Higher-aspect-ratio trenches for vertical transistor interconnections are more difficult to create and fill.

“But the act of stacking does not mean that they're not interconnected. What used to be connections on the top or even on the bottom, as you've seen from our press release, now has to go vertically in between the transistors, and that's a new challenge, because the aspect ratio of creating trenches and filling those trenches becomes more and more difficult.”