The present invention provides a new and improved overlay vernier that can increase the overlay measurement accuracy. The overlay vernier of the present invention comprises an inner square vernier and an outer square vernier. The outer vernier comprises a central square opening and four trapezoid-shaped openings surrounding the central square opening.
A method for wafer backside alignment overlay accuracy includes forming a buried layer on a front-side of a wafer; forming a conductive layer on the buried layer and patterning a first test structure and a second test structure therein; forming an etch stop layer on the conductive layer; etching through the wafer from the backside to perform an alignment process with the first test structure; and determining an overlay accuracy of the alignment process with the second test structure. The first test structure includes an optical vernier and the second test structure includes an electrical testing structure.
