At the recent International Memory Workshop (IMW 2024) in Seoul, South Korea, Kioxia discussed the technology and challenges of increasing the storage density of 3D NAND flash memory.
Kioxia predicts that storage density will reach 100 Gbit/mm2 by 2027 with line stacks of 1,000 words.
Reporting on Kioxia’s keynote, PC watch notes that a storage density of 100 Gbit/mm2 would mean that a 64 square millimeter silicon chip could hold 6.4 Tbit (about 800 GB). A pack of eight such chips would provide 6,400 GB, and four-pack flash storage would provide 25.6 TB. If it is sold as a 20TB SSD in 2028, it could cost between $250 and $350, making it competitive with the prices of 20TB HDDs.
Using molybdenum
Kioxia’s forecast to reach a thousand layers by 2027 is ambitious but plausible, given historical trends. However, forming channel holes that penetrate the stacked word lines becomes increasingly difficult as the layers increase.
Advanced etching techniques such as low-temperature RIE (Reactive Ion Etching) are needed to control the high aspect ratio of these deep holes. Furthermore, channel resistance and signal noise increase with depth, leading to a possible shift from polycrystalline to monocrystalline silicon using MILC (Metal Induced Lateral Crystallization) technology. This switch could double the cell current, improving performance.
Increasing the number of wordline stacks does not necessarily improve storage density due to the “staircase” used for vertical electrodes. Innovations such as combining vertical electrodes and switching from TLC (3 bits/cell) to QLC (4 bits/cell) can increase storage density. Density can also be significantly increased by promoting multi-level processing such as PLC (5 bits/cell), HLC (6 bits/cell) and HeLC (8 bits/cell).
Increasing the wordline stacks increases delay times due to resistance and capacitance. Reducing the stacking distance and changing the word line metal material from tungsten to molybdenum can alleviate these problems.
While the storage density of 3D NAND flash memory has steadily improved historically, Kioxia and rivals like Samsung are well aware that future developments will require new technologies and innovations to maintain this pace and realize the dream goal of petabyte SSDs.