ANODE LAYER PLASMA ACCELERATOR

INVENTOR:

Vadim Dudnikov, Ph.D.

J. Cross-Reference to Related Application

[0001] This application claims the benefit of U.S. Provisional Patent Application No.

61/578,223, filed December 20, 2011, which is hereby incorporated by reference in its entirety, as if set out below.

//. Field of the Disclosure

[0002] The present disclosure is generally related to a plasma generation system for high

current negative ion beam production and, in particular, to a large volume surface plasma source (SPS) with an anode layer plasma accelerator for high current negative ion beam production and for directed deposition by flux of sputtered neutrals and negative ions.

///. Summary

[0003] We have invented a plasma generation system that can be used for high current negative ion beam production and for directed deposition by flux of sputtered neutrals and negative ions. The main mechanism of negative ion formation in Surface Plasma Sources (SPS) is the secondary emission from a low work function surface bombarded by a flux of positive ion or neutrals. The emission of negative ions is enhanced significantly by introducing a small amount of cesium or other substance with a low ionization potential. In our inventive source, positive ions are generated by a Hall drift plasma accelerator (with an anode layer plasma accelerator, ALPA, or with an insulated channel, with a cylindrical or a race track configuration of emission slit), accelerated in a crossed ExB field and forming a conical ion beam bombarding the target-emitter. Negative ions are extracted from the target surface with geometrical focusing and accelerated by negative voltage applied between emitter and plasma, contacting with the plasma accelerator. The Hall drift ion source has a special design with a space for the passage of emitted negative ions and sputtered particles through the positive ion source.

[0004] The cesiation effect, a significant enhancement of negative ion emission from a gas discharge with a decrease of co-extracted electron current below the negative ion current was observed for the first time by placing into a planotron (plane magnetron) discharge chamber a compound with one milligram of cesium, as described, for example, in V. Dudnikov, The Method for Negative Ion Production, SU patent,

C1.H013/04, No 411542, Appl. 3/10/72. This observation became the basis for the development of Surface Plasma Sources (SPS) for highly efficient production of negative ions from the interaction of plasma particles with electrodes on which adsorbed cesium reduced the surface work- function, as described, for example, in Yu. I.

Belchenko, G. I. Dimov, and V. G. Dudnikov, Proc. Symp. Production and

Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1977 (BNL, Upton, NY, 1977), BNL- 50727, p. 79, Yu. Belchenko and V. Dudnikov, "Surface Negative Ion Production in Ion Sources", ESNIP, Belfast, 1991, pp. 47-66; Yu. Belchenko, Rev. Sci. Instrum., 64, 1385, (1993), V. Dudnikov, Rev. Sci. Instrum. 63, 2660 (1992), and V. Dudnikov, Rev. Sci. Instrum. 73, 992 (2002). The emission current density of negative ions was increased rapidly from j~ 0.01 A/cm ² to 3.7 A/cm ² with a flat cathode and up to 8 A/cm ² with an optimized geometrical focusing in the long pulse SPS, and up to 1 A/cm ² in direct current (DC) SPS. The intensity of negative ion beams was increased by cesiation up to 10 ⁴ times from mA to tens of Amperes. Now cesiation is routinely used in SPS for injection of negative ions into accelerators and into large fusion devices.

[0005] In the energy spectra of the H ^" ions from the planotron, two peaks divided by a gap were typically present. The position of the first low energy peak corresponds to the ions accelerated by the extraction voltage. The ion energy at the second peak was higher by a magnitude equal to additional acceleration by the discharge voltage W=eU _d , expressed in eV. The discharge voltage is concentrated in a thin layer near the cathode.

[0006] These experimental results led us to the conclusion that the main mechanism of the

enhanced negative ion generation was their emission from the surface of the cathode bombarded by the intense fluxes of particles from the gas discharge plasma. The adding of the cesium admixture enhances significantly the generation of negative ions on the surface of the emitter due to the capture of electrons from the surface on the electron affinity levels of the sputtered, reflected, and desorbed particles.

[0007] The flux of negative ions emitted from the cathode can be concentrated by a concave focusing surface of the emitter. This geometrical focusing was demonstrated in the SPS with a semiplanotron discharge configuration, as described, for example, in V.

Dudnikov, Yu. Belchenko, Preprint, INP 78-95, Novosibirsk, 1978, V. Dudnikov, Yu. Belchenko, Journal de Physique, 40, p. 477, (1979), Dudnikov V., Fiksel G., Journal de Physique, v. 40, p. 479 (1979), V. Dudnikov, Proc. Second Symp. Production and Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1980 (BNL, Upton, NY, 1980), BNL- 51304, p. 137, G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. Il l, 376 (1984); AIP Conf. Proc. 287, 239 (1992), G. E. Derevyankin and V. G. Dudnikov, Pribory i Techn. Exp. 30, 523 (1987), and G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. 158, 395 (1987).

