A method for and system of intrusion detection by using ultrasonic signals are provided. By comparing a reference current Iref and a drive current ID, a current-mirror circuit CM can monitor the voltage of a power supply VDD. Then portions of the first polishing pad are punched off to remove portions of the hard polishing material positioned on the surface of the first polishing pad so as to form holes penetrating the first polishing pad. The switching circuit, which may include a MOSFET switch, is coupled to the transformer and is configured to switch the transformer on and off.