[mikeash]

Note that 21 halvings doesn't take you from 14nm to 0.7nm, but rather to about 7 femtometers, or about 0.000007nm. For comparison, that's roughly 10x the size of a single proton.

I'd say a better estimate would be to assume density stops increasing around the point when feature size is the size of a silicon atom. I'm sure that'll be way off, but closer than estimating 21 more doublings.