Method for forming contact hole

A method for forming contact holes, comprising:

• providing a substrate, wherein the substrate includes a dense region and an isolation region;
• forming a material layer on the substrate;
• forming a plurality of sacrificial patterns on the material layer in the dense region, wherein there is a first opening between two adjacent sacrificial patterns;
• forming a spacer on each of two sides of each of the sacrificial patterns, wherein the spacers are separated from each other;
• removing the sacrificial patterns to form a second opening between two adjacent spacers;
• forming a planar layer filling up the second openings;
• forming a first straight narrow opening in the planar layer, wherein the first straight narrow opening exposes a portion of the material layer under the second openings, and the step of forming the first straight narrow opening in the planar layer comprises:
• forming a first patterned photoresist layer on the planar layer, wherein the first patterned photoresist layer has a second straight narrow opening and a fourth opening, the second straight narrow opening exposes a portion of the planar layer in the dense region and the fourth opening exposes a portion of the planar layer in the isolation region;
• and
• removing the portions of the planar layer exposed by the second straight narrow opening and the fourth opening by using the first patterned photoresist layer as a mask so as to form the first straight narrow opening in the planar layer in the dense region and to form a fifth opening in the planar layer in the isolation region;
• and
• removing the portion of the material layer exposed by the first straight narrow opening to form a plurality of third openings in the material layer.