By Paula Doe, SEMI
Ever growing volumes of data to be stored and accessed, and advancing process technologies for sophisticated control of deposition and etch in complex stacks of new materials, are creating a window of opportunity for an emerging variety of next-generation non-volatile memory technologies. While flash memory goes vertical for higher densities, resistive RAM and spin-transfer magnetic RAM technologies are moving towards commercial manufacture for initial applications in niches that demand a different mix of speed, power and endurance than flash or SRAM. This article delves into some of the topics that will be addressed at SEMICON West 2015.
Micron: Memory Needs to go Vertical
“Memory is going through a transformation, making it an exciting time to be in the sector, with both emerging opportunities and new challenges,” notes Naga Chandrasekaran, Micron Technology VP of process R&D, who will keynote the next-generation memory program at SEMICON West 2015. As new applications in the connected world drive demand for increased storage, bandwidth, and smart memory, and as conventional planar memory scaling faces more challenges, memory suppliers across the industry face a transformation, requiring new emerging memory types and a transition from planar to vertical technology.
“Memory needs to go vertical to meet growing demands placed on performance, and that means a new set of process and equipment requirements,” says Chandrasekaran. Scaling the vertical 3DNAND structures is no longer limited by the lithography, but instead is driven by the capability of the etch, film and characterization processes. “Metrology and structure/defect characterization is a holdup for the entire sector, which is slowing down the cycle time for development,” he notes. “In addition, there are challenges in materials, structural scaling, equipment technology, and manufacturability on the new roadmap that need to be resolved.”
Everspin Targets ST-RAM on GLOBALFOUNDRIES’ 40nm 300mm Process in a Year
Everspin Technologies’ recently introduced 64Mb spin transfer torque MRAM makes a big jump in density over the company’s earlier 16Mb device, as switching the magnetization by a current of electrons of aligned spin allows much better selectivity than applying a magnetic field. Manufacturing these spin-transfer devices has traditionally been a challenge, but the company claims it sees a clear roadmap to continue to increase the density. “We’re squeezing a 64Mb device on 90nm silicon out of the quarter-micron process equipment in our fab,” says VP of manufacturing Sanjeev Aggarwal, who will give an update on the technology at SEMICON West. The company is in the process of transferring the technology to a 40nm process on 300mm wafers at partner GLOBALFOUNDRIES in the next year, to significantly reduce the cell size and spacing.
Aggarwal notes that the layers in the magnetic stack of the spin-transfer torque device (ST RAM) are similar in thickness to those of the earlier magnetic-field switched MRAM devices, which have already shipped some 50 million units. In the 28nm version of the ST-RAM, targeted for a couple of years out, the company plans to switch from an in-plane to a perpendicular structure, which will significantly improve efficiency to cut power consumption by an order of magnitude, though the material stack and processing will remain very similar.
Current deposition tools can provide the layer uniformity required for the many ultrathin layers of these magnetic stacks, and etching technology being developed with a vendor for cleanly removing these non-volatile magnetic material looks promising for 40nm, says Aggarwal. Key is the company’s IP for depositing the tunnel barrier MgO and for stopping the etch uniformly on the tunnel barrier when etching the magnetic stack. “These deposition and etch technologies should extend to 1Gb without much change, though at 16Gb we may need something new,” he adds. “In the next several years we will need help from vendors on better ways to clean up the etch residue, such as by ion milling after RIE, or encapsulating the stack to protect it before the next round of etching.”
Demand for the 64Mb ST-RAM is coming from buffer storage applications, such as high-end enterprise-class solid state drives, where an array of the fast-writing, non-volatile chips holds the data until it can be more permanently filed and stored, and where the high volumes of data require better endurance than flash, reports Terry Hulett, Evergreen VP Systems Engineering and GM Storage Solutions. “As our products increase in density, we expect to serve the same function for bigger storage systems, like a whole rack of solid state drives,” he projects. The company also targets applications for potential power savings for the instant-on persistent memory, such as powering off the display buffer between every refresh cycles for mobile devices, or shutting down the server between operations.
