Manufacturing method of semiconductor device

A manufacturing method of a semiconductor device, comprising:

• forming a first recess in a first region of a semiconductor substrate;
• forming a second recess in a second region of the semiconductor substrate, wherein a bottom surface of the first recess is lower than a bottom surface of the second recess in a vertical direction, and the first recess and the second recess are formed concurrently;
• forming a first gate oxide layer and a second gate oxide layer concurrently, wherein at least a portion of the first gate oxide layer is formed in the first recess, and at least a portion of the second gate oxide layer is formed in the second recess;
• performing a removing process for removing a part of the second gate oxide layer, wherein a thickness of the second gate oxide layer is less than a thickness of the first gate oxide layer after the removing process;
• forming a pad oxide layer on the semiconductor substrate before the first recess and the second recess are formed, wherein a first portion of the pad oxide layer is formed on the first region of the semiconductor substrate, a second portion of the pad oxide layer is formed on the second region of the semiconductor substrate, and a third portion of the pad oxide layer is formed on a third region of the semiconductor substrate;
• and
• removing the third portion of the pad oxide layer after the first gate oxide layer and the second gate oxide layer are formed and before the removing process.