A semiconductor fabrication method is provided for forming an opening in a dielectric layer, which can help downsize the critical dimension of the resulting opening through the use of a photoresist layer with silylated sidewall spacers. By this method, the first step is to coat a base photoresist layer over the dielectric layer. Next, a photolithographic process is performed to remove a selected part of the base photoresist layer. Then, a conformational coating process is performed to coat a silylatable photoresist layer over the base photoresist layer to a controlled predefined thickness. Subsequently, a silylation process is performed on the silylatable photoresist layer so as to form a silylated photoresist layer over all the exposed surfaces of the base photoresist layer. After this, a first etching process is performed on the silylated photoresist layer, with the remaining portions of the silylated photoresist layer serving as silylated sidewall spacers on the base photoresist layer. Then, with the combined structure of the base photoresist layer and the overlying silylated sidewall spacers serving as mask, a second etching process is performed on the dielectric layer to etch away the unmasked part of the dielectric layer to form the intended opening in the dielectric layer.