Profile control of isolation structures in semiconductor devices

A method, comprising:

• forming first and second fin bases on a substrate;
• forming first and second nanostructured layers on the first and second fin bases, respectively;
• forming a first isolation structure between the first and second fin bases;
• forming a second isolation structure on the first isolation structure and between the first and second nanostructured layers;
• forming a polysilicon structure on the first and second nanostructured layers;
• and
• forming a third isolation structure, comprising:
• forming first and second isolation portions of the third isolation structure in the substrate;
• forming third and fourth isolation portions of the third isolation structure on opposite sides of the first isolation structure;
• forming fifth and sixth isolation portions of the third isolation structure on opposite sides of the second isolation structure;
• and
• forming a seventh isolation portion of the third isolation structure in the polysilicon structure.

A semiconductor device, comprising:

• a substrate;
• first and second fin bases disposed on the substrate;
• first and second nanostructured layers disposed on the first and second fin bases, respectively;
• first and second gate structures surrounding the first and second nanostructured layers, respectively;
• first and second shallow trench isolation (STI) regions disposed on opposite sides of the first and second fin bases;
• first and second isolation structures disposed on the first and second STI regions, respectively;
• and
• a third isolation structure, comprising:
• a first isolation portion disposed between the first and second gate structures;
• a second isolation portion disposed between and in contact with the first and second isolation structures;
• a third isolation portion disposed between and in contact with the first and second STI regions;
• and
• a fourth isolation portion disposed in the substrate, wherein the first isolation portion is wider than the second and third isolation portions.