In reactive ion etching, it is found that CCl4 discharge impedance is reduced during aluminum etching and changes markedly at the final aluminum interface. This phenomenon was monitored by measuring the target electrode voltage with a 2‐in. aluminium‐coated Si wafer. This measurement is compared with the 261.6‐nm Al–Cl optical emission spectrum and verified with SEM photographs of the etched wafer. As a result the end points detected by these two methods are consistent with each other and discharge impedance monitoring is applicable as an in‐process monitoring method.

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