Alignment performance vs. mark quality

A procedure is described for preparing relief alignment marks with precisely degraded quality that are then used to calibrate alignment performance. Alignment degrades with mark quality, eventually failing when the marks are no longer found. Using conventional processes it is difficult to accurately fmd this threshold and virtually impossible to experimentally establish the functional relationship between alignment mark quality and alignment precision. Marks that simulate a full range of process conditions, including planarization and granularity, are formed utilizing the continuous tone relief response of I-line photoresist to lambda = 248nm dose, thereby avoiding the complication of fabricating wafers through selective steps of the actual semiconductor manufacturing process. The usual box in box overlay measurement problem, caused by boxes formed by different processes, is avoided by printing high contrast overlay evaluation structures regardless of the alignment mark quality. Overlay is measured and plotted as a function of mark quality and the lithography engineer knows precisely the condition of the alignment system. For example; It is easily established by direct measurement the alignment system's ability to control magnification as the relief of the alignment marks change.