Study of power coupling experiments in a large-area transformer coupled plasma source

Abstract

A large-area transformer coupled plasma (TCP) source has been designed and constructed. In this design, a plasma generation chamber and a RF (Radio-Frequency) antenna chamber have been separated with dielectric material, and differentially pumped to accommodate large-area, relatively thin dielectric window against mechanical pressures. With large diameter (78cm) chamber, low frequency (4-5MHz) RF source has been chosen. By calculating plasma impedance from TCPRP code based on 2D heating theory, the diameter of a single-turn copper coil antenna was optimized to provide high-density plasmas in large area. Also the impedance matching circuit of this large-area TCP source has been designed from the calculated plasma impedance. Large-area helium plasma was successfully generated and used to study power coupling in this source using the single-turn antenna. E-H transition phenomenon was observed, and threshold current was measured. Threshold current and power were measured to be lower at low pressure than at high pressure, and look to be linear with pressure. In H-mode antenna impedance and reactance decrease as RF power increases. This means that increased mutual inductance between antenna and plasma loop decreases primary side inductance as plasma density increase. Power transfer is more efficient at low pressure than high pressure. From these results, optimum pressure range is observed to be 1-10mTorr.