Method for fabricating high density pillar structures by double patterning using positive photoresist

A method of making a semiconductor device, comprising:

• forming a first photoresist layer over an underlying layer;
• patterning the first photoresist layer into a first photoresist pattern, wherein the first photoresist pattern comprises a plurality of spaced apart first photoresist features located over the underlying layer;
• etching the underlying layer using the first photoresist pattern as a mask to form a plurality of first spaced apart features;
• removing the first photoresist pattern;
• forming a second photoresist layer over the plurality of first spaced apart features;
• patterning the second photoresist layer into a second photoresist pattern, wherein the second photoresist pattern comprises a plurality of second photoresist features covering edge portions of the plurality of first spaced apart features and wherein the plurality of second photoresist features are arranged over the plurality of first spaced apart features such that three second photoresist features form an equilateral triangle covering three edge portions of each first spaced apart feature;
• etching exposed portions of the plurality of first spaced apart features using the second photoresist pattern as a mask, such that a plurality of spaced apart edge portions of the plurality of first spaced apart features remain;
• and
• removing the second photoresist pattern.