Yasushi Sakakibara, Geniti Katagiri, Makoto Toraguti and Tadahiro Sakuta*

Product Development Laboratory, Fuji Electric Corporate R&D, Ltd.
2-2-1 Nagasaka, Yokosuka, Kanagawa 240-01, Japan
sakakibara-yasushi@fujielectric.co.jp

*Department of Electrical & Computer Engineering, Kanazawa University

Among several types of a thermal plasma torch used for processing, the ICP (Inductively Coupled Plasma) torch has an advantage that the plasma is generated without any electrodes vapor contamination. The volume of the ICP has been limited to about 6cm in diameter by the applied high frequency of a power supply such as MHz order, while a more wide area plasma is required for higher rate of processing. Also, the energy efficiency which is defined as a ratio of absorbed power into the plasma to the plate power was considerably low because the oscillator operates with vacuum bulbs.

In order to solve these problems, an attempt to generate the ICP was made by using a low frequency (a few hundred kHz) magnetic fields. Because the skin depth of such low frequency magnetic fields is a few cm, it is expected that a wider plasma in diameter is produced steadily. A semiconductor device instead of vacuum bulbs can be now applicable as the power supply with such a low frequency for plasma generation.

Our new ICP generation system consists of a MOSFET (Metal Oxide Semiconductor Field Transistor) inverter, an ICP torch and a vacuum chamber. The inverter is operated at 450kHz in frequency. The ICP torch has a 6-turn coil, 20cm in diameter and 10cm in length, and a quartz tube of 16cm in inner diameter. (Fig.1) In this system, both an Ar plasma and a N2 plasma were successfully produced at an input power of 15kW(Ar) and 55kW(N2) under atmospheric pressure. The energy efficiency of the system was increased from 35% (conventional ICP system) to 85% by using the MOSFET inverter.(Fig.2)

Fig.1 Electrical circuit of ICP generation system
Fig.2 Energy balance of ICP generation system