Semiconductor structures and the manufacturing method thereof are disclosed. An exemplary manufacturing method includes providing a workpiece that includes a substrate, first channel members and second channel members over the substrate, a first gate structure engaging the first channel members, a second gate structure engaging the second channel members, a dielectric fin disposed between the first and second gate structures, an isolation feature disposed under the dielectric fin. The method also includes forming a metal cap layer at the frontside of the workpiece and depositing a dielectric feature on the dielectric fin. The dielectric feature dividing the metal cap layer into a first segment and a second segment. The method also includes etching the isolation feature to form a trench at the backside of the substrate, depositing a spacer on sidewalls of the trench, etching the dielectric fin from the trench, and depositing a seal layer in the trench.