A thermal oxidation process is used to fill trenches with an oxide; however, the oxidation process consumes some of the silicon. The embodiments herein advantageously apply this tendency for the oxidation process to consume silicon so as to convert all the silicon substrate material between the multiple trenches into an oxide. Therefore, because all of the silicon between the multiple trenches is consumed by the oxidation process, the multiple smaller trenches are combined into a single larger trench filled with the oxide.