Color mixing in overlay metrology for greater accuracy and robustness

The on-product overlay roadmap demands an aggressive overlay requirement in the advanced node. Currently the on-product overlay is dominated by effects coming from wafer processing and overlay target detectability. Processing effects such as symmetric stack variation and asymmetric overlay target deformations are expected to become limiting for accurate overlay measurements in future nodes. Increased accuracy requirements and overall complexity in product stacks require a sensor with a higher flexibility. To address this an advanced metrology system is introduced in the fab, providing full flexibility in the selection of measurement wavelengths. On top of the wavelength flexibility, the increased wavelength switching speed enables the use of asymmetry robust recipes by combining multiple wavelength measurements at each overlay target. In this paper we will introduce a method to select the most accurate multi-wavelength recipe that provides significant improvement in accuracy compared to the best single wavelength recipe. We will introduce KPIs to monitor the health of the multi-wavelength measurement. The KPIs are reported per site indicating the accuracy for every measured point. Additionally we will show our steps towards the recovery of the points flagged by multi-wavelength KPI by a combination of measuring more wavelengths and an accuracy guided region of interest selection.