High-Density Plasma Diagnosis Using Quasi-Optical Millimeter-Wave Systems

Abstract

This report proposes a quasi-optical millimeter-wave system based on a noninvasive measurement technique to diagnose high-density plasmas. The millimeter wave in the F-band (90–140 GHz) can interact with high-density plasma (1013–1014 cm−3) in the quasi-optical system, such that the plasma characteristics (e.g., plasma density and effective collision frequency) obtained using the Drude model can be measured with a high signal-to-noise ratio. High-density inductive radio frequency (RF) discharge argon plasma is generated in a helix of inductively coupled plasma using RF power of up to 1000 W at 13.56 MHz with a pressure of a few Torr. The millimeter-wave signals can be analyzed to estimate the plasma-density distribution by using the proposed axisymmetric 2-D multilayered plasma modeling. The experimental results agreed well with the model calculations. Qualitatively, the plasma density tends to increase with increasing RF power and gas pressure. The effective collision frequency decreases only with increasing gas pressure. The proposed measurement technique will also be applicable to measuring localized regions (with areas of a few square millimeters) of higher density plasma (1014–1017 cm−3) by using terahertz waves for various applications.