[0008] A discharge configuration of semiplanotrons is shown, for example, in Figure 1. The discharge in crossed ExB fields is localized in the cathode groove with a mm scale. Positive ions and atoms from the discharge are bombarding the negative surface of the groove initiating a secondary emission of negative ions. These negative ions are accelerated near the cathode potential drop and are focused by the concave surface of the groove to the emission aperture. The plasma density in this discharge can be very high (10 ¹⁴ cm ^"3 ). The mean free path of H ^" ions becomes very small (~<lmm).

[0009] Very interesting emission characteristics were observed in this semiplanotron SPS, as shown, for example, in Figure 1, and as described, for example, in G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. Il l, 376 (1984); AIP Conf. Proc. 287, 239 (1992), G. E. Derevyankin and V. G. Dudnikov, Pribory i Techn. Exp. 30, 523 (1987), and G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. 158, 395 (1987). At low discharge current, the emission of negative ions from the cathode surface increases rapidly with discharge current, but the destruction of negative ions in the plasma reduced the emitted beam intensity exponentially after the optimal value of the discharge current, and at higher discharge current it started rising again. This N-shaped emission curve was formed due to generation of negative ions on the surface near the emission slit caused by a flux of fast atoms. This atomic flux originated from accelerated negative ions stripped in the plasma. This process increases negative ion emission after the minimum, but the rise was several times slower than in the beginning of the emission curves. These compact SPSs have high gas efficiency with pulsed gas valves, as described, for example, in G. E. Derevyankin, V. G. Dudnikov, and P. A. Zhuravlev, Prib. Tekii. Eksp., 5, 168 (1975).

[0010] Another class of SPS is the large volume SPS (LV SPS) with discharge volume of up to a hundred liters. The first LV SPS, developed in the Lawrence Berkeley Laboratory (LBL), as described, for example, in K.N. Leung and K.Ehiers, Rev. Sci. Instrum., 53, 803 (1982), was similar to the semiplanotron scaled up a hundred times. Now, such a version of SPS (Surface Conversion SPS, or SC SPS) is used for charge exchange injection into a storage ring of the LANSCE neutron source, as described, for example, in K.N. Leung and K.Ehlers, Rev. Sci. lustrum., 53, 803 (1982), and O. Tarvinen et al., Rev. Sci. Instrum. 79, 02A501 (2008), and in KEK . This H ^" SC SPS is composed of a plasma chamber and a negatively-biased converter electrode, as illustrated, for example, in Figure 2.

[0011] The gap between the emitter and extractor aperture is very large (8-12 cm) and the plasma and gas density must be kept a hundred times lower to prevent negative ion destruction. LV SPSs use hot filaments, RF coils, or microwave discharge and multicusp magnets for plasma production at low gas density, as described, for example, in O. Tarvinen et al., Rev. Sci. Instrum. 79, 02A501 (2008), and J. Sherman et al., AIP Conf. Proc, 763 (2005) 254. LV SPSs have a low power density and can be used for dc operation. The emission current density is only about 20 mA/cm ² and the brightness is not so high.

[0012] The negative ion beam intensity in the surface conversion source is limited by negative ion stripping in gas and plasma between the negative ion emitter and the emission aperture. It is important to decrease the gas and plasma thickness with a high intense bombardment of the emitter surface by positive ions and neutrals.

[0013] Our inventive sputtering/deposition system may be used for high current negative ion beam production and directed deposition by flux of neutrals and negative ions. The main mechanism of negative ion formation in Surface Plasma Sources (SPS) is the secondary emission from a low work function surface, bombarded by a flux of positive ions or neutrals, as described, for example, in Yu. I. Belchenko, G. I. Dimov, and V. G. Dudnikov, Proc. Symp. Production and Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1977 (BNL, Upton, NY, 1977), BNL- 50727, p. 79, and Yu.

Belchenko and V. Dudnikov, "Surface Negative Ion Production in Ion Sources", ESNIP, Belfast, 1991, pp. 47-66; Yu. Belchenko, Rev. Sci. Instrum., 64, 1385, (1993). The emitter work function is lowered by introducing a small amount of cesium or another substance with a low ionization potential. In our inventive source, positive ions are generated by a Hall drift plasma accelerator (anode layer or with insulated channel, with cylindrical or race track configuration of the emission slit), accelerated in a crossed ExB field and forming a conical ion beam bombarding a target-emitter. Negative ions are extracted from the target surface and accelerated by negative voltage applied between emitter and plasma, contacting with a plasma accelerator. The Hall drift ion source has a special design with a space for the passage of emitted negative ions and sputtered particles through the positive ion source. A cross-section of the ion source is shown, for example, in Figure 3. The negative ion source comprises a negative ion emitter (7) and an anode layer plasma accelerator (ALP A) generating a flux of positive ions (6). This negative ion emitter (7) is bombarded by positive ions (6), generating the secondary negative ion beam (10). Negative ions are accelerated by voltage applied between the negative ion emitter (7) and the plasma flux (6) and further accelerated by extraction voltage between a suppressor (8) and an extraction electrode (9). The extraction of co-extracted electrons is suppressed by the magnetic field of the suppressor (8). The anode layer plasma accelerator (ALP A) comprises a magnetic pole (1) serving as a cathode and an anode (2). The anode (2) is supported by anode supports (5) supplying cooling and current. The magnetic field in the emission slit is created by a permanent magnet (3) or coil, a magnetic yoke and magnetic poles-cathodes (1). A working gas is injected into the anode layer plasma accelerator (ALP A) uniformly all along the source. Positive ions are formed and accelerated in a discharge with a closed drift of electrons in crossed ExB fields. The negative ion emitter (7) has a cylindrical or a spherical emitting surface for the geometrical focusing of the emitted negative ions. A cesium flux is supplied to the negative ion emitter (7) surface to enhance the secondary emission of negative ions. [0015] Different versions of anode layer plasma accelerators (ALPA sources) are described, for example, in V. Dudnikov and A. Westner, Rev. Sci. Instrum. 73, 729 (2002), and V. V. Zhurin, H. R. Kaufman, and R. S. Robinson, Plasma Sources Sci.Technol.8, Rl (1999).

