Mirror plane in 3D

I am working on calculating the E-field distribution for a rectangular 3D geometry. I require a very fine mesh and therefore I run into memory limitations of my computer. I therefore would like to take advantage of the symmetries of my geometry i.e. it is symmetric about the x-y plane. So if I could define a mirror plane at y=0, I would only need to calculate the field distribution for half the computational cell. Is this possible for a 3D structure in comsol?

I have looked a bit a 2D axial symmetry but the symmetry does not seem to be extruded from a line to a plane when extruding the geometry to 3D.

Cheers,

Claus