Electron Information in Single- and Dual-Frequency Capacitive Discharges at Atmospheric Pressure

Sanghoo Park,Jian Jun Shi,Se Youn Moon,Wonho Choe

doi:10.1038/s41598-018-25892-w

Abstract

Determining the electron properties of weakly ionized gases, particularly in a high electron-neutral collisional condition, is a nontrivial task; thus, the mechanisms underlying the electron characteristics and electron heating structure in radio-frequency (rf) collisional discharges remain unclear. Here, we report the electrical characteristics and electron information in single-frequency (4.52 MHz and 13.56 MHz) and dual-frequency (a combination of 4.52 MHz and 13.56 MHz) capacitive discharges within the abnormal α-mode regime at atmospheric pressure. A continuum radiation-based electron diagnostic method is employed to estimate the electron density (ne) and temperature (Te). Our experimental observations reveal that time-averaged ne (7.7–14 × 1011 cm−3) and Te (1.75–2.5 eV) can be independently controlled in dual-frequency discharge, whereas such control is nontrivial in single-frequency discharge, which shows a linear increase in ne and little to no change in Te with increases in the rf input power. Furthermore, the two-dimensional spatiotemporal evolution of neutral bremsstrahlung and associated electron heating structures is demonstrated. These results reveal that a symmetric structure in electron heating becomes asymmetric (via a local suppression of electron temperature) as two-frequency power is simultaneously introduced.

Highlights

Low-temperature radio-frequency discharges at low pressure have been extensively studied[1,2,3,4] and applied in the microelectronics industry
A dual-frequency operational approach was proposed to achieve the selective control of ion energy and density separately at low-pressure plasmas
The spatiotemporal evolution of neutral bremsstrahlung and associated electron heating structures showed that electron heating via ohmic heating at the plasma-sheath boundary is suppressed at certain rf phases and electrons lose their kinetic energy via hf oscillations, which results in the spatiotemporal asymmetric structure of electron heating

Summary

Introduction

Low-temperature radio-frequency (rf) discharges at low pressure have been extensively studied[1,2,3,4] and applied in the microelectronics industry. One well-known technique involves capacitive discharges driven by two rf power sources This so-called dual-frequency operation was introduced to control the ion energy and ion flux that are individually coupled by low and high frequencies to electrodes or targets, respectively, for specific uses. Many of the underlying principles of low-pressure plasmas are known to be inconsistent with atmospheric-pressure plasmas because of their extremely high collisionality Undiscovered anomalies, such as resonance heating and abnormal mode transitions in low-pressure dual-rf capacitive discharges, can occur in highly collisional plasmas because of its nonlinear nature. To date, no experimental evidence of electron characteristics, electron density (ne) and temperature (Te), in dual-rf capacitive discharges at atmospheric pressure have been reported, and only a few theoretical approaches have been presented. We report the discharge characteristics (including electron information) of single- and dual-rf-driven argon capacitive discharges generated at atmospheric pressure. Considering practical engineering applications, the insights provided here will be useful for parametric optimization without heuristics or product-oriented approaches in various applications

Methods

Results

Conclusion