Conference-ACSIN-10-A New Metal-Ion Source with An Electron-Beam Evaporator for Surface Modification

A metal-ion source equipped with an EB (Electron Beam) gun for the evaporation of solid materials was designed, and its prototype source was constructed. The prototype produced the source plasma successfully by PIG discharge with the metal vapors of titanium, zinc, and manganese without feeding any ignition gas. As a preliminary results, the extracted total currents composed of Ti , Zn and Mn ions were measured as unsaturated currents of about 50 , 115 and 155 μA·dc, respectively, at an extraction voltage of 6 kV. [DOI: 10.1380/ejssnt.2010.131]


I. INTRODUCTION
The purpose of this work is to develop useful ion sources capable of extracting intense large-area metal-ion beams with low energy (below several tens of keV).Ion implantation with metal ion sources is a promising technique for the modification of surfaces and interfaces.Ion implantation has distinct features characterized by the nonthermal equilibrium process as known widely [1,2].The process of ion implantation is notable on the following points regarding surface modification: 1) the ion species and the concentration of implanted impurities on the surface layers can be controlled strictly in the non-thermal equilibrium state, 2) the metastable states of the surface layers are created momentarily, 3) the spatial fine difference of the surface energy is induced with the aid of the impurities implanted to the surface, and 4) sputtering, amorphization, and defect formations occur.
In order to make use of these characteristics for improving physical, chemical, electrical, and biological properties of material surfaces, many kinds of ion species are implanted into various kinds of materials such as metals, alloys, ceramics, plastics, and films.Instead of ion implantation, dry processes are also used; reacting ions such as N for nitridation, C for carburization, and O for oxidation are accelerated with a bias voltage of only a few kVs applied to the samples against plasma, while ion implantation normally requires energy over 10 keV.These techniques are simple and effective on surface modification, and their mechanisms have been studied by ab initio molecular orbital simulation by Inoue [3][4][5][6].
The authors had a great deal of concern regarding how to use ion implantation as a means of surface improvements.The use of ion implantation may be roughly classified as; 1) direct surface modification, 2) IBAD (Ion Beam Assisted Deposition), and 3) ion cascade mixing.
In the first manner, ion implantation was used as a simple means to introduce impurities to bring about some specific property changes on the surfaces.As a result, a prominent surface improvement with a variety of functions was achieved.However, the modified layers were too thin to use in practice, since the mean projected range, for instance, of N ions implanted to a Fe plate at energy of 30 keV was calculated to be about 0.5 µm by LSS theory.In the second technique, ion beams played a role of the "in-situ treatment", in ion-beam-assisted deposition for coating or crystallization.In the last one, an energetic ion beam consisting of inactive ions; (He, Ar etc.), was used to bombard the coated surface, after the coating process, as an "after-treatment" for mixing interfaces.The authors have previously reported that the new application of the use of ion implantation with metal ions as a "pretreatment" is effective on nitridation performed as the main process.In the pretreatment, ion implantation acted for obtaining chemical affinity on the surface by arranging the distribution of surface energy with the assistance of impurities.Ion implantation with one of such light or heavy ions as; Ar, N, Ni, Mo and W into pure aluminum was carried out as the pretreatment at energy of 25 keV, and afterwards, nitridation was performed as the main process.The result was that the effects of ion implantation on nitriding pure aluminum depended sensitively on the ion species and the fluence [7][8][9].
In order to develop these primitive but aggressive uses of ion implantation in the laboratory scale into the industrial scale, it is indispensable to provide a high-end metal ion source that outputs an intense and large crosssectional ion beam of various ion species [10,11].In this article, the overall design of a new metal ion source equipped with an EB gun to evaporate raw materials is introduced, and the preliminary results obtained by operating the prototype were described.The prototype was first constructed to verify whether it is possible to cause the PIG (Penning Ion Gauge)-type discharge with metal vapor diffused from the hearth of the EB gun, with or without adding the ignition gas.Such a discharge was introduced by an abrupt increase of free electrons produced by the ionization of the vapor containing electrons moving freely between the cathode and the unti-cathode while a positive potential over about 100 V was biased to the anode set between the two electrodes.

