Method of forming isolation film in semiconductor device

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

• sequentially forming a tunnel oxide film, a conductive film and a hard mask film on a semiconductor substrate;
• forming a photoresist pattern on the hard mask film, through which an isolation region is opened;
• forming a trench having a sidewall by etching the hard mask film, the conductive film, the tunnel oxide film and the semiconductor substrate by an etch process using the photoresist pattern as an etch mask;
• stripping the photoresist pattern;
• performing a first cleaning process for removing a polymer film, which is generated during the etch process for forming the trench, on a sidewall of the trench to improve the stabilization of the roughness of the sidewall;
• performing a second cleaning process to uniformly form a sidewall oxide film through subsequent processes after the first cleaning process;
• performing a sidewall oxidation process to form a sidewall oxide film on the sidewall of the tunnel oxide film, conductive film and within the trench;
• and
• depositing a field oxide film on the entire structure, performing a polishing process using the hard mask film as a stop layer and then stripping the hard mask film to form the isolation film.