Darkfield imaging for enhancing optical detection of edges and minimum features

A method for optically measuring lithographic process bias of a minimum feature formed by a lithographic process, the method comprising:

• a) creating by the lithographic process an array of elements formed in a pattern on a substrate, the elements having spaces therebetween, each element in the array having a nominal length and a nominal width and each space having a nominal width, the nominal width of each element and of each space corresponding to the minimum feature and the nominal length being larger than the minimum feature;
• b) measuring a created length of the lithographically created elements in the array by:
• i) generating a darkfield optical image of the array of elements;
• ii) detecting the darkfield optical image with an optical detector;
• iii) generating electronic information corresponding to the optical image;
• and
• iv) processing the electronic information to determine the created length of the elements;
• c) calculating a difference between the element created length and the nominal length;
• and
• d) determining lithographic process bias of the minimum feature from the difference between the created length and the nominal length.