A. Total design
The elemental conditions required for designing a new metal ion source are; (1) to use the EB gun as an evaporator, and (2) to produce a metal source plasma by PIGdischarge without feeding any ignition gas.Other conditions are; (3) to raise up the density and the volume of the source plasma by connecting other discharge systems such as arc electrodes and rf-coils to the PIG-electrode system, (4) to be convenient for operation and dismantlement, and (5) to have a long life.
An EB gun is the best choice as an evaporator in terms of evaporating materials quickly and conveniently.However, it cannot work under a high gas pressure over about 10 −2 Pa.Meanwhile, the plasma density for extracting an intense ion beam is on the order of 10 12 ions/cm 3 , and the arc discharge is desired to be applied for the production of such an intense source plasma.However, the arc discharge requires a higher gas pressure over 10 −1 Pa.Thus, PIG discharge was utilized first for ionizing metal vapor, since the common gas pressure range for the operation of the EB gun and PIG-discharge is narrow.The relations between the types of discharge and the gas pressure are drawn figuratively in Fig. 1 [12].From the figure, it is es- timated that PIG-discharge might have possibly occurred without arcing in the EB gun in the gas pressure range of 10 −2 Pa.
To satisfy condition (3), some optional techniques were prepared: superposing magnetic fields and/or rf-fields on the discharge chamber, and arraying an anode electrode to introduce arc discharge between unti-cathode and plasma electrodes by using the seed plasma generated by PIGdischarge.Other problems related to conditions (4) and ( 5) have prospected to be solved with common knowledge related to ion source technology.Consequently, the new type of metal ion source was designed as shown in Fig. 2. http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 8 (2010) FIG.3: The conceptual drawing of the prototype metal-ion source.The axial length of discharge chamber (4) was about 7 cm.The PIG-type electrode configuration was constituted with a thermal cathode of filament (7a) and two ring electrodes; the PIG anode and the unti-cathode with inner diameters of 13 and 10 mm, respectively.The metal vapor exit (11a) and the diaphragm (11b) were designed to work as evacuation registers.n: metal vapor, +: ions, and −: electrons.

B. Prototype
In the design explained above, a critical problem to be solved is to determine whether PIG-discharge is fired practically with the vapor diffused from the EB gun (JEBG-203UB).It was suspected that the EB gun might introduce arc discharge in the electron acceleration region when a high vapor pressure was required for plasma ignition.The prototype source with a structure simpler than the apparatus explained above was then built to test out the occurrence of PIG discharge and the extraction of ion beams.The conceptual structure and the explanation of the prototype source are shown in Fig. 3.
The second challenge was to collect enough amount of metal vapor to ignite PIG discharge in the plasma chamber, without breaking out arcing in the EB gun.So, two diaphragms were built in as evacuation resistors; one was set up as a metal vapor exit (11a) between the discharge chamber (4) and the hearth (2) as shown in Fig. 3, and another was shown as a diaphragm (11b) between the hearth and the electron-beam acceleration zone in the gun in Fig. 3.The former was made with a stainless steel cylinder of 50 mm in length and 40 mm in diameter, and the latter was a copper plate having a 50×40 mm 2 hole cut off as the entrance of the 10-keV electron beam to the hearth on operation of the EB gun.The vertical part of the diaphragm (11b) in Fig. 3 was shown in Fig. 4 as "Diaphragm".The PIG-electrode configuration was constituted with a hot filament cathode of 10 mm in diameter and two ring electrodes as the anode and the FIG.4: Photograph of hearth and diaphragm.A foil attached at the center of the "diaphragm" (11b, shown in Fig. 3) having a cut off space of 5×4 cm 2 turns out an entrance of the 10-keV electron beam when the EB gun was operated.The discharge chamber (4) with the metal vapor exit (11a) and the ion extraction electrode system (5) were set up above the "hearth", coaxially.
unti-cathode with diameters of 13 and 10 mm, respectively.The plasma electrode (12), the ion extraction-( 13) and the deceleration-electrode (14) coaxially set facing the unti-cathode electrode (7c) had apertures with diameters of 5.0, 4.8 and 5.0 mm, respectively, for ion beam extraction.Magnetic fields superposed to arcing are useful in general for the ignition and the increase of discharge currents, but this was put aside in this case, since the behavior of electron beams near to the hearth and of metal vapor fully ionized in the hearth was supposed to be very sensitive to magnetic fields.

