In this work, the effect of increasing Si content in the absorber layers (CrAlSiNx/CrAlSiOyNx) of solar selective absorbers upon their selectivity and thermal stability are studied. The two optical stacks presented consist of four magnetron 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 software depended 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, high solar absorptance, alpha = 95.9%, and low emissivity, epsilon = 9.7% (calculated for 400 degrees C), with higher thermal stability at 600 degrees C in vacuum, for 650 h. Additionally, with the annealing at 600 degrees C an increase of surface roughness was found, which was smaller for sample with higher Si content.