Integrated fiber-optic coupler for very large scale integration interconnects.

The performance of very-high-speed integrated cir­ cuits and very-large-scale integrated circuits (VLSIC's) is impaired by limitations in the communi­ cation capacity among gates, chip, and boards.! Fi­ ber-optic interconnects are widely recognized as a po­ tential solution to this communication problem.27 However, no satisfactory means exists to couple a fiber to a detector on a VLSIC chip. We demonstrate a new optical interconnection technique that, relative to pre­ vious methods, offers several advantages such as high packing density, accurate alignment, and mechanical­ ly stable coupling. The advantages of optical over electronic intercon­ nections include increased transmission bandwidth, immunity to mutual interference, freedom from ca­ pacitive loading, and freedom from planar con­ straints.5 The large information capacity of optical fibers can relieve the electronic interconnection bot­ tleneck known as the "pinout problem." Optical in­ terconnections also have the potential for reconfigura­ ble switching and optically controlled electronic logic.3 Standard fiber-coupling techniques such as butt coupling or silicon V groove are not suitable for VLSIC interconnections. Butt coupling may be mechanically unstable, provide poor alignment, and not be suitable for high packing densities. V-groove coupling is not easily aligned with a detector array on a chip, and there are problems associated with multigroove splic­ ing.s The new optical interconnect technique provides excellent alignment and mechanical stability, requires minimal area on the surface of the chip (approximate­ ly 100 JIm2), and is well suited for high packing densi­ ties. Furthermore, interconnections are not confined to the perimeter of the chip but can be located at any point on the chip surface. The basic idea (see Fig. 1) is to etch a cylindrical hole, slightly larger than a single­ mode optical-fiber core, into a semiconductor wafer. A p-n junction, which serves as a detector, is then formed on the inside of the hole. A single-mode fiber core is inserted into the hole and affixed using epoxy or a simple mechanical fit. Light emanating from the fiber core is collected by the reverse biased p-n junc­ tion and converted into an electrical signal. This sig­ nal is utilized by the circuitry on the surface of the wafer.