[0016] The discharge and beam characteristics for different N ₂ gas flows (different gas

pressures) in a vacuum chamber with the ALPA source are shown, for example, in Figure 4. Plotted are: Vd— discharge voltage, in kV,10xM— discharge current, in A, Zc/10— collector current, in mA, and Vc/100— floating potential of collector, in V.

[0017] The ion source, as described, for example, in V. Dudnikov and A. Westner, Rev. Sci.

Instrum. 73, 729 (2002), has a racetrack emission slit of 6 cm long straight parts and 3 cm radius semicircles on the ends. The slit width is 3 mm and the discharge gap between the cathode and the anode is 3 mm. In the high voltage mode of operation with pressure variation P = 1.4 mTorr (calibration for nitrogen), the discharge voltage Vd can be changed from 3 to 1 kV with a discharge current variation Id = 0.12 A. The ion beam current to the collector at a distance L = 35 cm from the source increases up to Ic = 50 mA, and the floating potential of the insulated target is Vc = 50 V.

[0018] An ion beam with a small angle divergence has formed in the high voltage mode of operation. The smallest observed divergence of a beam is 0.1 rad. With a further increase of gas flux (to P > 1.4 mTorr), a transition to a low voltage mode has been observed. The discharge voltage Vd drops to 0.4 kV, and the discharge current Id jumped to 0.28 A. The discharge voltage and discharge current have a weak dependence on gas pressure variations in the range 1.4 mTorr < P < 3 mTorr. The low voltage (400 V) mode of operation is most suitable for SPS operations. As an emitter material, it is possible to use a compound with a low work function such as LaB ₆ , for B ₂ ^~ production, compounds of phosphorus with an admixture of metals and lanthanides, and compounds of As, Sb, Ge with lanthanides for the production of clusters of negative ions of P, As, Sb, and Ge, for example. Catalysts with a low ionization potential (such as Cs or Rb) may be deposited onto the emitter surface and/or implanted into the emitter to decrease a surface work function and increase the yield of the secondary emission of negative ions. Our inventive source of negative ions can be used for the production of high currents of negative ions and fast neutrals for directed deposition of thin films into nanostructures with a high aspect ratio and for implantations. Cesium control and diagnostics, as described, for example, in V. Dudnikov, P. Chapovsky, and A.

Dudnikov, Rev. Sci. Instrum., 81, 02A714 (2010), may be used for SPS operation optimization and stabilization.

[0019] Some applications of negative ion deposition are discussed, for example, in J.Ishikawa, Rev. Sci. Instrum., 63,2357 (1992).

[0020] Uniform sputtering of the emitter with conservation of the focusing properties of the surface of the emitter may be important for long time operation.

[0021] In a particular embodiment, a device is disclosed that includes means for providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator. The device also includes means for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam production.

[0022] In another particular embodiment, a method is disclosed that includes steps for

providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator. The method also includes steps for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam production.

IV. Brief Description of the Drawings

[0023] The following figures form part of the present specification and are included to further demonstrate certain aspects of the present invention. The present invention may be better understood by reference to one or more of these drawings in combination with the description of embodiments presented herein.

[0024] Consequently, a more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, wherein:

[0025] Figure 1 is a diagram illustrating emission characteristics of semiplanotrons with

different discharge cell configurations with a 0.5x10 mm ² slit, where (7) is for a Penning discharge SPS; [0026] Figure 2 is a diagram illustrating a simplified conceptual scheme of a surface conversion ion source (SCIS);

[0027] Figure 3 is a diagram illustrating a simplified conceptual scheme of a surface plasma negative ion source with an anode layer plasma accelerator (ALPA, planar design), where Figure 3a shows a vertical cross-section, and Figure 3b shows a front view;

[0028] Figure 4 is a diagram illustrating the dependence of discharge and beam characteristics versus gas pressure in the vacuum chamber of the anode layer plasma accelerator (ALPA) source, as described, for example, in V. Dudnikov and A. Westner, Rev. Sci. Instrum. 73, 729 (2002);

[0029] Figure 5 is a diagram illustrating an embodiment of an apparatus including means for providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator and means for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam production; and

[0030] Figure 6 is a flow diagram of an illustrative embodiment of a method including steps for providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator and steps for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam production.

