Spacers for semiconductor devices including a backside power rails

A device comprising:

• a transistor structure;
• a front-side interconnect structure on a front-side of the transistor structure;
• and
• a backside interconnect structure on a backside of the transistor structure, the backside interconnect structure comprising:
• a first conductive line electrically coupled to a first source/drain region of the transistor structure by a first backside via;
• a first dielectric spacer contacting a side surface of the first conductive line;
• and
• an air gap adjacent the first dielectric spacer, wherein a side surface of the first dielectric spacer defines a first boundary of the air gap and a side surface of the first conductive line defines a second boundary of the air gap.

A method comprising:

• forming a first transistor on a first substrate;
• exposing a first epitaxial material, wherein exposing the first epitaxial material comprises thinning a backside of the first substrate;
• replacing the first epitaxial material with a first backside via, the first backside via being electrically coupled to a first source/drain region of the first transistor;
• forming a first dielectric layer over the first backside via;
• forming a first conductive line in the first dielectric layer over the first backside via, the first conductive line being electrically coupled to the first backside via;
• forming a first spacer over the first dielectric layer adjacent the first conductive line;
• removing the first dielectric layer to form a first recess exposing a sidewall of the first conductive line;
• and
• sealing the first recess to form an air spacer.