1362 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 4, AUGUST 2008

Structure in RF Hydrogen PlasmaInduced by Magnetic Field

E. V. Barnat, Member, IEEE

Abstract—Images of the spatial structure of a capacitively cou-pled hydrogen discharge are presented for various strengths ofapplied magnetic field. With increasing magnetic field, we findthat not only does the distribution of emission change becauseof the confinement of the electrons by the magnetic field, but wealso find “dark-bands” regions that form in the discharge. Byusing narrowband interference filters (∼10 nm bandwidth), weexamine how the relative optical emission centered on Hα and Hβ

(with respect to the total optical emission) change with the appliedmagnetic field.

Index Terms—Double layer, hydrogen plasma, magnetic field,radio frequency discharge.

W E PRESENT images of a 25-mtorr 75-W 13.56-MHzhydrogen discharge influenced by the presence of a

magnetic field. As discussed and characterized in our earlierstudy [1], a pair of coils are placed about a Gaseous Electron-ics Conference reference cell. A dc current through the coilsgenerates a (mostly) horizontal magnetic field, parallel to the100 mm electrode. For the images presented here, we changethe spacing between the powered (lower) electrode and thegrounded (upper) electrode from ∼20 to ∼50 mm.

Images of optical emission are obtained of the dis-charge as the magnetic field strength is increased from 0 to150 G using a color digital camera (Sony Mavica) and agated intensified charge coupled devices (ICCD) (Andor Istar).Images obtained with the digital camera are presented foraesthetics while images obtained with the ICCD are presentedfor quantitative comparison and analysis. Images of plasma-induced emission (PIE) centered on Hα (656 nm), Hβ (486 nm),and Hγ (434 nm) are obtained using narrowband (<10 nm, full-width at half-maximum) interference filters.

Fig. 1 demonstrates the impact the applied magnetic fieldhas on the hydrogen discharge. The first column contains thefull color images for increasing the magnetic field. Only thecontrast and brightness are changed to highlight “key” featuresof the plasma. The second column corresponds to images ofHβ emission, normalized to images of Hα. Each image is firstnormalized to the peak intensity of the PIE measured withoutany external magnetic field applied. We present the data in thismanner to highlight changes in the excitation characteristics

Manuscript received November 28, 2007. This work was supported in partby Applied Materials.

The author is with Sandia National Laboratories, Albuquerque, NM 87185-1423 USA (e-mail: evbarna@sandia.gov).

Digital Object Identifier 10.1109/TPS.2008.917788

of the discharge. Images of (Hγ/Hα) behave in a mannercomparable to (Hβ/Hα), but are not shown here.

For the 0 G case, we observe a diffuse glow discharge, wherethe peak in the emission occurs in a region of 20 to 40 mmabove the powered electrode. In this region there is little changein the ratio of Hβ emission to Hα emission. In the “dark”sheath region between the powered electrode and the diffuseglow, we see a reduction in the Hβ emission compared to Hα.This reduction becomes more pronounced closer to the poweredelectrode.

For intermediate magnetic field strengths of 20 and 40 G, wesee a constriction of the plasma to a region closer to the poweredelectrode. In general, the structure of the plasma remains quali-tatively similar to that observed in the 0 G case. Specifically,there is a “dark” sheath region near the powered electrodewhere emission from Hβ is reduced compared to Hα. However,for the 20 G case, this “dark” region appears to contain asecondary brighter region within it. While not apparent in theimage of (Hβ/Hα), this second region of excitation was seenin the image of (Hγ/Hα). Finally, above the bright plasmaregion, there is a very faint glow (not clearly seen in the images)dominated by Hα emission.

For higher magnetic field strengths of 80 and 150 G, thereis a substantially different structure introduced to the plasmathan had been observed for the lower magnetic field strengths.Here, we observe a clear separation of the plasma, delineated by“dark bands” starting at the edges of the powered electrode andmoving diagonally upwards toward the center of the discharge.Comparison of Hβ to Hα clearly accentuates the presence ofthese “dark-bands,” where Hβ emission is greatly reduced inthese regions. The inner region of the plasma bounded by these“dark-bands” has similar (Hβ/Hα) emission characteristics ofthe bulk plasma observed in the 0 G case, while the “outerwings” of the plasma shows augmented (Hβ/Hα) emission.

We suspect that these “dark-bands” are double layers that areseparating different plasma regions. Double layers are regionsof electric fields inside the plasma that form because of (abrupt)changes in the local characteristics of the plasma, such asspace charge densities or plasma potentials. These double layerscan be induced by changes in discharge geometry [2], [3] orthey can be induced by biased electrodes placed in the plasma[4], [5].

As the discharge becomes magnetized, we may have a situ-ation where the charged particle loss rate in the central plasmabounded by the “dark-bands” and in intimate contact with thepowered electrode may be significantly different than the lossrate of the “outer wings.” A potential between these two regionsmay be induced to compensate for these differences.

0093-3813/$25.00 © 2008 IEEE

BARNAT: STRUCTURE IN RF HYDROGEN PLASMA INDUCED BY MAGNETIC FIELD 1363

Fig. 1. Comparison of images of PIE produced in a 25-mtorr 75-W capacitively coupled hydrogen discharge driven at 13.56 MHz, influenced by a magneticfield. The left column corresponds to full color images obtained with a digital camera while the right hand column compares ICCD images of Hβ emission toICCD images of Hα emission. ICCD images are normalized to peak intensities measured in the “bulk plasma” region measured for the 0 G case.

REFERENCES

[1] E. V. Barnat, P. A. Miller, T. Panagopoulos, and A. M. Paterson, PlasmaSources Sci. Technol., submitted for publication.

[2] J. G. Andrews and J. E. Allen, “Theory of a double sheath between twoplasmas,” Proc. R. Soc. Lond. A, vol. 320, no. 1543, pp. 459–472, 1970.

[3] M. A. Lieberman, C. Charles, and R. W. Boswell, “A theory for formationof a low pressure, current-free double layer,” J. Phys. D, Appl. Phys.,vol. 39, no. 15, pp. 3294–3304, Aug. 2006.

[4] N. Hershkowitz, G. L. Payne, C. Chan, and J. DeKock, “Weak doublelayers,” Plasma Phys., vol. 23, no. 10, pp. 903–925, 1981.

[5] D. Tang and P. K. Chu, “Anode double layer in magnetized radio frequencyinductively coupled hydrogen plasma,” J. Appl. Phys., vol. 94, no. 3,pp. 1390–1395, 2003.