Suppression of nonlinear standing wave excitation via the electrical asymmetry effect

Abstract

The electrical asymmetry effect (EAE) enables separate control of the ion flux and the mean ion energy in capacitively coupled plasmas (CCP). While a variety of plasma processing applications benefit from this, large-area, very-high-frequency CCPs still suffer from lateral nonuniformities caused by electromagnetic standing wave effects (SWE). Many of such plasma sources are geometrically asymmetric and are operated at low pressure so that high frequency nonlinear plasma series resonance (PSR) oscillations of the RF current are self-excited. These PSR oscillations lead to the presence of short wavelength electromagnetic waves and a more pronounced SWE. In this work, we investigate the influence of the EAE on the nonlinear standing wave excitation in a geometrically asymmetric, low pressure capacitively coupled argon plasma driven by two consecutive harmonics (30 MHz and 60 MHz) with an adjustable phase shift, θ. We use a hairpin probe to determine the radial distribution of the electron density in combination with a high-frequency B-dot probe to measure the radial distribution of the harmonic magnetic field, which in turn is used to calculate the harmonic current density based on Ampere’s law. Our experimental results show that the asymmetry of the discharge can be reduced electrically via the EAE. In this way the self-excitation of high frequency PSR oscillations can be attenuated. By tuning θ, it is, therefore, possible to switch on and off the nonlinear standing wave excitation caused by the PSR and, accordingly, the plasma uniformity can be optimized.

Fundamental study towards a better understanding of low pressure radio-frequency plasmas for industrial applications

“…Due to continuous growth in both the driving frequency and the wafer size, plasma non-uniformities caused by the standing wave effect have become a major challenge in the material etching and film deposition processes. To optimize the plasma uniformity, several methods have been put forward, such as the use of specially shaped electrodes, [163][164][165][166][167] phase-shift control, [168][169][170] the use of the dual-frequency or multi-frequency source, [133,138,139,171,172] discharge symmetry control, [173,174] etc.…”

“…However, the underlying mechanism of the influence of discharge symmetry on the nonlinear harmonics and standing waves was not clarified. To this end, Zhao et al [174] experimentally investigate the influence of the EAE on the nonlinear standing waves induced by the PSR-enhanced harmonics in a geometrically asymmetric, capacitive argon discharge driven by two consecutive harmonics (30 MHz and 60 MHz) with an adjustable phase shift. To bring the effects of nonlinear harmonic excitations on the plasma uniformity into clear focus, a hairpin probe was used to determine the radial distribution of the electron density while a high-frequency magnetic probe was used to measure the radial distributions of the harmonic magnetic field and harmonic current density (see Fig.…”

Control of electron velocity distributions at the wafer by tailored voltage waveforms in capacitively coupled plasmas to compensate surface charging in high-aspect ratio etch features

“…As the wavelength of the GHz oscillations is comparable, or even smaller than the size of typical plasma processing chambers, radial non-uniformity could be induced due to their self-excitation according to the non-linear standing wave effect [47,50]. An accurate description of the spatial distribution of the plasma parameters would require higher dimensional and electromagnetic simulations, which is out of the scope of the present work.…”

“…Its ability to control plasma properties has already been demonstrated by simulations [27][28][29][30][31][32][33][34] and in laboratory experiments [35][36][37]. VWT was demonstrated to provide control of (i) the dc self-bias voltage via the Electrical Asymmetry Effect to improve the control of ion properties [8,[38][39][40][41][42], (ii) the sheath dynamics to control the electron power absorption dynamics and the electron energy distribution function [43][44][45][46], and (iii) nonlinear electromagnetic effects to optimize plasma uniformity [47][48][49][50][51]. Based on existing multi-frequency impedance matchings [52][53][54], VWT is a modular technology.…”

Equivalent Test Method for Strong Electromagnetic Field Radiation Effect of EED

“…e test method to measure the induced current, voltage, or power cannot effectively evaluate the electromagnetic safety test of the EED under microwave radiation. Under the radiation frequency exceeding GHz, the induced current will form a standing wave distribution on the lead wire of the EED, and the current amplitude at different positions will be altered noticeably [31]. As impacted by the measuring probe, bridge wire thickness, and lead electromagnetic induction, the induced current at the bridge wire position is difficult to be accurately measured; besides, under skin effect, the bridge wire resistance and ignition current change with frequency, and its electromagnetic safety cannot be accurately evaluated [32][33][34][35][36].…”