Method for forming an isolation layer in a semiconductor device

A method for forming an isolation layer in a semiconductor device comprising the steps of:

• sequentially forming a pad oxide film and a first nitride film on a silicon substrate, and then forming a photoresist pattern on said first nitride film to define a field region;
• sequentially etching a portion of said first nitride film, said pad oxide film and said silicon substrate, using said photoresist pattern as an etch mask, to form a trench;
• removing said photoresist pattern, and then growing a sacrifice oxide film on the silicon substrate that is exposed at the interior of said trench;
• depositing a second nitride film on said sacrifice oxide film and said first nitride film, and then depositing a polysilicon layer on said second nitride film;
• etching said polysilicon layer to form a polysilicon spacer on said second nitride film of the inner wall of said trench;
• etching a portion of said second nitride film, using said polysilicon spacer as an etch mask, thereby forming a nitride spacer between said sacrifice oxide film and said polysilicon spacer;
• forming an isolation layer by oxidizing said polysilicon spacer and said exposed silicon substrate;
• and
• sequentially removing said first nitride film, said nitride spacer and said pad oxide film from said silicon substrate.