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In this paper, the effect of the capacitance of a variable capacitor connected on the ground side of a ferrite-enhanced internal U-type inductively coupled plasma (ICP) source on the electrical characteristics of an ICP antenna and the plasma characteristics is investigated. When the capacitance of the variable capacitor satisfies C = 2/ω2L (where ω is the angular power RF frequency and L is the antenna inductance), the highest plasma density of 2.74 × 1011/ cm3 and the lowest plasma potential of about 19.5 eV are obtained at a 13.56-MHz RF power of 400 W and 10-mtorr Ar. In addition, under these conditions, the best plasma uniformity of about 6% over a substrate area of 300 mm and the most stable operating conditions, due to the lowest heating of the ferrite installed on the ICP antenna, are obtained. The capacitance condition of C = 2/ ω2L is related to the minimization of the capacitive coupling to the plasma due to the lowest peak RF voltage being obtained along the antenna line. It is also related to the maximization of the power-transfer efficiency to the plasma by minimizing the power loss to the ferrite installed on the ICP antenna, due to the lowest peak RF current being obtained along the antenna line.