Performance of Capacitively Loaded RF Electrodes on SiO2/Si Substrates for High-Speed Electro-Optic Modulators
We experimentally investigate conventional and capacitively loaded traveling-wave RF electrodes fabricated on thermally grown SiO$_2$ on high-resistivity silicon substrates for high-speed electro-optic modulators. The capacitively loaded electrodes exhibit RF attenuation coefficients as low as approximately $0.23~\mathrm{dB/cm}/\sqrt{\mathrm{GHz}}$, comparable to values reported for electrodes fabricated on insulating quartz substrates, while retaining the silicon handle and avoiding substrate removal or undercut processing. By varying the SiO$_2$ buried-oxide (BOX) thickness and the capacitive-loading geometry, the microwave effective index can be engineered over a broad range while maintaining low RF propagation loss. This enables microwave-optical velocity matching over a wide range of optical wavelengths, from the ultraviolet to the telecom C-band. These results demonstrate that thermally oxidized silicon provides a simple and scalable substrate platform for low-loss traveling-wave electrodes in thin-film lithium niobate and thin-film lithium tantalate electro-optic modulators, as well as for high-speed RF interconnects in integrated transceivers and co-packaged optical systems.