Method For Forming Gate Oxide

A method for forming a gate oxide film for a transistor, comprising a plurality of steps:

• step 1:
• forming a hard mask layer on a surface of a semiconductor substrate, sequentially etching the hard mask layer and the semiconductor substrate in areas of shallow trenches, and forming an active region in the semiconductor substrate between two adjacent ones of the shallow trenches, and wherein portions of the hard mask layer remain on a surface of the active region;
• step 2:
• performing an tilt-angle ion implantation into an upper area of side surfaces of each of the shallow trenches to form an upper doped region, wherein the upper doped region is located on an edge of an outer side of the active region;
• step 3:
• filling a field oxide layer into the shallow trenches and removing the hard mask layer;
• and
• step 4:
• performing thermal oxidation to form the gate oxide film on the surface of the active region, wherein ions implanted into the upper doped region increase a thermal oxidation rate in the upper doped region, resulting a larger gate oxide film thickness on the surface of the upper doped region than a gate oxide film thickness on the surface of the active region, and wherein the gate oxide film in the upper doped region has a rounded morphology.