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NAND flash manufacturers showcase new technologies

Get up to speed on new NAND flash technologies, including Toshiba Memory's XL-Flash, startup Yangtze Memory's Xtacking, and SK Hynix's so-called 4D NAND.

NAND flash manufacturers laid out their roadmaps for next-generation products and architectures at the 2018 Flash...

Memory Summit this month.

As expected, Intel, Micron, SK Hynix and Toshiba talked up 3D NAND flash chips that can store four bits of data per cell, known as quadruple-level cell (QLC). They also spotlighted their 96-layer 3D NAND and outlined roadmaps that extend to 128 layers and beyond to further boost density.

NAND flash manufacturers introduced new efforts to speed performance, raise density and lower costs. Toshiba launched a low-latency option called XL-Flash. Chinese startup Yangtze Memory Technologies Co. (YMTC) hopes to catch up to the flash chip incumbents with its "Xtacking" architecture that can potentially increase performance and bit density. And South Korea-based chipmaker SK Hynix harbors similar aspirations with its so-called "4D NAND" flash that industry experts say is a misnomer.

Key NAND flash manufacturer Samsung was notably absent from the Flash Memory Summit keynotes, a year after discussing its Z-NAND technology at the conference. Z-NAND is another attempt to reduce costs by shifting periphery logic to a place that doesn't take up space on the flash chip, said Jim Handy, general director and semiconductor analyst at Objective Analysis.

Here are some of the new technologies that NAND flash manufacturers showcased at last week's Flash Memory Summit:

Toshiba's XL-Flash

Toshiba's XL-Flash is based on the company's single-level cell (SLC) 3D NAND bit column stacked (BiCS) technology and enables optimization for multi-level cell (MLC) flash. The XL stands for excellent latency, according to Shigeo (Jeff) Ohshima, a technology executive in SSD application engineering at Toshiba Memory Corporation.

Ohshima said XL-Flash requires no additional process and is fully compatible with conventional flash in terms of the command protocol and interface. The read latency of XL-Flash could be 10 times faster than conventional TLC flash devices, according to Ohshima.

He said the company has "a lot of room" to do more with its current 3D NAND BiCS flash technology before new nonvolatile memories such as resistive RAM (ReRAM), magnetoresistive RAM (MRAM), and phase change memory ramp up in volume and become dominant.

"So it ain't over 'til it's over," Ohshima said.

Ohshima said a combination of XL-Flash and denser QLC flash could handle a broad range of application workloads and improve overall system performance over the classic storage architecture of DRAM and HDDs. He noted the performance gap between XL-Flash and QLC flash is considerably smaller than the differential between DRAM and HDDs. And, although XL-Flash is slower than DRAM, it costs less and offers higher capacity.

Industry analysts view Toshiba's XL-Flash and Samsung's Z-NAND as a low-latency, flash-based response to 3D XPoint memory technology that Intel and Micron co-developed. Intel last year began shipping 3D XPoint-based SSDs under the brand name Optane, and this year started sampling persistent memory modules that use the 3D XPoint technology. Micron has yet to release products based on 3D XPoint.

David Floyer, CTO and co-founder of Wikibon, said Toshiba's XL-Flash and Samsung's Z-NAND will never quite reach the performance of Optane SSDs, but they'll get "pretty close" and won't cost anywhere near as much.

Handy expects XL-Flash and Z-NAND to read data at a similar speed to Optane, but he said they "will still be plagued by the extraordinarily slow write cycle that NAND flash is stuck with because of quantum mechanics."

Startup takes on incumbent NAND flash manufacturers

YMTC hopes to challenge established NAND flash manufacturers with Xtacking. YMTC claims the new architecture can improve efficiency and I/O speed, reduce die size and increase bit density, and shorten development time.

"It really takes courage to go down that path because we know that it's not easy to make that technology work," YMTC CEO Simon Yang said.

