New Materials Solve Key 3D NAND Issue

imec III-V 3D NAND channelAt the IEEE’s IEDM conference last week Belgian research consortium imec showed an improved “gate first” 3D NAND that replaced the conventional polysilicon channel with InGaAs, Indium Gallium Arsenide, a III-V material.  This new technique opens the door to higher layer counts in 3D NAND, allowing denser parts to be made in support of further cost reductions.

For those unfamiliar with the term, the “gate first” approach is the foundation of Toshiba’s BiCS NAND, and presumably Micron’s floating gate 3D NAND.

imec explains that “Replacing poly-Si as a channel material is necessary, as it is not suitable for long-term scaling.”  Further they report that on-state current (ION) and transconductance (gm) of the III-V channel was better than that of polysilicon devices, without any programming, erase, or endurance degradation.  The device’s characteristics are shown in this post’s graphic.

The consortium reports that the current through the poly-Si channel in existing 3D NAND designs decreases in proportion to the number of memory layers, rendering it unsatisfactory for long-term scaling. The channel current decreases because the polysilicon’s conduction is impaired by scattering at the boundaries of polysilicon’s random-sized gains, an issue that doesn’t exist with the monocrystalline silicon used to make planar NAND flash.  A thorough discussion of the problem can be found in Andrew Walker’s fine 3DInCites posts on polysilicon channel issues.

Interestingly, the new material has been proven to work with holes as small as 45nm, which is about half the diameter of the ~80nm channel hole in Samsung’s current 3D NAND chips.  Hole diameters limit 3D NAND’s ability to use lithographic scaling.

Although the addition of new materials presents new challenges, the use of III-V materials in the channel is one step toward allowing 3D NAND to scale beyond its current 48-layer level.  The Memory Guy would like to remind readers, however, that there are many other issues that must be resolved before 3D NAND can become cost competitive with today’s planar chips.  These will be the subject of an upcoming post.

4 thoughts on “New Materials Solve Key 3D NAND Issue”

  1. Great insight, guy.

    I hope soon we achieve parity with physical drives. HDDs have already lost the capacity game thanks to Samsung’s tremendous efforts with a ridiculous 16tb SSD… In a 2.5 inch form factor! The only issue seems to be the cost. Which, to be fair, is quite reasonable currently for that drive at 5k-7k. But of course, it is nowhere near an HDD’s pp/GB, currently it is at a factor of 9-10 but hopefully, very soon, new advancements in affordable reliable memory technology will finally render HDDs totally obsolete. Seems like Samsung will lead the way.

    1. Thanks for the compliment!

      Samsung wasn’t the first to ship SSDs with higher capacities than HDDs in a 2.5″ HDD form factor. A number of companies have been doing that for years, most notably SMART, Storage, prior to its acquisition by SanDisk, but also BiTMICRO and a few others. I think that the bigger question is whether or not there’s a market for such a product. I wrote a little post on The SSD Guy blog that was inspired by these high-capacity SSDs to estimate the maximum possible capacity of an SSD:

      Nobody should expect for SSD prices to fall below HDD prices, at least not for a very long time. I discussed that in another post – also on the SSD Guy.



  2. Dear Jim,
    first of all thank you for the post! great insight!

    “new materials presents new challenges”:
    how does it look like with defects, polycristallinity and the other issues that effectively hampered III-V/Si integration for optoelectronic devices until now?
    Thank you again,

    1. Marcello, Thanks for the comment.

      Unfortunately I don’t know enough about these materialsto answer your question, and the companies who are developing these technologies dislike sharing that kind of information.

      Perhaps some other reader might be able to help out.


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