Experimental and modeling investigation on charged particle dynamics and electron power absorption mode in 400 kHz/27.2 MHz dual-frequency capacitively coupled argon discharges

Abstract Dual-frequency capacitively coupled plasmas driven by a low frequency less than 1 MHz have recently received growing attention due to their ability to produce higher ion energy, narrower energy spread, and enhanced ion flux (Zhou et al 2024 Appl. Phys. Lett. 124 064102). Here, the dependences of the charged particle dynamics and electron power absorption mode on the low-frequency (LF) voltage amplitude ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ), high-frequency (HF) voltage amplitude ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ), and gas pressure ( p ) in 400 kHz/27.2 MHz DF capacitively coupled argon discharges have been investigated by experimental diagnostics and particle-in-cell/Monte Carlo collision simulations. A hairpin probe is used to measure the plasma density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) at the discharge center, while phase-resolved optical emission spectroscopy allows for the measurements of the time- and space-averaged optical emission intensity at 750.4 nm ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:mn>750</mml:mn> <mml:mo>,</mml:mo> <mml:mi>ave</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) and the spatiotemporally resolved electron-impact excitation rate. Compared to a single-frequency 27.2 MHz discharge, the addition of the LF source is found to significantly affect the electron-impact excitation/ionization dynamics via frequency coupling and secondary electron emission. At <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>p</mml:mi> <mml:mo>=</mml:mo> </mml:mrow> </mml:math> 40 Pa and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> </mml:mrow> </mml:math> 225 V, both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:mn>750</mml:mn> <mml:mo>,</mml:mo> <mml:mi>ave</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> experience a moderate decline followed by a dramatic rise with the increase of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , corresponding to an electron power absorption transition from α -mode to a hybrid α - γ -mode. The decline in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:mn>750</mml:mn> <mml:mo>,</mml:mo> <mml:mi>ave</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> versus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 300 V is caused by a reduced bulk electron power deposition and a stronger frequency coupling effect at a higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml: