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NAND flash manufacturers showcase new technologies

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发表时间:2018-08-24 09:44

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

NAND flash manufacturerslaid out their roadmaps for next-generation products and architectures at the2018 Flash Memory Summit this month.

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

NAND flash manufacturersintroduced new efforts to speed performance, raise density and lower costs.Toshiba launched a low-latency option called XL-Flash. Chinese startup YangtzeMemory Technologies Co. (YMTC) hopes to catch up to the flash chip incumbentswith its "Xtacking" architecture that can potentially increaseperformance and bit density. And South Korea-based chipmaker SK Hynix harborssimilar aspirations with its so-called "4D NAND" flash that industryexperts say is a misnomer.

Key NAND flashmanufacturer 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 aplace that doesn't take up space on the flash chip, said Jim Handy, generaldirector and semiconductor analyst at Objective Analysis.

Here are some of the newtechnologies that NAND flash manufacturers showcased at last week's FlashMemory Summit:

Toshiba's XL-Flash

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

Ohshima said XL-Flash requires no additionalprocess and is fully compatible with conventional flash in terms of the commandprotocol and interface. The read latency of XL-Flash could be 10 times fasterthan conventional TLC flash devices, according to Ohshima.

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

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

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

Industry analysts view Toshiba's XL-Flash andSamsung's Z-NAND as a low-latency, flash-based response to 3D Xpointmemory technology that Intel and Micron co-developed. Intel last yearbegan shipping 3D XPoint-based SSDs under the brand name Optane, and this yearstarted sampling persistent memory modules that use the 3D XPointtechnology. 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 theperformance of Optane SSDs, but they'll get "pretty close" andwon't cost anywhere near as much.

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

Startup takes on incumbent NAND flash manufacturers

YMTC hopes to challenge established NANDflash manufacturers with Xtacking. YMTC claims the new architecture can improveefficiency and I/O speed, reduce die size and increase bit density, and shortendevelopment time.

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

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

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

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

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

YMTC started 3D NAND development in 2014 witha nine-layer test chip and later co-developed a 32-layer test chip withSpansion, which merged with Cypress Semiconductor. YMTC moved the chip intoproduction late last year, but Yang said the company held back on volumeramp-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'scost competitive."

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

SK Hynix 4D NAND flash

SK Hynix came up with a new name to describeits latest NAND flash technology. The company said its "4D NAND" putsthe periphery circuitry under the charge-trap-flash-based 3D NAND cellarray to reduce chip size, cut the number of process steps and lower overallcost over conventional NAND, in which the periphery circuitry is generallyalongside the NAND cell.


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