Abstract: A 3D integrated circuit, the circuit including: a first wafer including a first crystalline substrate, a plurality of first transistors, and first copper interconnecting layers, where the first copper interconnecting layers at least interconnect the plurality of first transistors; and a second wafer including a second crystalline substrate, a plurality of second transistors, and second copper interconnecting layers, where the second copper interconnecting layers at least interconnect the plurality of second transistors, where the second wafer is bonded face-to-face on top of the first wafer, where the bonded includes copper to copper bonding, and where the second crystalline substrate has been thinned to a thickness of less than 5 micro-meters.