ISSN 10637850, Technical Physics Letters, 2009, Vol. 35, No. 6, pp. 511–513. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.A. Burdovitsin, A.S. Klimov, E.M. Oks, 2009, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2009, Vol. 35, No. 11, pp. 61–66.
On the Possibility of ElectronBeam Processing of Dielectrics Using a Forevacuum Plasma Electron Source V. A. Burdovitsin*, A. S. Klimov, and E. M. Oks Tomsk State University of Control Systems and Radio Electronics, Tomsk, 634050 Russia *email:
[email protected] Revised manuscript received January 22, 2009
Abstract—An insulated target was irradiated by an electron beam generated by a forevacuum plasma electron source operating in the pressure range of 5–15 Pa. Measurements of the target potential showed that plasma formed in the region of electron beam transport ensured the almost complete neutralization of charge accu mulated on the target. This effect results in the possibility of direct electronbeam processing of nonconduct ing materials, including the melting and welding of ceramics. PACS numbers: 52.40.Mj DOI: 10.1134/S1063785009060091
As is known, the electronbeam processing of non conducting materials is only possible provided that conditions are created that ensure the neutralization of the negative charge accumulated on the target sur face bombarded by accelerated electrons. These con ditions can be realized, for example, by acting on the target surface simultaneously with electrons and ions, immersing the target in plasma, or heating the dielec tric to a temperature at which it exhibits significant conductivity, ensuring the charge leakage. It should be noted that ceramic materials, which are of most inter est among dielectrics as objects for electronbeam processing, acquire significant conductivity at tem peratures above 1000°C. In all cases mentioned above, it is obviously necessary to use special equipment, which leads to the significant complication of the pro cessing technology and, hence, makes it more expen sive. Plasma electron sources, which employ the princi ple of taking electrons from gasdischarge plasma, offer the principal possibility of effective operation at elevated pressures of the residual gas medium [1]. Investigations performed in recent years allowed us to expand the range of working pressures of these elec tron sources to the socalled forevacuum level of 5⎯15 Pa [2, 3]. In the forevacuum plasma electron sources, the generation of an electron beam is accom panied by the formation of plasma in the beam trans port region, the concentration of which (1010 ⎯1011 cm–3) [4] is probably sufficient to achieve the effective neutralization of the negative charge that arises on the surface of nonconducting targets under the action of an electron beam. Thus, plasma electron sources operating under forevacuum conditions can yield the possibility of direct electronbeam processing of nonconducting materials or insulated targets with
out using special equipment and technologies to neu tralize the beaminduced charge. The present study was aimed at experimentally ver ifying the possibility of direct electronbeam process ing of dielectrics using a plasma electron source oper ating under forevacuum conditions. Figure 1 shows a schematic diagram of the experi mental arrangement intended to measure of the potential of an insulated target irradiated by an elec tron beam at elevated pressures. Plasma source 1 employing hollowcathode discharge, which was spe cially developed to generate an electron beam under forevacuum conditions [5], was mounted on a flange of vacuum chamber 2. Electron beam 3 was focused by a magnetic lens 4 and directed at insulated target 5, the potential of which was measured by voltmeter 6. The beam of electrons with an energy of 2–4 keV propa gating in a residual atmosphere of the vacuum cham ber created plasma 7 along the trajectory. In addition to measurements of the target potential, we also mea
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Fig. 1. Schematic diagram of the experimental arrange ment (see text for explanations).
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BURDOVITSIN et al. ϕt , V −40
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Fig. 3. Alundum ceramics welded by an electron beam under forevacuum conditions.
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amounted to 1–2 eV. On this basis, the plasma poten tial could be estimated as ϕp = ϕf + (2–3)kTe, accord ing to which the target potential was lower than the plasma potential by no more than 10 V.
