Semiconductor device and method for manufacturing the same

A method of manufacturing making a semiconductor device having a combination structure of a horizontal oxide layer structure and a vertical oxide layer structure, the method comprising:

• a) etching from an upper surface of a semiconductor substrate to inside of the semiconductor substrate to form a trench;
• b) depositing an oxide in the trench to form a vertical oxide structure;
• c) etching the vertical oxide structure from an upper surface thereof to decrease a height of the vertical oxide structure to form the vertical oxide layer structure, and to make a top surface of the vertical oxide layer structure be below the upper surface of the semiconductor substrate, in order to expose side surfaces of the trench;
• d) forming, by an oxidation process, the horizontal oxide layer structure to cover a part of the upper surface of the semiconductor substrate and the upper surface of the vertical oxide layer structure;
• and
• e) wherein at a first side of the combination structure whereby the horizontal oxide layer structure extends horizontally beyond the vertical oxide layer structure, a junction between the horizontal oxide layer structure and the vertical oxide layer structure is smoother than a junction between the upper surface of the semiconductor substrate and the combination structure at a second side opposite the first side.