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The Technical Explanation
The most desirable device structure for high density nonvolatile memory (NVM) is the crossbar architecture composed of two electrode lines (bit-line and word-line) perpendicular to each other with memory materials sandwiched in between. This provides the highest density attainable in a 2-dimensional (2D) planar crossbar architecture (4F2 where F = minimum feature size). Furthermore, this planar structure is conducive to multi-layer integration, wherein multiple levels of 2D arrays are fabricated on top of each other forming a highly dense 3D memory array.
At the same time, the crossbar structure can lead to undesirable misreading of the switching state within the intended cell if the neighboring cells are in the ON-state (low resistance state). This is often called “crosstalk” between adjacent cells, and it must be eliminated to achieve reliable memory states for each cell.
In order to overcome the crosstalk problem, diverse integration architectures have been suggested such as one diode-one resistor (1D-1R) stacks, one transistor-one resistor (1T-1R) stacks, one selector-one resistor (1S-1R) stacks, or complementary resistive switches (CRS). Among them, the 1D-1R is a simplest design for 3D (multiple-layer) stackability without the crosstalk issue. Therefore, 3D stackable switchable crossbar architectures that are composed of simple one-diode and one-resistor configurations offer the desired structure with ultrahigh density of individual memory cells, which is the most desirable high density architecture of next-generation memory technology.
Prof. Tour has demonstrated unipolar NVM with SiOx. SiOx is the most common and lowest cost material in the semiconductor industry. These SiOx memory elements have shown desirable performance metrics such as extremely high ON-OFF ratio, multi-bit capability, low energy consumption, and X-ray hardened property by conducting nano-filaments forming at a sub-5-nm scale. The devices have fast switching speed and good stability suitable for future memory applications.
The novelty in Weebit’s nanoporous SiOx memory
The nanoporous-based material is a new concept for a memory structure. The achieved performances are born from the unique nano-porosity of SiOx, in contrast to all other memory platforms based on non-porous materials. Until now, nanoporous-based memory devices have never been demonstrated.
Through the nanoporous material, many of the critical requirements for future nonvolatile memory can be met such as reliable nano-scale memory filament formation, high memory stability, low power consumption, and multi-bit capability. Greatly improved rewriting times, low power consumption, multi-bit memory state were demonstrated.
Advantages
Preliminary results have demonstrated remarkably fast switching speed in the range of 1000 times faster than the most advanced NAND Flash technology. Weebit cell shows the ability to meet the demand for the future device requirements.
As Flash technology is scaled down, it’s cycling endurance is dramatically reduced, which created severe reliability issues. To compensate for these flaws, expensive error correction and wear leveling circuitry is required. Endurance tests on Weebit devices have demonstrated great stability which potentially places Weebit future products in a high reliability specification range as required by the automotive and aerospace markets.
Weebit’s memory cell will require significantly lower power during device operation. This lower power will be a great advantage increasing battery life not only in any battery operating device, but also in computing and datacenters to save energy on heat dissipation solutions.
Weebit’s ReRAM has the potential of further scaling beyond the Flash limit, thus providing smaller cells which enable to store more information (higher density) and reduce production cost.
Weebit’s memory cell is based on SiOx material which is the most common, studied and available dielectric material in the semiconductor industry, thereby avoiding industry re-tooling costs.
One of the weaknesses of the traditional Flash technology is its radiation sensitivity. High energy radiation easily causes electrons to escape from the floating gate thus creating errors, which require costly solutions and limits the density of radiation hardened devices.
Weebit’s ReRAM (unlike Flash) does not confine electrons in a floating gate therefore it is a natural radiation hardened memory cell. This capability was demonstrated in the International Space Station (ISS) as Prof. Tour memory samples showed great performance after 2 years in space.
