OPO Measurement Improvement in Advanced DRAM with Tunable Wavelength Imaging

As DRAM process nodes keep shrinking, the overlay budget becomes tighter, and overlay error showing a more significant effect on yield. This calls for a more accurate and more robust on-product overlay (OPO) measurement approach. In many cases, engineers tune their process to improve yield, especially in the research and development (R&D) phase of a product, but overlay measurability is sensitive to process variation and the measurement window could drift or even disappear after process changes. In the method described in this paper, we use features on the Archer™ imaging-based overlay (IBO) measurement system - such as illumination with tunable wavelength (wave tuner, WT) to optimize illumination wavelength, and dynamic focus mode (DFM) to select best focus position - to produce a more accurate and robust OPO measurement on critical layers in advanced DRAM. With WT, the overlay target has better illumination conditions, resulting in residual improvement of ~60%, and more stable measurements from wafer to wafer and lot to lot. DFM improves measurement accuracy with a more accurate focus position. For products both in R&D and high-volume manufacturing (HVM) phases, WT and DFM are demonstrated to be critical knobs to improve measurability as a function of wavelength and focus position. These features allow further information, such as accuracy heatmaps, residual landscape maps, and focus offset maps to help the user identify key process variations.