Reprinted from
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Nuclear Instruments and Methods in Physics Research A 439 (2000) 31-44
High-current pulse sources of broad beams of gas and metal ions for surface treatment N.V. Gavrilov3*, E.M. Oksb 'Institute ofElectrophysics, RAS, 34 Komsomolskaya St., Yekaterinburg 620049, Russia ^Institute of High-Current Electronics, RAS, 4 Akademichesky Ave., Tomsk 634055, Russia Received 6 April 1999; received in revised form 26 July 1999; accepted 6 August 1999
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Section A
Nuclear Instruments and Methods in Physics Research A 439 (2000) 31-44
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High-current pulse sources of broad beams of gas and metal ions for surface treatment N.V. Gavrilov3-*, E.M. Oks b ''Institute of Electrophysics, R,4S, 34 Komsomolskayu St., Yekaterinburg 620049, Russia Institute of High-Current Electronics, RAS, 4 Akademichesky Ave., Tomsk 634055, Russia Received 6 April 1999; received in revised form 26 July 1999; accepted 6 August 1999
Abstract This paper reviews the experimental study, development, and improvement of various types of processing ion sources undertaken in association with the joint program performed in recent years by the Institute of Electrophysics and the Institute of High-Current Electronics of the Russian Academy of Sciences. The beam parameters (type and energy of ions, current density, cross-sectional area of the beam, permissible content of impurities, etc.) should meet the requirements of particular ion beam treatment conditions, while the ion source itself should be simple and reliable in operation. Technical and service characteristics of the developed ion sources permit their use for ion-beam modification of materials, preparation of surfaces and ion-assisted deposition of thin films, and in some other applications. The sources under consideration employ high-current glow discharges with a hollow cathode or in crossed electric and magnetic fields, and low-pressure arc discharges and vacuum arc. Cold cathodes enhance reliability of the ion sources when they work at a high residual gas pressure or in the reactive gas media. The repetitive pulse mode of the plasma and beam generation provides optimum conditions for stable operation of the discharge, control of the average beam current over a wide range, and formation of homogeneous large-cross-section beams. The paper describes techniques used to realize high-current discharges at a reduced pressure, methods for producing a stable, dense and homogeneous plasma in a large volume, techniques of formation of large-cross-section homogeneous beams, and also findings on the mass-charge composition of the plasma and beams produced. Some design versions of the sources are given. At voltages from 10 to 100 kV. the pulse duration of 10 to 1000 us. and the pulse repetition rate of 1 to 500 Hz these sources provide the current density of ~ 1-10 mA/cnr in beams having the cross-sectional area of a few hundreds of square centimeters. The achieved parameters of the sources permit their use both in commercial and research applications. The use of these sources for modification of surface properties of materials has been exemplified. © 2000 Elsevier Science B.V. All rights reserved. PACS: 07.77. + p; 41.75. - I; 52.75. - d Keywords: Ion beam; Plasma; Ion source; Glow discharge; Low-pressure constricted arc: Vacuum arc
1. Introduction * Corresponding author. Tel.: (3432)-499-185; fax: (3432)-741353 E-mail address:
[email protected] (N.v. Gavrilov)
T h e
development of commercial ion-beam technologies for surface modification of materials is impossible without highly efficient, simple and
0168-9002/00/$-see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 O 0 2 ( 9 9 ) 0 0 8 9 5 - 5
32
N. V. Gavrilov, EM. Oks I Nuclear Instruments and Methods in Physics Research A 439 (2000) 31-44
dependable ion sources. These sources should produce nonseparated broad beams (BBs) with the ion energy of 10-100 keV and operate stably in an imperfect vacuum. Since the result of ion beam treatment depends not only on such basic parameters as the type, energy and fluence of ions but also on the conditions (current density, temperature of the treated article, mass-charge composition and impurity content of the beam, vacuum quality, etc.) that are determined by the operating principle and the operation mode of the source, the sources to be designed by all means are subject to careful preliminary certification. Moreover, creation of new types of the sources whose beam parameters differ from those of the earlier sources (high density of the beam current or unique mass-charge composition) open up new horizons of physical research and development of respective novel technologies. Therefore development of the ion sources is intimately