Abstract This paper deals with real-time FTIR (Fourier Transform Infrared Reflectometry) etch depth measurements performed on passive integrated silicon substrates. High-density trench capacitors are non-destructively characterized using an FTIR Michelson type spectrometer. Based on effective medium approximations, an effective index associated to the capacitor layer is introduced which allows a good evaluation of the capacitor hole depth. Obtained results correlate well with those from SEM (Scanning Electron Microscopy) measurements performed on cross-sections, on a range going from 12µm to 30µm depth.