The NAND flash industry may seem like an alphabet soup of terms today, but this is simply the result of continuous market evolution.
Bits Per Cell – An Evolution
NAND flash started with the standard planar process with all circuitry fabricated on the surface of a wafer. Today, this is called 2D NAND or 2 dimensional NAND. For the first generations of NAND flash, a memory cell transistor only stored two charge states. A cell was either in the programmed or erased state. Therefore, a cell could store 1 bit of information. This was called SLC for single level cell. However, it was recognized that a cell could store more than two charge states. Four charge states enabled the memory cell to store 2 bits of information. This was named MLC for multilevel cell. By storing two bits per cell, the total memory capacity could be doubled without changing the chip size. Future generations of NAND include 3 bit per cell or triple level cell (TLC) and 4 bit per cell or quadruple level cell (QLC). However, the tradeoff of increasing the number of levels per cell was reduced write/erase (W/E) cycle endurance, longer memory cell read and programming times, and increased error correction requirements.
While 2D NAND evolved to higher bits per cell, the cell size was also decreasing. Lithography shrinking from 350nm to 15nm resulted in ever-decreasing memory cell sizes and increasing chip densities. Each new generation essentially enabled a doubling of the number of cells.
The Recipe for Revolution: Building Flash Memory Skyscrapers
The solution for higher density NAND was to go up. In 2D NAND, all transistors are on the surface of the wafer (the ground level). KIOXIA’s invention of 3D NAND1 is like a skyscraper in which layers of memory cells are built in a stack. The memory cells are still connected in series as in the original 2D NAND flash, but the memory cell string is now formed vertically in a column.
Within its BiCS FLASH™ 3D flash memory family, KIOXIA makes a variety of NAND for different applications.
NAND Flash Going Forward: A Lid for Every Pot
With that being said, one might think that 2D NAND is no longer made, but there is in fact still a market for it. KIOXIA continues to make low density SLC NAND in package capacities from 1Gb to 256Gb. The endurance is from 50k-100K W/E cycles. 15nm MLC continues to be made as well for lower density products.
The market size of NAND flash and the types of NAND flash have grown tremendously over the years to address specific requirements in the marketplace. From the early days of SLC and planar 2D NAND, to the advanced 3D architectures and high-density solutions of today, each innovation has played a role in shaping the industry. Hopefully, this historical perspective has helped make the "alphabet soup" of acronyms a little easier to understand.
For more flash memory insights, be sure to take a look at our ‘Ask the Expert’ series of educational and informative videos: https://americas.kioxia.com/en-us/about/flash35/ask.html
Notes:
1: KIOXIA first presented the concept for BICS Flash 3D flash memory at the IEEE VLSI Symposium in 2007.
Read and write speed may vary depending on various factors such as host devices, software (drivers, OS etc.), and read/write conditions.
All other company names, product names and service names may be trademarks of their respective companies.
Definition of capacity: KIOXIA Corporation defines a megabyte (MB) as 1,000,000 bytes, a gigabyte (GB) as 1,000,000,000 bytes and a terabyte (TB) as 1,000,000,000,000 bytes. A computer operating system, however, reports storage capacity using powers of 2 for the definition of 1GB = 2^30 bytes = 1,073,741,824 bytes and 1TB = 2^40 bytes = 1,099,511,627,776 bytes and therefore shows less storage capacity. Available storage capacity (including examples of various media files) will vary based on file size, formatting, settings, software and operating system, and/or pre-installed software applications, or media content. Actual formatted capacity may vary.
Product density is identified based on the density of memory chips(s) within the product, not the amount of memory capacity available for data storage by the end user. In terms of product capacity, available user storage capacity (including examples of various media files) will vary based on file size, formatting, settings, software and operating system, pre-installed software applications, media content, and other constraints. Actual formatted capacity may vary. KIOXIA Corporation defines a gigabit (Gb) as 1,073,741,824 bits, a megabyte (MB) as 1,000,000 bytes, a gigabyte (GB) as 1,000,000,000 bytes and a terabyte (TB) as 1,000,000,000,000 bytes. However, a computer operating system, reports storage capacity using powers of 2 for the definition of 1 GB = 2^30 bytes = 1,073,741,824 bytes and 1 TB = 2^40 bytes = 1,099,511,627,776 bytes.