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Pulsed ion sources for surface modification of materials
HON B
Nuclear Instruments and Methods in Physics Research B80/81 (1993) 242-245 North-Holland
e.g."m Irlt®riCtiorls with Ante rLis "Atoms
Pulsed ion sources for surface modification of materials S . Korenev
Joint Institute for Nuclear Research, Dubna. Me- region, 141980, Russian Federation
A series of pulsed ion sawn-, working on the basis of explosive ion emission, used for surface modificati- of different materials is described in tnis report. The ion fources consist of a vacuum diode and a high-voltage pulse generator . The pulse duration of the ion current i P is 0.1-10.0 p,s and ..he kinetic energies of the ions from conducting materials (C, Cu, Nb, Ti, Mo and others) are between 2 and 600 keV . Emission characteristics of these ion sources are discussed .
1. Introduction At the present time pulsed for beams find wide applications in modern technologies involving nonlinear processes . The surface modification of materials by pulsed high current ion and electron beams belongs to these technologies. High-current pulsed ian beams of conducting materials were generated by two pulse supplies [1] . First a pulse of negative voltage is used for forming the plasma, and then a positive pulse is applied to extract the ion beam. However, progress in the high current pulse electronics suggested that sources of charged particles with one pulse supply are the most promising for these technologies . After successful experiments of forming a plasma with a positive voltage pulse on the anode, we used thie method to produce pulsed ion beams [2] . The new type of ion sources on the basis of explosive pulsed ion emission used for surface modification of different materials is described in this report .
2 . The mechanism of pulsed explosive ion emission The main idea of this ion emission is to form at first an anode plasma and then to extract ions from this plasma with one pulse of positive voltage . A high-voltage electric field applied to the diode evaporates anode materials starting from microheterogeneities on the anode surface . The atoms of this vapour are . ion.. ized and form an anode plasma which is the emitter of ions . Ionization can take place in a couple of ways : 1) by ions from autoemission; 2) by the electrical field around microheterogeneities; 3) by electrons from cathodes .
For forming an anode plasma the intensity of the electrical field E in the vacuum diode must exceed the threshold Ethr : EP
KU = - D> Ethr= 10' V/cm,
E, = -
KU > E1hr = 10 7 V/cm, r ln( R/r)
(1) (2)
where EP and E. are the electrical field strengths in the planar and coaxial diode, respectively, K is the amplification factor of the electrical field on the initiator of the anode plasma, U is the voltage amplitude on the diode, D is the distance between anode and cathode, r and R are the radius of the anode and the cathode, respectively, and Ethr = 10' V/cm is valid for P = 5 x 10 - ' Torr [3]. To get sufficient electrical field to form the anode plasma, one should vary the parameter D in eqs . (1) and (2). K is determined by the geometry and the properties of the material of the anode plasma initiator. In our case of the vacuum diode the ion curren, is described by the Child-Langmuir law . The anode plasma moves to the cathode with velocity V., P. = (1-5) x 10' em/s. The ion beam pulse duration is limited by the time. of movement of the plasma from anode to cathode. To get ion beams of microsecond duration, special methods to decrease V.t,P, are necessary. These methods arc described below . The new type of ion emission has some advantages . At first it has a large evaporation rate of material of the anode plasma initiator which depends on the energy of the vacuum discharge, on the properties of this material and on the explosive mechanism of starting the formation of the anode plasma. In comparison with evaporation of explosive electron emission, the evapo-
0168-583X/93/$06 .00 C 1993 - Elsevier Science Publishers B .V. All rights reserved
S. Korenec / Pulsed ionsources forsurface modifications
ration rate is 10-100 times higher for many materials (AI, Ti, O, Ni, Nb, Mo and others). This phenomenon can be used for modification of materials: ion plating ; mixing ; and others. The magnetic system is necessary for separating the different kinds of ions of the vapour material of the anode plasma initiator . Second, it is a nonlinear load of the high-voltage generator . The load is due to the anode plasma and the ion beam. It leads to oscillations of the ion-beam current. The main technical problem consists of the formation of a homogeneous anode plasma if large diameter ion beams are needed. 