Kioxia’s working on 7-bit NAND for future SSDs
Kioxia’s one of the world’s leading NAND manufacturers, and at the International Memory Workshop (IMW) the company confirmed that they are working on 7-bit NAND, Hept-Level Cell (HLC) NAND.
Today, most SSDs currently use Triple-Level Cell (TLC) or Quad-Level Cell (QLC) NAND, and last year, Kioxia demonstrated their ability to create 6-bit Hex-Level Cell NAND. The ability to fit more bits of information into NAND cell allows SSD manufacturers to create higher capacity SSDs, as fitting more information into each cell gives the entire SSD more storage capacity. With Hept (7-layer) Cell NAND, each SSD cell can store 75% more bits of data than today’s QLC NAND, enabling the creation of huge SSDs.
Every extra bit makes SSDs two times as complex
So how do SSD manufacturers fit more data into a NAND chip? The answer is voltage states. A bit of NAND can either be a 1 or a 0, and NAND uses readable voltage states to difference between these bits of information. For single-level cell (SLC) NAND, two voltage states are requires to read either a 1 or a 0. For Multi-Level Cell (MLC/2-bit) NAND, two pieces of information are stored. For 2-bit NAND, there are four data options, 00, 01, 10, and 11. This requires four voltage states, making the NAND twice as complex.
With TLC NAND, there are eight voltage states to define 000, 001, 011, 111, 100, 110, 101, and 010. As you add more bits of information to NAND, the NAND’s required voltage states double every time, with QLC NAND requiring 16 voltage states.
Don’t expect SSDs with 7-cell NAND anytime soon
With Kioxia’s 7-cell NAND, 128 voltage thresholds are required. This makes 7-cell NAND eight times as complex as today’s QLC NAND. To make this NAND possible, Kioxia’s experimental NAND needed to be used at 77 degrees kelvin (-196 degrees celcius) to reduce data read noise. As such, we should not expect to see this NAND enter the consumer market anytime soon.
To commercialise 7-bit NAND, Kioxia needs to create a version of this NAND that can operate at room temperature and beyond. This will be a tricky task, and it will take years for Kioxia to make this NAND functional outside of a laboratory.
Today, QLC NAND is growing in popularity, with each new generation of QLC NAND minimising its weaknesses when compared to TLC NAND. While today’s QLC NAND offers poor write performance when compared to TLC NAND, newer QLC SSDs are starting to achieve higher performance levels. Even so, TLC NAND remains the most popular option for high-end SSDs.