Rawat Arti, Ganguli A, Narayanan Ramesh, Tarey R D
Centre for Energy Studies, Indian Institute of Technology, Delhi, New Delhi 110016, India.
Rev Sci Instrum. 2020 Sep 1;91(9):094705. doi: 10.1063/5.0009015.
An accurate, non-invasive ex situ diagnostic technique for analyzing plasma generated harmonics in radio frequency (RF) discharges is presented utilizing a broadband Dual Directional Coupler (DDC) that measures accurately both forward and reflected voltage signals in a transmission line. For usual applications such as monitoring forward and reflected power, the DDC is placed between the RF generator and the matching network (MN). However, the MN reflects all plasma generated harmonics back toward the plasma. Hence, no harmonics reach the generator side of the MN. Thus, for monitoring the harmonics, it is necessary to place the DDC between the impedance matching unit and the plasma, which was used for the first time in an asymmetric, parallel plate RF discharge at 13.56 MHz, 10 W-50 W at 200 mTorr (argon). The analysis of DDC data yields voltage, harmonic power contents, complex load impedance, plasma reflection coefficient, Voltage Standing Wave Ratio (VSWR), etc., for the fundamental frequency. For instance, at 10 W net input power, the computed plasma impedance is Z = R + jX, with R = 16.8 Ω and X = -81.9 Ω, yielding VSWR ≈11. Additionally, for 50 W input power, the third harmonic (72.31 mW) is dominant, followed by the second (8.28 mW) and fourth harmonics. In contrast, the literature states that the second harmonic is usually dominant, possibly due to the invasive nature of the diagnostics. Because harmonics are an important signature of processes taking place within the plasma, the proposed diagnostic can be effectively used for calibration and verification of theoretical models/simulations for resolving relevant physics issues.
本文提出了一种精确的非侵入式异位诊断技术，用于分析射频（RF）放电中产生的等离子体谐波。该技术利用宽带双向耦合器（DDC），它能够精确测量传输线中的正向和反射电压信号。对于诸如监测正向和反射功率等常规应用，DDC放置在射频发生器和匹配网络（MN）之间。然而，匹配网络会将所有等离子体产生的谐波反射回等离子体。因此，没有谐波到达匹配网络的发生器侧。所以，为了监测谐波，有必要将DDC放置在阻抗匹配单元和等离子体之间，这是首次在13.56 MHz、200 mTorr（氩气）下10 W - 50 W的非对称平行板射频放电中使用。对DDC数据的分析可得出基频的电压、谐波功率含量、复负载阻抗、等离子体反射系数、电压驻波比（VSWR）等。例如，在10 W的净输入功率下，计算得到的等离子体阻抗为Z = R + jX，其中R = 16.8 Ω，X = -81.9 Ω，VSWR≈11。此外，对于50 W的输入功率，三次谐波（72.31 mW）占主导，其次是二次谐波（8.28 mW）和四次谐波。相比之下，文献表明二次谐波通常占主导，这可能是由于诊断方法具有侵入性。由于谐波是等离子体内发生的过程的重要特征，所提出的诊断方法可有效地用于校准和验证用于解决相关物理问题的理论模型/模拟。
