STI forming method for improving STI step uniformity

An STI forming method for improving STI step uniformity in a semiconductor device, said method comprising the steps of:

• providing a substrate;
• forming a pad oxide layer on said substrate;
• forming a pad nitride layer on said substrate, so as to constitute an intermediate structure including the substrate, pad oxide layer and pad nitride layer;
• forming shallow trench isolations in said intermediate structure, said shallow trench isolations having different depths;
• forming an oxide layer on the whole structure, the shallow trench isolations being filled with said oxide layer, causing the portions of the oxide layer at the shallow trench isolations having different depths to have different heights;
• forming a planarization material layer on said oxide layer to offset said different heights such that said planarization material layer has a substantially planar top surface;
• and
• performing planarization process to remove said planarization material layer and planarize the top surfaces of said oxide layer and said pad nitride layer.