Semiconductor mask double take


By Hank Hogan

Two recent announcements regarding semiconductor photomasks are good news for the industry, with one being a contamination-related breakthrough. The first of the two concerns defect levels on mask blanks for extreme ultraviolet (EUV) lithography, a technology that will be deployed several years from now. The second involves a new tool that should help in the manufacture of masks for double patterning, a lithography technique that could see action fairly soon.

Sematech’s (Albany, NY) director of lithography, Michael Lercel, notes the EUV mask blank result is critical to the industry’s eventual adoption of the technology. Research and development on EUV lithography is moving along, which means the necessary infrastructure has to be in place. “People are already starting to get to the point where they want some defect-free blanks within the next few years,” he says.

In February, Sematech demonstrated EUV mask blanks with a record low of only eight defects. That number includes both the substrate and the 80 layers deposited on top of it that form the multilayer reflector. Since the mask blanks serve as the raw material for the masks that contain the device pattern for each layer, the blanks have to have as low a defect level as possible.

Photomask blank in preparation for patterning. Photo courtesy of SEMATECH.
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Sematech achieved the low defect level through a combination of multilayer deposition enhancements, better substrate cleaning, improved substrates, and state-of-the-art inspection. Capable of finding defects as small as 53 nm on the blanks, the inspection process helped evaluate the other changes and sped the improvement process along.

Though an important accomplishment, Lercel says this defect level isn’t enough. “Your goal is to try to get it down to zero, at least at some yield.”

He feels, though, that the potential for producing blanks that hit that target is good. At the same time, Lercel readily acknowledges that not all the technology showstoppers have been solved.

The mask metrology tool could be useful in EUV lithography, although it will first be used in double patterning lithography. An extension of today’s technology, double patterning could be deployed fairly soon because it’s fairly easy to do and allows the processing of smaller features.

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In double patterning, part of a circuit layer is patterned on a wafer, which then is removed from the lithography tool and processed. The wafer then comes back, has photoresist put on it, and the rest of the layer is patterned.

This approach allows smaller features to be imaged, but demands very precise placement of the pattern on the mask. That’s where the PROVE™ tool, developed by Carl Zeiss SMT (Jena and Oberkochen, Germany) with the support of Sematech, comes in. The tool enables the needed measurements to be made. Zeiss product manager, Dirk Beyer, notes that most registration and overlay measurement systems today use 365-nm illumination, something the new tool improves upon.

“We are using 193-nm illumination,” says Beyer, while listing the new tool’s biggest advantages.

That smaller wavelength enables the required placement measurement accuracy at the minimum feature size to be printed, which is increasingly demanded by all semiconductor photolithography. The new tool will see production in the second half of 2009.