Currently, the performance of overlay metrology is evaluated mainly based on random error contributions such as precision and TIS variability. With the expected shrinkage of the overlay metrology budget to < 0.5nm, it becomes crucial to include also systematic error contributions which affect the accuracy of the metrology. Here we discuss fundamental aspects of overlay accuracy and a methodology to improve accuracy significantly.We identify overlay mark imperfections and their interaction with the metrology technology, as the main source of overlay inaccuracy. The most important type of mark imperfection is mark asymmetry. Overlay mark asymmetry leads to a geometrical ambiguity in the definition of overlay, which can be ~1nm or less. It is shown theoretically and in simulations that the metrology may enhance the effect of overlay mark asymmetry significantly and lead to metrology inaccuracy ~10nm, much larger than the geometrical ambiguity. The analysis is carried out for two different overlay metrology technologies: Imaging overlay and DBO (1st order diffraction based overlay). It is demonstrated that the sensitivity of DBO to overlay mark asymmetry is larger than the sensitivity of imaging overlay.Finally, we show that a recently developed measurement quality metric serves as a valuable tool for improving overlay metrology accuracy. Simulation results demonstrate that the accuracy of imaging overlay can be improved significantly by recipe setup optimized using the quality metric. We conclude that imaging overlay metrology, complemented by appropriate use of measurement quality metric, results in optimal overlay accuracy.The standard evaluation of the capability of an overlay metrology tool relies mostly on a Total Measurement Uncertainty (TMU), which includes effects of precision, TIS variability and tool matching. Overlay control requirements of future nodes lead to the very tight requirement of TMU < 0.5nm. It is known, however, that small TMU does not guarantee that the overlay metrology budget is met, because some metrology errors are not taken into account in the TMU. The most important additional errors are associated with process (litho, etch, CMP, etc.) induced overlay mark imperfections and their interaction with the metrology technology, which may lead to inaccurate overlay measurement. Such inaccuracy may be reflected, for example, in bias between after develop and after etch measurements, difference between measurements carried out with different wavelengths or different focus positions of the metrology tool. In extreme cases, the resulting inaccuracy can be much larger than 1nm, and can consume the whole overlay control budget.In this work we explain the origin of overlay mark related inaccuracy, and show that a measurement quality metric developed by KLA-Tencor is a valuable tool to address this problem. We show that using this quality metric it is possible to eliminate outlier measurements and to select a recipe setup which enables an accurate overlay metrology.We start by realizing that in overl...
No abstract
In order to fulfill the ever tightening requirements of advanced node overlay budgets, overlay metrology is becoming more and more sensitive to even the smallest imperfections in the metrology target. Under certain circumstances, inaccuracy due to such target imperfections can become the dominant contribution to the metrology uncertainty and cannot be quantified by the standard TMU contributors. In this paper we describe a calibration method that makes the overlay measurement robust to target imperfections without diminishing its sensitivity to the target overlay. The basic assumption of the method is that overlay measurement result can be approximated as the sum of two terms: the accurate overlay and the measurement inaccuracy (independently of the conventional contributors). While the first term (the "real overlay") is robust it is known that the overlay target inaccuracy depends on the measurement conditions. This dependence on measurement conditions is used to estimate quantitative inaccuracy by means of the overlay quality merit which was described in previous publications. This paper includes the theoretical basis of the method as well as experimental validation.
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