3-D IC Technology and Modular Computation
3D IC Technology is growing at an amazing pace. The ability to take advantage of the z-axis in IC chip fabrication provides another dimension to Moore’s law, and thereby provides a means to further mitigate the slowing returns of Moore’s law.
One obstacle in 3D IC chip design is the fabrication of inter-connects from one layer to the other. It is not only that such fabrication is difficult, but each vertical connection eliminates circuit area in all IC layers sandwiched between the connection. Ironically, with 3D IC technology, it makes sense to minimize connections between IC layers. Binary circuits that require close connection of many inputs to outputs, are especially problematic for taking advantage of 3D layout while also minimizing inter-layer interconnect.
One solution to the problem is to use modular circuits instead of binary circuits. With modular circuits, each layer of the IC can be used to implement each modulus of a register word, and because there is no carry between digits, there is less connections between adjacent layers. One set of required connections is the so-called “cross-bar bus”. Since this bus shares all the same connections in the same places at each layer of the IC, it can be easily and efficiently synthesized. Thus, not only is modularity an advantage, but it leads to self-similar and symmetrical circuit typologies and is advantageous for 3D IC implementation.