An trench isolation structure and method for manufacturing the trench isolation structure are disclosed. An exemplary trench isolation structure includes a first portion and a second portion. The first portion extends from a surface of a semiconductor substrate to a first depth in the semiconductor substrate, and has a width that tapers from a first width at the surface of the semiconductor substrate to a second width at the first depth, the first width being greater than the second width. The second portion extends from the first depth to a second depth in the semiconductor substrate, and has substantially the second width from the first depth to the second depth.