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Title: Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff

Abstract

Simple kinematic considerations indicate that, under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on ion velocities upstream, at the presheath/sheath boundary. By measuring this amplitude, the velocities at which ions exit the presheath can be determined and long-standing controversies regarding presheath transport can be resolved. Here, IEDs measured in rf-biased, inductively coupled plasmas in CF{sub 4} gas determined the presheath exit velocities of all significant positive ions: CF{sub 3}{sup +}, CF{sub 2}{sup +}, CF{sup +}, and F{sup +}. At higher bias voltages, we detected essentially the same velocity for all four ions. For all ions, measured velocities were significantly lower than the Bohm velocity and the electropositive ion sound speed. Neither is an accurate boundary condition for rf sheaths in electronegative gases: under certain low-frequency, high-voltage criteria defined here, either yields large errors in predicted IEDs. These results indicate that many widely used sheath models will need to be revised.

Authors:
; ;  [1]
  1. National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (United States)
Publication Date:
OSTI Identifier:
22590616
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 109; Journal Issue: 2; Other Information: (c) 2016 U.S. Government; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; AMPLITUDES; BOUNDARY CONDITIONS; CARBON TETRAFLUORIDE; CATIONS; ELECTRIC POTENTIAL; ELECTRODES; ENERGY SPECTRA; ERRORS; GASES; PEAKS; PLASMA; RADIOWAVE RADIATION; SOUND WAVES; VELOCITY

