FABRICATION OF SEMICONDUCTOR DEVICE

PROBLEM TO BE SOLVED: To enhance breakdown voltage of gate and the like, by absorbing defective matters through oxidation after cleaning and depositing an oxide in a second element region after removing the oxide thereby preventing the oxide from becoming thin without increasing the number of fabrication steps. SOLUTION: Silicon nitride possibly produced during selective oxidation using an anti-oxidation mask is absorbed through oxidation after cleaning. Furthermore, when an oxide 10 is grown at a bipolar transistor part BT using a second anti-oxidation mask, silicon nitride produced at an MOS transistor part MT is absorbed through oxidation after cleaning and then a gate oxide 35 is grown at the MOS transistor part MT. Since the silicon nitride 31 and the thin part have been removed entirely at the time of growing the gate oxide 35, the gate oxide 35 can be grown uniformly with a desired thickness without producing a thin part.