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Murray Hill, N.J. - A modern semiconductor-fabrication line is a control-system designer's nightmare. As a wafer travels through a series of processing tools-each adding a wrinkle to the complex, multilevel design-slight variations in physical conditions can introduce errors that depress yield and drive up chip-production costs.
A project at Lucent Technologies Inc.'s Bell Laboratories has addressed one of the critical links in the processing chain-plasma etching-to get a better grip on process-control parameters. The researchers devised a combined neural-network- and genetic-algorithm-based system to analyze and control plasma-etch procedures. Computer simulations indicate a resultant potential reduction in film-thickness variation from 730 A to 220 A.
"The typical fab line involves around 450 processing steps, which use a variety of tools," observed Lucent's Ed Rietman. "Smaller device dimensions mean that the allowable statistical variations in each step have to be reined in." But the process is so complex that no one has found a way to gauge the impact of individual control improvements for one or...