Resistive Memory - ReRam

The memory tech that will eventually replace NAND flash, finally in market

What is ReRam?

ReRam is Resistive random-access memory (RRAM or ReRAM) is a type of non-volatile (NV) random-access (RAM) computer memory that works by changing the resistance across a dielectric solid-state material often referred to as a memristor. The biggest advantage of ReRAM technology is its good compatibility with CMOS technologies.

It is under development by a number of companies, and some have already patented their own versions of the technology. The memory operates by changing the resistance of special dielectric material called a memresistor (memory resistor) whose resistance varies depending on the applied voltage.

What makes ReRam?

From the viewpoint of the material choice, the advantage of ReRAM is evident. It is possible to fabricate MOM structures easily by using the oxides widely used in the current semiconductor technologies. Low-current ReRAM operation was reported in the CuOx-based MOM structure. The CuOx layer was grown by the thermal oxidation of the 0.18-μm Cu. NiO and CoO are being intensively studied as oxide materials for ReRAM, and these transition metal elements are also used in metal silicides employed as gate materials. Recently, the good scaling feasibility of ReRAM was demonstrated in an HfOx-based memory with a cell size of 30 nm. The devices in a 1-kbit array exhibited a high device yield (~100%) and robust cycling endurance (>106) with a pulse width of 40 ns. The memory cell consisted of a TiN/Ti/HfOx/TiN structure. Here, the Ti overlayer played the role of oxygen gettering for better ReRAM operation. The gettering effect has already been investigated in HfOx as a high-k material for the gate dielectric films in CMOS devices. The academic and technological knowledge about high-k materials will be very useful in the design of the stacking structure for a ReRAM device.

How ReRam Works?

RRAM is the result of a new kind of dielectric material which is not permanently damaged and fails when dielectric breakdown occurs; for a memresistor, the dielectric breakdown is temporary and reversible. When voltage is deliberately applied to a memresistor, microscopic conductive paths called filaments are created in the material. The filaments are caused by phenomena like metal migration or even physical defects. Filaments can be broken and reversed by applying different external voltages. It is this creation and destruction of filaments in large quantities that allows for storage of digital data. Materials that have memresistor characteristics include oxides of titanium and nickel, some electrolytes, semiconductor materials, and even a few organic compounds have been tested to have these characteristics.

The principal advantage of RRAM over other non-volatile technology is high switching speed. Because of the thinness of the memresistors, it has a great potential for high storage density, greater read and write speeds, lower power usage, and cheaper cost than flash memory. Flash memory cannot continue to scale because of the limits of the materials, so RRAM will soon replace flash memory.

Panasonic with ReRAM mounted microcomputers

Panasonic Corporation today announced that it will start the world's first mass-production of microcomputers with mounted ReRAM, a type of non-volatile memory, in August 2013. Now, Panasonic is taking ReRAM into the mass market with the news that it has become the first company to begin mass production of a product based around the technology. Dubbed the MN101LR series, the microcomputers are being produced from August at a rate of a million units per month using the company's newly-developed 0.18µm ReRAM modules.

Designed for embedded use, the systems are eight-bit microcomputers running at 10MHz with just a few kilobytes of ReRAM available to the user.

This development has the following features:

- The use of the newly developed 0.18 µm ReRAM in microcomputers and low power-consumption processes contributes to longer operational times for customers' products.
- The high-speed, low power-consumption by byte rewriting can easily reduce the amount of EEPROM [3] previously required as part of an external attachment, thereby reducing the system cost.
- The ReRAM to be produced this time around is based on the rewriting principle of a redox reaction of a metal oxide, in which high-speed rewriting and high reliability can be achieved, making it ideal for industrial applications.

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Is ReRAM the end of NAND flash?

A primary storage technology: ReRAM.

NAND flash stores data in a little cloud of electrons in a quantum well. The presence or absence of charge - or the strength of the charge - tells us what bits are stored.

ReRAM stores data through changes in the resistance of a cell. There are a variety of ReRAM technologies in development, including phase-change memory (PCM) and HP's memristors, based on at least a half-dozen competing materials.

Expect healthy competition as the industry and buyers sort out the details.

Advantages

While different implementations have different specs, all ReRAM has key advantages over today's common NAND flash.

• Speed. ReRAM can be written much faster - in nanoseconds rather than milliseconds - making it better for high-performance applications.
• Endurance. MLC flash - the most common - can only handle about 10,000 writes. ReRAM can handle millions.
• Power. Researchers have demonstrated micro-Amp write power and expect to get in the nano-Amp range soon, which makes ReRAM much more power efficient than NAND flash, which requires voltage pumps to achieve the 20 volts required for writes.

The Storage Bits take

NAND flash will retain advantages in cost and density for the foreseeable future, meaning that it will be here for decades to come. So where will ReRAM fit in the storage hierarchy?

• Data integrity. Losing a snapshot is no big deal. Losing your checking account deposit is. Mission critical applications will prefer ReRAM devices - and can afford them.
• Performance. Today's SSDs go through many contortions to give good performance - and don't succeed all that well. A fast medium removes complexity as well as increasing performance.
• Mobility. Depending on how the never-ending tug-of-war between network bandwidth and memory capacity develops, consumers may come to prefer large capacity storage on their mobile devices. If so, ReRAM's power-sipping ways will be an asset on high-end products.

Toshiba is well-positioned to enter these high-end markets with SSDs analogous to today's 15k disks. It may not be a huge market, but the margins will make it worthwhile.

Other vendors, including Panasonic, Micron and Samsung, are also working on ReRAM products. Another interesting question: to what extent will fast ReRAM replace DRAM in systems?

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