SERS substrate containing SERS-active array of Ag, Au and Ag50Au50 nanoparticles with normal size distribution, a mirror layer and a separating SiO2 layer was optimized. A layer of amorphous carbon was used as the object of study. It is shown that such a structure enhances the Raman signal both due to the localized surface plasmon resonance and due to interference. The average size of nanoparticles, the reflection coefficient of mirror layer, and the thickness of SiO2 layer influence the amplification of the Raman signal. To provide amplification the thickness of SiO2 layer should be calculated for the appropriate wavelength in order to get constructive interference. It is revealed that additional amplification of the Raman signal occurs if the mirror layer as well as the array of particles is made of plasmon metal and thickness of separating SiO2 layer is quite small (up to 30 nm).