The effect of metal masks on the matching of the lower electrode with a high-frequency bias generator during selective reactive-ion etching through the mask of massive substrates in freon-14 has been studied theoretically and experimentally. It is shown that masks with a substrate coating above 30% lead to an increase in the reactive power component at distances from the center close to the substrate radius. The absence of influence on the specific reactive power of the thickness and material of the masks is established. It is experimentally shown that masks with any practically significant coating coefficient of the substrate, connected to the lower electrode through the substrate holder, improve the matching, reducing the power reflection coefficient.