Development of VHF inductively coupled plasma source and study on the frequency effect of plasma parameters

Abstract

A large-area inductively coupled plasma (ICP) source capable of securing azimuthal plasma uniformity at a 40.00 MHz has been developed. The antenna, referred to as a capacitor distributed resonance antenna, minimizes the azimuthally non-uniform antenna capacitive field with eight distributed vertical capacitors. The antenna was designed to maximize the antenna current using L-C series resonance. Based on plasma diagnostics with a 13.56 MHz conventional ICP, comparative analyses were performed in terms of the plasma density, electron temperature, and frequency characteristics of the electron energy probability function (EEPF). In addition, the frequency dependency of the EEPF was found in the collisional ($ν_{en}$ > ω), normal skin ($v_{th} /δ ≪ (ω^2 + ν_{en}^2)^{1/2}$) regime and the physical causes were examined. Also, a cooling mechanism for bulk electrons, according to increased capacitive coupling, was discovered in a high frequency discharge using inductively coupled plasma. It was found that when antenna voltage increases by high frequency driving, the threshold energy required for wall loss in the sheath collapse phase decreases, and electrons with lower energy can participate in the wall loss. This expansion of the depletion range for high energy electrons decreases the effective temperature for bulk electrons. To confirm this, we carried out Electron Energy Probability Function (EEPF) measurement and particle-in-cell (PIC) simulation according to increased driving frequency.