A method of changing a formation rate of silicon oxide includes providing a substrate, wherein two conductive lines are disposed on the substrate and a recess is between the conductive lines. Later, a cleaning process is performed to clean the substrate and the conductive lines using diluted hydrofluoric acid. After the cleaning process, a silicon oxide layer is formed to cover a sidewall and a bottom of the recess, wherein a formation rate of the silicon oxide layer at the bottom of the recess is greater than a formation rate of the silicon oxide layer at the sidewall of the recess.