Ellipsometry is a characterization method allowing the determination of the optical index and of the thickness of thin films. We have used this method to control, during their growth, the realization of diode RF sputtered soft X-ray multilayers. Three parameters limit the multilayer reflectivity : compactness defects, thickness drift and interface roughness. Our ellipsometers' accuracy being 0.1 Å on thickness and 10-3 on optical index, we have been able to optimize the deposit thickness, to control the growth rate and to understand the interface formation mechanism. This has enabled us to produce soft X-ray optics of good quality for wavelengths down to 44.7 Å. Thus C/W 200 layer stacks with period from 40 Å to 80 Å and Si/W 100 layer stacks with period from 20 Å to 30 Å showing good reflectivities have been achieved