3D buzz centers on through-silicon vias

TSV to the rescue
TSV technology remains immature and expensive. Heat dissipation in the silicon stacks is a major problem. A few companies are sampling TSV products, but none have shown the potential to replace today's entrenched wire-bonding techniques or new advances in package-on-package technology.

"Electromigration is a well-known root cause for reliability problems" in 3D integration, said Riet Labie, a researcher at IMEC, a Belgium-based R&D group.

IBM, Intel, Elpida, Samsung, Toshiba and a growing list of companies have talked about or demonstrated devices based on TSV technology. The list continues to grow, as a number of entities at IITC presented ways to propel the technology into the mainstream.

For example, silicon foundry giant Taiwan Semiconductor Manufacturing Co. Ltd. (TSMC) claims to have developed a production process in the arena. In a paper at IITC, entitled "Production Worthy 3D Interconnect Technology," TSMC proposes a method that would fabricate the vias in the fab or in the front-end-of-the-line (FEOL).

The so-called ''via first'' approach is somewhat competitive to proposals, which would fabricate the vias in the backend or in an IC-packaging house.

TSMC also proposes to fabricate the vias with two methods: wafer-to-wafer and die-on-wafer. Each method would use a copper-to-copper thermo-compression metallization scheme to bond the wafer to the wafer or the die to the wafer.

TSMC's TSVs, which were filled with both copper and tungsten interconnect materials, are said to have high-aspect ratios of 8:1 and 15:1.

Others are taking a different approach to the technology, but all agreed there are some major issues with TSVs. "Everyone talks about cost and reliability," said Armin Klumpp, a physicist with the Fraunhofer Institute for Reliability and Microintegration.

At IITC, Fraunhofer described the latest findings within a European project, dubbed "Through-Silicon Via Technologies for Extreme Miniaturized 3D Integrated Wireless Sensor Systems.''

The three-year project, also called e-Cubes, proposes to devise a tiny wireless sensor node. The projected started in 2006. A demonstration product is due out by 2009. The project includes CEA, Infineon, Alcatel, Honeywell, Philips, SensoNor, IMEC and others.

Fraunhofer also described the development of a tire pressure monitor, based on a TSV scheme. To devise this product, Fraunhofer described a technology called ICV-SLID (solid-liquid-interdiffusion) soldering.

The R&D group uses a chip-to-wafer bonding method. A 3D chip has been fabricated by bonding the chip on the wafer with 10-um thin soldering pads. The TSVs were processed in the front-end via a copper/tin metal system. In one experiment, the group claims to have devised 2 x 10-micron² and 20-micron deep TSVs.

A more exotic approach is being taken Tohoku University, which proposes a 3D technology based on a chip-to-wafer bonding technique, dubbed ''super-chip'' integration.

''After wafer probing and dicing many known good dies (KGDs) with TSVs for the first layer of 3D, LSIs are simultaneously aligned using a self-assembly technique and bonded to the chips on a LSI wafer through the metal micro-bumps,'' according to a paper from the university.

The self assembly process is done by growing silicon dioxide films on a wafer. Then, so-called hydrophilic areas are patterned on a wafer via lithography. Then, aqueous solutions are dropped on the hydrophilic areas. The silicon films are then aligned on the wafer via a self-assembly process, said Mitsu Koyangai, a professor at Tohoku University, at IITC.

Silicon chips with sizes of 1-mm² to 5-mm² square are assembled with alignment accuracy of less than 0.5-micron, he said at the event.