Direct interferometric phase measurement using an aerial image measurement system

Abstract

Accurate phase measurement is required to fabricate manufacturing-quality phase shifting masks. Phase error can severely limit the total process window as the k1 factor becomes smaller and smaller. Existing phase measurement techniques use either a specialized phase interferometer or an aerial image measurement system (AIMS) with image-plane analysis. In this presentation we report a direct interferometric phase measurement technique that can be implemented on an AIMS tool with a few inexpensive accessories. The method is based on analyzing far-field interference patterns of two double-slit features on the mask. In one double-slit feature, the left slit is phase- shifted; in the other feature the right slit is phase- shifted. By measuring a relative shift of position between the interference patterns one can calculate phase shift directly. Advantages of the method over other published methods based on image plane measurements include: 1) significantly better accuracy, and 2) no aerial image simulation is required, thus eliminating a source of measurement error. Compared wit lateral shearing interferometry used in dedicated phase interferometers, the current method can be applied to smaller geometries without adding complexity. In addition, results obtained with current method are effective phase shifts that may correlate directly with lithographic performance of the mask. We will present the experiment setup, phase extracting algorithm, and experimental result on an alternating phase shifting mask. Result will be compared with that of a step height measurement.