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In this paper, first measurements with a particularly designed quantum-cascade-laser (QCL) arrangement for application in semiconductor industrial environments for in situ wafer-to-wafer etch monitoring are reported. The combination of QCLs and infrared absorption spectroscopy (QCLAS) opens up new possibilities for plasma process monitoring and control. In silicon etch plasmas, concentrations of the etch product SiF4 were measured real time in an industrial-production environment. The comparison of the results with inline data of the processed wafers shows a correlation between the amount of produced SiF4 and the measures of the trench depth and the bottom void. Furthermore, it is shown that the characteristics of the refractive index of Si and SiO2 in the mid-infrared can be used to determine etch rates of SiO2 and Si wafers during the processing.