V. Detailed Description

[0031] Illustrative embodiments of the present invention are described in detail below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of the present disclosure. [0032] Particular embodiments of the present disclosure are described with reference to the drawings. In the description, common features are designated by common reference numbers.

[0033] Referring to Figure 1, a diagram illustrating emission characteristics of semiplanotrons with different discharge cell configurations with a 0.5x10 mm ² slit, where (7) is for a Penning discharge SPS, is depicted and indicated generally, for example, at 100.

[0034] The cesiation effect, a significant enhancement of negative ion emission from a gas discharge with a decrease of co-extracted electron current below the negative ion current was observed for the first time by placing into a planotron (plane magnetron) discharge chamber a compound with one milligram of cesium, as described, for example, in V. Dudnikov, The Method for Negative Ion Production, SU patent,

C1.H013/04, No 411542, Appl. 3/10/72. This observation became the basis for the development of Surface Plasma Sources (SPS) for highly efficient production of negative ions from the interaction of plasma particles with electrodes on which adsorbed cesium reduced the surface work- function, as described, for example, in Yu. I.

Belchenko, G. I. Dimov, and V. G. Dudnikov, Proc. Symp. Production and

Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1977 (BNL, Upton, NY, 1977), BNL- 50727, p. 79, Yu. Belchenko and V. Dudnikov, "Surface Negative Ion Production in Ion Sources", ESNIP, Belfast, 1991, pp. 47-66; Yu. Belchenko, Rev. Sci. Instrum., 64, 1385, (1993), V. Dudnikov, Rev. Sci. Instrum. 63, 2660 (1992), and V. Dudnikov, Rev. Sci. Instrum. 73, 992 (2002). The emission current density of negative ions was increased rapidly from j~ 0.01 A/cm ² to 3.7 A/cm ² with a flat cathode and up to 8 A/cm ² with an optimized geometrical focusing in the long pulse SPS, and up to 1 A/cm ² in direct current (DC) SPS. The intensity of negative ion beams was increased by cesiation up to 10 ⁴ times from mA to tens of Amperes. Now cesiation is routinely used in SPS for injection of negative ions into accelerators and into large fusion devices.

[0035] In the energy spectra of the H ^" ions from the planotron, two peaks divided by a gap were typically present. The position of the first low energy peak corresponds to the ions accelerated by the extraction voltage. The ion energy at the second peak was higher by a magnitude equal to additional acceleration by the discharge voltage W=eU _d , expressed in eV. The discharge voltage is concentrated in a thin layer near the cathode. [0036] These experimental results led us to the conclusion that the main mechanism of the enhanced negative ion generation was their emission from the surface of the cathode bombarded by the intense fluxes of particles from the gas discharge plasma. The adding of the cesium admixture enhances significantly the generation of negative ions on the surface of the emitter due to the capture of electrons from the surface on the electron affinity levels of the sputtered, reflected, and desorbed particles.

[0037] The flux of negative ions emitted from the cathode can be concentrated by a concave focusing surface of the emitter. This geometrical focusing was demonstrated in the SPS with a semiplanotron discharge configuration, as described, for example, in V.

Dudnikov, Yu. Belchenko, Preprint, INP 78-95, Novosibirsk, 1978, V. Dudnikov, Yu. Belchenko, Journal de Physique, 40, p. 477, (1979), Dudnikov V., Fiksel G., Journal de Physique, v. 40, p. 479 (1979), V. Dudnikov, Proc. Second Symp. Production and Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1980 (BNL, Upton, NY, 1980), BNL- 51304, p. 137, G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. I l l, 376 (1984); AIP Conf. Proc. 287, 239 (1992), G. E. Derevyankin and V. G. Dudnikov, Pribory i Techn. Exp. 30, 523 (1987), and G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. 158, 395 (1987).

[0038] A discharge configuration of semiplanotrons is shown, for example, in Figure 1. The discharge in crossed ExB fields is localized in the cathode groove with a mm scale. Positive ions and atoms from the discharge are bombarding the negative surface of the groove initiating a secondary emission of negative ions. These negative ions are accelerated near the cathode potential drop and are focused by the concave surface of the groove to the emission aperture. The plasma density in this discharge can be very high (10 ¹⁴ cm ^"3 ). The mean free path of H ^" ions becomes very small (~<lmm).

[0039] Very interesting emission characteristics were observed in this semiplanotron SPS, as shown, for example, in Figure 1, and as described, for example, in G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. I l l, 376 (1984); AIP Conf. Proc. 287, 239 (1992), G. E. Derevyankin and V. G. Dudnikov, Pribory i Techn. Exp. 30, 523 (1987), and G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. 158, 395 (1987). At low discharge current, the emission of negative ions from the cathode surface increases rapidly with discharge current, but the destruction of negative ions in the plasma reduced the emitted beam intensity exponentially after the optimal value of the discharge current, and at higher discharge current it started rising again. This N-shaped emission curve was formed due to generation of negative ions on the surface near the emission slit caused by a flux of fast atoms. This atomic flux originated from accelerated negative ions stripped in the plasma. This process increases negative ion emission after the minimum, but the rise was several times slower than in the beginning of the emission curves. These compact SPSs have high gas efficiency with pulsed gas valves, as described, for example, in G. E. Derevyankm, V. G. Dudnikov, and P. A. Zhuravlev, Prib. Tekh. Eksp., 5, 168 (1975).

