Dielectric fins with air gap and backside self-aligned contact

A method, comprising:

• providing a structure having two fins extending from a substrate and an isolation structure adjacent to lower portions of the fins, wherein each of the fins includes first semiconductor layers and second semiconductor layers alternately stacked one over another;
• forming a cladding layer over top and sidewalls of the fins and exposing the isolation structure;
• recessing the isolation structure using the cladding layer as an etch mask to expose the substrate;
• forming a seal layer over the substrate, the isolation structure, and the cladding layer;
• forming a sacrificial plug filling space between the seal layer over opposing sidewalls of the two fins, wherein a top surface of the sacrificial plug is below a topmost layer of the first semiconductor layers;
• depositing a dielectric top cover over the sacrificial plug;
• and
• forming a high-k dielectric helmet over the dielectric top cover and the seal layer, resulting in dielectric fins comprising the seal layer, the dielectric top cover, and the high-k dielectric helmet.

A method, comprising:

• providing a structure having two fins extending from a substrate and an isolation structure adjacent to lower portions of the fins;
• forming a cladding layer over top and sidewalls of the fins and exposing the isolation structure;
• recessing the isolation structure using the cladding layer as an etch mask to expose the substrate;
• after the recessing of the isolation structure, depositing a seal layer over the substrate, the isolation structure, and the cladding layer;
• forming a sacrificial plug over the seal layer and between the two fins;
• depositing a dielectric top cover over the sacrificial plug and laterally between the two fins;
• thinning down the substrate from a backside of the structure until the fins are exposed;
• forming a backside dielectric layer over the backside of the structure;
• performing a chemical mechanical planarization process to the backside of the structure until the sacrificial plug is exposed;
• removing the sacrificial plug, resulting in a trench from the backside of the structure;
• and
• sealing the trench with a dielectric bottom cover, resulting in an air gap surrounded by the seal layer, the dielectric top cover, and the dielectric bottom cover.