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Title: Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths

Abstract

Many of today's processing plasma tools are operated at low pressures to achieve high etch directivity and reduce side erosion on the wafer. At these pressures electron-neutral collisions are rare and the electrons cannot gain energy through the ohmic heating process. Instead, the heating mechanism is attributed to a stochastic process between the electrons and the sheath electric field. Theoretical models of this stochastic process include the hard wall approximation and the pressure heating effect. The former is inconsistent with electron current conservation at the sheath, while the latter shows a difference in power absorption when electron loss to the electrodes is considered. This article examines the effects of electron current on a capacitive sheath by controlling this current with an additional dc bias on the electrode. Experimental and particle-in-cell model results for a low-pressure argon plasma are compared and presented. Results show that the electron power absorption is more effective when the electron current is removed. The model also shows a high harmonic content on the sheath voltage which is attenuated by removing the electron current. These high-frequency harmonics are measured in the experiment with a floating probe, and their correlation with the electron current is in agreement withmore » the model results.« less

Authors:
;  [1]
  1. National Centre for Plasma Science and Technology, School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9 (Ireland)
Publication Date:
OSTI Identifier:
20982626
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 101; Journal Issue: 2; Other Information: DOI: 10.1063/1.2422748; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; APPROXIMATIONS; ARGON; ELECTRIC FIELDS; ELECTRIC POTENTIAL; ELECTRODES; ELECTRON LOSS; ELECTRONS; EROSION; ETCHING; PLASMA; PLASMA HEATING; PLASMA PRESSURE; PLASMA SHEATH; PLASMA SIMULATION; RADIOWAVE RADIATION; STOCHASTIC PROCESSES; WALL EFFECTS

Citation Formats

Gahan, D., and Soberon, F. Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths. United States: N. p., 2007. Web. doi:10.1063/1.2422748.
Gahan, D., & Soberon, F. Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths. United States. doi:10.1063/1.2422748.
Gahan, D., and Soberon, F. Mon . "Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths". United States. doi:10.1063/1.2422748.
@article{osti_20982626,
title = {Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths},
author = {Gahan, D. and Soberon, F.},
abstractNote = {Many of today's processing plasma tools are operated at low pressures to achieve high etch directivity and reduce side erosion on the wafer. At these pressures electron-neutral collisions are rare and the electrons cannot gain energy through the ohmic heating process. Instead, the heating mechanism is attributed to a stochastic process between the electrons and the sheath electric field. Theoretical models of this stochastic process include the hard wall approximation and the pressure heating effect. The former is inconsistent with electron current conservation at the sheath, while the latter shows a difference in power absorption when electron loss to the electrodes is considered. This article examines the effects of electron current on a capacitive sheath by controlling this current with an additional dc bias on the electrode. Experimental and particle-in-cell model results for a low-pressure argon plasma are compared and presented. Results show that the electron power absorption is more effective when the electron current is removed. The model also shows a high harmonic content on the sheath voltage which is attenuated by removing the electron current. These high-frequency harmonics are measured in the experiment with a floating probe, and their correlation with the electron current is in agreement with the model results.},
doi = {10.1063/1.2422748},
journal = {Journal of Applied Physics},
number = 2,
volume = 101,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • A capacitively coupled, rf biased electrode mounted in an inductively coupled plasma reactor is used to investigate rf power coupling through the resulting capacitive sheath. Carefully calibrated current and voltage probes provide the electrical measurements necessary for this investigation. An external negative dc bias enables control over the electron component of the total current arriving at the electrode through the capacitive sheath. Sheath models generally assume that electron loss at the end of the rf cycle is negligible but recent theoretical work indicates that there is a significant difference in power coupled through the sheath when electron loss is permitted.more » The experimental study presented here confirms this result. Retardation of electron loss to the electrode is accompanied with an increase in rf power absorbed by the electrons in the capacitive sheath. Comparing the current signals with and without electron loss establishes that the measured increase in power is associated with the electron loss phase of the rf cycle, which is consistent with predictions of the earlier theoretical work.« less
  • Negative hydrogen ion sources, for instance for fusion devices, currently attract considerable attention. To generate the precursors—highly rovibrationally excited hydrogen molecules—for negative hydrogen ions effectively by electron excitation, a thin dielectric layer is introduced to cover the surface of the electrically grounded electrode of two parallel metal plates in a low-pressure hydrogen capacitive discharge driven by combined rf and pulse power sources. To understand the characteristics of such discharges, particle-in-cell simulations are conducted to study the effects that the single dielectric layer would bring onto the discharges. The simulation results show that the dielectric layer leads to a much highermore » plasma density and a much larger production rate of highly vibrationally excited hydrogen molecules compared to discharges without the dielectric layer on the electrode. Further investigation indicates that the nonlinear oscillation of the electrons induced by the nanosecond-pulse continues until it is finally damped down and does not show any dependence on the pulse plateau-time, which is in stark contrast to the case without the dielectric layer present. The physical reason for this phenomenon is explored and explained.« less
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