Method for forming trench isolation having sidewall oxide films of different thickness

PURPOSE: A method for forming the trench isolation having the sidewall oxide films of different thickness is provided to have the thickness different each other at an isolation area of the N-channel and the P-channel MOS transistor in order not to surround the holes around the trench isolation area of the P-channel MOS transistor for a compensation type MOS transistor. CONSTITUTION: The first and the second trench(131,132) are formed on the first and the second isolation area(I,II) separated each other. A silicon film including the nitrogen in a side part and a bottom part of the second trench is formed by implanting the nitrogen into the second trench. The first sidewall oxide film(161) having the first thickness and the second sidewall oxide film(162) having the second thickness thinner than the first thickness are formed on the first trench by carrying out an oxidation process. A stress buffering liner(170) is formed on the surfaces of the first and the second sidewall oxide film. The inside of the first and the second trench is buried with an insulation material.