A. Ion beam
An electron beam of several hundreds of mAs accelerated to 10 keV in the gun bombarded and quickly melted the surfaces of solid metals such as Ti, Mn, and Zn filled in the hearth.The size of the hearth was 30 mm in diameter and 20 mm in depth.The gas pressure required for the ignition of PIG-discharge was obtained in a few minutes even with Ti whose surface temperature was estimated to be over about 1300 • C [12].The discharge was ordinarily ignited nonlinearly when the anode voltage reached over 120 V, under the gas pressure over 1×10 −2 Pa, and sus-FIG.7: XPS profile of Si substrate implanted with Ti-ions.
tained stably at about 40-60 V.The discharge currents were sensitively variable with the gas pressure and the filament current.The maximum current was about 10 A when using a W-Re filament with a diameter of 0.6 mm.
The preliminary experiments of ion bam extraction were carried out.The extracted ion currents were measured with a Faraday cage connected in parallel electrically to the decal-electrode.The cage was positioned 15 cm away from the ion exit aperture: the deceleration electrode, and its active area was 3 cm in diameter.The total unsaturated ion currents of metal: Ti, Mn, and Zn, were 50, 115 and 155 µA•dc, respectively, at the ion extraction voltages of 6 kV constant (Fig. 5).The extracted current densities from the source plasma were calculated by multiplying the values by about 5.5, since the ion extraction aperture bored in the ion extraction electrode was about 0.48 cm in diameter.With regards to focussing of ion beams, the core beam with halo was distinguished by applying the deceleration potential of about +1.5 kV to the decal-electrode against the ion extraction electrode, on extracting Mn-ion beams.Any other beam focusing system was not built in the apparatus.Formation of a large-area metal-ion beam was tested also with Mn-ion beams accelerating to 6 kV toward a simple plate as a beam dumper.The focussing state of Mn-ion beams was visible in the ion drifting region, as shown in Fig. 6, even under the low gas pressure below 5×10 −3 Pa.

B. XPS
An XPS profile of the surface layer of the Si (100) substrate implanted with Ti-ions at 6 kV is shown in Fig. 7.A series of X-ray photoelectron (hν: 1486.6 eV) spectroscopy with Ar-ion sputtering has been performed to probe the depths of the implanted ions.From the changes in the spectra, it was clarified that the maximum depth of an implanted Mn-ion reached about 30 nm, and the intensity profiles varied with the depth.

IV. CONCLUSION
In the preliminary experiment, it was verified that a metal-ion source constituted with an EB gun as the evaporator and the PIG discharge configuration as the ionizer worked well.The total beam current of the extracted Ti + , Zn + , and Mn + were 50, 115 and 155 µA•dc, respectively, measured as unsaturated currents against 6 kV.

FIG. 1 :
FIG. 1: Gas pressure for operation of EB gun and the types of discharge.From the figure, it is estimated that PIG-discharge might have possibly occurred without arcing in the EB gun in the gas pressure range of 10 −2 Pa.EBG: Electron Beam Gun, P.I.G.: Penning Ion Gauge type discharge, E.B.P.: Electron-Beam -Plasma type discharge, e bomb: electron bombardment type ionization, Arc discharge: Arc type discharge, Duo-pls: Duoplasmatron type arc discharge, RF: RF-type discharge.

FIG. 5 :
FIG. 5: Total ion current vs ion extraction voltage.Each total ion current was not yet saturated against the ion extraction voltage at 6 kV.An ion drifting distance to a Faraday cage of 3 cm in diameter was about 15 cm.