Selector-only memory gains advocates, including SK Hynix

Feature articles | March 15, 2024

By Peter Clarke

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A novel type of solid-state memory, the selector-only memory (SOM) or self-selecting memory (SSM), could be about to rise from the ashes of Intel’s failed 3D XPoint non-volatile memory.

IMEC, SK Hynix, Samsung and others have been publishing learned papers, blogs and lay articles on the topic, which could be adopted for use with Compute Express Link (CXL) interconnect as well as elsewhere. CXL is used for high-speed communication between the central processing units (CPUs) and device memory or accelerators within data centers and therefore is of interest due to the AI boom/bubble.

As a result SOMs or SSMs could succeed in applications where other emerging memory technologies – the various forms of metal-oxide ReRAM and phase-change memory – have failed to gain traction to date.

The selector-only-memory is likely to be based on variants of the phase-change material systems in Ovonic Threshold Selector devices (OTS) that were used in 3D XPoint memories and that are used for various ReRAM devices.

The alternative to an OTS is a transistor as a memory cell selector switch. A transistor is a three-terminal device that takes up additional area compared with the two-terminal OTS. As a result while transistors can be used with embedded memories some form of two-terminal device is considered superior, if not necessary, for a discrete, stackable memory.

An OTS is usually a germanium-selenium diode-switch based on an electrode cross-point so it can sit under or above the memory cell being selected. The selector serves a key role in that it selects the memory cell being addressed and prevents “sneak paths” through the array that would bypass the selected cell. However, research has shown that the OTS could perform the tasks of both memory cell and selector.

Research

In a ‘primer’ article published recently in EDN, Daniele Garbin, an R&D engineer at IMEC, and Gouri Sankar Kar, IMEC’s vice president of memory, discuss the advent of the SoM (see The promise of OTS-only memories for next-gen compute).

The authors state that during research into the switching mechanism of SiGeAsSe OTS selectors, published in 2021, IMEC investigators noticed that the threshold voltage of the OTS device varied depending on the polarity of the previous pulse.

This threw up the possibility of dense OTS-only memories, albeit ones that are not non-volatile. The OTS-only memory effect, as observed then, lasted for several hours, but periodic refresh makes this acceptable for many applications. It is also reported that data retention without power has been considerably improved in other SOM devices. So non-volatile SOM is possible.

The OTS-only memory has the obvious advantage of being simpler than the PCM-OTS stack used in 3D XPoint. It should also require much less current than that needed for switching PCM cells, resulting in a more energy efficient memory technology. This should also allow stacking of arrays which was a problem with 3D XPoint memory because of PCM’s susceptibility to heat and thermal cross-talk.

The authors report IMEC has manufactured OTS-only memories using SiGeAsSe material between carbon bottom and top electrodes on 300mm-diameter wafers. The endurance is 10^8 cycles with read and write operation of 10ns and a write current of <15 microamps.

They add that further research is required to try and exclude toxic arsenic and selenium from the material systems; to improve the endurance to 10^12 and to reduce cell-to-cell variability and threshold voltage drift over time.

The authors admit that the physical mechanism, or mechanisms, that drives the polarity effect in OTS-only memories and threshold voltage drift are not fully understood. Switched vacancy modulation at internal material surfaces is one of the latest ideas being proposed to try and explain observed effects. However, the possibility of multiple voltage-driven effects co-existing continues to frustrate observers and researchers.

SK Hynix is preparing SOM

SK Hynix researchers could see the potential application for self-selecting memories and reported the performance of a 32Mbit SSM at the International Electron Devices Meeting in December 2022 in the paper Extremely high performance, high density 20nm self-selecting cross-point memory for Compute Express Link. The single-cell stack that acts as both memory and selector in bi-directional operations overcomes the scaling limitations of conventional PCM.

Cross section transmission electron microscopy (TEM) of an SSM cell stack (left) and plan-view scanning electron microscopy (SEM) of eight mats making up a 32 Mbit array (right). Source: SK Hynix.

The operational mechanism of SK Hynix’ SSM is thought to be related to atomic migration, according to Hyejung Choi, a researcher at SK Hynix’s Revolutionary Technology Center, writing in a blog in May 2023. However, Choi stated that the atomic migration model combined with conventional transport theory fails to explain the observed voltages. Again the conclusion is that further research is needed.

In a blog in September 2023 Jaeyun Yi, another researcher at SK Hynix’s Revolutionary Technology Center, concluded that “Selector-only memory is the future for ultra-fine processes.” This, he asserts, is because of limitations to scaling that SK Hynix researchers had observed in a four-deck 256Gbit 3DXP chip implemented in 20nm process and presented at the 2023 VLSI Technology Symposium.

“As a result of these limitations, SK Hynix is preparing SOM as an alternative solution for the next generation of storage class memory,” said Yi. He added: “Therefore, SOM is set to be a leading solution for next-generation applications with scaling longevity beyond 1z nm technology.”

IEDM

At the International Electron Devices Meeting held in San Francisco in December 2023 there were two papers on SOM.

Enhancing Se-based Selector-only Memory with Ultra-fast Write Speed (~10 ns) and Superior Retention Characteristics (>10 years at RT) via Material Design and UV Treatment Engineering from researchers from Pohang University of Science and Technology aka Postech.

Enhanced Endurance Characteristics in High Performance 16nm Selector Only Memory (SOM) was contributed by Il-Mok Park of Samsung Electronics. The paper reports on a 64Gbit ovonic threshold switch (OTS)-based selector-only memory (SOM) with a cell dimension of 16nm. The paper reports that SOMs are vulnerable to external events such as spike current at turn-on and process-induced damage and also reports on the steps Samsung has taken to address these issues.