Direct Connection of Optical Fibers To

Introduction Conventional metallic conductors carrying electronic signals between integrated circuits are susceptible to electromagnetic coupling which can result in parasitic capacit~~ce, inductance, and noise pickup. Consequently, overall system performance suffers, especially at high bit rates. Signals transmitted photor.ically are immune to these effects, which accounts for ~~ch of the current interest in optical interconnections. Ideally, one would like to interconnect integrated circuits with optical fibers instead of metallic wires. These circuits could be located on the same chip, on the same circuit board, or on different boards; they could also be physically separated by a signific~~t distance. Several technological hurdles must be overcome in order to achieve this goal. For example, a suitable means must be found to directly connect an optical fib€ r to ar. integrated circuit in a manner analogous to wire bonding or solder-bump technology. It must feature a srr~ll real-estate footprint, be assembled reliably, and be mechanically stable. This paper describes a new technique which satisfies these requirements. The Intearated Fiber-Optic Coup:er Concept For directly coupling optical fib€ rs to integrated circuits, an integrated fiber-optic courler (IFOC) concept has been developed. 1 ,2 In the IFOC, a cavity is formed in the integrated circuit, and a photon detector or e~itter is fabricatec at the bottorr of the cav:ty. AJ. optical fiber is inserted into the cavity and, in a receiver structure, the light e".a.:-.at inc; frorr. the fiber generates a photocurrer.: in the detector. In an o?tical SO:..lrce struct~re, light emitted by the transmitter is launched into the fiber. For fro~t-side connected from the bottorr of the cavity to the front-side circuitry. For backside conne:::tions, the cavity extends to just below the front surface on which the detector/emitter structure is fabricated. This is illustrated in FiS. 1. If the emitter were an LED and the cavity much larger in diameter, the structure ~ould resemble a Burrus-type LED. 3 The IFOC concept satisfies the requirements for the direct connection of optical fibers described above. The cavity approach insures accurate and reproducible alignment between the optical f~r and the detector/emitter, and also serves to mechanically stabilize the connection against vibration or shock. The vertical nature of the interconnect also implies the s~allest possible real-estate footprint for the connection. In fact, since the cavity d~a~eter need only be as large as the optical fiber cladding (or smaller fo" front-side con.. ections), the projected footprint of the optical fibe" jacket …