Feature Article - Future Memory Technologies, Part 2 - By Christopher Henderson

Another contender as a future non-volatile memory technology is Resistive Random Access Memory, sometimes shortened to RRAM or ReRAM. A prototype RRAM chip is shown in Figure 1. Several major manufacturers are working on RRAM, including: Samsung, Micron, Macronix, and Elpida Memories. The technology is somewhat similar to Conductive Bridging RAM and Phase Change Memory, which we discussed in the previous issue. IMEC in Belgium has also done extensive research into this technology.

The basic concept is that one can create a conducting path through a dielectric layer by applying a high voltage (see Figure 2). The conducting path or filament can be reset, restoring the high resistance path and reformed to create a low resistance path at will. This filament path may actually be multiple paths, according to researchers. There are different types of materials that can exhibit this behavior. They include perovskites, chalcogenides, and transition metal oxides. Some of the leading transition metal oxides include nickel oxide, titanium oxide, tungsten oxide, hafnium oxide, and heterostructures such as aluminum oxide/titanium oxide. Ironically, even silicon dioxide can be used for this application. The failure mechanism engineers try to avoid in standard CMOS, time-dependent dielectric breakdown, is the mechanism by which the programming occurs.

RRAM has the potential to become the front runner for future memory technologies. RRAM can operate much faster than Phase Change Memory. The switching time can be on the order of 10ns. Compared to MRAM, RRAM has a much smaller cell size. The cell size is less than 8F², where F is the smallest feature size. It can also function at lower operating voltages than standard flash memories. RRAM also has the potential to scale down below 30nm. While standard flash memory is now below 30nm, the cell size is larger, so RRAM can still accommodate more cells in the same silicon area. Researchers believe that the mechanism might involve oxygen motion, which might allow for scaling down to as low as 2nm. The filament dimensions during the forming process are also key factors to a stable, reliable device(1)

(1) G. Bersuker, et. al., "Diode-less Nano-scale ZrOx/HfOx RRAM Device with Excellent Switching Uniformity and Reliability for High-density Cross-point Memory Applications," Proc. Int. Elec. Dev. Meeting, 2010 pp. 452-455.