Photoresist structure, semiconductor device comprising the same, and method for fabricating the semiconductor device comprising the same

A method for fabricating a semiconductor device, comprising:

• providing a substrate, forming an under layer on the substrate, and forming a bottom photoresist layer on the under layer;
• forming a top photoresist layer on the bottom photoresist layer, wherein a coefficient of thermal expansion of the top photoresist layer is greater than a coefficient of thermal expansion of the bottom photoresist layer;
• performing an exposure process to form un-exposed portions and exposed portions of the bottom photoresist layer and the top photoresist layer;
• performing a post-exposure baking process to the bottom photoresist layer and the top photoresist layer;
• performing a developing process to remove the exposed portions of the bottom photoresist layer and the top photoresist layer and form a recess exposing the under layer;
• conformally forming a layer of spacer material on sidewalls of the top photoresist layer and the bottom photoresist layer, on a top surface of the top photoresist layer, and on a top surface of the under layer;
• performing a thermal treatment to expand the top photoresist layer;
• and
• performing an etching process to partially remove the layer of spacer material and form a spacer on the sidewalls of the top photoresist layer and the bottom photoresist layer.