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4DS Memory Limited has broken its radio silence to lay out its go-forward plans for its Resistive RAM (ReRAM) technology.
The company said its interface switching capabilities based on PCMO (Praseodymium, Calcium, Manganese, Oxygen) delivers significant advantages over other filamentary ReRAM technologies, making its high-bandwidth, high-endurance persistent memory suitable for AI, big data and neural net applications.
In a briefing with EE Times, chief strategy officer Peter Himes said 4DS’ ReRAM requires no refresh within its persistence window and can be “refreshed” within the DRAM operating window, which makes it able provide high bandwidth and high endurance while using less energy.
He said the company’s roadmap includes a development agreement with Belgium-based imec for a 20-nm Mb chip with 1.6B elements to be run at imec in 2024.
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Himes said 4DS’ use of PCMO makes its ReRAM different from other ReRAM makers, in that the switching mechanism is based on the interface characteristics of the cell—the entire interface area is involved in the switching. Other ReRAM makers use a filamentary wire, which proves long cell retention, he said. “But it has drawbacks in terms of endurance.”
In PCMO ReRAM, oxygen ions are moved in and out of the cell by the electric field pulse. When this oxygen is present, the cell conducts, and it is said to be SET. When the oxygen is removed, the current path is lost, and it is said to be RESET.
A key advantage of 4DS’ PCMO-based interface is that the pulse response is very fast, according to Himes. “We’ve shown speeds up to or down to 4.7 nanoseconds in terms of programming response of the cell,” he said. “That’s significantly faster than DRAM.”
Endurance is higher, Himes added, because the programming is over the entire area or interface of the cell. “You don’t have these concentrated current spikes through the filaments.” 4DS’ ReRAM technology is also backend integrated into any advanced CMOS process. “It requires only a few additional layers, so it’s low cost,” he said.
Near term focus on making competitive ReRAM
4DS is focused on two goals this year: It’s continuing to work with imec to fab out a 20-nm cell to make it competitive with other ReRAM technologies, and the company sees no point in waiting to strike potential partnerships and start new application discussions.
“We still think that there’s a lot of interest around this persistent memory concept that was started by Intel with the Optane program that they promoted several years ago,” Himes said.
One application example Himes gave was using ReRAM as a very fast cache for quick recovery, reboot and system protection in super large clusters of GPUs for AI. As models get larger, the challenges of compute data backup and recovery have also grown, and persistent ReRAM is part of the solution, he said. “We’re not trying to be a DRAM replacement. We’re not trying to be NAND flash replacement.”
In an interview with EE Times, Jim Handy, principal analyst at Objective Analysis, said that using praseodymium is a unique choice by 4DS, and the company could encounter issues getting a praseodymium-based process to a maturity level that will allow it to be put into mass production and drive out the costs.
All ReRAM makers are in the same race
Along with Thomas Coughlin of Coughlin Associates, Handy publishes an annual report on emerging memories, including ReRAM. Last year’s edition, “Emerging Memories Branch Out,” noted that there are a few ReRAM devices available at present for special applications, with Fujitsu Semiconductor and Renesas offering standalone products.
Weebit Nano began working to commercialize SiOx ReRAM technology developed by Rice University, with the goal of avoiding troubles other technologies had by using materials that would not create issues in a standard CMOS logic fab, such as silver or magnetic materials. The company has advanced its technology several new generations and is no longer purely SiOx. In early 2020, Weebit Nano said it was looking to ramp up its discrete ReRAM efforts based on customer demand.
Weebit Nano has collaborated with CEA-Leti to develop a two-terminal selector for discrete high-density ReRAMs and to commercialize its products. In 2022, the company publicly demonstrated a working ReRAM array and announced the tape-out of a demonstration chip with embedded ReRAM for production in the SkyWater foundry.
Among the other ReRAM players, TSMC offers both a 40-nm and 22-nm ReRAM process, according to the Handy Couglin report. CrossBar has also sampled a 40-nm ReRAM, made by its foundry partner SMIC, and focused its efforts using its ReRAM technology to enable cryptographic physical unclonable function (PUF) keys.
The report said internet of things (IoT) applications may drive adoption of ReRAM devices to take advantage of the lower costs and energy that ReRAM promises. In addition, current applications prefer ReRAM either because of its low power consumption or its radiation tolerance, which is valued in satellites and surgical instrumentation, including arthroscopic cameras that undergo frequent sterilization using high doses of X-rays.
Handy said established memory technologies have been doing a very good job of holding emerging memories like ReRAM at arm’s length. “The real place where the established memory technologies have not been able to continue is in the embedded memories and microcontrollers.”
He said all ReRAM makers are in the same race to get their wafer volume high enough that they can drive the costs out of it before anybody else does. “Because it’s memory, it’s a commodity, and these guys are going to win or lose based on whether or not they can get the cost out.”
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