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TSMC OIP 2013: Tackling FinFET Parasitics, Cell Pin Access, and Wire Resistance Challenges at 16nm

Comments(0)Filed under: Cadence, EDA tools, Brian Fuller, FinFET, TSMC, finfet process, finfet advantages, EDA vendors, semiconductor design, extraction, IC design, FinFETs, Fuller View, Open Innovation Platform, OIP, Paul Cunningham, Rachid Salik

The emergence of 16nm FinFET process technology opens a world of promise to electronics systems wrestling with Moore's law. It also comes with its share of peril.

Consider exploding design-rule complexity and skyrocketing resistance. Then there's the "silent killer:" the difficulty for routers to connect efficiently to pins on standard cells amidst the quagmire of conflicting and complex design rules.

And there's the challenge of extracting parasitics from FinFETs, which is significantly different from regular planar CMOS devices—all without growing old in the process.

Paul Cunningham, Cadence vice president R&D and Azuro co-founder, and Rachid Salik, vice president of R&D at Cadence, addressed these challenges in a presentation on October 1 at the annual TSMC OIP event here in San Jose.

"The 16nm FinFET process has presented an awesome challenge for us," said Cunningham.

Wire delays have been dominated by increased net resistance, while net capacitance has remained relatively constant, and at 16nm, it's only getting exponentially worse, he added.

Hence, the effective use of low-resistance upper metal layers (M9/10) to buffer timing-critical nets is vital to achieve good timing closure. The wire delays of these upper metal layers can be 10 times less than those on the lower layers, resulting in a big timing gain from routing long/critical nets on these upper layers, according to Cunningham.

"However, there are a limited number of routing resources on these upper metal layers due to the presence of power/special nets, which restricts the assignment of signal nets, and causes potential congestion and routing issues down the line if not precisely dealt with."

"What is needed is a route-aware optimization engine that identifies long timing-critical nets, performs an accurate congestion analysis to make sure there is space available on the upper layers, and then re-buffers these nets on the upper layers to improve timing."

Optimizing in GigaOpt

Cunningham called out the GigaOpt route-driven optimization technology in the Encounter Digital Implementation System. GigaOpt is a multi-threaded, ROI-driven optimization engine that provides optimal performance, power, and area, while accelerating turnaround time.

Cunningham said: 

"We look at the entire solution space to optimize the critical net. We evaluate different strategies, and for every strategy, do a cost analysis. What's the impact of timing and congestion? What are the resources that are available? What are the area and the power costs? Anything we do must converge in the flow."

And as if there aren't enough metrics to be concerned about, please welcome pin access as a new critical design-closure metric (see images nearby). 

Double patterning techniques-—critical to ultra-deep submicron fabrication—are leveraged to get the maximum possible density of tracks in lower metal layers.

But this makes it harder to undertake graceful via spacing and via cuts.

Cunningham said: 

"It's getting harder and harder to cut them close together. You get a larger and larger halo around each via cut before we can cut a neighboring via. If we're trying to turn a corner on the same layer, to escape from a via then we have to have a certain minimum distance. We can't turn a sharp corner."

When it comes to pin access, it's going to be very hard "to via down to the pin. As you via down to one pin, you're going to create a halo that locks pin-access to the neighboring pin," Cunningham said.

This and other complexities can make it "just incredibly difficult to get the design to route. The congestion... can be low but if your local pin density gets out of hand, the design won't close, it won't route."

"As a result, careful control of pin densities during cell placement, and global pin-access planning during detail routing, can have a big impact on achievable design area." 

The extraction challenge

And then there's extraction at 16nm FinFET.

First off, you have to build electrical models that will represent parasitics on the FinFET. And then those models have to be compact to minimize design-turnaround time.

There are challenges with FinFET that we have to address:

  • Establish and extend FinFET RC parasitic models to be closer to those extracted using a field solver (2.5D versus 3D)
  • Compact RCs around FinFET not to explode design TATs
  • Convergence between pre-layout and post-layout by generating good-estimation parasitic RC models of FinFET

"It's one thing to accurately model RCs, but if we have to explode the design time by 10X, that's not an efficient way to do it," Salik said.

Cadence is also driving towards giving FinFET designers an earlier estimation of RC impact to converge their designs, reducing the gap between pre-layout and post-layout.

"Designers can no longer afford to assess the impact of parasitics after layout. Early simulation during schematic design in the Virtuoso custom design platform or in the optimization loop of the Encounter digital platform is required," he noted. This means tight integration of signoff extraction models of those implementation platforms.

He added:

"Moving to 16nm FinFET means more functionality on the chip. So we had to work on extraction side to not be the bottleneck at signoff—mainly how we can improve runtimes and build other methodologies like incremental extraction."

Vertically challenged

The 3D nature of FinFETs raises capacitance challenges; in fact from 28nm to 16nm, capacitance increases 25 times.

"We have to extract an accurate model but extract it in a compact way," Salik said.  

He added:

"There is the electrical interaction, capacitance between gates and how we connect to transistors. The poly is also a 3D device, so all of these bring a bigger challenge to accurately extract capacitances."

Salik said the "obvious solution" is to build a field solver, but runtimes can be slow. 

"So we have to extend our 2.5D models to be almost 2.9 to be as accurate as a field solver and to keep as extraction runtime as low as possible compared to 28nm," he added.  

Another key is to bring the RC of the FinFET into a designer's platform as early as possible, since RCs have twice the impact on delay at 16nm as they do at 20nm.

Cadence has been working with TSMC on some reference flows in which the RC estimate within the FinFET is segmented, "so you have an early estimation of the design parasitics to reduce the gap between pre-layout and post-layout," he added.

Brian Fuller

Related stories:

EDPS Workshop—a Review of FinFET Parasitic Extraction Challenges

TSMC 2013 Symposium: Progress in 20nm, 16nm FinFET, and 3D-IC Technologies

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