Manufacturers Struggle to Improve QLC NAND as Yields Sit Below 50 Percent

When Micron announced that it would bring QLC (quad-level cell) NAND to market earlier this year, we were genuinely surprised to hear it. While stuffing more bits of data into each NAND cell is one of the most fundamental ways to improve NAND storage capacity, the industry went through significant teething problems on TLC (triple level cell) and didn’t start shipping TLC drives in high volume until the shift away from planar NAND built on 20nm nodes to 3D NAND built on older 40nm nodes. QLC NAND had seemed out-of-reach under such conditions — and maybe still are.

According to a report from DigiTimes, multiple manufacturers are struggling to yield QLC. The publication also makes some interesting references to low TLC yields from earlier this year that may have caused supply chain struggles. This could be related to efforts to scale up the number of manufacturing layers in 3D NAND products.

As the number of layers in 3D NAND increases, so does the manufacturing difficulty. While DigiTimes doesn’t explicitly link any TLC issues to 3D NAND density increases, it’s the most likely reason, especially given the way density has been improving year-on-year — and the report heavily implies that the TLC NAND coming out of the industry in 2018 hasn’t been very good. This isn’t the first time we’ve heard reports of low yield on QLC, however — a story from Tweaktown at the end of August claimed that QLC NAND yields at Micron were below 50 percent as well.

Low yields would explain why Micron launched enterprise QLC drives before its consumer counterparts. Because there’s a trade-off between how many bits of data you store per cell of NAND flash and the durability of the drive, TLC NAND debuted in consumer systems first, before making its way towards higher-end markets. When we spoke to Intel at the launch, the company stated that it was able to put QLC into enterprise products because better NAND flash profiling and the sheer capacity of higher-end products made it possible to deal with the reduced number of P/E (program/erase) cycles without compromising enterprise-class reliability. Both of these statements are likely true, but poor yields could still have made an enterprise launch more financially attractive.

One important point is that we don’t know exactly what “low yield” means in this context. NAND flash can be reconfigured on-the-fly to store varying amounts of data per cell — that’s how TLC and QLC drives are able to dynamically allocate an SLC cache to improve performance today. As the amount of free space available on the drive shrinks, the amount of space dedicated to SLC caching decreases as well and overall performance drops. Low yield could mean that manufacturers aren’t getting usable NAND at all — or it could mean that they’re getting NAND that works beautifully in MLC or TLC configurations but can’t store enough voltage levels to retain QLC data. But regardless of short-term teething problems, all of the major manufacturers are planning to introduce QLC 3D NAND drives for both consumer and enterprise applications over the next 12 months.

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