3D NAND

# Why Etching More Layers Gets Progressively Harder

## 12. 12. Why Etching More Layers Gets Progressively Harder

The channel-hole etch described in the previous step does not stay equally difficult as the industry adds more layers to chase higher capacity — it gets harder in a way that planar lithography's difficulties never quite matched, because the etch has to punch through a taller stack using the same basic tool. A plasma etch removes material by directing ions down into the hole being formed, and as the hole gets deeper relative to its diameter — a ratio the industry calls aspect ratio — ions lose energy colliding with the hole's own sidewalls on the way down, arriving at the bottom of a very deep hole with less directed energy than they started with. Etch rate slows near the bottom of a tall stack, etch chemistry behaves differently at depth than near the top, and the conditions that cleanly open a hole through the first twenty layers are not quite the same conditions needed to finish that same hole cleanly through the hundredth layer. Every additional layer added to chase higher capacity makes the single hardest step in the whole process a little harder still, and this is the central tension layer-count scaling runs into: it dodges the planar wall from steps 1 and 2, but it has its own wall, and this one shows up directly in the etch.