Gate Isolation Regions and Fin Isolation Regions and Method Forming the Same

A method includes forming a gate stack on a semiconductor region, etching the gate stack to form a first trench separating the gate stack into a first gate stack portion and a second gate stack portion, and forming a gate isolation region filling the first trench. The gate isolation region includes a silicon nitride liner, and a silicon oxide filling-region overlapping a first bottom portion of the silicon nitride liner. The method further includes etching the gate stack to form a second trench and to reveal a protruding semiconductor fin, and etching the protruding semiconductor fin to extend the second trench into the bulk semiconductor substrate. A fin isolation region is formed to fill the second trench. The fin isolation region includes a silicon oxide liner, and a silicon nitride filling-region overlapping a second bottom portion of the silicon oxide liner.