The present invention relates to a method of forming a contact hole on the semiconductor wafer. The semiconductor wafer comprises, in ascending order, a substrate, a silicon nitride layer, a silicon oxide layer, and a photo-resist layer. There is a hole in the photo-resist layer. The method comprises: (1) performing a first anisotropic etching process in a downward direction to remove the silicon oxide layer under the hole down to the surface of the silicon nitride layer to form a recess; (2) performing an in-situ plasma cleaning process to entirely remove the polymer material remaining at the bottom of the recess; (3) performing an in-situ second anisotropic etching process in a downward direction to remove the silicon nitride layer from the bottom of the recess down to the surface of the substrate to form the contact hole; (4) performing another in-situ cleaning process to entirely remove the polymer material remaining at the bottom of the contact hole.