Achieving optimum diffraction based overlay performance

Diffraction Based Overlay (DBO) metrology has been shown to have significantly reduced Total Measurement Uncertainty (TMU) compared to Image Based Overlay (IBO), primarily due to having no measurable Tool Induced Shift (TIS). However, the advantages of having no measurable TIS can be outweighed by increased susceptibility to WIS (Wafer Induced Shift) caused by target damage, process non-uniformities and variations. The path to optimum DBO performance lies in having well characterized metrology targets, which are insensitive to process non-uniformities and variations, in combination with optimized recipes which take advantage of advanced DBO designs. In this work we examine the impact of different degrees of process non-uniformity and target damage on DBO measurement gratings and study their impact on overlay measurement accuracy and precision. Multiple wavelength and dual polarization scatterometry are used to characterize the DBO design performance over the range of process variation. In conclusion, we describe the robustness of DBO metrology to target damage and show how to exploit the measurement capability of a multiple wavelength, dual polarization scatterometry tool to ensure the required measurement accuracy for current and future technology nodes.