Method of forming isolation

A method of forming an isolation for isolating an active region at which a FET to be formed from other regions on a surface of a silicon substrate, comprising the steps of:

• forming a first underlaid oxide layer by oxidizing the silicon substrate;
• forming a silicon layer on the first underlaid oxide layer;
• forming a first silicon nitride layer on the silicon layer;
• forming a thickness-keeping oxide layer on the first silicon nitride layer;
• removing a portion of the silicon substrate at which an isolation is to be formed to a depth which regulates an encroaching length of bird's beak of the isolation and a drop of a threshold voltage of the FET to be formed by etching the thickness-keeping oxide layer, the first silicon nitride layer, the silicon layer, the first underlaid oxide layer and the silicon substrate, using a mask of a pattern whose aperture corresponds to the isolation region;
• oxidizing the silicon substrate and the silicon layer to form a second underlaid oxide layer on surfaces thereof;
• forming a second silicon nitride layer entirely over the substrate on which the second underlaid oxide layer is formed;
• anisotropically etching the second silicon nitride layer so as to remove at least the second silicon nitride layer in the isolation region and so as to leave silicon nitride side walls of more than 25 nm in thickness which are made of the second silicon nitride layer at a side part of the first silicon nitride layer, at a side part of the silicon layer, and at a side part of a step part of the silicon substrate;
• forming the oxide layer to be the isolation by selectively oxidizing the silicon substrate, using as a mask the first silicon nitride layer and the second silicon nitride layer
• removing the thickness-keeping oxide layer after the formation of the oxide layer to be the isolation;
• removing the first and second silicon nitride layers and the silicon layer;
• and
• removing the first underlaid oxide layer by etching.