3D NAND

# Hole Bowing and Twisting at Extreme Depth

## 13. 13. Hole Bowing and Twisting at Extreme Depth

Beyond simply slowing down, a channel-hole etch pushed to very high aspect ratio can go geometrically wrong in ways that a shallower etch rarely does: the hole can bow outward partway down before narrowing again, or drift off a perfectly vertical path, a defect generally called bowing, and in more severe cases the hole's cross-section can twist or deform rather than staying a clean circle. Both defects come from the same underlying cause described in the previous step — ions and etch byproducts behaving differently at different depths inside an extremely narrow, extremely deep channel — but their consequences are specific and serious. A bowed or twisted hole produces a macaroni channel with inconsistent diameter along its length, which means the transistor formed at each word-line layer along that channel no longer has uniform electrical characteristics from the top of the stack to the bottom; cells near a bow may behave measurably differently from cells on a straight section of the same string. At the layer counts the industry has reached, controlling this geometry precisely enough, across every one of the many thousands of holes etched into a single die, became one of 3D NAND's genuinely hard, ongoing manufacturing problems — and it is the direct motivation for the solution the next step in this series introduces.