Method and apparatus for determining photomask alignment

Abstract

This describes a method for electrically evaluating the overlay error of a photolithographic tool. In this process a reusable substrate bearing a fixed reference mark has a photolithographic tool defined metal liftoff pattern formed thereon to provide a pair of conductive lines by measuring the relative resistance of the lines with respect to one another the alignment of the tool defined pattern with regard to the reference mark may be determined and thus the overlay error of the tool established. The reference mask is formed of an L-shaped recess in a substrate so that when a metal structure is deposited on the surface of the substrate across the mask it will be made discontinuous by the mark. To form the metal structure photoresist is applied to the substrate and is exposed and developed in a L-shaped liftoff mask pattern larger than the reference mark and arranged on both sides of the mask so that when a conductive layer is deposited over the exposed and developed photoresist the remaining photoresist and overlying conductor subsequently removed, there will remain on the substrate surface a pair of conductive lines. One reference edge of this conductive line is defined by the recess forming the reference mask and the second edge is defined by the lifting off of the developed photoresist created by the photo tool defined liftoff pattern. By measuring the resistance of predetermined segments of the deposited metal remaining on the substrate the difference in line width caused by misalignment of the photo tool with respect to the reference mask can be determined. Any such difference in linewidth represents the overlay mismatch caused by the tool. After measurements are completed the conductive lines can be stripped from the substrate and the substrate reused.