Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a substrate including a semiconductor fin;
• and
• a dummy fin structure spaced from the semiconductor fin and including:
• a first dielectric fin layer;
• a second dielectric fin layer above the first dielectric fin layer and having a material different from a material of the first dielectric fin layer;
• and
• a third dielectric fin layer above the second dielectric fin layer and having a dielectric constant higher than a dielectric constant of the first dielectric fin layer.

A semiconductor device comprising:

• a substrate including a semiconductor fin;
• a dummy fin structure above the substrate, spaced from the semiconductor fin, and having a width larger than a width of the semiconductor fin;
• and
• a source/drain feature grown on a top of the semiconductor fin.