3D silicon circuits bring denser computer chips closer to reality

By stacking transistors on top of one another, rather than laying them side by side on a flat chip, many electronic engineers are hopeful that vast amounts of computing power could be packed into tiny spaces, all while cutting energy use. So far, however, the ability to build these monolithic 3D integrated circuits has proven stubbornly difficult, largely because the fabrication processes required can damage the layers already in place.