July, 2011
www.us-tech.com
Upping Performance with Through Silicon Vias (TSVs)
By Tom Adams
recent years, and are now beginning to appear in production. The concept, though, is not new. William Shockley, one of the
T
inventors of the transistor, described and patented TSVs (without calling them that) in U.S. Patent #3,044, - 909, filed in 1958 and issued in 1962. But it took half a century for the pro- duction technology to reach the level
hrough Silicon Vias (TSVs) have been at the center of dis- cussions of 3D integration in
research, and particularly in devel- oping techniques for fabricating TSVs, is the GigaScale Integration program at Georgia Tech. Dr. Muhannad Bakir, principal investi- gator in the program, points out that a few products using TSVs are already in production, and that sev- eral companies have announced that they will shortly be releasing prod- ucts that use TSVs.
A Few Microns Across The diameters of TSVs vary
from a few microns to many tens of microns, depending on where they are used. By far the smallest are the die-to-die TSVs made in fabs. It might be tempting for back-end man- ufacturers to think about doing their own TSVs, but Dr. Bakir thinks that this technology will remain firmly in the hands of the IBMs and Intels of
the world. The technology is difficult, and it would be very difficult for com- panies from other disciplines to try to catch up. The TSVs used in silicon inter-
posers, however, are considerably larger and easier to form, and Dr. Bakir points out that many silicon interposer applications are currently in development. A silicon interposer
Continued on next page Nordson DAGE X-ray Inspection Systems
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Computer model of a tapered Mechanically Flexible Interconnect (MFI), a tiny,
coiled, tapered spring that can be formed at the wafer level.
of expertise that would actually per- mit making TSVs. The idea of a TSV is simple
enough. Holes are etched through sil- icon die, generally in wafer form, and then plated with copper. The copper makes it possible to connect one device with another device placed on top of it, and eliminates the need to use wires. Instead of placing two die —
memory and a processor, for example —in different locations on a board and interconnecting them with wires and traces, they are stacked on top of each other and connected by TSVs. It is currently possible to stack
up to thousands of TSVs per square centimeter. Solder bumps are typical- ly used to join the two TSVs, although a very different technology, referred to as MFIs, is on the horizon. Communication between the
two die is much faster because the distances are so much shorter. A sec- ond advantage: regions of circuitry that frequently talk to each other can be placed at opposite ends of the
To make a TSV, holes are etched through silicon die, generally in wafer form, and then plated with cop- per, making it possible to connect one device with another placed on top of it.
same TSV. And stacking two or more thinned die in this way means that applications that demand higher speed, greater performance and smaller size can benefit greatly from the much higher interconnect densi- ty that TSVs provide. One of the leaders in TSV
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