Method for forming trench including a first and a second layer of photoresist

A method for forming a symmetric trench, comprising:

• providing a semiconductor substrate;
• forming a first inter-metal dielectric (IMD) layer on said semiconductor substrate;
• forming a mask layer on said first IMD layer;
• forming a second IMD layer on said mask layer;
• performing a photolithography process to transfer a photo mask pattern onto said second IMD layer;
• etching said second IMD layer according to the photo mask pattern to form a first opening inside said second IMD layer wherein said mask layer is used as an etching stop layer;
• forming a second photoresist layer on said first photoresist layer and inside said first opening sidewall;
• removing a portion of said second photoresist layer to expose a portion of said mask layer within the first opening;
• etching said mask exposed layer and said first IMD layer thereunder to form a second opening until said semiconductor substrate is exposed;
• and
• stripping simultaneously said first photoresist layer and said second photoresist layer to form a symmetric trench having said first opening and said second opening wherein a size of said first opening is larger than said size of said second opening.