of the oxide layer 500 this condition can make the sharp corners which are formed by using traditional technology on both sides of the oxide layer 500 be pared off gradually by the increased probability of the molecule collision and the increased etching rate of the sharp corner place therefore the present invention can improve the characteristic of the film in the follow up process referring to fig1 after the oxide layer 500 which is over deposited on the nitride oxide layer 300 keeping the etching rate of the second step etching process to remove the nitride oxide layer 400 in other words the etching rate of nitride is still higher than the etching rate of oxide referring to fig1 because the nitride oxide layer 400 is not exist on the nitride layer 300 in the follow up removing nitride layer 300 process the over etching process will be proceeded enough to remove the nitride oxide layer 400 completely in the process if part of the nitride oxide layer remains on the nitride layer the nitride layer will not be removed easily and will retain on the silicon layer 200 this over etching process will also decrease the thickness of the oxide layer 500 to affect the ability of the electrically insulating layer in resisting the diffusion ions in the follow up process but the thickness of the oxide layer 500 which is fabricated by.