Structure of isolation feature of semiconductor device structure

A semiconductor device structure, comprising:

• a first isolation feature formed over a semiconductor substrate with a fin structure, wherein the fin structure is adjacent to and protrudes above the first isolation feature;
• a first source or drain structure and a second S/D structure extending into the fin structure by a first depth from a top surface of the fin structure and separated from each other by a distance;
• a second isolation feature extending into the fin structure between the first S/D structure and the second S/D structure by a second depth from a top surface of the fin structure;
• and
• a gate stack covering the first isolation feature and the second isolation feature,
• wherein the second isolation feature has a thickness substantially equal to a thickness of the fin structure and a width less than the distance, and wherein the second depth is greater than the first depth.

A semiconductor device structure, comprising:

• a dummy gate stack and first and second active gate stacks formed over a fin structure of a semiconductor substrate, wherein the dummy gate stack is between the first and second active gate stacks;
• a first source or drain (S/D) structure formed in the fin structure between the first active gate stack and the dummy gate stack;
• a second S/D structure formed in the fin structure between the second active gate stack and the dummy gate stack;
• a first isolation feature formed in the fin structure between the first S/D structure and the second S/D structure, wherein the first isolation feature has a first sidewall surface that is aligned with a first sidewall surface of the fin structure between the first S/D structure and the second S/D structure;
• and
• a second isolation feature formed on the semiconductor substrate to cover the first sidewall surface of the first isolation feature and the first sidewall surface of the fin structure between the first S/D structure and the second S/D structure.