Shallow trench isolation with improved structure and method of forming

A method for forming a semiconductor structure, the method comprising:

• forming a recess in a semiconductor substrate, the recess having a first width at a top surface of the semiconductor substrate, the recess narrowing from the top surface of the semiconductor substrate to a depth below the top surface of the substrate, the recess having a second width at the depth, the recess having a lower surface at the depth;
• forming a widened recessed portion through the lower surface of the recess, the widened recessed portion having a third width that is greater than the second width;
• and
• forming an isolation material in the recess and the widened recessed portion, the isolation material having a uniform composition from a first semiconductor sidewall of the recess to a second semiconductor sidewall of the recess.

A method for forming a semiconductor structure, the method comprising:

• forming a first isolation structure in a first region of a semiconductor substrate, the forming the first isolation structure comprising:
• forming a first recess to a first depth in the first region of the semiconductor substrate, the first recess having a first lower surface at the first depth, the first recess having a first width at the first lower surface;
• forming a first widened recessed portion through the first lower surface, the first widened recessed portion having a second width greater than the first width;
• and
• forming a first isolation material in the first recess and the first widened recessed portion;
• and
• forming a second isolation structure in a second region of the semiconductor substrate, the forming the second isolation structure comprising:
• forming a second recess to a second depth in the second region of the semiconductor substrate, the second recess having a second lower surface at the second depth, the second recess having a third width at the second lower surface;
• forming a second widened recessed portion through the second lower surface, the second widened recessed portion having a fourth width greater than the third width;
• and
• forming a second isolation material in the second recess and the second widened recessed portion;
• wherein at least one of:
• (i) the first depth being different from the second depth, (ii) the second width being different from the fourth width, (iii) an aspect ratio of the first isolation structure being different from an aspect ratio of the second isolation structure, and (iv) a combination thereof.