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Running out of space? Go 3D

Posted by Zenobia HegdeSeptember 14, 2016

Where higher performance is required eMMC-based solid-state storage just isn’t as fast or robust as a full solid-state drive. New 3D Flash memory technology could change all that.

In contrast to earlier NAND Flash memory, where cells are formed on a two-dimensional silicon substrate, this new technology involves stacking Flash memory cells vertically on a silicon substrate to give significant density improvements.

A number of silicon vendors are beginning to manufacture 3D NAND memories, and several solid state disk drives based on these ICs have been announced already. For example, Toshiba announced the first prototype 3D flash memory technology in June 2007; the company is actively promoting BiCS FLASH to meet demand for larger capacity with smaller size. The company’s latest device incorporates three-bits-per-cell (triple-level cell, TLC) technology and achieves a 256-gigabit (32 gigabytes) capacity, says Andrew Pockson, Divisional Marketing manager, Semiconductors at Anglia.

As BiCS FLASH technology gets more refined, the next milestone on the development roadmap is a 512-gigabit (64-gigabytes) device, also with 64 layers. The 64-layer stacking process enables 40% larger capacity per unit chip size than 48-layer stacking process, reduces the cost per bit, and increases the manufacturability of memory capacity per silicon wafer. Upcoming 64-layer BiCS Flash can meet demanding performance specs; so the early beneficiaries will probably be datacentres, where SSD modules can provide massive capacity in a small space with low power requirements.

Figure 3 3D structure of Toshiba's 48-layer BiCS Flash memory (Source: http://www.toshiba.com/taec/adinfo/technologymoves/3d-nand.jsp)

Figure 3 3D structure of Toshiba’s 48-layer BiCS Flash memory
(Source: http://www.toshiba.com/taec/adinfo/technologymoves/3d-nand.jsp)

Storage based on 3D NAND Flash will be useful for applications that include enterprise and consumer SSD, smartphones, tablets and memory cards as well as IoT nodes.

MRAM promises zero-power standby, instant-on

Another area where the IoT may benefit from newer memory techniques is the emerging technology of MRAM – Magnetoresistive random-access memory. Instead of storing data as electric charge or current flows, data in MRAM is stored by magnetic storage elements formed from two ferromagnetic plates separated by a thin insulating layer.

One of the two plates is a permanent magnet set to a particular polarity; the other plate’s magnetisation can be changed to match that of an external field to store memory. This so-called magnetic tunnel junction is the basic building block of an MRAM bit. MRAM devices are built from a grid of these blocks.

There are a number of techniques for writing data to MRAM cells. The simplest “classic” design, places each cell between a pair of write lines arranged at right angles to each other, parallel to the cell, one above and one below the cell. When current is passed through them, an induced magnetic field is created at the junction, which the writable plate picks up. This approach requires a fairly substantial current to generate the field, however, which makes it less-than-ideal for low-power applications.

A newer technique, spin transfer torque (STT) or spin transfer switching, uses spin-aligned (“polarised”) electrons to directly torque the domains. Specifically, if the electrons flowing into a layer have to change their spin, this will develop a torque that will be transferred to the nearby layer. This reduces the amount of current needed to write the cells, making it about the same as the read process.

As a fast-write, non-volatile memory, MRAM potentially has many advantages in support of the Internet of Things. For one: many IoT applications operate in intermittent access or batch mode, making MRAM the perfect solution where fast-write working memory data must be continually updated yet preserved between batch read accesses or when power is lost.

Another advantage is where there are long or frequent periods of standby, yet instant-on is required. MRAM enables instant access to data as well as critical code. In addition, it preserves data for up to 20 years, yet retains very fast write times.

Figure 4 Magnetic tunnel junction memory cell composed of a fixed magnetic layer, a thin dielectric tunnel barrier and a free magnetic layer. (Source: https://www.everspin.com/mtj-storage-element)

Figure 4 Magnetic tunnel junction memory cell composed of a fixed magnetic layer, a thin dielectric tunnel barrier and a free magnetic layer. (Source: https://www.everspin.com/mtj-storage-element)

Finally, in IoT nodes where devices spend a lot of time in deep sleep mode, MRAM can be powered down completely with zero energy consumption, yet data are non-volatile and MRAM has a very fast power-up write time.

MRAM pioneers Everspin reckon that over 50million of its MRAM and ST-MRAM products are currently deployed in datacentre and cloud storage facilities and across energy, industrial, automotive, and transportation markets. OEMs building IoT nodes have a choice of parallel, SPI or DDR3 interfaces, at densities from 128kb to 64Mb.

Technologies like 3D NAND Flash and MRAM may be in their infancy, but could deliver substantial benefits for IoT data storage. Meanwhile, significant improvements in more conventional Flash memories are meeting current demands for data storage in globally connected Things.

The author of this blog is Andrew Pockson, Divisional Marketing manager, Semiconductors at Anglia

About the Author:

Andrew Pockson is Divisional Marketing manager for Semiconductors at Anglia. He has over 20 years of distribution experience, in technical and marketing support roles. Previous positions have included Technical Marketing manager and Field Applications engineer gaining extensive knowledge and experience using passive, e-mech and semiconductor products.

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Zenobia Hegde

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