Multigate device with air gap spacer and backside rail contact and method of fabricating thereof

A device comprising:

• an isolation feature disposed over a substrate, wherein the isolation feature is disposed between a first fin portion and a second fin portion extending from the substrate;
• a dielectric gate isolation fin disposed over the isolation feature, wherein the dielectric gate isolation fin;
• a first gate structure around a first channel layer and a second channel layer over the first channel layer, wherein the first channel layer and the second channel layer are disposed over the first fin portion and extend between a first source feature and a first drain feature;
• gate spacers abutting sidewalls of a portion of the first gate structure, the gate spacers being disposed on the second channel layer;
• an air gap between the second channel layer and the first channel layer and between the first gate structure and the first source feature;
• and
• a dielectric material residue between the first channel layer and the second channel layer adjacent the air gap.

A method, comprising:

• providing a structure having a frontside and a backside;
• forming a gate-all-around (GAA) device on the frontside of the structure, wherein the forming the GAA device includes:
• etching a plurality of alternating layers to form a fin structure;
• forming inner spacers including a dummy dielectric material and a low-k dielectric material between channel layers of the fin structures;
• epitaxially growing a source/drain feature adjacent a first side of the inner spacers;
• releasing the channel layers in a channel region of the fin structure adjacent a second side of the inner spacers, the second side opposing the first side;
• and
• forming a metal gate structure between the channel layers;
• after forming the GAA device, flipping the structure to remove a portion of substrate exposing the dummy dielectric material;
• removing the dummy dielectric material to form an air gap;
• and
• depositing an insulating material layer on the backside of the structure over the air gap.