Finfet device having flat-top epitaxial features and method of making the same

A method of forming a semiconductor device, comprising:

• forming first and second fins extending from a substrate;
• depositing an isolation structure over the substrate and on sidewalls of the first and second fins;
• etching the first and second fins to form first and second trenches, respectively;
• growing first epitaxial features from the first and second trenches;
• growing second epitaxial features over the first epitaxial features in a first growth condition using a first etching gas at a first flow rate and a deposition gas, wherein the second epitaxial features merge and form a merged portion having a top surface with a dip;
• after the second epitaxial features merge, switching to a second growth condition by reducing the first flow rate of the first etching gas to a second flow rate lower than the first flow rate, wherein the second growth condition provides a growth rate inside the dip that is higher than a growth rate outside the dip;
• and
• continuing the growing of the second epitaxial features in the second growth condition until the dip is filled and the merged portion has a flat top surface.

A method of forming a semiconductor device, comprising:

• providing a substrate, an isolation structure over the substrate, and first and second fins extending from the substrate and through the isolation structure;
• forming a dielectric layer on portions of sidewalls of the first and second fins;
• etching the first and second fins to form first and second trenches, respectively;
• growing first epitaxial features in the first and second trenches;
• growing second epitaxial features over the first epitaxial features in a first growth condition until the second epitaxial features merge into a merged portion having a top surface with a dip, wherein a lower portion of the second epitaxial features extends into each of the first and second trenches and is sandwiched by the first epitaxial features in a cross section perpendicular to a lengthwise direction of the first and second fins;
• after the merged portion reaches a target critical dimension, changing the first growth condition to a second growth condition that differs from the first growth condition in at least one of a deposition precursor and an etching gas, wherein the second growth condition grows the second epitaxial features inside the dip and outside the dip simultaneously at a growth rate inside the dip higher than a growth rate outside the dip;
• and
• continuing the growing of the second epitaxial features in the second growth condition until the dip is filled and the merged portion has a flat top surface that is parallel to a top surface of the substrate.