Fin isolation regions with improved depth distribution and methods forming the same

Abstract

A method includes forming a gate stack on a semiconductor region, wherein the semiconductor region is over a bulk semiconductor substrate. The gate stack is etched to form a first trench, wherein a plurality of protruding semiconductor fins are revealed to the first trench. The plurality of protruding semiconductor fins are etched to form a plurality of second trenches extending into the bulk semiconductor substrate. The plurality of second trenches are underlying and joined to the first trench. The plurality of second trenches include a first outmost trench having a first depth, a second outmost trench, and an inner trench between the first outmost trench and the second outmost trench. The inner trench has a second depth equal to or smaller than the first depth. A fin isolation region is formed to fill the first trench and the plurality of second trenches.