How to add a rigid body to a electromechanical simulation ?

I would like to model a MEMS device but have one part be rigid. In the solid mechnics modul, one can simply define a rigid domain, by clicking on Material Models>Rigid Domain.

However, when I use the electromechanics modul, I cannot find the rigid domain option.

Do you know how I can fix this ?

My COMSOL version: 3.5