A method of forming a low-k layer includes forming a layer by providing a silicon source, a carbon source, an oxygen source, and a nitrogen source onto a substrate. The forming of the layer includes a plurality of main cycles, and each of the main cycles includes providing the silicon source, providing the carbon source, providing the oxygen source, and providing the nitrogen source, each of which is performed at least one time. Each of the main cycles includes sub-cycles in which the providing of the carbon source and the providing of the oxygen source are alternately performed.