Semiconductor device with air-gap spacers

A method of forming a semiconductor device, comprising:

• forming a fin protruding from a substrate;
• forming a gate stack over the fin;
• forming a seal spacer on a sidewall of the gate stack;
• depositing a sacrificial spacer layer on a sidewall of the seal spacer and on a top surface of the fin;
• depositing a first dielectric layer over the sacrificial spacer layer;
• depositing a second dielectric layer over the first dielectric layer;
• etching the second dielectric layer, the first dielectric layer, and the sacrificial spacer layer, thereby forming a trench exposing a bottom portion of the sacrificial spacer layer;
• removing the sacrificial spacer layer by applying an etchant in the trench, thereby forming an air gap with an opening at a bottom portion of the trench;
• and
• depositing a third dielectric layer in the trench, thereby sealing the opening of the air gap.

A method of forming a semiconductor device, comprising:

• forming a fin protruding from a substrate;
• forming a sacrificial gate stack across the fin;
• forming a gate spacer layer on sidewalls of the sacrificial gate stack;
• growing an epitaxial feature adjacent the gate spacer layer;
• after the growing of the epitaxial feature, thinning the gate spacer layer;
• depositing a sacrificial layer on sidewalls of the gate spacer layer and on a top surface of the epitaxial feature;
• depositing a first dielectric layer over the sacrificial spacer layer;
• replacing the sacrificial gate stack with a metal gate stack;
• etching the first dielectric layer and the sacrificial layer to expose a sidewall of the sacrificial layer;
• removing the sacrificial layer to form an air gap under the first dielectric layer;
• and
• depositing a second dielectric layer on sidewalls of the first dielectric layer, wherein the second dielectric layer seals the air gap.