A study on diffraction-based overlay measurement based on FDTD method

Abstract

As the semiconductor manufacturing critical dimension continues to shrink, the requirement placed on overlay control becomes much more stringent. Due to the fact that the absolute overlay tolerances are approaching 2-3 nm, process induced errors can be a major contribution to the overlay error. For instance, chemical mechanical polishing (CMP) are found to cause 1-3 nm overlay measurement error, which is of the same magnitude to the total overly budget. Because of this, efforts are being made to investigate the mechanism of overlay shift caused by process variations. In this paper, we present a study of the Diffraction based Overlay (DBO) metrology with a model based on the Finite-Difference Time-Domain (FDTD) method on the impact of CMP process to overlay measurement. Measurement error caused by CMP are discussed. Our investigation shows that the impact of the CMP process can cause the +/- diffraction orders to become asymmetric, which will confuse DBO measurement signals. This study has been performed across the visible illumination spectrum and the result of our study will be illustrated.