Images in photoresist for self-interferometric electrical image monitors

Self-interferometric electrically measurable image focus monitors have been designed and tested in photoresist in a double exposure implementation. The Pattern-and-Probe monitors for this experiment were placed on a multiple student phase shift mask, and were validated by means of SEM images in photoresist on wafers shot at 193nm with an NA of 0.85, with 40nm focus steps. A large number of monitor parameters were varied in the experiment, such as the number of rings, linewidth and center probe size, and recommendations for the optimal combination of parameters for the design are given. Misalignment is accounted for by placing multiple patterns on the mask, translated a small amount so that at least one will be centered exactly over the line. The results exhibit sufficient sensitivity to produce an open circuit after 0.3 - 0.6 Rayleigh Units defocus, and thus detect when the process is at the edge of the process window.