AbstractIn this study, we report on structural variation of ultrathin of p‐type Porous Silicon (PS) films during the early stage growth process using X‐Ray Reflectometry (XRR). The PS samples were firstly characterized by Fourier Transform Infrared spectroscopy (FTIR) measurements. The homogeneity and surface roughness of the PS films, which enable characterizing the PS films by XRR, were investigated by UV‐visible spectroscopy and atomic force microscopy (AFM) measurements. The PS layers were then analyzed by specular X‐Ray Reflectivity (XRR). XRR reveals a linear thickness behavior as a function of etching time, while the porosity increases before reaching a constant value around 72%. These results were compared to those obtained from other characterization. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)