3. The construction of ion sources The general scheme of ion sources includes a highvoltage pulse generator, the plasma diode with vacyum system and the elementary diagnostics system . The high-voltage pulse generator has a few construction types depending on the required kinetic energy of the ions : I) U = 2-5 kV generator with a double forming LC-line with an optic thyristor commutator; 2) U = 5-20 kV generator on the basis of an RC-circuit with a thyratron commutator; 3) U = 20-100 kV generator based on the pulse transformer; 4) U = 100-600 kV generator of the Arkadiev-Marx type . '1 he voltage-pulse duration is 0 .1-10 Ws . In tor 4 gas discharges were used for the commutation of the capacitors . The arrangement of polyethylene tubes is needed for electrical insulation of the dischargers . The plasma diode consists of a vacuum chamber from stainless steel, a high-voltage insulator a caprolon or teflon, and anode and cathode units. The residual gas pressure in the vacuum chamber is P = 5 X 10 - " Torr. The cathode is a metal net or plate with an opening in the center . The anode unit consists of an anode leg and an initiator of the anode plasma . For the generation of different kinds of ions different anode plasma initiators are used. 3.1 . Anode plasma initiators for ion sources in the nanosecond range For the production of ion beams in the nanosecond range a vacuum diode with an explosive ion emission initiator of the anode plasma is used with a special geometry to get the necessary amplification factor of the electrical field in the diode . For producing carbon ions we use a fiber carbon anode [3]; for copper ions multipin systems of Cu needles which are prepared by the heavy ion track technique are used [4] (the needle density ranges from
243
Fig. 1. The construction of the diode with cryogenic anode: (1) vacuum chamber, (2) high-voltage insulator, (3) cryostat, (4) liquid nitrogen, (5) substrate (anode leg), ((,) condensing ini tiator of anode plasma, (7) catnode, (8) leak valve. and (9) vessel with liquid .
10 3 to 10 9 needles/CMZ) ; for titanium ions thin-walled tubes with a thickness of about 0 .1-0 .25 mm are used . The pin systems are necessary for conducting materials in order to get a sufficient amplification factor K for the realization of conditions (1) and (2). In the case of the coaxial ion source initiator of the anode plasma, a roll of thin foils (Ni, . , lie, Al and others) with shims is used . This variant of an ;on source with longitudinal magnetic field can form a tubelike ion beam (diode with magnetic insulation) . This type of ion emission permits one to form multicomponent ion beams which are very effective for surface modification of materials . For example, for êcneratin!! double. ~em, nnent ion beams an anode plasma initiator on the basis of Nb and Ti fibers of a superconductivity cable was used . The copper matrix was etched in sulfuric scid for 20-30 min . The initiator made with this method is a bunch of Nb-Ti fibers with a length of 0.5-2 rnm . The diameter of the fibers depends on the type of cable. For generating ions of liquid materials a special initiator of the anode plasma was elaborated. Fig. 1 shows the cross-section view of the diode with a cryogenic anode. The main idea of this device is the following: the condensate (6) (material of initiator of the anode plasma) forms en the metallic substrate (5), which is cooled by a cryostat (3) with liquid nitrogen (4). It forms at a specific pressure in the diode and temperature of the metallic substrate (5) (the thermodynamical triple point). The condensate (6) is easily flashed and ionized. The anode plasma contains elements from the liquid from vessel (9). The vapour pressure is regulated with the leak valve (8). We used vapour of water and xylene with a saturation pressure of 8 X 10 -^ Torr. Forming of the condensate takes about 5-6 s. This type of ion source was used for the generation of oxygen ions . We found that the metallic substrate with the multipin surface formed the most stable anode plasma. Ila. METAL MODIFICATION (a)
244
S. 1Yarenev / Pulsed ioi, --fi, ,svrfre modificatiom
a
b
Fig. 2. (a) The construction of the anode unit: (1) metal adapter, (2) contact electrode, (3) resistor (for reduce current) (TVO45 type, N = 25-50, R = 270 0), (4) transition connector, and (5) anode plasma initiator. ;b) The construction of the anode unit: (1) metal adapter, (2) contact electrode, (3) dielectric resin, wid (4) initiator of anode plasma on the basis of resistor type TVO-2 with cut-off part.
Fig . 4. Emission characteristics of the diode for Al, Ti, and In ions for a distance between the anode and the cathode of 1 cm.
3.3. Diagnostic devices 3.2. Anode plasma initiators for ion sources in the microsecond range To get ion pulses to the microsecond range, we restricted the discharge current by resistors placed in the current circuit of the initiator of the anode plasma. The construction of anode units are presented in figs. 2a and 2b. The anode plasma initiator is fabricated from bunches of Nb end Ti fiber:. or sharpened electrodes made of other metals. The first initiator was made of the superconductivity cable type NT-50 (fig . 2a). The second anode unit used for the production of carbon ions was made from nr?phite resistors of the TVO-2 type (see fig. 2b) . Ir the case of microsecond ion beams the value of the ion current decreases corresponding to the law of balance of energy.
