aluminium as a groun plane and silicon as a substrate

You can just define the No.0 layer with the conductivity of alumninum and the No.1 substrate with the Er of silicon in Basic Parameters.

If you know the Er of the silicon can you share to me


Thanks

According to this Dielectric Constant Reference Guide it is between 11.0 and 12.0 but you really need to verify the exact value with the foundry as it would depend on their process.