Crossbar, the makers of new non-volatile RAM said that their new memory is ready to move from a prototype to a fabrication facility, where 1TB chips the size of a postage stamp. The Silicon Valley start-up expects that its 3D resistive RAM or shortly called as 3D RAM, will be out in early 2016 as memory in wearable devices, with high-density storage devices like solid-state drives arriving within 18 months after that.
The first look of RRAM was presented to the world by a team of experts from Crossbar in August 2013. The concept is simply advantageous over NAND Flash, which has been approaching a density dead end. RRAM is natively denser than NAND, with high performance. It also has a high native endurance, with the ability to sustain 100,000 write cycles, according to Sylvian Dubios, Vice President of Marketing and Business Development, Crossbar.
It is because of its greater density, RRAM can be used as silicon wafers that are half the size used by current NAND flash fabricators. In a single chip, it has nearly 10 times the capacity of NAND flash and uses 20 times less power to store a bit of data. It also sports 1000 times lower latency than NAND flash meaning performance is massively improved. And because RRAM is fully compatible with the standard manufacturing process already used in NAND fabrication, no changes will be needed in manufacturing facilities.
Now, to those who think, RRAM is only filled with advantages, here’s an update. The major challenge to RRAM is “error causing electron leans between memory cells”. Crossbar refers this error as a “sneak path current” which is inherent in RRAM memory.
Generally, this is a common problem in non-volatile memory and even in today’s NAND Flash Solid-state drives. As the size of transistors shrinks below 20 nanometers and chip density is increased, bits stored in tiny cells leak through to adjacent cells, creating data errors.
Samsung, Micron, Intel and other SSD manufactures have increased error connection code on their devices to address the problem. And more than one company has turned to 3D NAND, which stacks cells up to 32-layers high to increase density, offering some capacity breathing room without requiring a further cell size reduction.
The densest process of creating silicon flash memory cells to store data on planar (2D) NAND is between 10 nanometer and 19nm in size. To give some idea of how small that is, a nanometer is one-billionth of a meter, and a human hair is 3K times thicker than NAND flash made with 25nm process technology. There are 25 million nanometers in an inch.
NAND Flash uses transistors or a charge to trap and store a bit of data in a silicon cell. By comparison, RRAM uses tiny conductive filaments that criss-cross and connect silicon layers to represent a bit of data.
In RRAM, the top metal layer creates a conductive electrode, the middle is an amorphous silicon switching medium, and the lower layer is nonmetallic. When the programming voltage is applied between the two electrodes, the nano- particles of the top electrode diffuses in the switching material and creates a filament. It is then the memory cell becomes conductive when the filament contacts the bottom electrode. When a reverse voltage is applied between the two electrodes, the filament is pushed back and disappears. The memory cell is non-conductive.
Recently, Crossbar has found a solution to its ‘sneak path current’ issue. It invented a way to hide adjacent cells from those being programmed to store data, thereby insulating them from unintentional changes. It did that by setting a specific voltage range for cells. Cells programmed between -1 and +1 volt are ignored and anything outside that range can be programmed to hold new data.
This technique is called as Field-Assisted Super-linear Threshold (FAST) selector device and it has suppressed the sneak path current. Thus, this marked another significant milestone needed to commercialize RRAM memory for high-density data applications. It is predicted that RRAM has the potential to influence the storage industry to the utmost level, when its commercial version starts to take the market.
More details will be published in 1Q of 2015.