Multi-layer silicon photonic devices for on-chip optical interconnects
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Large on-chip bandwidths required for high performance electronic chips will render optical components essential parts of future on-chip interconnects. Silicon photonics enables highly integrated photonic integrated circuit (PIC) using CMOS compatible process. In order to maximize the bandwidth density and design flexibility of PICs, vertical integration of electronic layers and photonics layers is strongly preferred. Comparing deposited silicon, single crystalline silicon offers low material absorption loss and high carrier mobility, which are ideal for multi-layer silicon PIC. Three different methods to build multi-layer silicon PICs based on single crystalline silicon are demonstrated in this dissertation, including double-bonded silicon-on-insulator (SOI) wafers, transfer printed silicon nanomembranes, and adhesively bonded silicon nanomembranes. 1-to-12 waveguide fanouts using multimode interference (MMI) couplers were designed, fabricated and characterized on both double-bonded SOI and transfer printed silicon nanomembrane, and the results show comparable performance to similar devices fabricated on SOI. However, both of these two methods have their limitations in optical interconnects applications. Large and defect-free silicon nanomembrane fabricated using adhesive bonding is identified as a promising solution to build multi-layer silicon PICs. A double-layer structure constituted of vertically integrated silicon nanomembranes was demonstrated. Subwavelength length based fiber-to-chip grating couplers were used to couple light into this new platform. Three basic building blocks of silicon photonics were designed, fabricated and characterized, including 1) inter-layer grating coupler based on subwavelength nanostructure, which has efficiency of 6.0 dB and 3 dB bandwidth of 41 nm, for light coupling between layers, 2) 1-to-32 H-tree optical distribution, which has excess loss of 2.2 dB, output uniformity of 0.72 dB and 3 dB bandwidth of 880 GHz, 3) waveguide crossing utilizing index-engineered MMI coupler, which has crossing loss of 0.019 dB, cross talk lower than -40 dB and wide transmission spectrum covering C-band and L-band. The demonstrated integration method and silicon photonic devices can be integrated into the CMOS back-end process for clock distribution and global signaling.