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Title: Alkali-halogen plasma generation by dc magnetron discharge

Abstract

An alkali-halogen plasma is generated by a dc magnetron discharge using thermal cathodes under a uniform magnetic field. Alkali-salt vapor is dissociated and ionized by ExB-drift electron impact, and alkali positive ions and halogen negative ions are produced. A magnetic-filter region is situated at an exit of the discharge region and electrons are removed from the plasma. The electron emission and E/B fields are optimized, resulting in the alkali-halogen plasma with the ion density of 3x10{sup 8} cm{sup -3} at B=0.2 T.

Authors:
; ;  [1]
  1. Department of Electronic Engineering, Tohoku University, Sendai 980-8579 (Japan)
Publication Date:
OSTI Identifier:
20779289
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 88; Journal Issue: 19; Other Information: DOI: 10.1063/1.2202723; (c) 2006 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; ANIONS; CATHODES; CATIONS; ELECTROMAGNETIC FIELDS; ELECTRON DRIFT; ELECTRON EMISSION; ELECTRONS; HALOGENS; HIGH-FREQUENCY DISCHARGES; ION DENSITY; IONIZATION; MAGNETIC FIELDS; MAGNETIC FILTERS; MAGNETRONS; PLASMA; PLASMA DENSITY; SALTS; VAPORS

Citation Formats

Oohara, W., Nakahata, M., and Hatakeyama, R.. Alkali-halogen plasma generation by dc magnetron discharge. United States: N. p., 2006. Web. doi:10.1063/1.2202723.
Oohara, W., Nakahata, M., & Hatakeyama, R.. Alkali-halogen plasma generation by dc magnetron discharge. United States. doi:10.1063/1.2202723.
Oohara, W., Nakahata, M., and Hatakeyama, R.. Mon . "Alkali-halogen plasma generation by dc magnetron discharge". United States. doi:10.1063/1.2202723.
@article{osti_20779289,
title = {Alkali-halogen plasma generation by dc magnetron discharge},
author = {Oohara, W. and Nakahata, M. and Hatakeyama, R.},
abstractNote = {An alkali-halogen plasma is generated by a dc magnetron discharge using thermal cathodes under a uniform magnetic field. Alkali-salt vapor is dissociated and ionized by ExB-drift electron impact, and alkali positive ions and halogen negative ions are produced. A magnetic-filter region is situated at an exit of the discharge region and electrons are removed from the plasma. The electron emission and E/B fields are optimized, resulting in the alkali-halogen plasma with the ion density of 3x10{sup 8} cm{sup -3} at B=0.2 T.},
doi = {10.1063/1.2202723},
journal = {Applied Physics Letters},
number = 19,
volume = 88,
place = {United States},
year = {Mon May 08 00:00:00 EDT 2006},
month = {Mon May 08 00:00:00 EDT 2006}
}
  • The precise determination of the relative concentration of negative ions is very important for the optimization of magnetron sputtering processes, especially for those undertaken in a multicomponent background produced by adding electronegative gases, such as oxygen, to the discharge. The temporal behavior of an ion acoustic wave excited from a stainless steel grid inside the plasma chamber is used to determine the relative negative ion concentration in the magnetron discharge plasma. The phase velocity of the ion acoustic wave in the presence of negative ions is found to be faster than in a pure argon plasma, and the phase velocitymore » increases with the oxygen partial pressure. Optical emission spectroscopy further confirms the increase in the oxygen negative ion density, along with a decrease in the argon positive ion density under the same discharge conditions. The relative negative ion concentration values measured by ion acoustic waves are compared with those measured by a single Langmuir probe, and a similarity in the results obtained by both techniques is observed.« less
  • The electron drift phenomenon is investigated in the downstream region of an unbalanced dc magnetron argon discharge. The spatially resolved measurements of the electron velocity distribution function (EVDF) using a planar probe reveal the existence of a strong on-axis electron drift parallel to magnetic field in spite of a very small axial variation less than 1 V in the plasma potential. The average drift velocities calculated from the asymmetry of the measured EVDFs show that there exists a significant electron drift from cathode to substrate with a maximum speed of about 1x10{sup 6} m/s, which is comparable to the bulkmore » electron temperature. The magnetic mirror force which is driven by the axial gradient of the magnetic field (i.e., the parallel {nabla}B force) is suggested as a possible source for the parallel electron drift. Carrying out a scaling of current densities with the measured data, it is found that the parallel {nabla}B force can produce the electron current enough to balance the discharge current, implying that the electron transport in the downstream region is determined not by the classical diffusion model in which electron motion toward the anode is diffusion and mobility dominated but by the modified diffusion model in which electron motion is drift dominated.« less
  • The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350 kHz) and reduced duty cycle (90%-50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350 kHz andmore » 60% duty cycle, the total power density is 83 mW/cm{sup 2}, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probe measurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [J. W. Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.« less
  • This paper investigates the spatial and temporal variation in plasma electron density over a region between 5 and 10 cm above the race-track region of a pulsed magnetron sputtering target. The pulse operation is performed using an asymmetric bipolar pulsed dc power supply, which provides a sequence of large negative ''on-phase'' voltage (-350 V) and a small positive ''reverse-phase'' voltage (+10 V) for 55% of the pulse duration (10 {mu}s). The electron density is measured using a floating microwave hairpin resonance probe. The results show electron expulsion from the target in the initial on phase, which propagates with a characteristicmore » speed exceeding the ion thermal speed. In the steady state on phase, a consistent higher density is observed. A quantitative model has been developed to explain the resultant density drops in the initial on phase. While in the reverse phase, we observed an anomalous growth in density at a specific location from the target (d>7 cm). The mechanism behind the increase in electron density has been attributed to the modulation in spatial plasma potential, which was measured earlier in the same apparatus using a floating emissive probe [J. W. Bradley et al., Plasma Sources Sci. Technol. 13, 189 (2004)].« less
  • The time-resolved negative oxygen ion density n{sub -} close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O{sup -} density remains pretty constant during the driven phase of the discharge at values typically below 5x10{sup 14} m{sup -3}; however, in the off-time, the O{sup -} density growsmore » reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) {alpha}=n{sub -}/n{sub e} being higher in the off phase (maximum value {alpha}{approx}1) than in the on phase ({alpha}=0.05-0.3). The authors also see higher values of {alpha} at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative electron attachment of highly excited oxygen molecules in the cooling plasma. The results show that close to the magnetic null the photodetached electron density decays quickly after the laser pulse, followed by a slow decay over a few microseconds governed by the negative ion temperature. However, in the magnetized regions of the plasma, this decay is more gradual. This is attributed to the different cross-field transport rates for electrons in these two regions. The resonance hairpin probe measurements of the photoelectron densities are compared directly to photoelectron currents obtained using a conventional Langmuir probe. There is good agreement in the general trends, particularly in the off-time.« less