An integrating Rogovski transformer and a Faraday cup are used for ion-beam measurements. The high resistance divider was used for the pulsed voltage measurements. 4 . Emission characteristics of the ion sources 'the emission characteristics of the ion sources are presented in figs. 3 and 4. These characteristics showed that the ion current yield corresponds to the ChildLangmuir law . So we can vary the ion-beam intensity corresponding to this law. The dimensions of the ion beams were determined by the size of the anode plasma initiator. For diffc-rert ion suutces, ion beam dimensions ranged from 10 to 100 CMZ. 5. Conclusion The described pulsed ion sources on the basis of explosive ion emission are comparably simple devices. The ion-beam density has been varied by means of changing the distance between the anode unit and the cathode in accordance with the Child-Langmuir law. The parameters of the ion beams and the kinds of vapour used for the anode plasma initiator of this type of ion source are acceptable for wide applications in the field of surface modification of materials .
150
100 N 'E a 50
0
200
400
600
800
U(KV) Fig . 3. Emission characteristics of a high current ion diode for a distance between the anode and the cathode of 1 cm .
Acknowledgements The author would like to thank I. Brown, G. Mesyac, D . Proskurovski, I . Enchevich and M . Mikhov for useful discussions about this type of ion source .
S. Koreneu / Pulsed ion sources for surface modifications References Ill A .N . Didenko, A.E. Ligachov and I .B. Kurakin, Charged Particle Beams Action on the Surface of Metal and Alloy (Energoatomizdat, Moscow, 1987). [2] S.A . Korenev, Proc . 2 Scrn ;~.ar *t Young Scientists by
245
JINR in the Field of Experimental Physics, JINR, NR 15-85-862, Dubna, JINR, 4 (1985). [3] S.A. Korenev, Preprint JINR N 13-89-389, JINR, Dubra (1989) . [4] G .N. Akapiev and S.A . Koreneu, Preprint JINR N 13-88347, JINR, Dubna (198F).
Ha. METAL MODIFICATION (a)
Nuclear Instruments and Methods in Physics Research B80/81 (1993) 242-245 North-Holland
e.g."m Irlt®riCtiorls with Ante rLis "Atoms
Pulsed ion sources for surface modification of materials S . Korenev
Joint Institute for Nuclear Research, Dubna. Me- region, 141980, Russian Federation
A series of pulsed ion sawn-, working on the basis of explosive ion emission, used for surface modificati- of different materials is described in tnis report. The ion fources consist of a vacuum diode and a high-voltage pulse generator . The pulse duration of the ion current i P is 0.1-10.0 p,s and ..he kinetic energies of the ions from conducting materials (C, Cu, Nb, Ti, Mo and others) are between 2 and 600 keV . Emission characteristics of these ion sources are discussed .
1. Introduction At the present time pulsed for beams find wide applications in modern technologies involving nonlinear processes . The surface modification of materials by pulsed high current ion and electron beams belongs to these technologies. High-current pulsed ian beams of conducting materials were generated by two pulse supplies [1] . First a pulse of negative voltage is used for forming the plasma, and then a positive pulse is applied to extract the ion beam. However, progress in the high current pulse electronics suggested that sources of charged particles with one pulse supply are the most promising for these technologies . After successful experiments of forming a plasma with a positive voltage pulse on the anode, we used thie method to produce pulsed ion beams [2] . The new type of ion sources on the basis of explosive pulsed ion emission used for surface modification of different materials is described in this report .
2 . The mechanism of pulsed explosive ion emission The main idea of this ion emission is to form at first an anode plasma and then to extract ions from this plasma with one pulse of positive voltage . A high-voltage electric field applied to the diode evaporates anode materials starting from microheterogeneities on the anode surface . The atoms of this vapour are . ion.. ized and form an anode plasma which is the emitter of ions . Ionization can take place in a couple of ways : 1) by ions from autoemission; 2) by the electrical field around microheterogeneities; 3) by electrons from cathodes .
