How to make a vacuum chamber in a capacitor?

Hi, I am trying to model a MEMS capacitor built on the silicon substrate. The thin top plate(membrane) of the capacitor is also made of silicon. The gap between the plates is vacuum sealed. So with zero bias voltage on the capacitor, this thin top plate (membrane) is deflected to the bottom substrate due to the surrounding air pressure. I don't konw how to make the vacuum chamber in the device to model the deflection. Emi physics is used in modeling.
Any suggestions is appreciated!