[0040] Referring to Figure 2, a diagram illustrating a simplified conceptual scheme of a surface conversion ion source (SCIS) is depicted and indicated generally, for example, at 200.

[0041] Another class of SPS is the large volume SPS (LV SPS) with discharge volume of up to a hundred liters. The first LV SPS, developed in the Lawrence Berkeley Laboratory (LBL), as described, for example, in K.N. Leung and K.Ehlers, Rev. Sci. lustrum., 53, 803 (1982), was similar to the semiplanotron scaled up a hundred times. Now, such a version of SPS (Surface Conversion SPS, or SC SPS) is used for charge exchange injection into a storage ring of the LANSCE neutron source, as described, for example, in K.N. Leung and K.Ehlers, Rev. Sci. Instrurn., 53, 803 (1982), and O. Tarvinen et al., Rev. Sci. Instrurn. 79, 02A501 (2008), and in KEK . This H ^" SC SPS is composed of a plasma chamber and a negatively-biased converter electrode, as illustrated, for example, in Figure 2.

[0042] The gap between the emitter and extractor aperture is very large (8-12 cm) and the plasma and gas density must be kept a hundred times lower to prevent negative ion destruction. LV SPSs use hot filaments, RF coils, or microwave discharge and multicusp magnets for plasma production at low gas density, as described, for example, in O. Tarvinen et al., Rev. Sci. Instrurn. 79, 02A501 (2008), and J. Sherman et al., AIP Conf. Proc, 763 (2005) 254. LV SPSs have a low power density and can be used for dc operation. The emission current density is only about 20 mA/cm ² and the brightness is not so high.

[0043] The negative ion beam intensity in the surface conversion source is limited by negative ion stripping in gas and plasma between the negative ion emitter and the emission aperture. It is important to decrease the gas and plasma thickness with a high intense bombardment of the emitter surface by positive ions and neutrals.

[0044] Referring to Figure 3, a diagram illustrating a simplified conceptual scheme of a surface plasma negative ion source with an anode layer plasma accelerator (ALPA, planar design) is depicted and indicated generally, for example, at 300. Figure 3a shows a vertical cross-section 310, and Figure 3b shows a front view 320.

[0045] Our inventive sputtering/deposition system may be used for high current negative ion beam production and directed deposition by flux of neutrals and negative ions. The main mechanism of negative ion formation in Surface Plasma Sources (SPS) is the secondary emission from a low work function surface, bombarded by a flux of positive ions or neutrals, as described, for example, in Yu. I. Belchenko, G. I. Dimov, and V. G. Dudnikov, Proc. Symp. Production and Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1977 (BNL, Upton, NY, 1977), BNL- 50727, p. 79, and Yu.

Belchenko and V. Dudnikov, "Surface Negative Ion Production in Ion Sources", ESNIP, Belfast, 1991, pp. 47-66; Yu. Belchenko, Rev. Sci. Instrum., 64, 1385, (1993). The emitter work function is lowered by introducing a small amount of cesium or another substance with a low ionization potential. In our inventive source, positive ions are generated by a Hall drift plasma accelerator (anode layer or with insulated channel, with cylindrical or race track configuration of the emission slit), accelerated in a crossed ExB field and forming a conical ion beam bombarding a target-emitter. Negative ions are extracted from the target surface and accelerated by negative voltage applied between emitter and plasma, contacting with a plasma accelerator. The Hall drift ion source has a special design with a space for the passage of emitted negative ions and sputtered particles through the positive ion source. A cross-section of the ion source is shown, for example, in Figure 3.

[0046] The negative ion source comprises a negative ion emitter (7) and an anode layer plasma accelerator (ALPA) generating a flux of positive ions (6). This negative ion emitter (7) is bombarded by positive ions (6), generating the secondary negative ion beam (10). Negative ions are accelerated by voltage applied between the negative ion emitter (7) and the plasma flux (6) and further accelerated by extraction voltage between a suppressor (8) and an extraction electrode (9). The extraction of co-extracted electrons is suppressed by the magnetic field of the suppressor (8). The anode layer plasma accelerator (ALPA) comprises a magnetic pole (1) serving as a cathode and an anode (2). The anode (2) is supported by anode supports (5) supplying cooling and current. The magnetic field in the emission slit is created by a permanent magnet (3) or coil, a magnetic yoke and magnetic poles-cathodes (1). A working gas is injected into the anode layer plasma accelerator (ALPA) uniformly all along the source. Positive ions are formed and accelerated in a discharge with a closed drift of electrons in crossed ExB fields. The negative ion emitter (7) has a cylindrical or a spherical emitting surface for the geometrical focusing of the emitted negative ions. A cesium flux is supplied to the negative ion emitter (7) surface to enhance the secondary emission of negative ions.