For forming an anode plasma the intensity of the electrical field E in the vacuum diode must exceed the threshold Ethr : EP
KU = - D> Ethr= 10' V/cm,
E, = -
KU > E1hr = 10 7 V/cm, r ln( R/r)
(1) (2)
where EP and E. are the electrical field strengths in the planar and coaxial diode, respectively, K is the amplification factor of the electrical field on the initiator of the anode plasma, U is the voltage amplitude on the diode, D is the distance between anode and cathode, r and R are the radius of the anode and the cathode, respectively, and Ethr = 10' V/cm is valid for P = 5 x 10 - ' Torr [3]. To get sufficient electrical field to form the anode plasma, one should vary the parameter D in eqs . (1) and (2). K is determined by the geometry and the properties of the material of the anode plasma initiator. In our case of the vacuum diode the ion curren, is described by the Child-Langmuir law . The anode plasma moves to the cathode with velocity V., P. = (1-5) x 10' em/s. The ion beam pulse duration is limited by the time. of movement of the plasma from anode to cathode. To get ion beams of microsecond duration, special methods to decrease V.t,P, are necessary. These methods arc described below . The new type of ion emission has some advantages . At first it has a large evaporation rate of material of the anode plasma initiator which depends on the energy of the vacuum discharge, on the properties of this material and on the explosive mechanism of starting the formation of the anode plasma. In comparison with evaporation of explosive electron emission, the evapo-
0168-583X/93/$06 .00 C 1993 - Elsevier Science Publishers B .V. All rights reserved
S. Korenec / Pulsed ionsources forsurface modifications
ration rate is 10-100 times higher for many materials (AI, Ti, O, Ni, Nb, Mo and others). This phenomenon can be used for modification of materials: ion plating ; mixing ; and others. The magnetic system is necessary for separating the different kinds of ions of the vapour material of the anode plasma initiator . Second, it is a nonlinear load of the high-voltage generator . The load is due to the anode plasma and the ion beam. It leads to oscillations of the ion-beam current. The main technical problem consists of the formation of a homogeneous anode plasma if large diameter ion beams are needed. 3. The construction of ion sources The general scheme of ion sources includes a highvoltage pulse generator, the plasma diode with vacyum system and the elementary diagnostics system . The high-voltage pulse generator has a few construction types depending on the required kinetic energy of the ions : I) U = 2-5 kV generator with a double forming LC-line with an optic thyristor commutator; 2) U = 5-20 kV generator on the basis of an RC-circuit with a thyratron commutator; 3) U = 20-100 kV generator based on the pulse transformer; 4) U = 100-600 kV generator of the Arkadiev-Marx type . '1 he voltage-pulse duration is 0 .1-10 Ws . In tor 4 gas discharges were used for the commutation of the capacitors . The arrangement of polyethylene tubes is needed for electrical insulation of the dischargers . The plasma diode consists of a vacuum chamber from stainless steel, a high-voltage insulator a caprolon or teflon, and anode and cathode units. The residual gas pressure in the vacuum chamber is P = 5 X 10 - " Torr. The cathode is a metal net or plate with an opening in the center . The anode unit consists of an anode leg and an initiator of the anode plasma . For the generation of different kinds of ions different anode plasma initiators are used. 3.1 . Anode plasma initiators for ion sources in the nanosecond range For the production of ion beams in the nanosecond range a vacuum diode with an explosive ion emission initiator of the anode plasma is used with a special geometry to get the necessary amplification factor of the electrical field in the diode . For producing carbon ions we use a fiber carbon anode [3]; for copper ions multipin systems of Cu needles which are prepared by the heavy ion track technique are used [4] (the needle density ranges from
243
Fig. 1. The construction of the diode with cryogenic anode: (1) vacuum chamber, (2) high-voltage insulator, (3) cryostat, (4) liquid nitrogen, (5) substrate (anode leg), ((,) condensing ini tiator of anode plasma, (7) catnode, (8) leak valve. and (9) vessel with liquid .
