Ed’s Threads 070928
Musings by Ed Korczynski on September 28, 2007

Who needs through-silicon vias?
Besides MEMS and opto-electronics, who really needs through-silicon vias (TSV) for commercial ICs? This was the burning question around which presenters danced for an afternoon at the International Wafer-Level Packaging Conference (IWLPC) held this September in San Jose, California. Starting with IC and wafer-level packaging technologies already in use, experts seem confident that technology integration can create a manufacturable TSV fab flow. However, while 3D-WLP is already commercially viable (pun intended), TSV do not seem to be needed for the near future; wire-bonding already can handle up to 16 chips, and 2 level connections can be easily flip-chipped for high-performance (like for a microprocessor cache).

Ken Gilleo of ET-Trends LLC discussed the “coming paradigm shift in packaging” caused by TSV and wafer-level packaging, asserting that significant technology development has occurred with unit processes in recent years such that the main technology hurdles remain with integration.

Leslie Lea, CTO and deputy CEO for STS, explained how deep reactive-ion etch (DRIE) for TSV on 300mm wafers will still use a derivative of the sequential “Bosch Etch” process, using the C4F8 plasma for polymer sidewall deposition, then SF6 plasma for etching. This process can produce vias to 80:1 aspect ratios, but sidewall scallops inevitably exist. Cu-TSV plating time shown was 4 hr for 50µm via, while 10µm via filled in 1 hr using NEXX systems and Enthone chemistry to create via fills without voids—with vias of 10-50µm depths all nicely filled on the same chip.

TSVs have been demonstrated in four different approaches and integration schemes: blind, poly, tungsten, and copper. Jim Walker, research vice president for Gartner Dataquest, suggests that we all should use the standard PCB term “blind vias” for essentially the same structures in silicon. Unlike the other three, ‘blind’ vias don’t include the conductor, but etch/drill out openings through an upper silicon chip, typically to allow a wire bonder to make connections to bond-pads on a lower silicon chip.

These are not new. Back in 1989 I developed a pilot process for a 3-level WLP using blind TSVs for an accelerometer chip for SenSym (Analog Devices’ designers were much smarter and their planar chip design was far more manufacturable and lower cost, so sadly for me at the time the chip was killed at pilot). Blind TSVs can be combined with flip-chip stacks and C4/C4NP bumping to get to three or more silicon layers with relatively low cost and minimal disruption of current packaging flows.

Blind TSVs are another way that wire bonders may continue to function as the ‘work-horses’ of packaging lines, working with KOH or EDPW wet-etches to form sloped openings along the crystalline planes in silicon. In an exclusive meeting with WaferNEWS, Giles Humpston, director of R&D for Tessera, explained that the company’s ~$100M investment in optical-WLP technology built on the acquired ShellCase technology for blind TSV applied to the unique requirements of image-sensors and quartz substrates.

Filled vias with poly, tungsten, or copper are the TSV ideal that many of us have conceived of for 3D ICs. If design and test software could handle it, and if integration can be as low as $200/wafer (EMC-3D goal), then these TSV might be first used to stack like devices like memory parts. Phil Marcoux, longtime packaging technology expert currently with Chip Scale/TPL Group, thinks that full integration won’t be ready for five years. Gilleo countered that in 2008, “some memory will use TSV.”

Citing first principles of electrical interconnection—going back to the use of copper in the first US printed circuit board patent in 1902—Gilleo is convinced that ultimately copper is the way to go for filled TSV. Used both for PCBs and on-chip interconnects, there is a tremendous amount of proven technology that can be borrowed to speed up TSV integration. “It’s well controlled in electroplating, and it has the right balance of chemical and mechanical properties,” informed Gilleo. It becomes the nature selection for the conductor. “It has almost everything you want for building conductor pathways.”

All of this was known to the early pioneers of the planar IC at Fairchild Semiconductor. And yet they chose aluminum over copper, because copper is more reactive and can more easily diffuse into silicon and damage transistors. Copper will always have a much higher expansion with temperature compared to silicon, and so high-temperature processes will inherently stress barrier layers. Polysilicon can be annealed and then have the same expansion with temperature as the silicon wafer. Of course, polysilicon conductivity is always lower than copper, so there are trade-offs in the TSV conductor choices.

While debating whether to consider integrating poly or copper or even tungsten plugs, a gold wire bonder has already made the connection. Packaging moves fast.

--E.K.

Labels: 3D, blind TSV, copper, IC, interconnect, stack, through-silicon via

posted by [email protected]
070928: Who needs through-silicon vias?

Post a Comment