Citation Formats

Sobolewski, Mark A., Wang, Yicheng, and Goyette, Amanda. Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff. United States: N. p., 2016. Web. doi:10.1063/1.4958836.
Sobolewski, Mark A., Wang, Yicheng, & Goyette, Amanda. Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff. United States. doi:10.1063/1.4958836.
Sobolewski, Mark A., Wang, Yicheng, and Goyette, Amanda. 2016. "Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff". United States. doi:10.1063/1.4958836.
@article{osti_22590616,
title = {Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff},
author = {Sobolewski, Mark A. and Wang, Yicheng and Goyette, Amanda},
abstractNote = {Simple kinematic considerations indicate that, under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on ion velocities upstream, at the presheath/sheath boundary. By measuring this amplitude, the velocities at which ions exit the presheath can be determined and long-standing controversies regarding presheath transport can be resolved. Here, IEDs measured in rf-biased, inductively coupled plasmas in CF{sub 4} gas determined the presheath exit velocities of all significant positive ions: CF{sub 3}{sup +}, CF{sub 2}{sup +}, CF{sup +}, and F{sup +}. At higher bias voltages, we detected essentially the same velocity for all four ions. For all ions, measured velocities were significantly lower than the Bohm velocity and the electropositive ion sound speed. Neither is an accurate boundary condition for rf sheaths in electronegative gases: under certain low-frequency, high-voltage criteria defined here, either yields large errors in predicted IEDs. These results indicate that many widely used sheath models will need to be revised.},
doi = {10.1063/1.4958836},
journal = {Applied Physics Letters},
number = 2,
volume = 109,
place = {United States},
year = 2016,
month = 7
}
  • The negative ion density profile in a low pressure oxygen rf plasma has been measured by a photodetachment technique. At an rf power of 10 W and a neutral pressure of 10 mTorr, a parabolic negative ion density profile is obtained with a peak density of 8[times]10[sup 15] m[sup [minus]3] and a maximum ratio of negative ion to electron densities [ital n][sub [minus]]/[ital n][sub [ital e]][approx]18. Under these conditions, the most abundant positive ion, determined by ion mass spectrometry, is O[sub 2][sup +] with O[sup +] being less than 10% of the positive ion density. The most abundant negative ionmore » is O[sup [minus]] with O[sub 2][sup [minus]] and O[sub 3][sup [minus]] being less than 20% of the total negative charge density. The maximum in the density profile of negative ions shifts closer to the powered rf electrode as the pressure is increased in the asymmetric system. Comparison of the results to theory indicates that the asymmetry follows from an enhancement of the ionization rate near the powered electrode sheath. The parabolic profile is also obtained in CCl[sub 2]F[sub 2] at low pressure. Simulations and measurements show a rapid drop in ion density near the sheath that may be related to the recently discussed stratification'' phenomenon in electronegative plasmas.« less
  • Unlike {alpha}- and {gamma}-mode operation, electrons accelerated by strong drift and ambipolar electric fields in the plasma bulk and at the sheath edges are found to dominate the ionization in strongly electronegative discharges. These fields are caused by a low bulk conductivity and local maxima of the electron density at the sheath edges, respectively. This drift-ambipolar mode is investigated by kinetic particle simulations, experimental phase-resolved optical emission spectroscopy, and an analytical model in CF{sub 4}. Mode transitions induced by voltage and pressure variations are studied.
  • We have measured the distributions of N{sub 2}{sup +} ion velocity components parallel and perpendicular to the electrode in the sheath of a radio-frequency nitrogen reactive ion etching discharge, using pulsed laser-induced fluorescence. Parallel to the electrode, the ions have throughout a thermal distribution that is found to be consistent with the rotational temperature of 355 K. In the perpendicular direction, we see clearly the acceleration of the ions towards the electrode, and our results agree well with theoretical predictions although an unexpected peak of unaccelerated ions persists. We have also determined the absolute ion concentrations in the sheath, whichmore » we have calibrated by analyzing the decay in laser-induced fluorescence in the plasma bulk after discharge extinction. At 20 mTorr, the bulk concentration of 1.0{times}10{sup 10} cm{sup {minus}3} falls to around 2{times}10{sup 8} cm{sup {minus}3} at 2 mm from the electrode. {copyright} {ital 1997 American Institute of Physics.}« less
  • We develop a global model for high pressure (0.1{endash}1 Torr) electronegative rf discharges and apply it to model a capacitively driven plasma etcher. The molecular gases considered consist of either pure chlorine species or a mixture of chlorine and helium species. The charged and neutral heavy particle densities together with the electron density and electron temperature are calculated by using the equations of particle balance and power balance for the input discharge parameters rf power or rf current, inlet pressure, gas flow rates, reactor diameter, and gap spacing. The power is deposited in the electrons via ohmic heating and inmore » those ions accelerated across the dc sheath potential. The voltage across the sheath is calculated self-consistently with the densities and the electron temperature by using a collisional Child law sheath model. Analytic scaling laws for the dependence of charged and neutral particle densities, electron temperature, rf voltage and current, sheath width, and plasma impedance on pressure and absorbed rf power are presented and used to explain the numerical results obtained from the global model. The model results are compared to recent experimental measurements in a chlorine discharge over a range of absorbed power P{sub abs}=20{endash}180W at an inlet pressure p{sub in}=0.4 Torr and a range of pressure 0.1{endash}1.6 Torr with a fixed input power of 100 W. We obtain reasonable agreement for P{sub abs}{lt}200W and for 0.2 Torr{lt}p{sub in}{lt}1Torr. {copyright} {ital 1997 American Vacuum Society.}« less
  • In this paper a continuum model is presented for low-pressure, radio-frequency (RF) electronegative discharges commonly encountered in reactive ion etching and plasma-deposition applications. The model is based on the moments of the Boltzmann transport equations. Local and convective acceleration terms are retained in the momentum equations for the electrons and ions as it allows nonlocal transport in weakly collisional regions. A stable numerical scheme to solve these equations is also presented. A chlorine discharge at 13.56 MHz is simulated here as a case study. The simulation results reproduce features observed experimentally in Cl{sub 2} discharges under similar conditions. Of particularmore » importance is the simulated excitation and ionization waveforms. In the bulk, the waveforms peak twice per cycle, which is essentially due to the modulation of electron temperature; in the sheath regions, the waveforms peak only during the anodic part of the cycle when the electrons are accelerated toward the electrode. The time-averaged ionization, excitation, and attachment rate profiles exhibit peaks near each sheath/plasma interface. The energy of the ions striking the electrodes is computed self-consistently. The ratio of ion energy to the average potential difference between the plasma and the electrodes is found to be 0.33.« less