presented a huge challenge." Buda notes that the data-acquisition system needed to monitor events that produced signals in the same frequency range as the plasma-generating gear. "There are lots of interesting things going on in processing," says Buda. "But we're not at a point where we can predict when an arc will occur so we can prevent it. That will come."
"There's a lesson in the project," says Buda, "Engineers need to get out and
look at the problems they must solve. You can't do that sitting at a desk and
running simulations. You need to see what's happening and what you have to work
Fast Update: The arc-detection circuit
compares the output of a power-supply monitor with a preset threshold
value. The comparator output gets sames every 33 nsec, ensuring every arc
gets detected and measured.
Paul Buda, Principal Technical Specialist for Schneider Automation, received an MSEE from Syracuse University, and holds 13 patents with several pending. He has applied his knowledge of electronics, controls, and digital signal processing to a range of problems, from radar and appliance control to welding and semiconductor processing.