Full-field wafer level thin film stress measurement by phase-stepping shadow Moiré

A wafer topography measurement system has been designed and demonstrated based on shadow Moiré. Three-step phase-stepping and phase unwarping techniques are also incorporated to enhance the system resolution. Wafer curvatures or bows can be achieved by analyzing the Moiré fringe patterns and film stress can be obtained subsequently by transforming this wafer curvature using a conversion equation such as Stoney's formula. Wafer bow of plasma enhanced chemical vapor deposition nitride and oxide coated wafers are measured by this shadow Moiré system and are subsequently verified by the KLA-Tencor FLX 2320 system. The discrepancy between both bow measurements is within 2 μm, regardless of the magnitude of the measurement. Therefore, this system is especially suitable for stress characterization of thicker, stiffer, or highly stressed films. In comparison with the traditional laser scanning method, wafer curvature obtained by shadow Moiré is based on full-field information and it would have a better accuracy. By integrating this system with a more accurate wafer curvature to film stress conversion formula, this system should also provide a better film stress characterization.