The Wheatstone bridge as an alignment test structure

A new test structure for the electrical measurement of level-to-level registration is introduced. This structure, based on the Wheatstone bridge measurement principle, offers improved accuracy over the conventional U-shaped test structure. With the described Wheatstone bridge method, the registration error (misalignment) is directly proportional to a single voltage. In contrast, the misalignment of the U-shaped structure is calculated from the difference of two voltages, an operation that can result in poor precision. The paper describes a two-by-twelve contact pad module, which includes two Wheatstone bridge structures to measure the misalignment in the X- and Y-directions, two calibration structures to determine the zero- and full-scale accuracy, and a Van der Pauw structure to measure the sheet resistance of the conducting material as required for the alignment measurement. The mathematical expressions are derived, and actual test results are discussed.