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In etch plasmas used for semiconductor processing, concentrations of the precursor gas NF3and of the etch product SiF4are measured online and in situ using a new diagnostic arrangement, the Q‐MACS Etch system, which is based on quantum cascade laser absorption spectroscopy (QCLAS). In addition, the etch rates of SiO2layers and of the silicon wafer are monitored including plasma‐etching endpoint detection. For this purpose the Q‐MACS Etch system is working as an interferometer arrangement. The experiments are performed in an industrial, dual‐frequency, capacitively coupled, magnetically enhanced, reactive ion etcher (MERIE), which is a plasma reactor developed for dynamic random access memory (DRAM) technologies. In the spectral range 1028 ± 0.3 cm–1, the absorption cross‐sections of SiF4and NF3are determined to be σ = (7.7 ± 0.7) × 10–18cm2molecule–1and σ = (8.7 ± 0.8) × 10–20cm2molecule–1, respectively.