Both Sanjeev Aggarwal (Everspin) and Naga Chandrasekaran (Micron Technology) will update SEMICON West attendees on the state of these emerging memory technologies in a TechXPOT. In addition, Wei D. Lu (Crossbar), Robert Patti (Tezzaron), and Jim Handy (Objective Analysis) will provide analysis and updates at the July 14 event in San Francisco:
Crossbar Aims for Embedded ReRAM IP Blocks from Foundry by End of Year
ReRAM suppliers, meanwhile, argue that their technology potentially offers better prospects for scaling and lower costs than either flash or spin-based MRAM, although it is still a ways from a commercial volume process. Crossbar Co-founder and chief scientist Wei Lu, who will also speak at SEMICON West, says the company plans to deliver its ReRAM technology to strategic partners as an IP block for embedded non-volatile memory on logic chips from a leading-edge manufacturing foundry by the end of the year. The company’s approach stores data by changing the resistance by forming a conductive metallic bridge through a resistive layer of amorphous silicon sandwiched between two electrode layers.
Lu says the devices are being made with two-mask steps on top of the CMOS transistors in a leading foundry. Key to improving performance to commercial levels and achieving very dense crossbar arrays, he notes, is the addition of a high speed selector device on top of the memory layer. This layer blocks unwanted sneak currents at low voltages and turns on at the threshold level to enable formation of the conduction bridge. “It’s like a volatile RAM stacked on top of the ReRAM, with nanosecond recovery time,” he explains. “This brings the on/off selectivity up to 108.”
Initial target market is chip makers who want to embed nonvolatile memory directly in the logic fab, for low-power applications like the IoT, with faster speed and higher endurance than flash. But ultimately the company targets the bigger market of stand-alone enterprise data storage with lower read and write latencies. “We expect to offer Gigabit-level density at faster speed than NAND flash by around 2017,” claims Lu. He figures ReRAM and STT RAM will both find their place in the more diverse memory market of the future, with SST RAM offering better endurance, and ReRAM offering higher density and lower cost.
Tezzaron Reports High ReRAM Yields from Repair and Remapping through Multilayer Stack
Tezzaron Semiconductor takes a different approach to ReRAM, storing data by moving oxygen vacancies instead of metal ions across the thin layers to change resistance. CTO Robert Patti, another SEMICON West speaker, credits the Tezzaron fab’s ALD technology for the tight control of layer uniformity required to build its 16-tiers of ReRAM cells on top of a CMOS transistor tier from another foundry. Controlling the chemistry of the layering and the reaction is a challenge, but the tiers allow dynamic repair and remapping of defective cells, which Patti claims can enable yields of up to 98%. “The possibility to repair across the vertical structure makes defect density less of an issue, and lets us deal with materials and processes that are less mature,” he notes.
Patti says his company’s aerospace/military customers, who need a non-volatile option with better endurance than flash memory, will likely move to ReRAM within a couple of years. Server makers are also starting to look at the potential for adding a new intermediate level of memory, between the solid state disk and the DRAM, which could potentially significantly improve server performance in analyzing big data by holding big chunks of data for faster access at lower power. It might also reduce system-level costs, although it will require changes in operating system architecture to use it effectively, and sophisticated programming algorithms to manage the memory to limit wear. Demands on the intermediate storage memory should be limited enough that the ReRAM target endurance of 109 cycles should be sufficient, though it remains lower than DRAM’s 1015. If ReRAM endurance reaches 1012 cycles, the nonvolatile, instant-on memory could become a viable replacement for mobile memory, Patti suggests.
“Vertical NAND is appealing because it’s more familiar, which has probably delayed interest in ReRAM. But ReRAM has a smaller cell size so may ultimately be easier to scale and more cost effective,” argues Patti.
Costs Remain the Challenge
“The only thing that ultimately matters in memory is cost,” argues Objective Analysis analyst Jim Handy, another speaker, pointing out that the target aerospace and enterprise storage applications remain small markets, and volumes are not high enough yet to build up deep understanding of the new materials used, so there will be bumps in the road to come. But as costs come down as MRAM and ReRAM scale to higher densities, he expects them to gradually take over more mainstream applications, starting with the highest cost memories, so first SRAM (especially SRAM with battery backup), then NOR flash, DRAM and finally NAND flash — perhaps by ~2023. “We have been predicting that 2017 is the earliest we’ll see significant penetration of 3D NAND into the planar NAND market,” he notes. “And now that some suppliers are saying it will be 2017, it makes me think it may be longer.”