Abstract: A method of forming a dielectric layer using a plasma enhanced atomic layer deposition technique includes: loading a semiconductor substrate having a three-dimensional structure into a reaction chamber; and repeatedly performing the following steps until a dielectric layer with a desired thickness is formed: supplying a source gas into the reaction chamber; stopping the supply of the source gas and purging the source gas remaining inside the reaction chamber; and supplying oxygen gas into the reaction chamber after purging the source gas, and applying RF power for oxygen plasma treatment, a level of the applied RF power and a partial pressure of the oxygen gas being increased concurrently with an increased aspect ratio of the three-dimensional structure.