Solidigm, the NAND research and manufacturing arm sold by Intel to SK hynix, took to the Flash Memory Summit 2022 to showcase a finished and working SSD prototype carrying the company's latest NAND technology: Penta-Level Cell (PLC). PLC builds upon NAND density by adding the capability for a single cell to store up to 5 bits of data (up from QLC's four bits per cell), increasing the amount of available space in each NAND flash chip.
The new technology thus paves the way for even cheaper $/GB ratios on PLC-based SSDs than what's currently possible with Quad-Level Cell (QLC) technology (let alone MLC and SLC tech, which have been mostly relegated to enterprise, datacenter, and other critical environments). Even as the amount of data produced by humanity increases at an astonishing pace (with predictions placing daily data creation sitting at 463 exabytes daily by 2025), storage technology will also need to evolve to accommodate this avalanche. With the increasing investments in supercomputing, the dawn of 5G and AI, all with deep data storage requirements of their own, Solidigm sees PLC as a way to scale while keeping costs in check.
We believe in the increasing benefits of QLC and PLC #NAND to meet the data-intensive needs of AI, machine learning, big data analytics, and to support 5G. We’re energized for the future and committed to seizing this moment. #TheNewParadigm #FMS2022 #SSD https://t.co/184jvKPGDE pic.twitter.com/AsKWjYujOTAugust 8, 2022
PLC works by adding additional voltage states, which represent the information bits in the cell. While SLC only had to deal with two voltage states (a bit was either written or not), MLC doubled that number to four voltage states (so each cell could contain any combination of both a 0 and a 1). PLC NAND technology ends up requiring 32 distinct voltage states to differentiate between all possible combinations of ones and zeros the cell can hold. This places additional strain on the cells and at the controller level, which has to account for the increased chance of voltage states being incorrectly written, corrupting the data. More powerful error correction algorithms thus have to be implemented at the controller level.
Each additional bit per cell requires a doubling of the voltage states, which has its own implications in the cell's life expectancy and requires much more fine-tuned reading and writing capabilities to deal with the increasingly smaller differences between states. This reduction in life expectancy for NAND cells as their bit storage capacity increases is part of the reason why QLC SSDs are generally considered to be less reliable (with cell "death" occurring earlier) compared to lower numbers of bits.
The laws of diminishing returns are pretty clear in scaling NAND density through additional bits per cell. Essentially, each additional bit of information on a single NAND cell leads to an increase of 100% in required voltage states (adding to its complexity), while the storage capacity increase itself is reduced in half.
| Row 0 - Cell 0 | SLC (1 Bit) | MLC (2 Bits) | TLC (3 Bits) | QLC (4 Bits) | PLC (5 Bits) | HLC (6 Bits, theoretical) |
| Delta Storage Capacity | Baseline | +100% | +75% | +50% | +25% | +12,5% |
| Delta Complexity Increase | Baseline | +100% | +100% | +100% | +100% | +100% |
"We are excited to show the industry's first PLC SSD in action today, " said Sanjay Talreja, SVP and General Manager of Solidigm's Client Storage Group. "It is an important milestone for Solidigm as a new company and an exciting moment for the future of storage technology with far-reaching implications."
Since Solidigm's PLC NAND is based on Intel technology, it uses a different type of cell than what other major NAND players - such as Micron - have been focusing on. Intel's NAND was based on floating gate transistors, which Solidigm says was suited to increasing bits per cell to enable the creation of PLC. Micron - alongside most of the NAND industry - has instead focused its manufacturing on charge trap technology, which has its own strengths and weaknesses.
It's currently unclear when exactly PLC-based drives will hit the market. Western Digital has gone on record saying they wouldn't hit the market before 2025 - yet here we are in 2022 with Solidigm's working prototype. But when they arrive, customers can look forward to NAND drives that have better $/GB ratios than what the cheapest QLC drives can ever hope to provide. And if you think Solidigm should be relegated to the $/GB equation, think again. One of the most efficient and fastest SSDs we ever tested, the SK hynix Platinum P41, belongs to Solidigms' parent company. PLC SSDs will likely have a hard time fighting on performance, but as controllers and buses mature, we'd expect them to provide great performance at a more affordable price.
