| CPC H10D 64/679 (2025.01) [H01L 21/28088 (2013.01); H10D 30/024 (2025.01); H10D 64/015 (2025.01); H10D 64/017 (2025.01); H10D 64/021 (2025.01); H01L 21/0206 (2013.01); H01L 21/02164 (2013.01); H01L 21/0217 (2013.01); H01L 21/02236 (2013.01); H01L 21/02255 (2013.01); H01L 21/02271 (2013.01); H01L 21/02274 (2013.01); H01L 21/0274 (2013.01); H01L 21/0332 (2013.01); H01L 21/0337 (2013.01); H01L 21/31053 (2013.01); H01L 21/31111 (2013.01); H01L 21/3212 (2013.01); H01L 21/76224 (2013.01); H10D 64/667 (2025.01)] | 20 Claims |

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11. A device, comprising:
a semiconductor fin extending from a substrate;
a gate structure extending across the semiconductor fin; and
a multilayer gate spacer abutting a sidewall of the gate structure, the multilayer gate spacer comprising:
a first spacer layer in contact with the sidewall of the gate structure;
a second spacer layer in contact with a sidewall of the first spacer layer oriented away from the gate structure;
a third spacer layer in contact with a sidewall of the second spacer layer oriented away from the second spacer layer; and
an air-filled region atop the third spacer layer;
a source/drain region over the semiconductor fin; and
a contact etch stop layer over the source/drain region, wherein the contact etch stop layer defines a vertical boundary of the air-filled region, and the contact etch stop layer has a top end not lower than a top end of the first spacer layer.
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