For years SanDisk has been presenting a memory roadmap (this post’s graphic is one rendition) that anticipates a move to ReRAM after 3D NAND has run through its natural life, which was expected to be as little as three generations. This has been backed by the idea that a 3D NAND stack would only be able to reach a certain number of layers before it would encounter difficulties caused by the need to etch a high aspect ratio hole through an increasing number of layers.
The aspect ratio issue is not hard to understand: Let’s assume that the hole in a 24-layer stack has an aspect ratio of 40:1, then a 32-layer hole would have an aspect ratio of about 50:1, and a 64-layer stack would be something close to 100:1. Today’s technology starts to have trouble etching holes with an aspect ratio higher than 60:1.
These high aspect ratios were thought to be the limiting factor that would prevent 3D NAND from continuing for more than three generations. 3D NAND could only have as many layers as the aspect ratio could support.
On a panel that I moderated at this year’s Flash Memory Summit one panelist, Dr. Myoung Kwan Cho of SK hynix, explained that although there is a limit Continue reading