Infrared multiple internal reflection (MIR) spectroscopy has been applied here to the characterization (exsitu) of thin amorphous silicon layers on crystalline silicon substrates. The specimens are tightly clamped against a Ge prism with 45° bevel angle for the entrance and exit sides, allowing for up to 25 multiple internal reflections. This greatly enhances the thin‐film absorption, and the stretching bands of SiHnbonds in amorphous Si are detected in a range of film thickness from 3 to 600 nm. Very thin films (with thickness less than 50 nm) exhibit a different hydrogen bonding compared to the thin ones, due to hydrogenated sublayers that are not visible in IR spectra for larger film thickness. Sublayers are found both at the film/substrate interface and at the film free surface. Another transitional layer, with increased concentration of SiH2groups and considerably less than 30 nm, is found at the film/substrate interface. MIR is also applied to study the step‐by‐step etching in CF4/O2of a 70‐nm‐thick amorphous silicon layer. This experiment is able to define the location of the sublayers. ©1995 American Institute of Physics.