Methods of forming integrated circuit devices include forming a gate electrode on a substrate and forming a nitride layer on a sidewall and upper surface of the gate electrode. The nitride layer is then anisotropically oxidized under conditions that cause a first portion of the nitride layer extending on the upper surface of the gate electrode to be more heavily oxidized relative to a second portion of the nitride layer extending on the sidewall of the gate electrode. A ratio of a thickness of an oxidized first portion of the nitride layer relative to a thickness of an oxidized second portion of the nitride layer may be in a range from about 3:1 to about 7:1.