| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. === Includes bibliographical references (p. 229-235). === The copper (Cu) interconnect has become the bottleneck for bandwidth scaling due to its increasing RC time constant with the decreasing gate line width. Currently, silicon based optical interconnect is widely pursued as the most promising technology to replace Cu in microprocessor chips. Silicon optical interconnect is based on integrated silicon nanophotonic technologies. It can leverage the large scale and low cost of CMOS technology and deliver higher bandwidth with no EMI and low heat dissipation. Passive photonic component, such as waveguides, couplers, filters, splitters, are the backbone of integrated photonic circuit. This thesis is dedicated to the development of low loss, high performance, high index contrast optical waveguides and couplers via materials, processes engineering, development, and device designs. We primarily focus on SOI single crystalline silicon (c-Si or SOI), PECVD amorphous silicon (a-Si:H, or simplified as a-Si), and PECVD silicon nitride (SiNxHy) based single mode channel waveguides.We have previously identified that sidewall roughness scattering is the dominant loss mechanism for the TE mode in high index contrast single mode channel waveguides. In this thesis, we provide a comprehensive understanding of the roughness scattering and its positive correlations with (1) sidewall optical intensity; (2) sidewall RMS roughness; and (3) sidewall index contrast. Novel processes and designs, such as hard mask and chemical oxidation, are developed based on the above understanding. In single mode, 500 x 200 nm2 c-Si channel waveguides, we have achieved world-record 2.7 dB/cm and 0.7 dB/cm transmission loss coefficients for the TE mode and the TM mode, respectively.For deposited waveguides, bulk absorption loss is also important for both TE and TM modes.For PECVD a-Si, we adapt hydrogen passivation to reduce dangling bond density. === (cont.) We also use a thin silicon nitride as the over cladding layer to help preserve H passivation and to reduce sidewall index contrast, acting as the graded index layer for a-Si waveguide core. We have accomplished the lowest reported loss coefficients in directly etched, single mode, 700 x 100 nm2 a-Si channel waveguides of 2.7 dB/cm for the TE mode, comparable to c-Si waveguide with similar dimensions. For the first time, damascene process has also been demonstrated as a promising process for a-Si waveguide fabrication. We have achieved a record-low loss of 2.5 dB/cm in 600 x 100 cm2 a-Si channel waveguides. Chemical-mechanical polishing (CMP) is the most critical step.For PECVD SiNxHy, we have previously identified that the absorption loss is due to the resonant absorption caused by N-H vibration. In this thesis, three different low temperature approaches have been developed and optimized to reduce NH concentration in as-deposited SiNxHY via (1) deposition chemistry; (2) post-deposition Ultraviolet light (UV) treatment; and (3) post-deposition, in-situ N2/Ar plasma treatment. All three processes are compatible with CMOS back-end processes, such as a-Si process. While changing deposition chemistry is the simplest method to obtain low NH containing SiNxHy, it comes with high SiH concentration and may have undesirable properties. Experimentally, for UV treatment, the highest H removal percentage is 60%; for plasma treatment, - 90%. UV treatment shows strong compositional dependence. The underlying mechanism of such dependence is identified and confirmed by Monte-Carlo modeling. Low loss and spectrally broadband optical couplers are indispensable optical components in an integrated photonic circuit. A high performance coupler should be capable of overcoming the mode-size mismatch, mode-shape mismatch, mode-position mismatch, and polarization mismatch, bridging different optical devices with minimal coupling loss. In this thesis, we have demonstrated a fiber-to-waveguide coupler based on asymmetric graded index taper and monolithically integrated cylindrical lens. === (cont.) It is capable of transforming single mode light between single mode fiber and waveguides with minimal coupling loss of 0.45 dB between 1520 nm and 1630 nm. We have also demonstrated a vertical waveguide-to-waveguide coupler that is based on complementary inverse tapers. This design is tolerant of large refractive index mismatch between the two waveguides and also of any fabrication variation that would affect the effective indices of the two waveguides. We have achieved a minimal coupling loss of 0.25 dB per coupler and excellent broadband behavior is also demonstrated. Slot waveguides are a newly developed class of waveguides with unique optical properties. Slot waveguides can achieve exceptional high optical field in nanometer sized low index regions. In this thesis, we have demonstrated low loss transmission of 6 dB/cm for the fundamental slot mode in horizontal slot waveguides at 1550 nm. The horizontal slot configuration removes the constraints of thin slot definition by lithography and allows an arbitrarily thin slot to be fabricated via deposition or oxidation. Because the resulting interface is much smoother than the etched interface, the transmission loss in horizontal slot waveguides is much lower than in vertical slot waveguides. We also demonstrated that multiple slot configurations result in higher optical confinement compared to single slot configurations with the same slot thickness. The low loss and high optical confinement in the low index slot region realized in horizontal slot waveguides promises many useful applications, such as Er-doped silicon-based light emitters. For integration of slot waveguides with conventional channel waveguides, we have designed and simulated mode couplers and polarization rotators for slot-slot, slot-channel waveguide mode transformations.Athermal operation is important for realizing stable passive, WDM optical network on silicon. Athermal design of silicon waveguide systems uses advanced polymer cladding of large negative TO coefficient to provide compensation for the large positive TO coefficient in silicon. The reduced thermo-optic (TO) effect is experimentally demonstrated by reducing TO coefficient from 85 pm/K to 11 pm/K using polymer films. === by Rong Sun. === Ph.D.
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