CPTF notebook: From restricted to ‘proscriptive’ design rules
From 14nm, possibly 10nm, we could be headed into a more controlled era for design where rule books tell designers what they can rather than what they cannot design, according to Gary Patton, vice president of IBM’s Semiconductor Research & Development Center.
Speaking at the Common Platform Technology Forum, Patton said the “proscriptive ground rules” would be necessary because of increasing pressure on lithography.
“Rules have become very complex, but we still leave a very large [design] window which can be full of different configurations and shapes,” he said. “There’s no way the litho team can simulate and validate all of those. And just one jog or some little line can take a whole design down.
“And we can’t have tens of thousands of rules.”
IBM and its Common Platform alliance partners, GlobalFoundries and Samsung, “hope” that commercially viable extreme ultraviolet (EUV) lithography will be available some time during the 14nm node, if not at the beginning.
“But we’re pursuing a dual path,” Patton added.
Apart from the proscriptive rules (which could even be necessary if EUV finally arrives), Patton also envisaged the alternative and continued use of double patterning, including a patented variant called sidewall image transfer (SIT).
An SIT layer is placed on top of a memory layer, according to IBM’s original filing: “The SIT layer is then patterned resulting in a SIT region. The SIT region is used as a blocking mask during directional etching of the memory layer resulting in a first memory region. Then, a side wall of the SIT region is retreated a retreating distance D in a reference direction resulting in a SIT portion.”
In addition, IBM is looking at EUV alternatives/stop-gaps such as self-aligned vias and ‘brute force’ litho-etch-litho-etch strategies.
After many years in the lab already, EUV still needs to overcome a number of significant obstacles. These include getting enough power in the scanner (an increase of roughly an order of magnitude remains necessary), securing effective resist materials and delivering masks that are defect-free because for EUV they will not have a protective pellicle layer.
The Common Platform companies are hoping many of these will be overcome through its various research links, including those with the EUV Center of Excellence now under construction in Albany, New York.
The idea of a ‘proscriptive’ rule deck is not entirely new, but has not been widely seen for digital design outside IDMs, which have much closer links to manufacturing process development.
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