Abstract: A semiconductor device having a shallow trench isolation (STI) structure, which reduces leakage current between adjacent P-FETs, and a manufacturing method thereof. The device comprises a semiconductor substrate having first and second trenches, the first trench being formed in a cell area; a first sidewall oxide layer formed on inner surfaces of the first and second trenches; a second sidewall oxide layer formed on a surface of the first sidewall oxide layer in the second trench; a first relief liner formed on the first sidewall oxide layer in the first trench; a second relief liner formed on the first relief liner in the first trench, and also formed on the second sidewall oxide layer in the second trench; and a dielectric material formed within the first and second trenches.