Alignment strategy for metal layers after W-CMP

CMP of Tungsten (W) in the wafer process flow pose a major challenge for robust stepper alignment, particularly at tighter design rules for 0.25 micron and beyond. Traditional alignment strategies based on optical diffraction often fail to provide alignment accuracy and consistency, as dictated by the tighter overlay budgets. Intensity based image analysis methods prove to be more successful in the alignment of metal CMP layers. In addition the noise reduction and signal processing capabilities of alignment sensors, the condition of the mark after W-CMP is vital for achieving good alignment. In this paper we report results from various alignment mark designs. The marks are designed to reduce the impact of CMP process variations on stepper alignment. Three types of alignment methods using laser diffraction, bright field imaging and laser interferometry techniques were investigated. The brightfield imaging alignment schemes provide the best results using a narrow trench mark separated by wider islands. Details of stepper alignment signals and overlay measurement results corresponding to some of the relevant mark design sand process variations are included in the paper.