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This work reports on a novel in situ sensing technique for monitoring the thickness of selectively grown Si epitaxial layers. The technique can be extended to detect selectivity loss when Si nuclei begin to appear on the insulator surface. In this technique, a quadruple mass spectrometer (QMS) monitors the ionized molecular hydrogen (H2+) signal, which is a by-product of the chemical-vapor deposition process. The thickness of the epitaxial layer is determined by evaluating the area under the hydrogen signal. We have deliberately used silane (SiH4) without HCl or Cl2 to achieve both nonselective and selective depositions. We also show that the amount of hydrogen produced by the deposition process is a strong function of the exposed Si area on the wafer and the effect can be accurately monitored by QMS. This finding was exploited to develop an in situ sensing method to detect the selectivity loss. When selectivity is lost, Si nuclei begin to form on the insulator surface increasing the effective Si area on the wafer. Consequently, the rate of hydrogen production increases rapidly as nuclei coalesce, resulting in a distinct change in the functional form of the hydrogen signal. The hydrogen signal was analyzed using an automatic edge detection procedure based on the wavelet transform modulus maxima representation. The technique facilitated the determination of selective film thickness from the time-integrated hydrogen (H2+) signal. To the authors' knowledge, This work represents one of the first applications of wavelets to in situ process monitoring and fault detection in semiconductor manufacturing. The authors expect the methodology presented in This work to be readily transferable to other selective deposition processes, including those that utilize dichlorosilane and disilane since hydrogen is a by-product of those processes as well.