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Chip stacks feature near-zero TSV keep-out zones

Posted: 20 Mar 2014 ?? ?Print Version ?Bookmark and Share

Keywords:chip stack? GlobalFoundries? TSV?

GlobalFoundries is earning acclaim for its work on making 3D chip stacks without a large keep-out zone around its through-silicon vias (TSVs), right when Moore's Law is slowly getting more costly.

The company will describe a middle-of-line (MoL) chip stack in a 20nm planar process that achieves a "near-zero" keep-out zone around its TSVs in a paper at the IEEE International Interconnect Technology Conference in May. Prior work used keep-out zones measuring 7?m or larger, wasting silicon space and driving up chip costs.

Globalfoundries will use the technique in its commercial 20nm process, said the paper's author and a member of technical staff at the company Mohamed A. Rabie, in an email exchange. He pointed to a description online of the foundry's via-middle approach.

A keep-out zone of just 3?m could take up 113?m2 on a chip, four times the 28?m2 of the TSVs used in the GlobalFoundries work, said Herb Reiter, a 3D IC expert who reviewed the paper. For large graphics chips expected to use 10 thousand TSVs or more, "the area savings of a zero keep-out zone... will remain very significant," Reiter added.

In addition, "Less strain between TSVs and surrounding silicon will also improve reliability of ICs, especially in regard to maintaining good contacts between TSVs and interconnect layers."

For years, engineers have been pursuing 3D chip stacks as a way to make smaller, faster devices. They aim to create an alternative to typical CMOS process shrinks, which are becoming more complex and costly due to the lack of new lithography tools. But the 3D efforts have been dogged by a variety of technical and business issuesincluding the large keep-out zones around TSVsthat makes the approach still costly for most uses.

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A cross section of GlobalFoundries' full stack with TSVs shows an oxide liner thickness of 278nm.

The MoL layer stack GlobalFoundries describes uses nitride, PMD oxide, and a contact protection layer with a high coefficient of thermal expansion. The CTE is about four times higher, and the elasticity modulus is a third lower than used in previous chemical-mechanical planarisation (CMP) stop layers. Thus the stop layer will "shrink at a faster rate than the underlying silicon, resulting in high tensile strain in the CMP stop layer," according to the paper.

The stop layer will "shrink at a faster rate than the underlying silicon, resulting in high tensile strain in the CMP stop layer," reads the paper. The main challenge was finding a stop-layer material that could "balance the TSV stress but not cause additional stress in silicon that might still affect active devices negatively," says Rabie.

Engineers have worked on similar approaches using stress-reduced annealing. Separately, Terrazon has shown a design using tungsten TSVs that have a high CTE and thus can eliminate the keep-out zone, Reiter notes.

GlobalFoundries did not provide cost estimates for using its annealing process, key to its overall viability in production, Reiter noted. In addition, the paper did not address how the technique would be used with FinFETs, a 3D transistor structure that GlobalFoundries, TSMC, and others will employ at 16nm and smaller nodes, he said.

Although the current paper lists only authors from GlobalFoundries, to date the company has been working with partners on 3D IC processes. By contrast, its larger rival, TSMC, is going it alone so far with an in-house process.

In other papers at the event, researchers at CEA-LETI will describe 50?m TSVs in a 14nm process, enabling dense chip stacks with multiple layers. Researchers at Imec will report on a fivefold improvement in carbon nanotubes as chip interconnects, approaching the performance of copper, which is widely used today.

- Rick Merritt
??EE Times

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