WD and Toshiba get ready for highest capacity 3D NAND

Western Digital and Toshiba have developed a 128-layer 3D NAND die with TLC (3bits/cell) cell formatting and 512Gbit capacity. 

A 128-layer die will be BiCS-5 in Toshiba flash generation naming terminology – BiCS-4 is 96-layer and BiCS-3 is 64-layer.

Adding 32 more layers should add a third more capacity – assuming the same process technology is used as in the two companies’ 96-layer 3D NAND. Accordingly, flash drives built with 128-layer dies could have a third more capacity than 96-layer drives. Alternatively they could be built to the same capacity at reduced cost. Product might appear in late 2020, with production ramping in 2021.


Senior Wells Fargo analyst Aaron Rakers has built a production model that assumes 66mm2 die size and a 7.8Gb/mm2 bit density rating. He thinks WD-Toshiba has the industry’s highest NAND density and models the suppliers at an 85 per cent wafer yield:

The die uses a Circuit-under-Array (CuA) design in which the logic circuits are built at the bottom of the chip with the data layers stacked above. Rakers says “his enables a 15 per cent smaller die size vs. non-CuA technology; we estimate a 23 per cent total die shrink vs. 96-layer.”

Partly through this freed-up space W. Digital was able to utilise four planes (vs. traditionally 2 planes) to increase performance by 2x. The die is divided into four planes or sections which be accessed independently and in parallel. Because of this it can reach a 132MB/sec program throughput level.

BL is bitline; a line of bits in a die. RC is Resistance-Capacitance.

The chart refers to bitlines and their place in the NAND world is explained here.

How 3D NAND is made conceptually by starting with 2D or planar NAND.

The 132MB/sec speed exceeds the 83MB/sec program throughput of a 110+ layer chip from Samsung which can operate at 1.2Gbit/s IO bandwidth and has a 45 microsecond read time.

WD is accessing data on the 128-layer die using 4KB pages and not the industry standard 16KB pages which limits electricity usage.

Lastly, this is a TLC die. A QLC (4bits/cell) version would have a third more capacity again – 682Gbit.

WD presented this technology last month at the International Solid State Circuits Conference in San Francisco.

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