Writing for Semiconductor Engineering, Michael Watts reports that Resistive RAM (ReRAM) appears to be gaining traction.
“Once considered a universal memory candidate—a replacement for DRAM, flash and SRAM—ReRAM is carving out a niche between DRAM and storage-class memory,” Watts explained. “ReRAM (known alternately as RRAM), is a type of non-volatile memory that began garnering attention in 2009 when startup Unity Semiconductor emerged from stealth mode.”
As Watts notes, Rambus purchased Unity in 2012 because it was one of several contenders for the next generation of memory technology, along with ferroelectric RAM (FeRAM) and Magnetoresistive RAM. To be sure, ReRAM also has been considered a possible replacement for 2D NAND, NOR flash and other memory types.
“What makes ReRAM so interesting is the limitation of other memory choices. There is DRAM for rapid access memory; NAND flash, which is three orders of magnitude slower; and there is storage-class memory in between,” said Watts. “Storage-class memory, a term first coined by IBM several years ago, could have a huge impact on computation efficiency.”
Indeed, according to VP of Rambus Labs Gary Bronner, two or three different types of ReRAM could potentially fill the salient gap between DRAM and flash.
“This would have very significant impact on the industry,” he confirmed. “[System] architects have been very clever at taking advantage of developments. They already take advantage of hierarchy of memory on chip and chip to chip.”
To further development of ReRAM, Rambus Labs is exploring the use of a multilayer metal oxide structure that changes resistance by injecting ions into the material. Another widely publicized approach to ReRAM involves phase-change materials, which depend on melting a material and then cooling, quickly or slowly, to create either crystalline or amorphous phases.
As Bronner points out, the “the physics of the phase change material is probably the best understood.” Nevertheless, thermal-based solutions have had somewhat of a difficult history in the semiconductor industry, including Heat Assisted Magnetic Recording (HAMR) and smectic liquid crystal displays.
The problem, Watt explains, is heat inevitably spreads, so there is “crosstalk” between neighbors and DC heating of the part that depends on duty cycle.
“Many people have favored other materials over phase change for these reasons,” Bronner told Semiconductor Engineering. “However, Intel and Micron have pioneered work in these materials, presumably because the device physics is much better understood. To even think about building a device of this size (128Gbit chip 3D XPoint memory) requires a very mature level of process and defect control.”
It should be noted that Rambus is partnering with licensees, such as Tezzaron, to create ReRAM-based embedded flash products. Rambus is also working with researchers at Tsinghua University in Beijing, China to help advance ReRAM adoption.
“Many semiconductor companies have expressed a strong interest in ReRAM,” Bronner added. “As such, our collaborative goal with Tsinghua researchers is to explore how to further improve the non-volatile memory so that it is ultimately suitable for consumer devices as well as more demanding platforms and environments.”