Micromasking of plasma etching due to bacteria: a yield detractor for ULSI

Continued size reduction of semiconductor microelectronic circuits increases their sensitivity to ultra small particulate contaminants. A form of self-replicating particulate is bacteria found in ultrapure water systems. The bacteria can micromask plasma etches causing electrical shorts between adjacent conduction paths, thereby providing a key yield loss mechanism when fabricating high density circuits. This paper discusses defects caused by water born bacteria in tight pitch repetitive patterns found in high density memory circuits. Using high resolution wafer inspection tools and scanning electron microscopy, intentionally introduced bacteria were located on wafers before plasma etching and the same locations were inspected after the etch was completed. While in some cases the defect formed followed the bacteria shape, in many instances the post-etch defects did not resemble the original bacteria in any way. Besides electrical shorts between conducting regions, due to the specifics of the technology, defects due to bacteria micromasking of dry etches can also cause failures between two layers of polysilicon. The "before" and "after" data provided by this work present a means of identifying a key defect source, namely bacteria, based on defect appearance.