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 metaloxide semiconductor back-end-of-line-compatible technology. The 0.5 mu m x 0.22 mu m a-Si: H and 1 mu m x 0.4 mu m AlN channel waveguides exhibit relatively low propagation losses of similar to 3.8 and similar to 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 similar to 3.5 and large TO coefficient (TOC) of similar to 2.60x10(-4) K-1, whereas the AlN layer is suitable for large-size temperature-insensitive devices because of its relatively small refractive index of similar to 2.0 and small TOC of similar to 3.56 x 10(-5) K-1. A cascade directional coupler structure is proposed for connection between these two layers, which provides coupling efficiency of similar to -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.