Method for forming a field oxide layer of a semiconductor integrated circuit device

A method for forming a field oxide layer of a semiconductor integrated circuit device comprising the steps of:

• thermally growing a pad oxide layer on a semiconductor substrate, and sequentially depositing a polysilicon layer and a nitride layer on said pad oxide layer;
• removing said nitride layer over a field region to define active regions adjacent said field region;
• forming spacers on the side walls of the remaining portions on said nitride layer;
• doping an impurity into said substrate in said field region using said spacers as a mask;
• growing a field oxide layer by thermally oxidizing the exposed portion of said substrate in said field region;
• depositing an insulating layer over the whole surface of the resultant structure;
• planarizing the upper portion of said field oxide layer by anisotropically etching said insulating layer;
• removing said remaining nitride layer and polysilicon layer;
• and
• improving the step coverage of said field oxide layer by anisotropically etching the whole surface of the resultant structure.