An Electromagnetic Study of Scalability of Si3N4 Optical Waveguides for 3D Nanophotonic Integration with CMOS Electronics in AI Era

We report computational electromagnetic (EM) investigation into the planar optical waveguide with the silicon nitride (Si3N4) core scaled down from 800 nm to 60 nm. The surrounding cladding of silicon dioxide (SiO2) is at least 2 micrometer thick. With the CMOS-compatible Si3N4 optical waveguide constructed on a resistive silicon (Si) substrate, EM fields of the whole structure are computed by solving Maxwell’s equations based on the finite element method (FEM). Electric field profiles of the fundamental mode are obtained at 900 nm wavelength (lambda) which is shorter than the bandgap wavelength of Si and hence optical loss in the substrate. It is found that in scaling down the Si3N4 core to sub-wavelength thickness (i.e. less than (lambda/n)), the evanescent electric field penetrates strongly through the SiO2 under-cladding into the resistive substrate. The field penetration is even stronger for core thickness of 100 nm and 60 nm, which are equivalent to 0.22(lambda/n) and 0.13(lambda/n) respectively. Apart from using SiO2 cladding of 4 micrometer or thicker, doubling the core width to increase the width-to-thickness ratio would provide alleviation. The results provide helpful guidelines about the Si3N4 thickness, width and spatial separation, in the design and fabrication of nanophotonic devices and circuits for potential 3D integration with CMOS electronics built on the Si substrate.