A wide slit nozzle having a slit as a discharge opening. A position sensor is configured to provide feedback position data to the controller indicative of the actual position of the capacitive object after it is moved. The phase of the modulated waveform is varied in proportion to the number of EMI generating sources in the electronic system. Thereby the surface treatment only effects in an area within the range of the targeting treatment area of the workpiece so as to reduce the treatment defect. Each concentration range defines a blocking range.