Transition between E-mode and H-mode in a cylindrical inductively coupled plasma reactor

An Inductively Coupled Plasma source was previously applied by us to surface treatment of a plastic material in moderate power conditions. In these conditions the coupling between the antenna and the plasma is capacitive and is usually referred to as E-mode. The machine has been modified to operate at working powers up to 1 kW, thus providing a real inductive coupling, referred to as H-mode. The plasma is generated by applying a 13.56 MHz radio-frequency to a / 4 antenna wrapped outside a quartz tube. The plasma chamber is evacuated by a rotary pump down to about 0.1 Pa and eventually fed with Argon at working pressures in the range from 1 to 100 Pa. In the present work the transition from capacitive to inductive coupling mode is investigated by means of electrical and optical methods. The two methods are compared and experiments reveal that the transition occurs at decreasing power levels in the range between 100 and 200 W, while pressure is increased from 1 to 100 Pa. The percentage of power that is delivered to the plasma is investigated, too, and appears to be proportional to the input power.