Fabrication of FinFETs with multiple fin heights

A semiconductor structure comprising:

• a semiconductor substrate;
• a first trench in the semiconductor substrate, wherein the first trench encircles a first semiconductor fin;
• a first oxide region underlying the first trench;
• a second trench in the semiconductor substrate, wherein the second trench encircles a second semiconductor fin, and wherein a bottom surface of the second trench is lower than a bottom surface of the first trench;
• and
• a second oxide region underlying the second trench.

A semiconductor chip comprising:

• a semiconductor substrate having a top surface;
• a first semiconductor strip in the semiconductor substrate;
• a second semiconductor strip in the semiconductor substrate, wherein top surfaces of the first and the second semiconductor strips are level with the top surface of the semiconductor substrate;
• a first insulating region encircling a bottom portion of the first semiconductor strip, wherein a first top edge of the first insulating region is recessed from the top surface of the semiconductor substrate by a first distance;
• and
• a second insulating region encircling a bottom portion of the second semiconductor strip, wherein a second top edge of the second insulating region is recessed from the top surface of the semiconductor substrate by a second distance greater than the first distance.