In some examples, a method includes etching a substrate to form a trench, wherein the trench includes sidewalls. The method further includes forming a first isolation region in the trench by growing a first layer of a first thickness on the sidewalls using a dry oxidation technique and depositing a second layer to fill a portion of the trench, the second layer contacting the first layer. The method further includes etching third and fourth layers atop the substrate to expose a first portion of the substrate. The method further includes growing a second isolation region in the substrate through the first portion by using a dry-wet-dry oxidation technique.