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Shape-Based Place and Route de Rigueur for Memory, Mixed-Signal Design

New layout requirements and routing in the nanometer age are forcing a fundamental shift in layout automation techniques. This shift is a move from the traditional gridded approach to placement and routing to the more general and flexible approach of gridless or so-called “shape-based” techniques.

Several factors account for why this technology is gaining significant momentum. The first is dictated by process and the required rules at nanometer dimensions, a well-reported phenomenon. Timing, power and yield are also reasons due to the geometry and positioning of the tracks and wires. The design rules are complex and constrained, and need to be for the automation to work accurately.

Curiously enough, this shift has taken some time to occur in the integrated circuit (IC) domain. A paper presented at the 22nd Design Automation Conference (DAC) in 1985 was the first ever public reference to gridless routing. And while its immediate application was to printed circuit boards, it has taken time for this technology to mature into production tools specifically built for IC design.

However, now IC designers are looking increasingly to shape-based technologies. This technology can handle the extreme and often conflicting constraints—geometrical, timing, electrical, yield—that demand careful attention to an incredible amount of detail and flexibility. It offers a quality more akin to the “art” in full custom layout.

Challenges are amplified in layout for memory, analog and mixed signal for which gridded techniques simply do not have the flexibility and spatial awareness to handle. The simple fact is that gridded technologies—while fast and offering high capacity—are not built for off-road conditions imposed by modern requirements of sophisticated layout. This would be similar to a standard road car that is not built to go off-road and on-country terrain.

Over time, being able to constrain layout has become more and more important. First came geometrical constraints, then timing and noise constraints. Constraint and control of automatic routers has become a key aspect in router technology. State-of-the-art layout systems are capable of adhering to hierarchy and a wide set of constraints that constitute part of the system specification and requirements. This aspect is most often seen in analog applications, for example, where two nets have to be perfectly electrically matched or a net must be tapered to adequately handle the electromigration effects of signal current flow.

For this reason, hierarchical and comprehensive constraint handling coupled with a shaped-based environment provides the engineer with the flexibility and control to perform modern layout.

Since the first publication in 1985, the Pulsic design team has been at the forefront of the development of constrained shaped-based technology. Indeed, one of the authors of the original gridless publication is a key architecture at Pulsic.
And today, the Pulsic shaped physical design environment couples these two key concepts of shape and constraint. It is providing a system that can produce layout quality and closure that has not been previously possible for analog and mixed-signal design.

Memory designers have been early adopters of the shaped approach, and old manual methods of design completion are fast disappearing to a new breed of automation through constrained shaped layout. In this application area, the benefit gained is the ability to accurately place and route in very narrow channels—in a spine style which is the most efficient routing style for this application. Users are able to realize productivity gains as well as area reduction and yield increase, all of which are positive economic factors for the application area.

“Shape” has finally come of age. With the appearance at the 42nd DAC of a number of new shaped-based design tools, the catch word this year will be “Shaped-Based,” making 2005 the year of shaped-based placement and routing. Dare I say, “It's the Shape of things to come!”?

About the Author
Kevin Steptoe is Pulsic's vice president of marketing and business development. He was most recently with Magma Design Automation as vice president of product engineering and, prior to this role, he held senior management positions at Avant and Cadence Design Automation. He has 20 years of experience in EDA, has held UK Government advisory posts and has published widely. Pulsic is based in Bristol, England. For more details, visit: http://www.pulsic.com.

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