- Email: [email protected]
Formation and destruction processes of negative ions in a bounded plasma
Thin Solid Films 316 Ž1998. 128]133
Formation and destruction processes of negative ions in a bounded plasma Motoi WadaU Department of Electronics, Doshisha Uni¨ersity, Kyotanabe 610-03, Japan
Abstract Fundamental atomic and molecular processes, including surface collisions determining the density of hydrogen negative ions in a bounded hydrogen plasma, are summarized. The results of the model calculations based on the fundamental reaction processes in plasma volume and at the surface of the container wall are compared with the experimental observations. The importance of the plasma]surface interaction in determining the density of negative ions in a bounded plasma is emphasized, and the discussion of the possible mechanism for the enhancement of the hydrogen negative ion density in a Cs introduced hydrogen discharge is given. Q 1998 Elsevier Science S.A. Keywords: Atomic processes; Molecular processes; Surface collisions; Hydrogen negative ions; Bounded hydrogen plasma
1. Introduction The presence of negative ions in a plasma can become important depending upon how the plasma is used. For a long time, negative ions are utilized for producing high energy particle beams. Tandem acceleration can easily realize the particle beam with twice the kinetic energy of the terminal voltage. Various kinds of negative ion sources have been developed to produce a beam of negative ions. Negative ion sources are categorized into several families and their principle of operation are introduced in textbooks w1,2x. In some types of ion source, a negative ion containing plasmas are produced to extract negative ions directly from the plasma. Negative ions are formed in the plasma and the final negative ion density is determined by the reaction rates of the related atomic and molecular processes occurring in the plasma. This process of negative ion production is often called the ‘volume process’ as the important reaction steps take place in the plasma volume.
U
Corresponding author. Tel.: q81 77 4656351; fax: q81 77 4656823; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00402-7
Intense beams of negative ions for particle injection can be also produced by generating negative ions at the low work function surface. Negative ions are produced at the surface of the low work function either by backscattering of incident ions or ion induced desorption of adsorbed atoms. The region of high negative ion density is created in front of the negative ion production surface, when the surface is immersed in the plasma. The density can become high when the mass of the negative ion is very high like in the case of Au negative ŽAuy. ions. Also, when the potential difference between the negative ion producing surface and the plasma is small, negative ions are extracted from the surface into the plasma with very low energy. This will also create a substantial negative ion density in the region adjacent to the surface. The surface of the plasma container can be a source of negative ion as well as the sink. Meanwhile, plasma parameters like electron temperature, electron density and plasma potential can often change due to a change of the surface condition of the plasma container. Therefore, the interaction between the plasma and the surface of the plasma container can be the important factor determining the negative ion density. The purpose of this paper is to state
M. Wada r Thin Solid Films 316 (1998) 128]133
129
which reaction process taking place in the plasma volume are related with the surface condition, and how the surface negative ion production can affect the negative ion density in the plasma volume. Several plasma]wall interaction processes are discussed for their importance in determining the final plasma density in the plasma. Another purpose of this report is to summarize how the plasma]surface interaction should affect the production and destruction processes in the plasma volume and at the surface of the plasma container. To postulate physical mechanisms governing the negative ion production, the discussion will be restricted to Hy production in a hydrogen plasma, as available data is abundant for hydrogen and the reaction process is straight forward compared to other larger and complex molecules. The application of the discussion in this report to plasma species other than hydrogen may be possible, provided enough fundamental data are known. 2. Production and destruction processes in a plasma Physics related with the negative ion formation in a plasma has been investigated in order to optimize the performance of the negative ion sources developed for accelerators and thermonuclear fusion experiments. Particularly, study of negative hydrogen ŽHy. ions in a hydrogen plasma had been advanced in the last two decades. The amount of Hy current extracted from an ion source developed for a neutral beam heating system of nuclear fusion experiment, is more than 20 A, and the corresponding current density is over 10 mArcm2 . Several papers reporting the status of the large Hy ion source developments have been already published w3,4x. To optimize the performance of the ion sources, attempts to theoretically predict the extractable Hy current from a hydrogen plasma had been started by Hiskes and co-workers w5]7x, and many researchers have made contribution in this field w8]12x. Negative ions can be produced in a plasma through the polar dissociation process, ABq e ª Aqq Byq e which shows a large cross section at relatively higher energy than a typical electron temperature of a laboratory plasma. However, the cross section for the electron impact detachment from Hy is large at energy where the cross section for the polar dissociation is large, it is quite difficult to realize a high Hy density in a hydrogen plasma making use of the polar dissociation process. Contrary to this process, the dissociative electron attachment, ABq e ª Aq By,
Fig. 1. Reaction rates important for Hy production and destruction in a plasma volume. The plotted reaction rate for dissociative attachment is for a hydrogen molecule excited to its eight vibrational level and the ground rotational level.
