Vertically Stacked Multilayer Photonics on Bulk Silicon Toward Three-Dimensional Integration

Vertically stacked hydrogenated amorphous silicon (a-Si:H) and aluminum nitride (AlN) photonic circuits are fabricated on bulk silicon using complementary metal-oxide semiconductor back-end-of-line-compatible technology. The 0.5 μm × 0.22 μm a-Si:H and 1 μm × 0.4 μm AlN channel waveguides exhibit relatively low propagation losses of ∼3.8 and ∼1.4 dB/cm at 1550-nm telecom wavelengths, respectively, thus enabling the realization of various high-performance photonic devices on these two layers, such as multimode interference power splitters, waveguide ring resonators, arrayed-waveguide gratings, etc. In particular, the a-Si:H layer is suitable for ultra-compact thermo-optic (TO) devices because of its large refractive index of ∼3.5 and large TO coefficient (TOC) of ∼2.60 × 10
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, whereas the AlN layer is suitable for large-size temperature-insensitive devices because of its relatively small refractive index of ∼2.0 and small TOC of ∼3.56 × 10
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. A cascade directional coupler structure is proposed for connection between these two layers, which provides coupling efficiency of ∼ –1.0 dB, as estimated from numerical simulations. The feasibility of stacking different photonic layers on bulk Si paves the way to realize complex 3-D photonic circuits on chip which are not possible in the conventional single-layer configuration.