Semiconductor device comprising a gradually increasing field dielectric layer and method of manufacturing a semiconductor device

A method of manufacturing a semiconductor device, the method comprising:

• forming a trench in a semiconductor substrate;
• forming an oxide layer over sidewalls and over a bottom side of the trench;
• performing an ion implantation process so as to damage the oxide layer disposed over the sidewalls and the bottom side of the trench;
• forming a covering layer in the trench over the damaged oxide layer;
• patterning the covering layer in the trench, thereby forming an uncovered area of the damaged oxide layer at a first area in the trench and a covered area of the damaged oxide layer at a second area in the trench, wherein the covered area of the damaged oxide layer is covered by a remaining portion of the covering layer;
• performing an isotropic etching process thereby removing the uncovered area of the damaged oxide layer in the trench and removing a portion of the covered area of the damaged oxide layer in the trench located adjacent to the uncovered area of the damaged oxide layer, wherein the removed portion of the covered area of the damaged oxide layer is located underneath the remaining portion of the covering layer;
• and
• removing the remaining portion of the covering layer.