System and method for decapsulation of plastic integrated circuit packages by providing a microwave generator, providing a Beenakker resonant cavity connected to the microwave generator, which cavity comprises a coupling antenna loop, providing the cavity with a tube or tubes for supply of plasma gas and etchant gas or gases and with means for igniting the plasma gas, and providing that the cavity is set at a predefined value of its Q factor by embodying the coupling antenna loop and/or a wire optionally attached to the coupling antenna loop in a metal or metal alloy, or providing that at least at part of its surface area the coupling antenna loop and/or the wire is coated with a metal or metal alloy different than copper and with a higher resistivity than copper.