[0047] Different versions of anode layer plasma accelerators (ALPA sources) are described, for example, in V. Dudnikov and A. Westner, Rev. Sci. Instrum. 73, 729 (2002), and V. V. Zhurin, H. R. Kaufman, and R. S. Robinson, Plasma Sources Sci.Technol. 8, Rl (1999).

[0048] Referring to Figure 4, a diagram illustrating the dependence of discharge and beam

characteristics versus gas pressure in the vacuum chamber of the anode layer plasma accelerator (ALPA) source, as described, for example, in V. Dudnikov and A. Westner, Rev. Sci. Instrum. 73, 729 (2002), is depicted and indicated generally, for example, at 400.

[0049] The discharge and beam characteristics for different N ₂ gas flows (different gas

pressures) in a vacuum chamber with the ALPA source are shown, for example, in Figure 4. Plotted are: Vd— discharge voltage, in kV,10xM— discharge current, in A, /c/10— collector current, in mA, and Vc/100— floating potential of collector, in V.

[0050] The ion source, as described, for example, in V. Dudnikov and A. Westner, Rev. Sci.

Instrum. 73, 729 (2002), has a racetrack emission slit of 6 cm long straight parts and 3 cm radius semicircles on the ends. The slit width is 3 mm and the discharge gap between the cathode and the anode is 3 mm. In the high voltage mode of operation with pressure variation P = 1.4 mTorr (calibration for nitrogen), the discharge voltage Vd can be changed from 3 to 1 kV with a discharge current variation Id = 0.12 A. The ion beam current to the collector at a distance L = 35 cm from the source increases up to Ic = 50 mA, and the floating potential of the insulated target is Vc = 50 V. [0051] An ion beam with a small angle divergence has formed in the high voltage mode of operation. The smallest observed divergence of a beam is 0.1 rad. With a further increase of gas flux (to P > 1.4 mTorr), a transition to a low voltage mode has been observed. The discharge voltage Vd drops to 0.4 kV, and the discharge current Id jumped to 0.28 A. The discharge voltage and discharge current have a weak dependence on gas pressure variations in the range 1.4 mTorr < P < 3 mTorr. The low voltage (400 V) mode of operation is most suitable for SPS operations. As an emitter material, it is possible to use a compound with a low work function such as LaB ₆ , for B ₂ ^~ production, compounds of phosphorus with an admixture of metals and lanthanides, and compounds of As, Sb, Ge with lanthanides for the production of clusters of negative ions of P, As, Sb, and Ge, for example. Catalysts with a low ionization potential (such as Cs or Rb) may be deposited onto the emitter surface and/or implanted into the emitter to decrease a surface work function and increase the yield of the secondary emission of negative ions. Our inventive source of negative ions can be used for the production of high currents of negative ions and fast neutrals for directed deposition of thin films into nanostructures with a high aspect ratio and for implantations. Cesium control and diagnostics, as described, for example, in V. Dudnikov, P. Chapovsky, and A.

Dudnikov, Rev. Sci. Instrum., 81, 02A714 (2010), may be used for SPS operation optimization and stabilization.

[0052] Some applications of negative ion deposition are discussed, for example, in J.Ishikawa, Rev. Sci. Instrum., 63,2357 (1992).

[0053] Uniform sputtering of the emitter with conservation of the focusing properties of the surface of the emitter may be important for long time operation.

[0054] Referring to Figure 5, a diagram illustrating an embodiment of an apparatus is depicted and indicated generally, for example, at 500. The apparatus 500 includes means for providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator 510 and means for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam

production 520.

[0055] Referring to Figure 6, a flow diagram of an illustrative embodiment of a method is depicted and indicated generally, for example, at 600. The method 600 includes steps for providing a large volume surface plasma source (SPS) with an anode layer plasma accelerator 610 and steps for operating the large volume surface plasma source (SPS) with the anode layer plasma accelerator for high current negative ion beam

production 620.

[0056] Attached herewith as an Appendix to this specification is a document describing more details about various illustrative embodiments, which Appendix to this specification is incorporated by reference as if set forth below. More details about various illustrative embodiments may be found by referring to the Appendix.

[0057] The present invention is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as those that are inherent therein. While the present invention has been depicted, described and is defined by reference to exemplary embodiments of the present invention, such a reference does not imply a limitation of the present invention, and no such limitation is to be inferred. The present invention is capable of considerable modification, alteration, and equivalency in form and function as will occur to those of ordinary skill in the pertinent arts having the benefit of this disclosure. The depicted and described embodiments of the present invention are exemplary only and are not exhaustive of the scope of the present invention.

