Terafab addresses mask requalification, but more’s needed

by M. David Levenson, Editor-In-Chief, Microlithography World

Feb. 15, 2008 – An odd thing happened on the way to 65nm chip technology: perfectly good photomasks started going bad after a few hundred wafer exposures — due, it turned out, to the deposition of crystal-like defects. Evidently, the 193nm exposure light desorbed material from the reticle, which combined with airborne molecular contamination to create… gunk!

A bad mask has zero yield, which can be a terrible shock to manufacturing. A way was needed to detect these mask defects before they were printed on wafers. If detected, those defects can be cleaned off and the $100k mask re-used. While mask shops have had technology suitable for finding these anomalies, those instruments were not optimal for wafer fab use.

KLA-Tencor has now produced a suite of three tools for requalifying masks in the wafer fab, thus avoiding yield crashes, but occasionally delaying production when the need for a week-long reticle clean is discovered. One key thing is that the company now claims to be able to handle sub-resolution assist features, which previously were registered as nuisance defects.

What’s still needed, though, is an efficient way to remove the printable defects and prevent their recurrrence. — M.D.L.

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