connected with the appropriate investigations in the physics of plasma, physical electronics, and the physics of the interaction between particles and a solid. One of the leads in development of the processing ion sources, which would have the required combination of high technical and service characteristics, involves the use of high-current glow or arc cold-cathode discharges. These discharges are less demanding with respect to the working atmosphere as compared to hot-cathode systems. Since the properties of cold- and hot-cathode discharges differ drastically, cold-cathode sources cannot utilize the principles used for hot-cathode ion sources, which have been brought to a high degree of perfection [1-4]. New concepts and approaches are needed to solve a number of physical problems related to initiation and stability of selfsustained high-current discharges at a low pressure, generation of a stable, dense and homogeneous plasma in a large volume, formation of a uniform plasma emitter of ions and homogeneous BBs, and decreasing contamination of the ion beam due to cold cathode sputtering. These problems have been largely solved thanks to the repetitive pulse mode (RPM) used to generate the plasma and the ion beam. The proper choice of the discharge current and the pulse duration ensures stable operation of both the low-current arc with a cathode-spot and
the high-current glow discharge. The average beam current can be adjusted within wide limits by varying the pulse repetition rate / In this case the conditions of the beam formation in a multiaperture ion optical system (IOS) do not change and therefore the beam remains homogeneous at the target. The studies have resulted in development of sources of gas or metal ions or mixtures thereof. The beam composition is adjustable within wide limits.
2. Electrode systems of the ion sources and basic properties of the plasma emission structures 2.1. Glow-discharge electrode systems Operation of the high-current glow discharge at the minimum possible gas pressure and generation of a homogeneous discharge plasma is ensured on condition of the proper choice of the form and dimensions of the discharge system electrodes, and the configuration and magnitude of the magnetic field. This provides for: (a) efficient ionization of the gas by fast electrons, which are formed as a result of the ion bombardment of the cathode and accelerated in the region of the cathodic potential drop; maximum energy relaxation of the electrons in the plasma; (b) maximum utilization of the ions, which were not extracted to the beam from the plasma, for maintaining the electron emission from the cathode; (c) oscillation of fast electrons in the electrode cavity under the action of electrostatic and magnetic fields, and (d) stable transport of the generated electrons to the anode without formation of a high-voltage near-anode layer or build-up of plasma instabilities. Two main types of the electrode systems used in the ion sources [5] can be cited: 1. a hollow-cathode discharge electrode system where the ion emitting plasma is generated in the cathode cavity, 2. an electrode system of a discharge with a hollow cathode and a hollow anode where the ion emitting plasma is produced in the anode cavity.
33
N. V. Gavrilov, E.M. Oks / Nuclear Instruments and Methods in Physics Research A 439 (201)0) 31-44 Hollow cathode
Anode = 0 . 0 4 ( 1 ) ; 0 . 3 2 ( 2 ) ; 0 . 7 2 (3) m T
10
20
30
1, cm
The type 1 systems are simplest in design. They comprise a cylindrical hollow cathode, whose cross-sectional area is nearly equal to the required cross-sectional area of the ion beam, and a smallsize rod anode placed inside the cathode (see Fig. 1). The surface area of the anode in this gas discharge system should IT; as small as possible [6], but sufficient to close the current of thermal electrons, which is equal to the discharge current [7], to the anode. The small-size anode and, correspondingly, a small surface area of the loss of fast electrons provides their oscillation inside the cavity. This favors a maximum energy relaxation of the fast electrons in the plasma and adds to the spatial homogeneity of the plasma. The static breakdown voltage of the gap, the operating voltage of the glow discharge U and the minimum operating pressure of the gas in this structure are reduced when a magnetic field is applied. Moreover, a weak magnetic field decreases the radial inhomogeneity of the plasma (Fig. 2). This is due to the increase in the density of fast particles and a more intense ionization at the periphery of the system. However, only a weak magnetic field can be used in the discharge system with a small-size anode. For example, the optimal value of the induction B is 0.2-2 mT in systems
ouu
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sA V 1
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