Method and Structure for FinFET

A method of forming a semiconductor device, comprising:

• receiving a substrate having an active fin projecting upwardly through an isolation structure over the substrate, an oxide layer over the active fin, a dummy gate stack over the isolation structure and the oxide layer, and a spacer feature over the isolation structure and the oxide layer and on sidewalls of the dummy gate stack;
• removing the dummy gate stack thereby forming a first trench, wherein the first trench exposes the oxide layer;
• partially removing the oxide layer in the first trench, resulting in a cavity under the spacer feature and a portion of the oxide layer over the active fin;
• depositing a dielectric material in the first trench and in the cavity;
• etching in the first trench so as to expose the active fin, leaving a first portion of the dielectric material under the spacer feature;
• and
• forming a gate stack in the first trench, the gate stack engaging the active fin.

A semiconductor device, comprising:

• a substrate having an active fin projecting upwardly through an isolation structure over the substrate;
• a gate stack over the isolation structure and engaging the active fin;
• a silicon nitride layer over the active fin and adjacent to the gate stack;
• and
• a spacer feature over the isolation structure, over the silicon nitride layer, and on sidewalls of the gate stack.