Material selection and nanofabrication techniques for electronic photonic integrated circuits

Abstract

Electronic-photonic integrated circuits have the potential to circumvent many of the performance bottlenecks of electronics. To achieve the full benefits of integrating photonics with electronics it is generally believed that wavelength-division multiplexing is needed; requiring an integrated optical device capable of multiplexing/demultiplexing operations. One such device is a bank of microring-resonator filters with precisely spaced resonant frequencies. In this work, a fabrication strategy based on scanning-electron-beam lithography (SEBL) is presented for precisely controlling the resonant frequency of microring-resonator filters. Using this strategy it is possible to achieve dimensional control, on the tens-of- picometer scale, as required for microring-resonator filter banks. To correct for resonant-frequency errors present after fabrication, two forms of postfabrication tuning, one dynamic and one static, are demonstrated. It is also shown that hydrogen silsesquioxane (HSQ) can be converted into a high-quality overcladding for photonic devices by optimizing the annealing process. Finally, a postfabrication technique of localized substrate removal is presented, enabling the integration of photonics with CMOS electronics. Second-order microring-resonator filter banks were fabricated using SiNx and Si as the high -index core materials. By controlling the electron-beam-exposure dose it is possible to change the average microring-waveguide width to a precision better than 75 pm, despite the 6 nm SEBL address grid. Using postfabrication tuning the remaining resonant-frequency errors can be reduced to less than 1 GHz.

(cont.) By annealing HSQ in a an 02 atmosphere using rapid thermal processing, it is possible to create thick overcladding layers that have essentially the same optical properties as SiO2 with the excellent gap-filling and planarization properties of HSQ. Using XeF2 to locally etch an underlying Si substrate, waveguides with a propagation loss of -10 dB/cm were fabricated out of polysilicon deposited on 50 nm of SiO2.