Isolation structure having different liners on upper and lower portions

An isolation structure for a substrate, the isolation structure comprising:

• a lower portion having a first U-shaped liner and a first dielectric material within the first U-shaped liner;
• and
• an upper portion over the lower portion, the upper portion having a second U-shaped liner and a second dielectric material within at least part of the second U-shaped liner,
• wherein a lower section of the second U-shaped liner contacts an upper surface of the first dielectric material.

A method, comprising:

• forming a first U-shaped liner within a trench extending through a semiconductor layer of a substrate;
• filling at least a portion of the trench with a first dielectric material;
• forming a second U-shaped liner in the trench over the first dielectric material in the trench and below an upper surface of the semiconductor layer;
• and
• filling at least part of the second U-shaped liner with a second dielectric material.