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Prasad: Wafer cleaning tools challenged as metallic materials enter at the 45-nm node

Posted on: 26-Apr-2007

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Sematech’s 2007 Surface Preparation and Cleaning Conference, which attracted about 300 researchers to Austin this week, comes as process engineers are faced with the likelihood that established forms of wafer cleaning will need to be replaced or radically altered at the 45-nm node and beyond.
For high-performance logic and memory manufacturers, the introduction of metal-based high-k dielectrics, such as hafnium oxide, and metal gate electrodes, will make it difficult to use traditional RCA clean methods, said Jagdish Prasad, the chief scientist at AMI Semiconductor (Pocatello, Idaho), the keynote speaker for the conference.
“Sixty percent of fab-related (yield) problems are related to cleans, and another 12 percent to etching steps,” said Prasad, who participates in the International Technology Roadmap for Semiconductors working group on cleaning issues.
RCA cleans, with their mix of hydrochloric acid, peroxide, and sulfuric acid, react badly with metals. “If companies are using metals at the gate, RCA cleaning is not an option. And there are other issues. Depending on the temperature cycles, these gate materials crystallize, and it becomes very difficult to remove contaminants and particles.”
Manufacturers will need to adopt new etch chemistries and cleans, he said. “At this point, I don’t think people have really figured out how to put two different metals (as the gate electrodes) in their process and get the cleaning steps right,” Prasad said.
For the time being, chip manufacturers will try to get by with modifications of existing cleaning methods, as they investigate brand new tools now being researched.
At the SPCC conference, researchers discussed a wide variety of new cleaning methodologies, such as laser-generated shock waves to move particles off the wafer, as well as improved megasonic techniques.
John Rosato and Rao Yalamanchili of Applied Materials, Inc. presented an investigation of damage caused in megasonic systems. One problem is that superheated liquid inside a bubble can cause cavitation. Another issue is that bubbles can coalesce, and then collapse. When a single bubble collapses, the resulting shock wave can result in a beneficial cleaning action. When a collection of bubbles collapses, the energy released is "asymmetrical."
That is not stopping Applied, which is looking at how megasonic clean systems with multiple transducers, arranged at the proper angle of incidence, could avoid multiple bubbles and produce “shock waves, not jets,” for 45-nm device cleaning, Rosato said.

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