Semiconductor devices with backside air gap dielectric

A semiconductor structure, comprising:

• a semiconductor feature;
• an isolation structure disposed on sidewalls of the semiconductor feature;
• a gate structure disposed on sidewalls and a top surface of the semiconductor feature and on a top surface of the isolation structure;
• first and second source/drain features sandwiching the semiconductor feature;
• first and second metal plugs through the isolation structure and contacting bottom surfaces of the first and second source/drain features, respectively;
• and
• a dielectric liner contacting sidewalls of the isolation structure and sidewalls of the first and second metal plugs, wherein an air gap between the first and second metal plugs is partially or fully surrounded by the dielectric liner.

A semiconductor structure having a frontside and a backside, comprising:

• an isolation structure at the backside;
• one or more transistors at the frontside, the one or more transistors having source/drain electrodes;
• two metal plugs through the isolation structure and contacting two of the source/drain electrodes from the backside;
• a dielectric liner contacting sidewalls of the isolation structure and sidewalls of the two metal plugs;
• and
• an interconnect structure over the dielectric liner at the backside,
• wherein an air gap between the two metal plugs is sealed by the dielectric liner and the interconnect structure.