CMOx shows its face in flash race

Another technology candidate to follow on from NAND

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Heard of CMOx? Here we are in the midst of a rush towards NAND flash replacing fast hard drives and successor technologies are already jostling for prominence. Unity Semiconductor came out of stealth last month and has unveiled its CMOx technology with claimed technology advantages over NAND and manufacturing advantages over other post-NAND candidates.

For a non-volatile technology, the NVRAM field looks very volatile indeed. So far we know about Phase Change Memory (PCM), Carbon-resistive RAM, Magneto -resistive RAM(MRAM), and Unity Semiconductor's CMOx (Conductive Metal Oxide x) technology. This is a resistive technology but not the same as other resistive RAM concepts. The company says it will yield products with 4x the density and 5-10x the write speed of today’s NAND flash.

Unity Semiconductor has devised a way of storing bits that is based, it says, on the movement of ionic charge carriers, with Oxygen ion distribution mentioned. It's used this to build a passive cross-point multi-layer memory array which does not have a transistor in each cell, unlike MRAM and PCM. That means, Unity says, they can't use a crosspoint, multi-layer design.

Unity says its CMOx technology uses a resistance change element, with quantum mechanical tunneling (trap-assisted tunneling) mentioned, but it is not a resistive RAM memory cell because conduction across the cell is uniform and not based on filaments. Oxygen ions move from a conductive metal oxide to an insulating one and back again under the influence of electric currents.

The CMOx technology has a fast write speed, though there is no actual comparison with NAND write speed available from Unity. There is a slide deck and technical paper (pdf) available describing Unity Semiconductor's approach.

Unity says: "The cross-point memory array architecture allows for the densest memory devices of all the next-generation NVM technologies. Further, it enables the physical stacking of multiple layers of memory. (Our CMOx) designs use 4 physical layers of (2-bit) multi-level cell (MLC) memory, and is the key to increasing the density of (our) storage-class memory products."

Apparently, memory cells are often compared on the basis of a measure termed F2 , or “feature size squared”. The smaller the F2 measure, the more memory cells per unit area. Unity says DRAM has a 6F2 - 8F2 cell size, single level cell NAND a 4.5F2 rating, and MLC NAND a 2.25F2 size. It says its CMOx cross-point arrays have a cell size of just 0.5F2. The Unity die size is four times smaller than NAND flash.

Unity has a second noteworthy aspect which is its manufacturing model idea, called BEOL (Back-End Of Line). It says that its CMOx memory array maufacture can be commenced in ordinary CMOS logic foundries. Custom memory fabrication plants are not needed because there is no requirement to put a transistor in each cell. Basic wafers can be built in existing CMOS fabs and then taken to a second fab for the many CMOx layers to be added, using a metallisation process. This 2-stage process enables cheaper base substrate manufacturing cost as older 90nm process plant can be used in the first stage.

This is a pitch to potential partners from fabless Unity. It wants to form a joint-venture with an IDM, an integrated (semiconductor) device manufacturing company, for volume manufacturing of its product technology. CMOS base wafers will be procured from existing CMOS logic foundries and then transferred to the joint venture's fab for the addition of the CMOx memory layers.

Unity Semiconductor, like a latecomer to a party where the big boys and girls are already paired up, needs to find a wallflower with resources and ambition that's willing to take a punt on its technology. Together, it's saying, we can build a better mousetrap - and take advantage of the problems people are foreseeing with NAND when it finds it can't go beyond a 22nm process shrink because of physics-type problems.

Then CMOx would have to compete with PCM, Carbon-resistive RAM, MRAM and STT-RAM products from the post-NAND big boys. Can Unity Semiconductor David get a big enough JV slingshot to take on these various Goliath-backed efforts? ®

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