Number 186 (Story #3), July 5, 1994 by Phillip F. Schewe and Ben Stein INTEGRATED CIRCUITS ARE MOSTLY TWO-DIMENSIONAL , whereas nature more efficiently works in three dimensions. The human retina, for example, is a massively-parallel, 3D imaging system consisting of a layer of sensing cells (rod and cone cells), two layers of processing cells (bipolar and ganglion) and two layers of interconnection cells. As for manmade circuits, one method for stacking two chips is to flip one over and attach it to the other using raised metal bumps which serve as a support and as electrical connectors. Such "flip chips" have been used in military infrared-detecting "smart Pixel" arrays. Some stacks with more than two layers have been made using metal interconnections that go all the way through the silicon wafer. Efforts are also underway to link up several stacked circuit layers with optical signals that pass through the wafers. One of the problems here is the lattice mismatch between light-emitting materials and the silicon substrate. One solution may be the use of "epitaxial liftoff" (ELO), a technique in which a specially-grown "epilayer" can be separated from an underlying growth substrate by etching away an intermediate sacrificial layer. The microns-thick epilayer can then be transferred to a different host substrate for further processing. (Optics & Photonics News, April 1994.)