In this work, the effect of increasing Si content in the absorber layers (CrAlSiNx/CrAlSiOyNx) of solar selectiveabsorbers upon their selectivity and thermal stability are studied. The two optical stacks presented consist of fourmagnetron sputtered layers on stainless steel substrates. In both cases, tungsten is used as back-reflector,CrAlSixN/CrAlSiOyNx as absorber layers with different Si/(Cr+Al+Si) atomic ratio (0.15 and 0.30, respectively)and finally the SiAlOx antireflection layer. The structures were theoretically designed by SCOUT softwaredepended on experimental transmittance (T) and reflectance (R) of thin single layers deposited on glass substrates.It is observed that optical stack coatings with higher silicon content show better selectivity values, highsolar absorptance, α=95.9%, and low emissivity, ε=9.7% (calculated for 400 °C), with higher thermal stabilityat 600 °C in vacuum, for 650 h. Additionally, with the annealing at 600 °C an increase of surface roughnesswas found, which was smaller for sample with higher Si content.