Glare gives silicon goose bumps

Fluorescent lighting in chip factories creates tiny, possibly troublesome welts on the silicon used to make microcircuits, new experiments suggest.

Scientists who have seen such nanoscale bumps form in their lab say that light might be causing defects in the commercial chips most densely packed with transistors. Chips with such defects are probably among those discarded during production.

As future chips become denser still, it will become more critical to avoid roughened silicon surfaces, says Hitohi Morinaga of Tohoku University in Sendai, Japan. The bumps can cause current to leak or flow poorly.

Morinaga and his Tohoku colleagues Kenji Shimaoka and Tadhiro Ohmi report on light-induced silicon roughness, including surface pits, in the July Journal of the Electrochemical Society.

The team was investigating the use of light to clean chips when “we found, by accident, that light had an adverse effect,” Morinaga recalls. The researchers’ experiments later revealed what they call hillocks when silicon is both illuminated by fluorescent lights and immersed in ultrapure water or in chemicals like those used commercially to etch and clean chips. The bumps result from light-induced oxidation followed by nonuniform etching of a silicon surface, the team found.

To prevent such roughening, chip factories “will require controlled illumination conditions,” predicts Takayuki Homma of Waseda University in Tokyo. Some other semiconductor specialists doubt that such a move is necessary.

Materials scientist Steven Verhaverbeke of Applied Materials in Sunnyvale, Calif., says that the findings might lead to some changes in production processes.

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