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ThruChip opts for wireless wafer stacking

Posted: 25 Feb 2014  Print Version  Bookmark and Share

Keywords:TSV  stack  coupling  wireless 

ThruChip Communications has an inductive coupling link in the works, which is said could save 40 per cent costs of through-silicon vias (TSVs), eliminating wired links that engineers craft for 3D stacks.

Veteran microprocessor designer Dave Ditzel was recently named CEO of the technology-licensing start-up. He believes that the technology pioneered by Keio University Professor Tadahiro Kuroda from Japan has potential for broad impact, especially for stacks of DRAM and flash chips.

Kuroda is "one of the most famous circuit-design professors in the world, and I've kept in touch with him over the years as a consultant and hired his students," says Ditzel who led CPU design teams at Intel, start-up Transmeta, and the former Sun Microsystems.

Today a growing group of companies is developing so-called 2.5D stacks that lay dies side-by-side, connected through wired links on a silicon interposer beneath them. Engineers are working on the next big leap, drilling tiny TSVs through dies stacked vertically, but so far the technique is mainly in a prototype phase, hampered by issues of cost and complexity.

ThruChip's wireless approach is simpler, uses less power, and is cheaper than using TSVs, says Ditzel who joined the company in December. He aims to hire a Silicon Valley support team to help companies design the coupling coils in their chips.

The start-up, founded in 2007, has designed 10 test chips over the decade Kuroda has published on the technique. It has shown signals going through 128 dies using repeaters in one test that packed a thousand 50-100 micron coils to create a terabit/second link. The links only carry data, relying on traditional wired links for power and ground signals.

Besides memory stacks, the technique could be used in other applications where engineers are currently working with TSVs. Uses including linking to memory chips a variety of mobile and cloud processors as well as camera backside imagers, Ditzel says.

One 3D stacking specialist expressed some scepticism. The wireless approach eliminates difficult steps needed for TSVs, such as extreme thinning of wafers, however it opens up new issues about aligning coils and eliminating interference between them, says Herb Reiter, principal of EDA2ASIC Consulting in Los Gatos, Calif.

The size of coils is another issue. "I question if these coils can be made small enough to compete in die area with TSVs—even including the TSV keep-out zone," says Reiter.

- Rick Merritt
  EE Times





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