IEDM news: Fujitsu updates work on ReRAMs, multilayer interconnects

December 14, 2007 – At this week’s IEDM, Fujitsu discussed its recent progress in developing a new resistive RAM (ReRAM) memory as a possible alternative to flash for embedded applications, and its work with multilayer interconnect technology for logic LSI devices at and beyond the 32nm node.

First, Fujitsu and Fujitsu Labs disclosed their work on high-reliability multilayer interconnects for 32nm and below logic LSIs. At 45nm and higher nodes, copper interconnects are protected by a barrier metal to prevent both Cu oxidation and diffusion into the insulating film. Thicker barrier metals improve reliability, but at 32nm and below they take up too much room (32nm Cu interconnects are just 50nm wide), increasing line resistance.

To solve this problem — reduce line resistance while maintaining high reliability — Fujitsu formed Cu with manganese additives on an ultrathin-film barrier metal. After forming the barrier metal on the insulating layer, a Cu seed layer containing manganese was formed and plated with the Cu interconnect material, and the structure was heated to >350°C.

The resulting manganese segregation layer not only prevents oxidation, but is one-third thinner than the previous (≥45nm node) barrier metal that wraps around the Cu interconnects, and reduces interconnect line resistance to 32nm requirements laid out in the ITRS, Fujitsu notes. And in terms of desired high reliability — endurance lifetime against electromigration (a cause for degradation over time) is increased by a factor of 47.

Also at IEDM, Fujitsu Labs said in its new ReRAM nonvolatile memory, it doped a nickel oxide film with titanium (Ti:NiO), to increase voltage for memory erasures. Operations required only 5ns, about 10,000X faster than before, and with resistance fluctuations reduced to 1/10 that of conventional ReRAMs. Optimizing the voltage applied to a transistor reduced the current needed to erase memory to ≤100μA.

The prototype ReRAM device offers low fluctuation of resistance value even during high-speed operation. The ReRAM technology “is amenable to miniaturization and can be manufactured inexpensively,” so it is seen as an alternative to flash, the company explained. “If further minute non-volatile memory can be realized using ReRAM, there is potential for higher performance of mobile devices.”


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