Evolution of plasma parameters in capacitively coupled He–O ₂ /Ar mixture plasma generated at low pressure using 13.56 MHz generator

Abstract

Abstract Low pressure capacitively coupled He–O 2 /Ar mixture plasma is investigated using optical emission spectroscopy and Langmuir probe (LP) techniques and the effects of discharge parameters i.e. radio frequency (RF) power, filling gas pressure and oxygen concentration on electron density ( n e ), electron temperature ( T e ), excitation temperature ( T exc ), plasma potential ( V p ) and electron energy probability function (EEPF) are monitored. It is noted that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:math> increases with increase in RF power and filling gas pressure, while it decreases with increasing O 2 concentration. The LP technique and Boltzmann plot method are employed to determine <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="italic">e</mml:mi> <mml:mi mathvariant="italic">x</mml:mi> <mml:mi mathvariant="italic">c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>.</mml:mo> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="italic">e</mml:mi> <mml:mi mathvariant="italic">x</mml:mi> <mml:mi mathvariant="italic">c</mml:mi> </mml:mrow> </mml:msub> </mml:math> estimated by both methods shows similar decreasing trend with increasing RF power and filling gas pressure. A sudden increase in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mspace width=".25em"/> </mml:math> and decrease in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> <mml:mspace width=".25em"/> </mml:mrow> </mml:msub> </mml:math> is noted with RF power, indicating a mode transition i.e. from alpha <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="(" separators=""> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:mfenced> </mml:math> to the gamma <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="(" separators=""> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:mfenced> </mml:math> mode. The threshold RF power for the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>γ</mml:mi> </mml:math> mode shifts from higher to lower value (150–120 W) with increasing gas pressure (0.3–0.5 mbar). This trend reverses and shifts from lower to higher RF value i.e. from 110 W (pure helium) to 150 W (8% O 2 ) with increase in O 2 concentration. Investigation of EEPF profile states evolution from Druyvesteyn-like to bi-Maxwellian distribution with increase in RF power and filling gas pressure; due to mode transition and decrease in the height of high energy tail of EEPF. Similarly, the effect of O 2 mixing on the shape of EEPF is also investigated. It is noted that in pure helium discharge the EEPF is bi-Maxwellian in nature, while addition of O 2 in the mixture results in the broadening of the EEPF. Moreover, the height of high energy tail of EEPF also increases. Finally, an increasing trend in atomic oxygen density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="]" open="[" separators=""> <mml:mrow> <mml:mi mathvariant="italic">O</mml:mi> </mml:mrow> </mml:mfenced> </mml:math>

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