Air gap formation between gate spacer and epitaxy structure

A device, comprising:

• a fin structure extending from a substrate;
• a gate structure extending across the fin structure;
• a first source/drain epitaxy structure and a second source/drain epitaxy structure on opposite sides of the gate structure, respectively;
• a first spacer layer spacing the first source/drain epitaxy structure apart from the gate structure, the first spacer layer and an inner sidewall of the first source/drain epitaxy structure defining a first air gap therebetween;
• a second spacer layer over the first source/drain epitaxy structure and sealing the first air gap;
• and
• a first dielectric residue in the first air gap, wherein the first dielectric residue forms an interface with a lower portion of the inner sidewall of the first source/drain epitaxy structure, and the interface is more linear than an upper portion of the inner sidewall of the first source/drain epitaxy structure.

A device, comprising:

• an n-type field-effect transistor (NFET) on a substrate, the NFET comprising a first n-type epitaxy structure, a second n-type epitaxy structure, and an NFET gate structure laterally between the first and second n-type epitaxy structures, wherein the NFET gate structure is separated from an inner sidewall of the first n-type epitaxy structure at least by a first unfilled gap;
• and
• a p-type field-effect transistor (PFET) on the substrate, the PFET comprising a first p-type epitaxy structure, a second p-type epitaxy structure, and a PFET gate structure laterally between the first and second p-type epitaxy structures, wherein the PFET gate structure is separated from an inner sidewall of the first p-type epitaxy structure at least by a second unfilled gap,
• wherein the inner sidewall of the first n-type epitaxy structure has a first residue-free region exposed in the first unfilled gap and a first residue-containing region below the first residue-free region, the inner sidewall of the first p-type epitaxy structure has a second residue-free region exposed in the second unfilled gap and a second residue-containing region below the second residue-free region, and the second residue-free region of the inner sidewall of the first p-type epitaxy structure is more linear than the first residue-free region of the inner sidewall of the first n-type epitaxy structure.