40 um^2 might cover the logic (although I think your logic transistor count is a factor of three low; and something like this is most likely to be made on a 45

40 um^2 might cover the logic (although I think your logic transistor count is a factor of three low; and something like this is most likely to be made on a 45 nm or bigger process), but doesn’t cover IO pads. If you’re willing to wire bond directly to a flex circuit you may be able to use pads on to order of 50 um x 50 um (each! Likely need 6 or 8 pads to be useful), but that’s a hell of a process, and you’d have to encapsulate afterwards, adding bulk. If you want to flip-chip mount it’s pretty hard to go under 1 mm x 1 mm for a useful microcontroller, although there’s some stuff out there at 600 um x 600 um or so from memory — but pad sizes under 300 um then bump up your resolution requirements for the flex circuit you’re bonding to.