Fin isolation structure for finfet and method of forming the same

A semiconductor device structure, comprising:

• a substrate having a fin structure protruding from the substrate, wherein the fin structure comprises a first fin, a second fin adjacent to and spaced apart from the first fin by a first distance and a third fin adjacent to and spaced apart from the second fin by a second distance that is different from the first distance;
• an isolation feature between and adjacent to the first fin and the second fin, and between and adjacent to the second fin and the third fin;
• a first insulating layer partially embedded in the isolation feature between the first fin and the second fin and between the second fin and the third fin;
• a second insulating layer having sidewall surfaces and a bottom surface that are covered by the first insulating layer between the first fin and the second fin;
• and
• a dual capping structure covering the second insulating layer and having a top surface that is substantially level with the top surface of the fin structure.

A method of forming a semiconductor device structure, comprising:

• patterning a substrate to form adjacent first and second fins and a first opening between the first and second fins;
• forming an insulating liner to cover the first and second fins and the first opening;
• forming a first insulating layer over the insulating liner to cover the first and second fins and the first opening and form a second opening between the first and second fins;
• filling a lower portion of the second opening with a second insulating layer;
• forming a first capping layer over the first insulating layer and the second insulating layer to cover the first and second fins and an upper portion of the second opening and form a third opening between the first and second fins, wherein the bottom of the third opening is lower than top surfaces of the first and second fins;
• forming a second capping layer over the first capping layer to cover the first and second fins and fill the third opening;
• successively polishing the second capping layer, the first capping layer, the first insulating layer, and the insulating liner until the first and second fins are exposed;
• and
• recessing the insulating liner after the polishing, so as to form an isolation feature.