can be the predominating process to form negative ions when the electron temperature of the plasma is lower than the threshold of the polar dissociation. The cross sections for dissociative electron attachment are usually decreasing functions against the electron energy with some thresholds. In the case of hydrogen, the cross section is larger and the threshold energy is lower as the vibrational excitation level of the molecule increases w13,14x. For the particular case of hydrogen, the reaction rates for these two processes are compared in Fig. 1. In Fig. 1, the reaction rate for dissociative attachment for the eighth vibrational level Ž ¨ s 8. is plotted. As shown in the figure, the dissociative attachment shows the reaction rate larger than that of polar dissociation by several orders of magnitude. On the other hand, the cross sections for fundamental processes efficiently produce highly vibrationally excited molecules Ž ¨ ) 4. usually show higher values for electron energy substantially higher than a typical laboratory plasma temperature w15,16x. As the dissociative electron attachment is a two-step process and high energy electrons are required to form vibrationally excited molecules, one would not expect high concentration of Hy in the hydrogen discharge for a single electron temperature plasma. However, as the energy distribution function of plasma electrons shows a high energy tale produced by the primary ionizing electrons w17x, vibrationally excited molecules are efficiently produced by this group of electrons. There-
130
M. Wada r Thin Solid Films 316 (1998) 128]133
fore, a plasma with two electron temperature component can produce Hy ions efficiently. The high energy part of the plasma electron distribution function contributes for the negative ion production through the production of highly vibrationally excited molecules, but they also contribute to destroy the produced negative ions in the plasma volume w18x. The reaction rate for this Hy destruction process is also plotted in Fig. 1. High energy electrons produce high concentration of protons, which shows a large reaction rate for Hy destruction w19]21x. Therefore, one should deal with the balance between the production of highly excited vibrational molecules and destruction of the produced Hy so long as it employs a single electron temperature system. A much better way to produce negative ions through vibrational excitation process is to divide the discharge container into two regions as shown in Fig. 2. In high temperature region, a dense plasma with energetic electrons is confined. Here, hydrogen molecules are efficiently excited by the impact of high energy electrons. In the low temperature region, which is separated by a localized magnetic field, only low energy electrons are confined. As the excited molecules do not have a charge, their transport across the localized field is not forbidden and they can make dissociative attachment reaction with low energy electrons in the region. The concept of this tandem approach w22x has been adopted successfully in designing a larger negative ion source of a neutral beam heating system for nuclear fusion experiments w23x. Model calculations trying to explain the physical processes influential on the Hy production have shown a good qualitative agreement with experiment, and one possibility drawn from the model is the existence of optimum length for the low electron temperature region w24x. Because the length of the region is finite, the effect of the wall collision, particularly, atom recombination at the wall becomes important in determining the final negative ion density. An
Fig. 2. Schematic illustrating the volume]production-type Hy ion source employing a tandem structure.
experiment to confirm the effect of the length of the discharge chamber, or the region where the vibrational excitation andror dissociative attachment takes place, had been conducted w25x. The result show a qualitative agreement with the prediction of the theoretical model. Thus, the size of the system is critically important in realizing a high concentration of negative ions. Also important is the degree of ionization of the plasma. It is not difficult to imagine that the perfectly ionized system does not contain substantial amount of negative ions as it does not contain molecules which are the primary source of negative ions. Therefore, the Hy current does not increase in proportion to the electron density for the fixed gas pressure in the chamber, but it increases with increasing gas pressure up to some electron density corresponding with the discharge current to sustain the plasma w26x. This effect has been successfully simulated by numerical models w27x. The concept of dividing the plasma source into two regions so as to separate the region for producing negative ions by dissociative attachment from that for producing highly excited vibrationally molecule can be extended to set up the scheme to enhance Hy production by controlling the electron temperature in a time-dependent way. In this scheme, highly vibrationally excited molecules are created during an intense discharge, and the produced excited molecules are attached with electrons to form negative ions during the afterglow phase. This effect is confirmed in hydrogen discharge w28,29x. The enhanced production of negative ions in after glow phase is also confirmed for reactive plasmas w30x. 3. Surface production of negative ions Prior to the findings that Hy ion concentration in a hydrogen discharge is higher and the electron current extracted with the Hy current can be minimized with the suitable magnetic field configuration near the extractor, surface conversion-type ion source had been developed for applications of particle accelerators and ion sources for fusion experiments w31x. The principle mechanism of the surface production of negative ions is the electron transfer from the metal to an atom leaving the surface. This electron transfer can be enhanced by minimizing the surface potential barrier. The produced negative ions should escape to an infinite distance without loosing electron. Lower work function enhances the probability for a negative ion to escape from the near surface region without loosing the electron in the affinity level. Particle fluxes leaving the solid surface can be created by surface collisions like reflection, sputtering and ion stimulated desorption of adsorbed atoms, and the dependence of
M. Wada r Thin Solid Films 316 (1998) 128]133
negative ion yields for these fundamental processes upon the work function of the negative ion production surface have been confirmed w32]35x. In these experiments, particle beams from ion sources are directed onto the surface. A much easier way to produce particle fluxes leaving the surface is to immerse the surface into a plasma. When the surface is biased negatively with respect to the plasma, the plasma particles strike the surface in the plasma and create the flux by reflection, sputtering, and ion induced desorption. If the surface is covered with Cs or Ba, the work function of the surface becomes low, and it efficiently produces negative ions. This negative ion production surface is called the converter, and typically biased 100]500 V negatively with respect to the plasma potential. Because the efficiency to produce this particle flux is higher for higher converter bias, surface production type ion source is operated with a converter bias higher than several hundred volt. The correlation among the work function of the surface of the converter in a plasma, converter bias and the produced Hy beam was experimentally investigated w36,37x. The result showed an exponential dependence of Hy production upon the work function decrease, and the Hy current increased by increasing the negative bias to the converter. Similar dependence has been found for surface production of Auy w38x, which are produced by sputtering at the Cs-covered Au surface. A high current density beam of heavy negative ions can be extracted from a surface conversion type ion source w39,40x. Negative ions produced at the surface in these sources travel inside the plasma creating a region where part of electrons are replaced with negative ions. The concentration should be particularly high when the current density of the produced beam is higher and the mass of the negative ion is larger. For example, 200 eV Auy beam of 1 mArcmy2 current density corresponds to about 4 = 10 9 cmy3 ion density. Another possibility for creating a substantial negative ion density based on surface produced neutral is to utilize the charge exchange reaction. Surface produced negative ions can give additional electron to neutral particles in a plasma, and make negative ions of the these neutrals. When the cross section for this charge exchange process is large, a negative ion rich area can be produced near the region where the parent negative ion beam is traveling.
