Software Aids Chip Design
Brent D. Johnson
According to the current roadmap for the semiconductor industry, by 2003 half of all chips will include transistors with gates smaller than the wavelength of radiation that the stepper used to create them. Because the old rule of "what you see is what you get" no longer applies in the production of such chips, designs must include optical correction to compensate for the distortion that occurs in the subwavelength regime.
When Motorola Corp. set out to create its latest PowerPC microprocessors, it turned to Numerical Technologies Inc. for assistance. The company's In-Phase software suite helps chip manufacturers apply phase-shifting processes that enable the fabrication of subwavelength
features. The three-module tool modifies a GDSII chip design, adding phase shift where necessary, simulates the lithography process to determine if further optical correction is needed and makes corrections where required.
Motorola Corp. used Numerical Technologies Inc.'s In-Phase design suite when producing its latest PowerPC processor. The modular software examines the layout data for a proposed integrated circuit and determines if optical correction is required to produce the desired subwavelength features. Where necessary, In-Phase adds correction to the design.
Motorola takes the revised designs and brings them into being by using reticle-enhancement technology at the fabrication facility. To perform the phase shifting, it employs alternating-aperture phase-shift masks designed using Numerical Technologies' software tools. Adjacent openings (or apertures) in the mask are etched so that the phase difference yields an enhanced contrast of the phase-shifted features, which offers better resolution and process control.
Can this be done another way? It depends on the stepper, said Warren Grobman of Motorola. A chip manufacturer may employ a shorter wavelength and a higher numerical aperture in lithography, but doing this would require significant capital expenditure. Similarly, he said, one may produce smaller gates by repeatedly etching wider ones, but this method lacks fine control and is not viable for producing very narrow features. "What phase shift gives you is small features, controllably," he said.
Phase-shifting is being used in the production of the microprocessors in Apple Computer Inc.'s new dual-processor 1.25-GHz Power Mac G4, which boasts speeds of more than 18 gigaflops.
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