Isolation layer having a bilayer structure for a semiconductor device and method for forming the same

A method for forming an isolation layer of a semiconductor device, comprising the steps of:

• defining a trench in a semiconductor substrate;
• forming a first liner nitride layer on the semiconductor substrate including a surface of the trench;
• forming a flowable insulation layer on the first liner nitride layer to fill the trench;
• removing a portion of the flowable insulation layer and a portion of the first liner nitride layer formed on the semiconductor substrate;
• removing a partial thickness of the flowable insulation layer to form a recess in the trench;
• forming a second liner nitride layer on the semiconductor substrate and in the trench on the flowable insulation layer and the first liner nitride layer;
• forming an insulation layer on the second liner nitride layer to completely fill the trench;
• and
• removing the portion of the insulation layer formed on the semiconductor substrate and over the trench.