On the feasibility of through-wafer optical interconnects for hybrid wafer-scale-integrated architectures

The desire to achieve a high degree of parallelism in multiwafer wafer-scale-integrated (WSI) based architectures has stimulated study of three-dimensional interconnect structures obtained by stacking wafer circuit boards and providing interconnections vertically between wafers over the entire wafer area in addition to planar connections. While the advantages of optical over electrical interconnects for conventional two-dimensional VLSI and wafer-scale-integrated circuits have not been clearly demonstrated, for dense multiwafer WSI or hybrid-WSI three-dimensional architectures, the ability to pass information optically between circuit planes without mechanical electrical contacts offers potential advantages. While optical waveguides are readily fabricated in the wafer plane, waveguiding vertically through the wafer is difficult. If additional processing is required for waveguides or lenses, it should be compatible with standard VLSI processing. This paper presents one method of meeting this criterion. Using optical devices operating at wavelengths beyond the Si absorption cutoff, low-loss through-wafer propagation between WSI circuit planes can be achieved over the distances of interest (≈ 1 mm) with the interstitial Si wafers as part of the interconnect "free-space" transmission medium. The thickness of existing VLSI layers can be readily adjusted in featureless regions of the wafer to provide antireflection windows such that >90 percent transmittance can be obtained through p-type silicon. Initial results show a 400-percent source-detector coupling enhancement is obtainable for these optical interconnections using VLSI process-compatible SiO2phase-reversal zone plate lenses.