Abstract: A method may include: forming a base layer on a substrate; forming a waveguide assembly on the base layer, where the waveguide assembly is surrounded by a cladding layer; forming a trench opening through the cladding layer and the base layer; forming an undercut void by etching the substrate through the trench opening, where the undercut void extends under the waveguide assembly and the base layer; and filling the trench opening with a filler to seal off the undercut void. Other embodiments are described and claimed.