Study of void observation technique using scanning electron microscopy

A non-destructive observation technique for sub-surface voids in Cu interconnect is required to enhance the effectiveness of metallization process development. In this work, the feasibility of high beam energy SEM is investigated through a case study of 25 nm wide Cu interconnects. Also an optimization of SEM conditions is carried out using a Monte Carlo method based three dimensional SEM simulating tool. The capability of SEM based method for sub-surface void observation is confirmed by comparing top-down SEM image with cross section analysis. According to the Monte Carlo based optimization, for interconnects with 20–35 nm line width, it is clarified that BSE imaging with beam energy of 8–10 keV leads optimum sub-surface void visibility. In addition, authors evaluated the possibility of void size expectation, and it is confirmed that void size is reflected as signal intensity in SEM images.

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► The capability of high beam energy SEM to observe sub-surface voids is confirmed.
► The optimum observation conditions for interconnect with 20–35 nm is clarified.
► BSE imaging with 8–10 keV beam energy produces high sub-surface void visibility.
► We confirmed that signal intensity of SEM images reflects size of sub-surface void.