10 3 to 10 9 needles/CMZ) ; for titanium ions thin-walled tubes with a thickness of about 0 .1-0 .25 mm are used . The pin systems are necessary for conducting materials in order to get a sufficient amplification factor K for the realization of conditions (1) and (2). In the case of the coaxial ion source initiator of the anode plasma, a roll of thin foils (Ni, . , lie, Al and others) with shims is used . This variant of an ;on source with longitudinal magnetic field can form a tubelike ion beam (diode with magnetic insulation) . This type of ion emission permits one to form multicomponent ion beams which are very effective for surface modification of materials . For example, for êcneratin!! double. ~em, nnent ion beams an anode plasma initiator on the basis of Nb and Ti fibers of a superconductivity cable was used . The copper matrix was etched in sulfuric scid for 20-30 min . The initiator made with this method is a bunch of Nb-Ti fibers with a length of 0.5-2 rnm . The diameter of the fibers depends on the type of cable. For generating ions of liquid materials a special initiator of the anode plasma was elaborated. Fig. 1 shows the cross-section view of the diode with a cryogenic anode. The main idea of this device is the following: the condensate (6) (material of initiator of the anode plasma) forms en the metallic substrate (5), which is cooled by a cryostat (3) with liquid nitrogen (4). It forms at a specific pressure in the diode and temperature of the metallic substrate (5) (the thermodynamical triple point). The condensate (6) is easily flashed and ionized. The anode plasma contains elements from the liquid from vessel (9). The vapour pressure is regulated with the leak valve (8). We used vapour of water and xylene with a saturation pressure of 8 X 10 -^ Torr. Forming of the condensate takes about 5-6 s. This type of ion source was used for the generation of oxygen ions . We found that the metallic substrate with the multipin surface formed the most stable anode plasma. Ila. METAL MODIFICATION (a)
244
S. 1Yarenev / Pulsed ioi, --fi, ,svrfre modificatiom
a
b
Fig. 2. (a) The construction of the anode unit: (1) metal adapter, (2) contact electrode, (3) resistor (for reduce current) (TVO45 type, N = 25-50, R = 270 0), (4) transition connector, and (5) anode plasma initiator. ;b) The construction of the anode unit: (1) metal adapter, (2) contact electrode, (3) dielectric resin, wid (4) initiator of anode plasma on the basis of resistor type TVO-2 with cut-off part.
Fig . 4. Emission characteristics of the diode for Al, Ti, and In ions for a distance between the anode and the cathode of 1 cm.
3.3. Diagnostic devices 3.2. Anode plasma initiators for ion sources in the microsecond range To get ion pulses to the microsecond range, we restricted the discharge current by resistors placed in the current circuit of the initiator of the anode plasma. The construction of anode units are presented in figs. 2a and 2b. The anode plasma initiator is fabricated from bunches of Nb end Ti fiber:. or sharpened electrodes made of other metals. The first initiator was made of the superconductivity cable type NT-50 (fig . 2a). The second anode unit used for the production of carbon ions was made from nr?phite resistors of the TVO-2 type (see fig. 2b) . Ir the case of microsecond ion beams the value of the ion current decreases corresponding to the law of balance of energy.
An integrating Rogovski transformer and a Faraday cup are used for ion-beam measurements. The high resistance divider was used for the pulsed voltage measurements. 4 . Emission characteristics of the ion sources 'the emission characteristics of the ion sources are presented in figs. 3 and 4. These characteristics showed that the ion current yield corresponds to the ChildLangmuir law . So we can vary the ion-beam intensity corresponding to this law. The dimensions of the ion beams were determined by the size of the anode plasma initiator. For diffc-rert ion suutces, ion beam dimensions ranged from 10 to 100 CMZ. 5. Conclusion The described pulsed ion sources on the basis of explosive ion emission are comparably simple devices. The ion-beam density has been varied by means of changing the distance between the anode unit and the cathode in accordance with the Child-Langmuir law. The parameters of the ion beams and the kinds of vapour used for the anode plasma initiator of this type of ion source are acceptable for wide applications in the field of surface modification of materials .
150
100 N 'E a 50
0
200
400
600
800
U(KV) Fig . 3. Emission characteristics of a high current ion diode for a distance between the anode and the cathode of 1 cm .
Acknowledgements The author would like to thank I. Brown, G. Mesyac, D . Proskurovski, I . Enchevich and M . Mikhov for useful discussions about this type of ion source .
S. Koreneu / Pulsed ion sources for surface modifications References Ill A .N . Didenko, A.E. Ligachov and I .B. Kurakin, Charged Particle Beams Action on the Surface of Metal and Alloy (Energoatomizdat, Moscow, 1987). [2] S.A . Korenev, Proc . 2 Scrn ;~.ar *t Young Scientists by
245
JINR in the Field of Experimental Physics, JINR, NR 15-85-862, Dubna, JINR, 4 (1985). [3] S.A. Korenev, Preprint JINR N 13-89-389, JINR, Dubra (1989) . [4] G .N. Akapiev and S.A . Koreneu, Preprint JINR N 13-88347, JINR, Dubna (198F).
Ha. METAL MODIFICATION (a)