TECHNICAL PHYSICS
VOLUME 44, NUMBER 7
JULY 1999
Initiation of a low-pressure glow discharge in a plasma electron source with a ribbon beam V. Ya. Martens Stavropol’ State Technical University, 355038 Stavropol’, Russia
~Submitted May 18, 1998! Zh. Tekh. Fiz. 69, 135–137 ~July 1999!
Results are presented from an experimental investigation of a low-pressure glow discharge with a wedge-shaped hollow cathode in a plasma electron source, where this discharge is initiated by reflex and magnetron discharges. © 1999 American Institute of Physics. @S1063-7842~99!02907-4#
tial drop reach the cathode potential drop at the opposite part of the hollow cathode and enter the cathode, not along the normal but at a certain angle to it since the surfaces of the wedge-shaped hollow cathode are not parallel. The normal velocity component of these electrons is such that they cannot overcome the cathode potential drop and reach the opposite part of the hollow cathode, and they are reflected by the electric field of the cathode potential drop without energy losses. This electron reflection can be repeated many times and in consequence, the average lifetime of the primary electrons and thus the gas ionization efficiency are increased substantially. This effect may mean that a discharge can exist at low pressure without being sustained by electrons injected from an auxiliary discharge. Initiation of the main discharge by a magnetron discharge takes place as follows ~Fig. 1b!. A switch S is set to position a, whereupon an auxiliary magnetron discharge is ignited between the cathodes 1 and 3, its anode being the wedge-shaped hollow cathode 3. The switch S is then switched to position p and the main discharge is ignited between the wedge-shaped hollow cathode formed by electrodes 1 and 3 and the anode 4. During the switching time no magnetron discharge burns since no voltage is taken from the anode 4 and at this time it is the anode for the magnetron discharge. The minimum required magnetron discharge current for which the main discharge can be initiated is higher than that in the first initiation system and is 200 mA for an argon flow rate of 6.8 m3 mPa/s. Figure 2 shows current–voltage characteristics of a discharge with a wedge-shaped hollow cathode initiated by reflex ~a! and magnetron ~b! discharges. We know4 that a lowvoltage hollow-cathode discharge can only burn stably at currents exceeding a certain critical level. A reduction in the discharge current specifically leads to expansion of the cathode potential drop zone, whose opposite sections can ultimately overlap inside the cavity. This is usually accompanied by an abrupt increase in the discharge burning voltage or the discharge is quenched, as occurs in our case. The points on the extreme left of the experimental curves in Fig. 2 correspond to the critical values of the main discharge current. As the gas flow rate increases and the discharge burning voltage U d decreases, the critical currents decrease.
One of the most effective methods for the heat treatment of semiconductor materials and devices is the electron beam method.1 In particular, cw ribbon electron beams of width 1–3 mm with power densities up to 1 kW/cm2 and electron energies less than 10 keV are suitable for recrystallizing polycrystalline silicon on an insulator.2,3 In order to deliver these beams with a small angle of convergence, the emitters should provide emission current densities up to 100 mA/cm2 . It is advisable to solve this problem using cw plasma emitters in which a plasma of the required density and homogeneity is generated in special gas-discharge structures. In this type of plasma electron source a linear emission channel may be fabricated in the anode or cathode electrodes of the plasma generator. In this case, the comparatively large channel cross section prevents a substantial pressure drop from being established between the working volume of the emitting plasma generator and the accelerating gap for the emitted electrons. Under these conditions the electrode structure of the plasma electron source should contain special initiating systems in the plasma generator to facilitate the ignition of a discharge at low pressure. Here we present results of an investigation of a lowpressure discharge in an electrode structure with a wedgeshaped hollow cathode, where this discharge is initiated by reflex and magnetron discharges ~Figs. 1a and 1b!. In the first case, a reflex ~auxiliary! discharge is initiated in a discharge cell formed by a planar cathode 1, an anode 2, and the planar outer part of a wedge-shaped hollow cathode 3. Electrons from the reflex discharge penetrate through a 3 mm diameter aperture to the inside of the wedge-shaped hollow cathode and initiate the main discharge. The minimum required auxiliary discharge current is 50 mA with a working gas ~argon! flow rate of 4.5 m3 mPa/s. A 6032.5 mm emission slit is provided in the main-discharge anode 4. After the main discharge has been ignited, the auxiliary discharge is quenched by removing the voltage U ad . For an argon flow rate of 4.5 m3 mPa/s, the pressure in the accelerating gap was ;1022 Pa and that in the wedge-shaped cathode was ;1021 Pa. A wedge-shaped hollow cathode was selected for the following reasons. The primary electrons leaving the cathode surface at a certain initial velocity as a result of ion-electron emission and accelerated in the cathode poten1063-7842/99/44(7)/2/$15.00
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© 1999 American Institute of Physics
Tech. Phys. 44 (7), July 1999
V. Ya. Martens
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FIG. 2. Current–voltage characteristics of discharge with wedge-shaped hollow cathode initiated by reflex ~a! and magnetron ~b! discharges. Working gas — argon; Gas flow rate Q, m3 mPa/s: 1 — 3.4, 2 — 4.5, 3 — 5.6, 4 — 5.1, 5 — 6.8, and 6 — 8.4.
FIG. 1. Schematic diagrams of a plasma electron source with a ribbon beam, where the main discharge is initiated by reflex ~a! and magnetron ~b! discharges: 1 — planar cathode, 2 — auxiliary discharge anode, 3 — wedgeshaped hollow cathode, 4 — main discharge anode, 5 — permanent ring magnet, and 6 — collector.
Using these plasma emitters we obtained emission currents I e up to 200 mA with an emission efficiency a 50.5– 0.7 ( a 5I e /I d , where I d is the main discharge current! and economy H 5 1.7–2.3 mA/W. A comparison between the characteristics of the main discharge initiated by reflex and magnetron discharges showed that in the first case, the critical currents are lower and a lower working gas flow rate is required although the design and power supply system are slightly more complex. The choice of a particular variant to develop a plasma electron source with a ribbon beam depends on the specific requirements for the source. The author would like to thank Yu. A. Burachevski for assistance with the experiment.
1
If the main discharges with different systems of initiation are compared, attention is drawn to the fact that in the second case ~Fig. 1b!, even at higher working gas flow rates, the critical currents are higher than in the first case ~Fig. 1a!. This is possibly because of differences in the magnitude and configuration of the magnetic field in the upper part of the wedge-shaped hollow cathode.
R. A. McMahon, H. Ahmed, D. J. Godfrey et al., Microelectron. J. 15~2!, 5 ~1984!. 2 L. R. Thompson, J. A. Knapp, C. A. Moore et al., Mater. Res. Soc. Symp. Proc. 107, 195 ~1988!. 3 C. A. Moore, J. D. Meyer, J. T. Fukumoto et al., Mater. Res. Soc. Symp. Proc. 107, 207 ~1988!. 4 S. P. Bugaev, Yu. E. Krendel’, and P. M. Shchanin, Large Cross-Section Electron Beams @in Russian#, E´nergoatomizdat, Moscow ~1984!, p. 43. Translated by R. M. Durham