131
plasma. From an early stage, the importance of interaction of plasma ions and neutrals with the surface or the plasma wall had been noticed and several parameters are included in the theoretical model to count for the effect of the surface collisions. Among them, the fraction of atomic reflection, and the fraction of deexcitation for vibrationally excited molecules had been considered most important. When Walther and co-workers w41,42x reported their observation that the amount of Hy current extracted from the ion source was increased by the introduction of Cs, the importance of the plasma]surface interaction on the Hy concentration in the plasma had been recognized again. Namely, the reduction of work function of the chamber wall due to Cs introduction into the discharge may increase the production of Hy at the chamber wall surface either by reflection of atomic hydrogen and plasma ions, or desorption of adsorbed hydrogen on the surface w43x. Results of experiments using a large ion source and a small ion source suggest the condition of the extraction electrode in contact with the plasma decides the amount of Hy current extracted from the source w44]46x. The fraction at which negative ions are formed at the surface from an incident low energy atomic hydrogen, which had been measured by Graham w47x, has been measured again w48,49x. The yield of Hy ions due to reflection of low energy positive hydrogen ions is also measured w50x, and compared with theory w51x. The results of these measurements of fundamental processes seem to suggest that the observed enhancement of Hy current from the Cs or Ba introduced discharge is attributable to surface production of Hy at the surface of the extraction electrode. To check if enough Hy ions are produced from the surface having a small potential difference with the plasma, the converter in a small ion source containing a Cs introduced hydrogen plasma was biased with a
4. Importance of plasma surface interaction The importance of the presence of the wall facing with the plasma can be seen in both the volume and the surface processes. For the volume process, the wall collisions changes the species composition in the
Fig. 3. The surface produced negative ion current plotted as a function of the converter voltage, which is applied between the negative ion production surface and the anode of the discharge. The plasma potential is from 1 to 2 V positive with respect to the anode.
132
M. Wada r Thin Solid Films 316 (1998) 128]133
voltage as low as 6 V. The amount of Hy current produced at the surface was measured after separated from other components by means of taking the energy spectrum of the extracted Hy current w52x. In Fig. 3, relative current from the converter is plotted as the function of the converter bias. As the plasma potential was 1]2 V positive with respect to the anode potential, the positive ions in the plasma are accelerated with the converter potential q1 to 2 eV. Though the Hy current is decreased by decreasing bias voltage, some amount of Hy is observed even at 6 V bias. Though the measurement below 6 V was not possible because Hy formed at the part in the ion source other than the converter overlapped on the energy spectrum of Hy produced at the converter surface, certain amount of Hy seems to be produced with the bias less than several eV. This result also suggests that Hy ions can be produced at the ion source wall. Thus, the enhancement of the Hy current from the ion source seems to be well explained by the Hy production from the surface of the low work plasma electrode. Particularly, atomic hydrogen are possibly converted to Hy by the surface reflection. However, the efficiency for the conversion is measured to be small, and enough flux of atomic hydrogen should be present if this mechanism is the principle source of the Hy enhancement. Besides, the dependence of the enhancement of Hy current upon the surface work function of the plasma electrode is weaker than the prediction from the theory, provided that Hy escape from the surface with velocity of thermal hydrogen atoms. If Hy ions are produced at the surface of the extraction electrode, they have to change the direction in the plasma to be extracted. In this case, additional interaction between the plasma and the surface should be present even if Hy ions are directly produced at the surface. Thus, several questions remain how the density in the ion source can be enhanced by the introduction of Cs into the discharge. For surface process, the surface condition including the work function is changed by the particle bombardment. The over-all negative ion concentration is then dependent upon the surface material of the plasma container, and the plasma parameters like electron temperature, plasma potential, electron density, degree of ionization, and chemical characteristics of species. Modification of the surface condition by plasma is another important realm in the plasma surface interaction. The absolute work function of Cs covered Cu and Mo in hydrogen discharge were measured with photoelectric method w36,37x. The results show some lower values than Cs covered Cu and Mo. Adsorption of hydrogen on the substrate metal surface is considered as the possible explanation, but the interaction of surface with other impurity species in the discharge may be also possible. There may be a
change in surface roughness due to sputtering. The surface of the wall of the ion source driven by hot W filaments should be covered with evaporated W. These may change the surface condition including work function and the atom reflection coefficients of the wall surface. So far, the discussion has been restricted to Hy in a hydrogen discharge. The situation should become far more complicated when one deals with a plasma of chemically active species, which are often used for plasma processing. For instance, when electronegative gas is contained in a plasma, it should increase the work function by being adsorbed on the surface. The higher work function of the surface will change the atom reflection coefficient andror coefficient for the deexcitation ratio of excited molecules. This will change the species condition in the plasma as the adsorption of the wall proceeds. The electron temperature and plasma potential may change depending upon the ratio of negative ion density to electron density, and the energy spectrum of the incoming ions onto the chamber wall surface will be altered. Through this process the condition for the particle adsorption on the surface will change. The links of the plasma condition and the surface condition are inevitable, and can be decisive in determining the final density of negative ions. 5. Summary Fundamental processes affecting the negative ion density in a bounded plasma are listed. In low temperature hydrogen plasma, dissociative attachment is believed to be the dominating formation process. When Cs is added into the discharge, some plasma]surface interaction takes place, and additional formation mechanism increases the Hy density in the hydrogen plasma. The plasma]surface interaction may become even more important when the species in the plasma is deposited on the surface. A modification of the surface parameter will cause a change of the plasma characteristics, and the negative ion concentration in the plasma. Acknowledgements The author is grateful to Professor Osamu Fukumasa at Yamaguchi University for his helpful discussion. This work was supported by the project, ‘Nanostructure Hybrid Devices and Their Properties’ at High Technology Center for Faculty of Engineering of Doshisha University. The project was supported by the Grant-in-Aid from Doshisha University and the aid from The Ministry of Education, Science and Culture.
