http://www.extremetech.com/computing/184946-14nm-7nm-5nm-how-low-can-cmos-go-it-depends-if-you-ask-the-engineers-or-the-economists
Specially last paragraph (my bold)
" — TSMC might declare 7nm to be the node at which it can offer III-V materials and a 3D stacked die with TSVs, rather than just basing the node declaration around physical characteristics."