Transistor isolation structures

A method, comprising:

• forming a fin structure on a substrate, wherein the fin structure comprises alternating first and second nanostructure elements;
• forming a sacrificial gate structure over the fin structure, wherein edge portions of the fin structure are not covered by the sacrificial gate structure;
• removing the edge portions of the fin structure;
• etching edge portions of the first nanostructure elements in the fin structure to form cavities;
• depositing a spacer material on the sacrificial gate structure and the fin structure to fill the cavities;
• treating the spacer material with a post-deposition treatment;
• removing a portion of the spacer material in the cavities to form an opening in the spacer material;
• and
• forming source/drain (S/D) epitaxial structures on the substrate to abut the fin structure so that the S/D epitaxial structures are in contact with the second nanostructure elements and isolated from the first nanostructure elements by the spacer material, wherein sidewall portions of the S/D epitaxial structures seal the opening in the spacer material to form an air gap in the spacer material.

A structure, comprising:

• a pair of epitaxial structures on a substrate;
• vertically-stacked nanostructures between the pair of epitaxial structures, wherein the vertically-stacked nanostructures are in contact with the pair of epitaxial structures;
• a gate structure surrounding first portions of the vertically-stacked nanostructures between the pair of epitaxial structures;
• a first gate spacer above the vertically-stacked nanostructures and on sidewall surfaces of the gate structure, wherein the first gate spacer has a first density;
• a second gate spacer on sidewall surfaces of the first gate spacer and in contact with the pair of epitaxial structures;
• and
• spacer structures between second portions of the vertically-stacked nanostructures and interposed between the gate structure and sidewall portions of the pair of epitaxial structures, wherein the spacer structures comprise:
• a spacer layer with a uniform thickness and having a second density different the first density;
• and
• an air gap surrounded by the spacer layer and the sidewall portions of the pair of epitaxial structures.