M. Wada r Thin Solid Films 316 (1998) 128]133
References w1x K.N. Leung, in: I.G. Brown ŽEd.., The Physics and Technology of Ion Sources, Ch. 17, Wiley, New York, 1989, p. 355. w2x G.D. Alton, in: H.S.W. Massey, E.W. McDaniel, B. Bederson ŽEds..rSheldon Datz ŽVol. Ed.., Applied Atomic Collision Physics, vol. 4, Ch. 2, Academic Press, Florida, 1983, p. 43. w3x Y. Okumura, Y. Fujiwara, T. Inoue, et al., Rev. Sci. Instrum. 67 Ž1996. 1092. w4x T. Kuroda, O. Kaneko, Y. Takeiri, et al., Rev. Sci. Instrum. 67 Ž1996. 1114. w5x J.R. Hiskes, A.M. Karo, M. Bacal, A.M. Bruneteau, W.G. Graham, J. Appl. Phys. 53 Ž1982. 3469. w6x J.R. Hiskes, A.M. Karo, J. Appl. Phys. 56 Ž1984. 1927. w7x J.R. Hiskes, A.M. Karo, P.A. Willmann, J. Appl. Phys. 58 Ž1985. 1759. w8x O. Fukumasa, J. Phys. D. 22 Ž1989. 1688. w9x O. Fukumasa, S. Ohashi, J. Phys. D 22 Ž1989. 1931. w10x C. Gorse, M. Capitelli, J. Bretagne, M. Bacal, Chem. Phys. 93 Ž1985. 1. w11x C. Gorse, M. Capitelli, J. Bretagne, M. Bacal, A. Lagana, Chem. Phys. 117 Ž1987. 177. w12x C. Gorse, R. Celiberto, A. Lagana, M. Capitelli, Chem. Phys. 161 Ž1987. 211. w13x J.M. Wadehra, J.N. Bardsley, Phys. Rev. Lett. 41 Ž1978. 1795. w14x J.N. Bardsley, J.M. Wadehra, Phys. Rev. A20 Ž1979. 1398. w15x J.R. Hiskes, J. Appl. Phys. 51 Ž1980. 4592. w16x J.R. Hiskes, J. Appl. Phys. 70 Ž1991. 3409. w17x K.F. Schoenberg, W.B. Kunkel, J. Appl. Phys. 50 Ž1979. 4685. w18x D.S. Walton, B. Peart, K.T. Dolder, J. Phys. B. 4 Ž1971. 1343. w19x R.J. Bieniek, A. Dargarno, Astrophys. J. 228 Ž1979. 635. w20x R.J. Bieniek, J. Phys. B. 13 Ž1980. 4405. w21x D.H. McNeill, J. Kim, Phys. Rev. A25 Ž1982. 2152. w22x K.N. Leung, K.W. Ehlers, M. Bacal, Rev. Sci. Instrum. 54 Ž1983. 56. w23x Y. Okumura, Y. Fujiwara, A. Honda, et al., Rev. Sci. Instrum. 67 Ž1996. 1018. w24x J.R. Hiskes, A.M. Karo, J. Appl. Phys. 56 Ž1984. 1927. w25x K.N. Leung, K.W. Ehlers, R.V. Pyle, Rev. Sci. Instrum. 56 Ž1985. 364. w26x Y. Okumura, M. Hanada, T. Inoue, H. Kojima, Y. Matsuda, Y. Ohara, M. Seki, K. Watanabe, A. Hershkovitch ŽEds.., Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 210, 1990, p. 169. w27x O. Fukumasa, E. Niitani, J. Allesi, A. Hershkovitch ŽEds..,
w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x w39x w40x w41x w42x w43x w44x w45x
w46x w47x w48x w49x w50x w51x w52x
133
Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 380, 1990, p. 187. M.B. Hopkins, W.G. Graham, J. Phys. D. 20 Ž1987. 838. M.B. Hopkins, M. Bacal, W.G. Graham, J. Appl. Phys. 70 Ž1991. 2009. T.H. Ahn, K. Nakamura, H. Sugai, Jpn. J. Appl. Phys. 34 Ž1995. L1405. K.W. Ehlers, K.N. Leung, Rev. Sci. Instrum. 51 Ž1980. 721. M.L. Yu, Phys. Rev. Lett. 40 Ž1978. 574. P.J. Schneider, K.H. Berkner, W.G. Graham, R.V. Pyle, J.W. Stearns, Phys. Rev. B23 Ž1981. 941. J.R. Hiskes, P.J. Schneider, Phys. Rev. B23 Ž1981. 949. M. Seidel, A. Pargellis, Phys. Rev. B26 Ž1982. 1. M. Wada, R.V. Pyle, J.W. Stearns, J. Vac. Sci. Technol. A1 Ž1983. 981. M. Wada, R.V. Pyle, J.W. Stearns, J. Appl. Phys. 67 Ž1990. 6334. Y. Okabe, M. Sasao, H. Yamaoka, M. Wada, J. Fujita, Jpn. J. Appl. Phys. 30 Ž1991. 1307. G.D. Alton, Y. Mori, A. Takagi, A. Ueno, S. Fukumoto, Nucl. Instrum. Methods A270 Ž1988. 194. G.D. Alton, Y. Mori, A. Takagi, A. Ueno, S. Fukumoto, Nucl. Instrum. Methods B40r41 Ž1989. 1008. S.R. Walther, K.N. Leung, W.B. Kunkel, J. Appl. Phys. 64 Ž1988. 3424. S.R. Walther, K.N. Leung, W.B. Kunkel, Appl. Phys. Lett. 54 Ž1989. 210. K. N Leung, S.R. Walther, W.B. Kunkel, Phys. Rev. Lett. 62 Ž1989. 764. Y. Mori, T. Okuyama, A. Takagi, D. Yuan, Nucl. Instrum. Methods A301 Ž1991. 1. T. Inoue, M. Hanada, M. Mizuno, Y. Ohara, Y. Okumura, Y. Suzuki, M. Tanaka, K. Watanabe, J. Allesi ŽEds..rA. Hershkovitch ŽVol. Ed.., Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 287, 1990, p. 316. K. Shinto, Y. Okumura, T. Ando, et al., Jpn. J. Appl. Phys. 35 Ž1996. 1894. W.G. Graham, Phys. Lett. 73A Ž1979. 186. T. Okuyama, Y. Mori, Rev. Sci. Instrum. 63 Ž1992. 2711. B.S. Lee, M. Seidl, Appl. Phys. Lett. 61 Ž1992. 2857. J.D. Isenberg, H.J. Kwon, M. Seidl, Rev. Sci. Instrum. 63 Ž1992. 5289. H.L. Cui, J. Vac. Sci. Technol. A9 Ž1991. 1823. M. Wada, H. Tsuda, Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 287, 1990, p. 746.
