Laser diagnostic studies of capacitively coupled rf discharges containing silane diluted in argon have received considerable attention in order to understand the dust particles' formation and behaviour. The formation mechanism of those macroscopic particulates needs to be clarified to enhance the product control in the surface processing industry. In situ Fourier Transform infrared absorption spectroscopy (FTIR) and Cavity Ring-Down Spectroscopy (CRDS) in the mid-infrared wavelength range have been performed to monitor their composition during the growth process. On the one hand, FTIR results particularly indicate time dependency of the solid-state vibrational absorptions of the SiH and SiH3 bands. They also show the silane dissociation after plasma ignition. CRDS, on the other hand, has been applied to the carbon monoxide molecule to investigate the potency of the diagnostic on future silane measurements. Some preliminary ring-down results point out a detection range for the absorbance in the order of 10¿5. These two diagnostic techniques prove to be valuable process-monitoring tools.