CMOS scaling requires materials advances

CMOS scaling requires materials advances
The challenges facing CMOS scaling increase with each technology node, presenting materials scientists, metrologists and process technologists with plenty of good work to do.

Several articles in this report discuss marrying silicon-on-insulator (SOI) with strained silicon, either by depositing an active silicon layer on a relaxed silicon germanium layer or through a layer-transfer method, such as Soitec's Smartcut.

Though SOI and strained silicon get most of the attention here, the article by Sematech senior fellows Peter Zeitzoff and Howard Huff looks at the wider obstacles, which they say will grow more challenging in 2007 and beyond. Gate lengths by then will be nearly half of what they are today. High-k oxides and probably metal gate electrodes will be needed, as will new process techniques.

Things are changing fast. Five years ago, who would have predicted strained silicon would be used in the commercial 90-nm processors due out next year?

The tensile and compressive strains that enable higher carrier mobility are tricky: Defects must be controlled and strain must be maintained during thermal steps. Gene Fitzgerald, a professor at the Massachusetts Institute of Technology, describes the most up-to-date thinking about strained silicon. IBM's Jeff Welser, another pioneer in the field, gives his thoughts as well.

And how about that back-to-the-future candidate, germanium? Europe's Interuniversity Microelectronics Center and other research hotbeds are considering whether to deposit germanium, rather than silicon, as the active layer. Because it is a rare element, germanium bulk wafers would be prohibitively expensive, so researchers are pursuing a germanium-on-oxide approach that would build transistors in a thin germanium layer on top of a thick oxide. That would save money and exploit the benefits of the reduced current leakage at the junctions.

Many of these materials require such thinness, and such uniformity across a fairly large 300-mm wafer, that metrology is a gating item. KLA-Tencor and other companies are meeting those challenges with improved, and perhaps someday entirely unique, measuring instruments-a fascinating subject.

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