Formation and destruction processes of negative ions in a bounded plasma Motoi WadaU Department of Electronics, Doshisha Uni¨ersity, Kyotanabe 610-03, Japan
Abstract Fundamental atomic and molecular processes, including surface collisions determining the density of hydrogen negative ions in a bounded hydrogen plasma, are summarized. The results of the model calculations based on the fundamental reaction processes in plasma volume and at the surface of the container wall are compared with the experimental observations. The importance of the plasma]surface interaction in determining the density of negative ions in a bounded plasma is emphasized, and the discussion of the possible mechanism for the enhancement of the hydrogen negative ion density in a Cs introduced hydrogen discharge is given. Q 1998 Elsevier Science S.A. Keywords: Atomic processes; Molecular processes; Surface collisions; Hydrogen negative ions; Bounded hydrogen plasma
1. Introduction The presence of negative ions in a plasma can become important depending upon how the plasma is used. For a long time, negative ions are utilized for producing high energy particle beams. Tandem acceleration can easily realize the particle beam with twice the kinetic energy of the terminal voltage. Various kinds of negative ion sources have been developed to produce a beam of negative ions. Negative ion sources are categorized into several families and their principle of operation are introduced in textbooks w1,2x. In some types of ion source, a negative ion containing plasmas are produced to extract negative ions directly from the plasma. Negative ions are formed in the plasma and the final negative ion density is determined by the reaction rates of the related atomic and molecular processes occurring in the plasma. This process of negative ion production is often called the ‘volume process’ as the important reaction steps take place in the plasma volume.
U
Corresponding author. Tel.: q81 77 4656351; fax: q81 77 4656823; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00402-7
Intense beams of negative ions for particle injection can be also produced by generating negative ions at the low work function surface. Negative ions are produced at the surface of the low work function either by backscattering of incident ions or ion induced desorption of adsorbed atoms. The region of high negative ion density is created in front of the negative ion production surface, when the surface is immersed in the plasma. The density can become high when the mass of the negative ion is very high like in the case of Au negative ŽAuy. ions. Also, when the potential difference between the negative ion producing surface and the plasma is small, negative ions are extracted from the surface into the plasma with very low energy. This will also create a substantial negative ion density in the region adjacent to the surface. The surface of the plasma container can be a source of negative ion as well as the sink. Meanwhile, plasma parameters like electron temperature, electron density and plasma potential can often change due to a change of the surface condition of the plasma container. Therefore, the interaction between the plasma and the surface of the plasma container can be the important factor determining the negative ion density. The purpose of this paper is to state
M. Wada r Thin Solid Films 316 (1998) 128]133
129
which reaction process taking place in the plasma volume are related with the surface condition, and how the surface negative ion production can affect the negative ion density in the plasma volume. Several plasma]wall interaction processes are discussed for their importance in determining the final plasma density in the plasma. Another purpose of this report is to summarize how the plasma]surface interaction should affect the production and destruction processes in the plasma volume and at the surface of the plasma container. To postulate physical mechanisms governing the negative ion production, the discussion will be restricted to Hy production in a hydrogen plasma, as available data is abundant for hydrogen and the reaction process is straight forward compared to other larger and complex molecules. The application of the discussion in this report to plasma species other than hydrogen may be possible, provided enough fundamental data are known. 2. Production and destruction processes in a plasma Physics related with the negative ion formation in a plasma has been investigated in order to optimize the performance of the negative ion sources developed for accelerators and thermonuclear fusion experiments. Particularly, study of negative hydrogen ŽHy. ions in a hydrogen plasma had been advanced in the last two decades. The amount of Hy current extracted from an ion source developed for a neutral beam heating system of nuclear fusion experiment, is more than 20 A, and the corresponding current density is over 10 mArcm2 . Several papers reporting the status of the large Hy ion source developments have been already published w3,4x. To optimize the performance of the ion sources, attempts to theoretically predict the extractable Hy current from a hydrogen plasma had been started by Hiskes and co-workers w5]7x, and many researchers have made contribution in this field w8]12x. Negative ions can be produced in a plasma through the polar dissociation process, ABq e ª Aqq Byq e which shows a large cross section at relatively higher energy than a typical electron temperature of a laboratory plasma. However, the cross section for the electron impact detachment from Hy is large at energy where the cross section for the polar dissociation is large, it is quite difficult to realize a high Hy density in a hydrogen plasma making use of the polar dissociation process. Contrary to this process, the dissociative electron attachment, ABq e ª Aq By,
Fig. 1. Reaction rates important for Hy production and destruction in a plasma volume. The plotted reaction rate for dissociative attachment is for a hydrogen molecule excited to its eight vibrational level and the ground rotational level.
