Embodiments of forming a waveguide structure are provided. The waveguide structure includes a substrate, and the substrate has an interconnection region and a waveguide region. The waveguide structure also includes a trench formed in the substrate, and the trench has a sloping sidewall surface and a substantially flat bottom. The waveguide structure further includes a bottom cladding layer formed on the substrate, and the bottom cladding layer extends from the interconnection region to the waveguide region, and the bottom cladding layer acts as an insulating layer in the interconnection region. The waveguide structure further includes a metal layer formed on the bottom cladding layer on the sloping sidewall surface.