Mentor builds models for CMP effects

Mentor Graphics has given its Calibre design-checking tool the ability to tune layouts using models of chemical-mechanical polishing processes to help avoid chip failures caused by changes in the thickness of metal lines.

Jeff Wilson, product marketing director at Mentor, claimed that, although metal lines have gradually become thinner as chipmakers have moved to more advanced processes, the percentage variation has increased. “The tolerance specification for the equipment has stayed pretty much around 10nm,” he said. “It was a 5 per cent variability impact at 90nm. At 32nm, we are now talking about 20 per cent.”

The variability is increasing not just due to the polishing process, in which a pad is spins on the wafer as an abrasive slurry is added. Etching of the top surface of the wafer before the copper metal is added also introduces some variation in surface profile, which also affect the metal thickness. Both sources of variation can be addressed by altering the design to reduce local difference. Polishing is sensitive the amount of metal in a given area and the etching profile is coming to depend on the ratio of area to perimeter of the metal features.

Wilson said the etching phenomenon has become more noticeable recently, with the introduction of 45nm processes: “Customers are giving us direction and guidance for improvements in the tool. They tell us that the etch rate is determined by the perimeter [of the metal features].”

Using the same approach as for polishing, in which dummy metal features are added to the layer where the density of metal lines connected to circuits is relatively low, the SmartFill software that plugs into Calibre can work out the best shape to use for etching resistance.

When Calibre selects a shape, it can select one that has uniformity with the perimeter values [of the circuits’ metal lines,” said Wilson.

The SmartFill tool uses either rules or simulation of CMP effects to work out how to add dummy metal. The models provide more accuracy but demand access to foundry data, or that the foundries create models that the tool can use.

For the initial release of the SmartFill tool, Mentor is providing a model calibrated for the 45nm process at Chartered Semiconductor Manufacturing.

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