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Fig. 2. Plots of the insulated target potential ϕt (a) versus pressure p for the electron beam currents Ib = 400 mA (1) and 600 mA (2) and (b) versus electron beam current Ib for a pressure of 4 Pa (1) and 6 Pa (2) at an electron energy of 3 keV.
sured the floating potential of separate probe 8 situated in the immediate vicinity of the target. The probe was provided with a screen that prevented electrons from the beam from striking the probe. Experiments showed that, during the action of an electron beam on the insulated target under forevac uum conditions, the floating potential ϕt of the target (despite an electron beam energy of several kiloelec tronvolts) was only a few dozen volts below the poten tial of the grounded chamber walls. The absolute value of ϕt somewhat increased with the beam current and decreased with growing pressure (Fig. 2). Evidently, these variations in the potential of the insulated target cannot produce any significant effect of the trajectory of accelerated electrons bombarding the target. The floating potential ϕf of probe 8 (Fig. 1) was 3–4 V above the target potential. The electron tempera ture Te determined from the probe characteristic
In order to elucidate the mechanism of establish ment of the target potential ϕt, we have used the results of theoretical analysis and experimental verification of the possible pathways of charge delivery to the target [6]. According to this, the ϕt value is determined by a balance of charges brought to the target by the electron beam and charged plasma species and those carried away by secondary electrons. In the range of pressures typical of plasma electron sources (~10–2 Pa), the plasma density in the region of accelerated electron beam transport is on the order of ne ≈ 108 cm–3. According to [6], both calculations and measurements for these conditions yield the values of ∆ϕ = ϕt – ϕp that almost coincide with the accelerating voltage for the electron beam. In this case, the electron beam is totally reflected. An increase in the pressure by two orders of magnitude on the passage to forevacuum conditions of plasma cathode operation provides for the corresponding growth in the plasma density (up to ~1010 cm–3). According to [6], this growth of ne is quite sufficient for the almost complete neutralization of the electron beam on the insulated target. It is this specific feature of electron beam generation at relatively high pressures that accounts for the possibility of electron beam processing of nonconducting dielectric materi als, in particular, ceramics.
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ON THE POSSIBILITY OF ELECTRONBEAM PROCESSING OF DIELECTRICS
With a plasma electron source operating under for evacuum conditions, the irradiation of glass, quartz, and Alundum (fused alumina) ceramic targets by an accelerated electron beam led to the local melting of the target material in the zone of beam action. In an Alundum ceramic target, the solidification of this melt resulted in the formation of a glassy region. In ceramic materials based on boron nitride, the electron beam action was accompanied by the sublimation of a target material, which made it possible to drill holes. The action of the electron beam on the joints of articles showed the possibility of welding glass, quartz, and Alundum ceramic parts (Fig. 3). Special tests showed that the welds in Alundum ceramics were vacuum tight at residual pressures of up to 0.001 Pa, while tensile testing showed a breaking strength of 15–30 MPa (analogous tests on the initial ceramics yielded 40–50 MPa). Thus, the generation of a dense plasma by an elec tron beam transported under forevacuum conditions ensures the almost complete neutralization of the beam charge on an insulated target, which makes the electronbeam processing of insulated targets and nonconducting dielectrics possible in principle with
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out a need to specially provide conditions for neutral izing the beaminduced charge.
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REFERENCES 1. E. M. Oks, Plasma Cathode Electron Sources: Physics, Technology, Applications (Wiley–VCH, 2006). 2. Yu. A. Burachevsky, V. A. Burdovitsin, A. S. Klimov, E. M. Oks, and M. V. Fedorov, Zh. Tekh. Fiz. 76 (10), 62 (2006) [Tech. Phys. 51, 1316 (2006)]. 3. I. S. Zhirkov, V. A. Burdovitsin, and E. M. Oks, Zh. Tekh. Fiz. 77 (9), 115 (2007) [Tech. Phys. 512, 1217 (2007)]. 4. I. S. Zhirkov, V. A. Burdovitsin, E. M. Oks, and I. V. Osipov, Zh. Tekh. Fiz. 76 (6), 106 (2006) [Tech. Phys. 51, 786 (2006)]. 5. V. A. Burdovitsin, Yu. A. Burachevskii, E. M. Oks, and M. V. Fedorov, Prib. Tekh. Eksp., No. 2, 127 (2003) [Instr. Exp. Techn. 46, 257 (2003)]. 6. V. Ya. Martens, Zh. Tekh. Fiz. 66 (6), 70 (1996) [Tech. Phys. 41, 559 (1996)].
Translated by P. Pozdeev
