High aspect ratio gate structure formation

A method, comprising:

• forming a fin over a substrate;
• depositing a mandrel layer over the fin;
• patterning the mandrel layer to form a first mandrel and a second mandrel across the fin, wherein in a cross section parallel to a top surface of the substrate and through a middle portion of the fin, each of the first mandrel and the second mandrel has a first width in proximity to the fin and a second width distal to the fin, and the second width is smaller than the first width;
• forming a sacrificial feature between the first mandrel and the second mandrel, wherein in the cross section, the sacrificial feature has a third width in proximity to the fin and a fourth width distal to the fin, and the fourth width is larger than the third width;
• removing the first mandrel and the second mandrel to form a first trench;
• depositing a dielectric layer in the first trench;
• removing the sacrificial feature to form a second trench;
• and
• forming a metal gate stack in the second trench.

A method, comprising:

• providing a structure having a substrate and a fin protruding from the substrate;
• forming mandrel patterns over the fin, wherein the mandrel patterns include a first footprint that has a protruding profile;
• depositing dummy gates in openings between the mandrel patterns, wherein the dummy gates include a second footprint that has a notch profile;
• removing the mandrel patterns to form trenches;
• growing epitaxial features in the trenches;
• and
• replacing the dummy gates with metal gate stacks.