Consequently, the present invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

[0058] The particular embodiments disclosed above are illustrative only, as the present

invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of composition or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and intent of the present invention. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood as referring to the power set (the set of all subsets) of the respective range of values, in the sense of Georg Cantor. Accordingly, the protection sought herein is as set forth in the claims below. [0059] The particular embodiments of the present invention described herein are merely exemplary and are not intended to limit the scope of this present invention. Many variations and modifications may be made without departing from the intent and scope of the present invention. Applicants intend that all such modifications and variations are to be included within the scope of the present invention as defined in the appended claims and their equivalents.

[0060] While the present invention has been illustrated by a description of various

embodiments and while these embodiments have been described in considerable detail, it is not the intention of the Applicants to restrict, or any way limit the scope of the appended claims to such detail. The present invention in its broader aspects is therefore not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of Applicants' general inventive concept.

Surface Plasma Source with Anode Layer Plasma Accelerator ³ '

Vadim Dudnikov ^b)

Muons, Inc., Batavia, IL 60510 USA

(Presented XXXXX, received XXXXX, accepted XXXXX, published online XXXXX)

Proposed plasma generation system can be used for high current negative ion beam production and for

directed deposition by flux of sputtered neutrals and negative ions. The main mechanism of negative ion formation in Surface Plasma Sources (SPS) is the secondary emission from low work function surface

bombarded by a flux of positive ion or neutrals. The emission of negative ions is enhanced significantly by introducing a small amount of cesium or other substance with low ionization potential. In the proposed

source, positive ions are generated by Hall drift plasma accelerator (with anode layer ALPA or with insulated channel, with cylindrical or race track configuration of emission slit), accelerated in crossed ExB field and forming a conical ion beam bombarding the target-emitter . Negative ions are extracted from the target

surface with geometrical focusing and accelerated by negative voltage applied between emitter and plasma, contacting with the plasma accelerator. Hall drift ion source has a special design with a space for passing of emitted negative ions and sputtered particles through the positive ion source.

The flux of negative ions emitted from the cathode can be

I INTRODUCTION

concentrated by concave focusing surface of the emitter.

Cesiation effect, a significant enhancement of negative This geometrical focusing was demonstrated in the SPS ion emission from gas discharge with decrease of co- with semiplanotrons discharge configuration ό ' T ' 8 '9 ' 10 ' 11 ' 12. extracted electron current below negative ion current was

observed for the first time by placing into a planotron

(plane magnetron) discharge chamber a compound with

one milligram of cesium ¹ . This observation became the

basis for the development of Surface Plasma Sources

(SPS) for highly efficient production of negative ions from

the interaction of plasma particles with electrodes on

which adsorbed cesium reduced the surface work- function ² ' ³ ' ⁴ ' ⁵ . The emission current density of negative

ions was increased rapidly from j~ 0.01 A/cm ² to 3.7

A/cm ² with a flat cathode and up to 8 A/cm ² with an

optimized geometrical focusing in the long pulse SPS, and

up to 1 A/cm ² in direct current (DC) SPS. Intensity of

negative ion beams was increased by cesiation up to 10 ⁴

times from mA to tens of Amperes. Now caesiation is

routinely used in SPS for negative ions injection into

accelerators and into large fusion devices. ¾, A

In the energy spectra of the H ^" ions from the planotron

FIG. 1. Emission characteristics of the semiiplanotrons with different two peaks divided by the gap were typically present. The discharge cell configurations with 0.5x10 mm ² slit (7)is for Penning position of the first low energy peak corresponds to the discharge SPS.

ions accelerated by the extraction voltage. The ion energy

at the second peak was higher by a magnitude equal to A discharge configuration of semiplanotrons is shown in additional acceleration by the discharge voltage W=eU _d , Fig. 1. Discharge in crossed ExB fields is localized in the expressed in eV. The discharge voltage is concentrated in cathode groove with mm scale. Positive ions and atoms thin layer near the cathode. from discharge are bombarding the negative surface of the

These experimental results led us to the conclusion that the groove initiating a secondary emission of negative ion. main mechanism of the enhanced negative ion generation These negative ions are accelerated by near cathode was their emission from the surface of the cathode potential drop and are focused by concave surface of bombarded by the intense fluxes of particles from the gas groove to the emission aperture. The plasma density in this discharge plasma. The adding of the cesium admixture discharge can be very high (10 ¹⁴ cm ^"3 ). A free mean pass enhances significantly the generation of negative ions on of H ^" ions becomes very small (~<lmm).

the surface of the emitter due to the capture of electrons Very interesting emission characteristics were observed in from the surface on the electron affinity levels of the this semiplanotron SPS as shown in Fig. 1 ^{10 11 12} . At low sputtered, reflected, and desorbed particles. discharge current the emission of negative ions from the

^Contributed (or Invited) paper published as part of the Proceedings of the 14th

International Conference on Ion Source, Giardini-Naxos, Sicily, Italy, September, cathode surface increases rapidly with discharge current, thickness with a high intense bombardment of the emitter but the destruction of negative ions in the plasma reduced surface by positive ions and neutrals.

