In an embodiment, a method includes forming a plurality of semiconductor fins over a substrate, the plurality of semiconductor fins comprising a first fin, a second fin, a third fin, and a fourth fin; forming a first dielectric layer over the plurality of semiconductor fins, the first dielectric layer filling an entirety of a first trench between the first fin and the second fin; forming a second dielectric layer over the first dielectric layer, the second dielectric layer filling an entirety of a second trench between the second fin and the third fin, the forming the second dielectric layer comprising: forming an oxynitride layer; and forming an oxide layer; and forming a third dielectric layer over the second dielectric layer, the third dielectric layer filling an entirety of a third trench between the third fin and the fourth fin.