ASR adoption as a holistic monitor solution in DRAM technology

YieldStar In-Device Metrology (YS IDM) and SEM can both be used for overlay after-etch inspection (AEI) on device structures. However, SEM faces challenges such as lower measurement speed, limited angled pattern measurements, and variable measurement results due to different focusing positions of the top and the bottom structure. YS IDM, an optical metrology, introduces Accuracy Self-Reference (ASR) mark to overcome these challenges. The ASR mark can avoid signals crosstalk by using programmed overlay in ASR sub-targets and provide various ASR KPIs for monitoring recipes. Besides, considering dual pupils helps create robust recipes by distinguishing process from true overlay signals. Moreover, scatterometry can provide rapid overlay measurement by analyzing the diffraction light, whereas SEM typically requires more time for all secondary electrons collection by a detector. The evaluation achievement in DRAM front-end layers shows IDM a.) is 40~50 times higher throughput than SEM. b.) has better accuracy, as demonstrated by significant yield improvement. c.) has a more robust recipe during process changes in the previous layer. d.) can be used for monitoring the quality of SEM recipe by ASR mark.