In this study, we present a very accurate method of determining the hydrogen concentration of amorphous silicon nitride (SiN) layers by employing Fourier Transform IR Spectroscopy (FTIR). Our method comprises a very effective means of rapid and non-destructive in-line monitoring and quality control for the semiconductor industry. Amorphous SiN layers are essential for the semiconductor industry as an encapsulant for silicon integrated circuits, as a diffusion barrier for H2O and sodium ions, as passivation films, oxidation masks, gate insulators and capacitor dielectrics, their quality being highly dependent on the hydrogen content which originates from the deposition reactants NH3, SiH4 and SiH2Cl2. In contrast to commercially available FTIR software and recommended procedures in the literature, which in principle use linear baselines to evaluate the areas of the N-H and Si-H absorbance peaks, we do a polynomial fit of the spectral background outside the absorption lines. This yields a much higher accuracy in determining the areas of the absorbance peaks and therefore the hydrogen concentration. Comparisons between the two methods of evaluation yield differences of up to 30 percent, thus emphasizing the importance of our method as advanced rapid failure analysis technique. We compare the hydrogen concentrations of SiN layers deposited by LPCVD and PECVD processes and we also show how different process parameters influence the hydrogen concentration.