can be the predominating process to form negative ions when the electron temperature of the plasma is lower than the threshold of the polar dissociation. The cross sections for dissociative electron attachment are usually decreasing functions against the electron energy with some thresholds. In the case of hydrogen, the cross section is larger and the threshold energy is lower as the vibrational excitation level of the molecule increases w13,14x. For the particular case of hydrogen, the reaction rates for these two processes are compared in Fig. 1. In Fig. 1, the reaction rate for dissociative attachment for the eighth vibrational level Ž ¨ s 8. is plotted. As shown in the figure, the dissociative attachment shows the reaction rate larger than that of polar dissociation by several orders of magnitude. On the other hand, the cross sections for fundamental processes efficiently produce highly vibrationally excited molecules Ž ¨ ) 4. usually show higher values for electron energy substantially higher than a typical laboratory plasma temperature w15,16x. As the dissociative electron attachment is a two-step process and high energy electrons are required to form vibrationally excited molecules, one would not expect high concentration of Hy in the hydrogen discharge for a single electron temperature plasma. However, as the energy distribution function of plasma electrons shows a high energy tale produced by the primary ionizing electrons w17x, vibrationally excited molecules are efficiently produced by this group of electrons. There-
130
M. Wada r Thin Solid Films 316 (1998) 128]133
fore, a plasma with two electron temperature component can produce Hy ions efficiently. The high energy part of the plasma electron distribution function contributes for the negative ion production through the production of highly vibrationally excited molecules, but they also contribute to destroy the produced negative ions in the plasma volume w18x. The reaction rate for this Hy destruction process is also plotted in Fig. 1. High energy electrons produce high concentration of protons, which shows a large reaction rate for Hy destruction w19]21x. Therefore, one should deal with the balance between the production of highly excited vibrational molecules and destruction of the produced Hy so long as it employs a single electron temperature system. A much better way to produce negative ions through vibrational excitation process is to divide the discharge container into two regions as shown in Fig. 2. In high temperature region, a dense plasma with energetic electrons is confined. Here, hydrogen molecules are efficiently excited by the impact of high energy electrons. In the low temperature region, which is separated by a localized magnetic field, only low energy electrons are confined. As the excited molecules do not have a charge, their transport across the localized field is not forbidden and they can make dissociative attachment reaction with low energy electrons in the region. The concept of this tandem approach w22x has been adopted successfully in designing a larger negative ion source of a neutral beam heating system for nuclear fusion experiments w23x. Model calculations trying to explain the physical processes influential on the Hy production have shown a good qualitative agreement with experiment, and one possibility drawn from the model is the existence of optimum length for the low electron temperature region w24x. Because the length of the region is finite, the effect of the wall collision, particularly, atom recombination at the wall becomes important in determining the final negative ion density. An
Fig. 2. Schematic illustrating the volume]production-type Hy ion source employing a tandem structure.
experiment to confirm the effect of the length of the discharge chamber, or the region where the vibrational excitation andror dissociative attachment takes place, had been conducted w25x. The result show a qualitative agreement with the prediction of the theoretical model. Thus, the size of the system is critically important in realizing a high concentration of negative ions. Also important is the degree of ionization of the plasma. It is not difficult to imagine that the perfectly ionized system does not contain substantial amount of negative ions as it does not contain molecules which are the primary source of negative ions. Therefore, the Hy current does not increase in proportion to the electron density for the fixed gas pressure in the chamber, but it increases with increasing gas pressure up to some electron density corresponding with the discharge current to sustain the plasma w26x. This effect has been successfully simulated by numerical models w27x. The concept of dividing the plasma source into two regions so as to separate the region for producing negative ions by dissociative attachment from that for producing highly excited vibrationally molecule can be extended to set up the scheme to enhance Hy production by controlling the electron temperature in a time-dependent way. In this scheme, highly vibrationally excited molecules are created during an intense discharge, and the produced excited molecules are attached with electrons to form negative ions during the afterglow phase. This effect is confirmed in hydrogen discharge w28,29x. The enhanced production of negative ions in after glow phase is also confirmed for reactive plasmas w30x. 3. Surface production of negative ions Prior to the findings that Hy ion concentration in a hydrogen discharge is higher and the electron current extracted with the Hy current can be minimized with the suitable magnetic field configuration near the extractor, surface conversion-type ion source had been developed for applications of particle accelerators and ion sources for fusion experiments w31x. The principle mechanism of the surface production of negative ions is the electron transfer from the metal to an atom leaving the surface. This electron transfer can be enhanced by minimizing the surface potential barrier. The produced negative ions should escape to an infinite distance without loosing electron. Lower work function enhances the probability for a negative ion to escape from the near surface region without loosing the electron in the affinity level. Particle fluxes leaving the solid surface can be created by surface collisions like reflection, sputtering and ion stimulated desorption of adsorbed atoms, and the dependence of
M. Wada r Thin Solid Films 316 (1998) 128]133
negative ion yields for these fundamental processes upon the work function of the negative ion production surface have been confirmed w32]35x. In these experiments, particle beams from ion sources are directed onto the surface. A much easier way to produce particle fluxes leaving the surface is to immerse the surface into a plasma. When the surface is biased negatively with respect to the plasma, the plasma particles strike the surface in the plasma and create the flux by reflection, sputtering, and ion induced desorption. If the surface is covered with Cs or Ba, the work function of the surface becomes low, and it efficiently produces negative ions. This negative ion production surface is called the converter, and typically biased 100]500 V negatively with respect to the plasma potential. Because the efficiency to produce this particle flux is higher for higher converter bias, surface production type ion source is operated with a converter bias higher than several hundred volt. The correlation among the work function of the surface of the converter in a plasma, converter bias and the produced Hy beam was experimentally investigated w36,37x. The result showed an exponential dependence of Hy production upon the work function decrease, and the Hy current increased by increasing the negative bias to the converter. Similar dependence has been found for surface production of Auy w38x, which are produced by sputtering at the Cs-covered Au surface. A high current density beam of heavy negative ions can be extracted from a surface conversion type ion source w39,40x. Negative ions produced at the surface in these sources travel inside the plasma creating a region where part of electrons are replaced with negative ions. The concentration should be particularly high when the current density of the produced beam is higher and the mass of the negative ion is larger. For example, 200 eV Auy beam of 1 mArcmy2 current density corresponds to about 4 = 10 9 cmy3 ion density. Another possibility for creating a substantial negative ion density based on surface produced neutral is to utilize the charge exchange reaction. Surface produced negative ions can give additional electron to neutral particles in a plasma, and make negative ions of the these neutrals. When the cross section for this charge exchange process is large, a negative ion rich area can be produced near the region where the parent negative ion beam is traveling.
