Method of removing an oxide and method of filling a trench using the same

A method of filling a trench, the method comprising:

• forming at least one first trench to extend from an upper surface of a substrate in a downward direction;
• forming a first oxide layer conformably on the substrate, such that the first trench is partially filled with the first oxide layer;
• removing a portion of the first oxide layer by reacting a reaction gas including fluorine and nitrogen with the first oxide layer to form a partially etched first oxide layer, the partially etched first oxide layer defining at least one second trench;
• and
• forming a second oxide layer on the partially etched first oxide layer, such that the second trench is filled with the second oxide layer,
• wherein removing a portion of the first oxide layer includes forming first byproducts via a reaction between the first oxide layer and the reaction gas, reacting the first byproducts with the reaction gas to form second byproducts, and removing the second byproducts.