Method for forming isolation layer in semiconductor device

A method for forming an isolation structure in a semiconductor device, the method comprising:

• forming a trench in a semiconductor substrate;
• forming a liner layer on an exposed surface of the trench, the liner layer comprising a liner nitride layer and a liner oxide layer;
• forming a flowable insulation layer to fill the trench and cover the liner nitride layer;
• recessing the flowable insulation layer to expose a portion of the liner nitride layer on an upper portion of the trench;
• loading the semiconductor substrate into a chamber after the recessing step;
• performing a first preheating process by supplying a first preheating gas to the chamber so as to release stress of the liner layer;
• performing a second preheating process by supplying a second preheating gas to the chamber so as to oxidize the exposed liner nitride layer;
• forming a buffer layer on the exposed liner nitride layer;
• etching the buffer layer by supplying an etch gas to the buffer layer so as to smoothen the exposed liner nitride layer;
• and
• depositing a buried insulation layer in the trench to foam an isolation structure.