131
plasma. From an early stage, the importance of interaction of plasma ions and neutrals with the surface or the plasma wall had been noticed and several parameters are included in the theoretical model to count for the effect of the surface collisions. Among them, the fraction of atomic reflection, and the fraction of deexcitation for vibrationally excited molecules had been considered most important. When Walther and co-workers w41,42x reported their observation that the amount of Hy current extracted from the ion source was increased by the introduction of Cs, the importance of the plasma]surface interaction on the Hy concentration in the plasma had been recognized again. Namely, the reduction of work function of the chamber wall due to Cs introduction into the discharge may increase the production of Hy at the chamber wall surface either by reflection of atomic hydrogen and plasma ions, or desorption of adsorbed hydrogen on the surface w43x. Results of experiments using a large ion source and a small ion source suggest the condition of the extraction electrode in contact with the plasma decides the amount of Hy current extracted from the source w44]46x. The fraction at which negative ions are formed at the surface from an incident low energy atomic hydrogen, which had been measured by Graham w47x, has been measured again w48,49x. The yield of Hy ions due to reflection of low energy positive hydrogen ions is also measured w50x, and compared with theory w51x. The results of these measurements of fundamental processes seem to suggest that the observed enhancement of Hy current from the Cs or Ba introduced discharge is attributable to surface production of Hy at the surface of the extraction electrode. To check if enough Hy ions are produced from the surface having a small potential difference with the plasma, the converter in a small ion source containing a Cs introduced hydrogen plasma was biased with a
4. Importance of plasma surface interaction The importance of the presence of the wall facing with the plasma can be seen in both the volume and the surface processes. For the volume process, the wall collisions changes the species composition in the
Fig. 3. The surface produced negative ion current plotted as a function of the converter voltage, which is applied between the negative ion production surface and the anode of the discharge. The plasma potential is from 1 to 2 V positive with respect to the anode.
132
M. Wada r Thin Solid Films 316 (1998) 128]133
voltage as low as 6 V. The amount of Hy current produced at the surface was measured after separated from other components by means of taking the energy spectrum of the extracted Hy current w52x. In Fig. 3, relative current from the converter is plotted as the function of the converter bias. As the plasma potential was 1]2 V positive with respect to the anode potential, the positive ions in the plasma are accelerated with the converter potential q1 to 2 eV. Though the Hy current is decreased by decreasing bias voltage, some amount of Hy is observed even at 6 V bias. Though the measurement below 6 V was not possible because Hy formed at the part in the ion source other than the converter overlapped on the energy spectrum of Hy produced at the converter surface, certain amount of Hy seems to be produced with the bias less than several eV. This result also suggests that Hy ions can be produced at the ion source wall. Thus, the enhancement of the Hy current from the ion source seems to be well explained by the Hy production from the surface of the low work plasma electrode. Particularly, atomic hydrogen are possibly converted to Hy by the surface reflection. However, the efficiency for the conversion is measured to be small, and enough flux of atomic hydrogen should be present if this mechanism is the principle source of the Hy enhancement. Besides, the dependence of the enhancement of Hy current upon the surface work function of the plasma electrode is weaker than the prediction from the theory, provided that Hy escape from the surface with velocity of thermal hydrogen atoms. If Hy ions are produced at the surface of the extraction electrode, they have to change the direction in the plasma to be extracted. In this case, additional interaction between the plasma and the surface should be present even if Hy ions are directly produced at the surface. Thus, several questions remain how the density in the ion source can be enhanced by the introduction of Cs into the discharge. For surface process, the surface condition including the work function is changed by the particle bombardment. The over-all negative ion concentration is then dependent upon the surface material of the plasma container, and the plasma parameters like electron temperature, plasma potential, electron density, degree of ionization, and chemical characteristics of species. Modification of the surface condition by plasma is another important realm in the plasma surface interaction. The absolute work function of Cs covered Cu and Mo in hydrogen discharge were measured with photoelectric method w36,37x. The results show some lower values than Cs covered Cu and Mo. Adsorption of hydrogen on the substrate metal surface is considered as the possible explanation, but the interaction of surface with other impurity species in the discharge may be also possible. There may be a