the emitted beam intensity exponentially after the optimal

B. LV SPS with anode layer plasma accelerator value of the discharge current, and at higher discharge

current it started rising again. This N-shaped emission Proposed sputtering/deposition system for used for high curve was formed due to generation of negative ions on the current negative ion beam production and directed surface near emission slit caused by a flux of fast atoms. deposition by flux of neutrals and negative ions. The main This atomic flux originated from accelerated negative ions mechanism of negative ion formation in Surface Plasma stripped in the plasma. This process increases negative ion Sources (SPS) is the secondary emission from low work emission after the minimum, but the rise was several times function surface, bombarded by a flux of positive ions or slower than in the beginning of emission curves. These neutrals ² ' ³ . The emitter work function is lowered by compact SPSs have high gas efficiency with pulsed gas introducing a small amount of cesium or other substance valves ¹³ . with low ionization potential. In the proposed source positive ion are generated by Hall drift plasma accelerator

II FEATURES OF LARGE VOLUME SPS

(anode layer or with insulated channel, with cylindrical or

A. General design of the LV SPS race track configuration of emission slit), accelerated in

for generating the secondary negative ion beam (10).

Negative ion beam intensity in the surface conversion Negative ions are accelerated by voltage applied between source is limited by negative ion stripping in gas and emitter (7) and plasma flux (6) and further accelerated by plasma between negative ion emitter and emission extraction voltage between suppressor (8) and extraction aperture. It is important to decrease the gas and plasma electrode (9). The extraction of co-extracted electrons is divergence of a beam is 0.1 rad. With the further increase suppressed by magnetic field of a suppressor (8). Anode of gas flux (to P>1 A mTorr), transition to low voltage layer plasma accelerator is comprised of a magnetic pole mode has been observed. The discharge voltage Vd drops (1) served as cathode and anode (2). The anode is to 0.4 kV, and the discharge current Id jumped to 0.28 A. supported by anode supports (5) supplying cooling and The discharge voltage and discharge current have a weak current. Magnetic field in the emission slit is created by dependence on gas pressure variations in the range 1.4 permanent magnet (3) or coil, magnetic yoke and mTorr<P<,3 mTorr. The low voltage (400 V) mode of magnetic poles -cathodes (1). A working gas is injected operation is most suitable for SOS operation. As emitter into anode layer plasma accelerator uniformly along the all material it is possible to use a compound with a low work the source. Positive ion are formed and accelerated in function such as LaB6, for B ₂ ^~ production, compounds of discharge with a closed drift of electrons in crossed field. phosphorus with admixture of metals and lanthanides, Emitter (7) has a cylindrical or spherical emitting surface compounds of As, Sb, Ge with lanthanides for production for the geometrical focusing of emitted negative ion. A of cluster negative ion of P, As, Sb, Ge production.

cesium flux is supplied to the emitter surface to enhance Catalysts with low ionization potential (Such as Cs or Rb) the secondary emission of negative ions. can be deposited to emitter surface or implanted to

ALPA source are shown in Fig. 4. There are: Vd— 9 V. Dudnikov, Proc. Second Symp. Production and Neutralization of discharge voltage, kV, Id— discharge current, A, Ic— Negative Hydrogen Ions and Beams, Brookhaven, 1980 (BNL,

Upton, NY, 1980), BNL- 51304, p. 137.

collector current, mA, and Vc— floating potential of 10 G. E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. I l l, 376 collector, V. (1984); ΑΓΡ Conf. Proc. 287, 239 (1992).

The ion source ¹⁷ has a racetrack emission slit of 6 cm long 11 G . E. Derevyankin and V. G. Dudnikov, Pribory i Techn. Exp. straight parts and 3 cm radius semicircles in the ends. The 30, 523 (1987).

12 G . E. Derevyankin and V. G. Dudnikov, AIP Conf. Proc. 158, 395 slit width is 3 mm and discharge gap between the cathode (1987).

and the anode is 3 mm. In the high voltage mode of 13 G. E. Derevyankin, V. G. Dudnikov, and P. A. Zhuravlev, Prib. Tekli. operation with pressure variation P=1 A mTorr Eksp., 5, 168 (1975).

(calibration for nitrogen), the discharge voltage Vd can be 14 K.N. Leung and K.Ehlers, Rev. Sci. Instrum., 53, 803 (1982).

15 O. Tarvinen et al., Rev. Sci. Instrum. 79, 02A501 (2008);

changed from 3 to 1 kV with a discharge current variation 16 J. Sherman et al., ΑΓΡ Conf. Proc, 763 (2005) 254.

ld=0.12 A. Ion beam current to the collector in distance 17 V. Dudnikov and A. Westner, Rev. Sci. Instrum. 73, 729 (2002). L=35 cm from the source increases up to Ic =50 mA, 18 V. V. Zhurin, H. R. Kaufman, and R. S. Robinson, Plasma Sources floating potential of insulated target is Vc=50 V. Sci.Technol. 8, Rl (1999).

19 V. Dudnikov, P. Chapovskyand and A. Dudnikov, Rev. Sci. Instrum., An ion beam with a small angle divergence has formed in 81, 02A714 (2010).

the high voltage mode of operation. The smallest observed 20 J.Ishikawa, Rev. Sci. Instrum., 63,2357 (1992).