Unlike conventional NAND, Xtacking separates the processing between the flash cell array and the periphery circuitry, or logic, onto different wafers. The startup claimed the high-voltage transistors that conventional NAND typically uses for the periphery circuit limit NAND I/O speed. YMTC claims Xtacking permits the use of lower voltage transistors that can enable higher I/O and more advanced functions, according to YMTC.

"We really can match the DDR4 I/O speed without any limitation," Yang said.

Yang said results have been encouraging. He said the flash chip yield is increasing, and the reliability of the memory bits through cycling looks positive. YMTC plans to introduce samples of the new Xtacking-based flash technology into the market early next year, Yang said.

"Hopefully, we can catch up with our friends and contribute to this industry," Yang said.

YMTC started 3D NAND development in 2014 with a nine-layer test chip and later co-developed a 32-layer test chip with Spansion, which merged with Cypress Semiconductor. YMTC moved the chip into production late last year, but Yang said the company held back on volume ramp-up because the first-generation product was not cost competitive.

"We are very much profit-driven," Yang said. He later added, "We only want to ramp into volume when it's cost competitive."

Handy expressed skepticism that YMTC will be able to meet its cost target, but he said YMTC's Xtacking efforts might help the company to get to market faster.

SK Hynix 4D NAND flash

SK Hynix came up with a new name to describe its latest NAND flash technology. The company said its "4D NAND" puts the periphery circuitry under the charge-trap-flash-based 3D NAND cell array to reduce chip size, cut the number of process steps and lower overall cost over conventional NAND, in which the periphery circuitry is generally alongside the NAND cell.

But, industry analysts say 4D NAND is merely a catchy marketing term and the approach not unique.

"YMTC is stacking a chip on top of the other, whereas Hynix is putting the logic on the same bit but just building it underneath," Handy said. "The cost of the chip is a function of how big the die is, and if you tuck things underneath other things, you make the die smaller. What Hynix is doing is a good thing, but I wouldn't call it an innovation because of the fact that it's the mainstream product for Intel and Micron."

Intel and Micron have touted their CMOS under the array (CuA) technology in both their 64-layer QLC and 96-layer TLC flash technologies that they claim reduces die sizes and improves performance over competitive approaches. Handy said Samsung has also discussed putting the logic under the flash chip.

Hyun Ahn, senior vice president of NAND development and business strategy at SK Hynix, said his company's charge-trap-based 4D NAND roadmap starts at 96 layers with a roadmap that extends to 128 layers and beyond using the same platform.

The first SK Hynix 4D NAND technology will begin sampling in the fourth quarter with 96 stacks of NAND cell, I/O speed of 1.2 Gbps per pin, and a mobile package of 11.5 by 12 mm. The chip size is 30% smaller, and 4D NAND can replace two 256 Gb chips with similar performance, according to SK Hynix.

The new SK Hynix 512 Gb triple-level cell (TLC) 4D NAND improves write performance by 30% and read performance by 25% over the company's prior 72-stack TLC 3D NAND, with 150% greater power efficiency.

Upcoming 1 terabit (Tb) TLC 4D NAND that SK Hynix will sample in the first half of next year fits into a 16 mm by 20 mm ball grid array (BGA) package with a maximum 2 TB for BGA. An enterprise U.2 SSD using the technology will offer up to 64 TB of capacity, according to SK Hynix.

SK Hynix plans to begin sampling 96-stack QLC 4D NAND, with 1 Tb density in a mono die, in the second half of next year. The company said the QLC 4D NAND would provide more than 20% higher wafer capacity than the TLC NAND that it has been producing since the second half of last year. The 72-stack, enterprise-class 3D NAND will represent more than 50% of SK Hynix NAND production this year, the company said.

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What are the most important new flash technologies that will have an impact on the industry?
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Our journey from 2D NAND (FG,Peri) to 3D NAND(stack up, CTF, Peri) to 4D NAND(PUC stacked with cell CTF) is amazing. If it really improves write performance by 30% and read performance by 25% over the company's prior 72-stack TLC 3D NAND, with 150% greater power efficiency, then that is the resort :).
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