Image analysis of alignment and overlay marks with compound structure

Decreasing of node size significantly increases requirement to overlay precision. Complex structure of target demands using of compound structures of overlay mark, which usually contain features with acute sidewall angles, coated by several layers. In this paper the possibility and limitations of image-based overlay with compound structures with overlay mark and coating layers are analyzed in detail. Dependence of overlay signal shape on overlay offset is considered. Structures with asymmetric sidewall angle, non-uniform thicknesses of layers and curved shape of layer borders are examined. Influence of thickness variation, difference between left and right sidewall angles of asymmetric shape and curvature of layer borders are investigated. For the simulation of such complex structures of overlay marks, our in-house simulator based on rigorous coupled-wave analysis (RCWA) module is used. Maximum allowed values of these parameters are studied in order to determine the limitations of image-based overlay. Results of this consideration can be used for improvement of overlay precision and elaboration of optimal overlay strategy in conditions of node shrinking in the semiconductor industry.