A structure provides a defect sensor for a cavity in an integrated circuit (IC). The structure includes a cavity defined in a substrate. A boundary is located where the cavity meets with a cavity-free area of the substrate. A metal line is arranged in a serpentine path in both a vertical and a horizontal direction and crosses the boundary. A controller may be provided that is configured to, in response to a change in an electrical characteristic of a signal through the metal line, generate an indication of the presence of a defect and/or change operation of at least one component of the IC. The structure may find application relative to a photonics integrated circuit (PIC) structure including an optical waveguide with a cavity under the optical waveguide.