Adding material to the face of the substrate

Hello, can anyone help me...? I want to design a new project. At the start of the project, I chose a box in which by default the material is vacuum. I want to design in which the two faces of the substrate in the z-axis to be copper but the spaces between the two faces to be vacuum.

If you consider your copper electrodes as perfect conductors (if you don't care about field penetration into the conductor or resisitive losses) you can simply assign perfect-E boundary condition to these faces.

You could also apply a "finite conductivity" boundary and set the properties to be that of copper.