change in surface roughness due to sputtering. The surface of the wall of the ion source driven by hot W filaments should be covered with evaporated W. These may change the surface condition including work function and the atom reflection coefficients of the wall surface. So far, the discussion has been restricted to Hy in a hydrogen discharge. The situation should become far more complicated when one deals with a plasma of chemically active species, which are often used for plasma processing. For instance, when electronegative gas is contained in a plasma, it should increase the work function by being adsorbed on the surface. The higher work function of the surface will change the atom reflection coefficient andror coefficient for the deexcitation ratio of excited molecules. This will change the species condition in the plasma as the adsorption of the wall proceeds. The electron temperature and plasma potential may change depending upon the ratio of negative ion density to electron density, and the energy spectrum of the incoming ions onto the chamber wall surface will be altered. Through this process the condition for the particle adsorption on the surface will change. The links of the plasma condition and the surface condition are inevitable, and can be decisive in determining the final density of negative ions. 5. Summary Fundamental processes affecting the negative ion density in a bounded plasma are listed. In low temperature hydrogen plasma, dissociative attachment is believed to be the dominating formation process. When Cs is added into the discharge, some plasma]surface interaction takes place, and additional formation mechanism increases the Hy density in the hydrogen plasma. The plasma]surface interaction may become even more important when the species in the plasma is deposited on the surface. A modification of the surface parameter will cause a change of the plasma characteristics, and the negative ion concentration in the plasma. Acknowledgements The author is grateful to Professor Osamu Fukumasa at Yamaguchi University for his helpful discussion. This work was supported by the project, ‘Nanostructure Hybrid Devices and Their Properties’ at High Technology Center for Faculty of Engineering of Doshisha University. The project was supported by the Grant-in-Aid from Doshisha University and the aid from The Ministry of Education, Science and Culture.
M. Wada r Thin Solid Films 316 (1998) 128]133
References w1x K.N. Leung, in: I.G. Brown ŽEd.., The Physics and Technology of Ion Sources, Ch. 17, Wiley, New York, 1989, p. 355. w2x G.D. Alton, in: H.S.W. Massey, E.W. McDaniel, B. Bederson ŽEds..rSheldon Datz ŽVol. Ed.., Applied Atomic Collision Physics, vol. 4, Ch. 2, Academic Press, Florida, 1983, p. 43. w3x Y. Okumura, Y. Fujiwara, T. Inoue, et al., Rev. Sci. Instrum. 67 Ž1996. 1092. w4x T. Kuroda, O. Kaneko, Y. Takeiri, et al., Rev. Sci. Instrum. 67 Ž1996. 1114. w5x J.R. Hiskes, A.M. Karo, M. Bacal, A.M. Bruneteau, W.G. Graham, J. Appl. Phys. 53 Ž1982. 3469. w6x J.R. Hiskes, A.M. Karo, J. Appl. Phys. 56 Ž1984. 1927. w7x J.R. Hiskes, A.M. Karo, P.A. Willmann, J. Appl. Phys. 58 Ž1985. 1759. w8x O. Fukumasa, J. Phys. D. 22 Ž1989. 1688. w9x O. Fukumasa, S. Ohashi, J. Phys. D 22 Ž1989. 1931. w10x C. Gorse, M. Capitelli, J. Bretagne, M. Bacal, Chem. Phys. 93 Ž1985. 1. w11x C. Gorse, M. Capitelli, J. Bretagne, M. Bacal, A. Lagana, Chem. Phys. 117 Ž1987. 177. w12x C. Gorse, R. Celiberto, A. Lagana, M. Capitelli, Chem. Phys. 161 Ž1987. 211. w13x J.M. Wadehra, J.N. Bardsley, Phys. Rev. Lett. 41 Ž1978. 1795. w14x J.N. Bardsley, J.M. Wadehra, Phys. Rev. A20 Ž1979. 1398. w15x J.R. Hiskes, J. Appl. Phys. 51 Ž1980. 4592. w16x J.R. Hiskes, J. Appl. Phys. 70 Ž1991. 3409. w17x K.F. Schoenberg, W.B. Kunkel, J. Appl. Phys. 50 Ž1979. 4685. w18x D.S. Walton, B. Peart, K.T. Dolder, J. Phys. B. 4 Ž1971. 1343. w19x R.J. Bieniek, A. Dargarno, Astrophys. J. 228 Ž1979. 635. w20x R.J. Bieniek, J. Phys. B. 13 Ž1980. 4405. w21x D.H. McNeill, J. Kim, Phys. Rev. A25 Ž1982. 2152. w22x K.N. Leung, K.W. Ehlers, M. Bacal, Rev. Sci. Instrum. 54 Ž1983. 56. w23x Y. Okumura, Y. Fujiwara, A. Honda, et al., Rev. Sci. Instrum. 67 Ž1996. 1018. w24x J.R. Hiskes, A.M. Karo, J. Appl. Phys. 56 Ž1984. 1927. w25x K.N. Leung, K.W. Ehlers, R.V. Pyle, Rev. Sci. Instrum. 56 Ž1985. 364. w26x Y. Okumura, M. Hanada, T. Inoue, H. Kojima, Y. Matsuda, Y. Ohara, M. Seki, K. Watanabe, A. Hershkovitch ŽEds.., Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 210, 1990, p. 169. w27x O. Fukumasa, E. Niitani, J. Allesi, A. Hershkovitch ŽEds..,
w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x w39x w40x w41x w42x w43x w44x w45x
w46x w47x w48x w49x w50x w51x w52x
133
Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 380, 1990, p. 187. M.B. Hopkins, W.G. Graham, J. