A method of manufacturing a spacer for a substrate having a gate structure formed thereon. The method comprises steps of forming a first oxide layer over the substrate and forming a nitride layer on the first oxide layer. A first asymmetric etching process is performed to remove a portion of the nitride layer until a portion of a top surface of the first oxide layer is exposed. A second asymmetric etching process is performed to remove a portion of the first oxide layer by using the remaining nitride layer as a mask until about 50% to 90% portion of the first oxide layer is removed. A quick wet etching process is performed to remove a portion of the remaining first oxide located on the top of the gate structure and on the substrate.