Forming a fin using double trench epitaxy

A structure comprising:

• an epitaxial semiconductor layer above a first dielectric layer on a substrate, a portion of the epitaxial semiconductor layer located in a trench in the first dielectric layer is in direct contact with a top surface of the substrate;
• epitaxial semiconductor fins extending upward from the epitaxial semiconductor layer;
• and
• a second dielectric layer on the epitaxial semiconductor layer and between the epitaxial semiconductor fins, a first portion of the second dielectric layer is directly above and aligned to the trench such that a width of the first portion of the second dielectric layer is greater than a width of the trench in the first dielectric layer, an upper surface of the first portion of the second dielectric layer is substantially flush with upper surfaces of the epitaxial semiconductor fins, and upper surfaces of second portions of the second dielectric layer are below the upper surfaces of the epitaxial semiconductor fins.