It has been technically possible and commercially sales of 3 dimensional computer RAM memory for some time and with various approaches. There have been problems with heat and higher costs.
An IBM scientist holds a thinned wafer of silicon computer circuits, which is ready for bonding to another circuit wafer, where IBM's advanced "through-silicon via" process will connect the wafers together by etching thousands of holes through each layer and filling them with metal to create 3-D integrated stacked chips
IBM is claiming a breakthrough chip-stacking technology in a manufacturing environment that paves the way for three-dimensional chips that will extend Moore’s Law beyond its expected limits. The technology – called “through-silicon vias” -- allows different chip components to be packaged much closer together for faster, smaller, and lower-power systems.
The difference this time is that IBM is getting a lot more advantages out of their chip stacking approach. Higher performance and not just higher density. They are also getting lower power.
The new IBM method eliminates the need for long-metal wires that connect today’s 2-D chips together, instead relying on through-silicon vias, which are essentially vertical connections etched through the silicon wafer and filled with metal. These vias allow multiple chips to be stacked together, allowing greater amounts of information to be passed between the chips.
The technique shortens the distance information on a chip needs to travel by 1000 times, and allows for the addition of up to 100 times more channels, or pathways, for that information to flow compared to 2-D chips.
IBM is already running chips using the through-silicon via technology in its manufacturing line and will begin making sample chips using this method available to customers in the second half of 2007, with production in 2008. The first application of this through-silicon via technology will be in wireless communications chips that will go into power amplifiers for wireless LAN and cellular applications. 3-D technology will also be applied to a wide range of chips, including those running now in IBM’s high-performance servers and supercomputers that power the world’s business, government and scientific efforts.
In particular, IBM is applying the new through-silicon-via technique in wireless communications chips, Power processors, Blue Gene supercomputer chips, and in high-bandwidth memory applications:
-- 3-D FOR WIRELESS COMMUNICATIONS TECHNOLOGY: IBM is using through-silicon via technology to improve power efficiency in silicon-germanium based wireless products up to 40 percent, which leads to longer battery life. The through-silicon via technology replaces the wire bonds that are less efficient at transferring signals off of the chip.