The degree of the capacitive coupling in the radio frequency inductively coupled plasma was estimated by measuring the dc shift of the potential at the central inner side of the dielectric window. Its value was -82.2 V for the condition examined (1.1 kW, 1.5 mTorr, C4F8 plasma, flow rate 20 sccm) and the dielectric window was severely damaged owing to this potential difference from the plasma potential, which could be confirmed with the quadrupole mass spectrometry. When an axial external magnetic field was applied, the mass spectrometer signals, which indicate the damage of the dielectric window, decreased drastically in addition to the improvement of the power transfer efficiency. This was mainly caused by the transition from the etching of the dielectric window to polymer formation due to the reduced window potential as a result of the application of magnetic field. © 1997 American Vacuum Society.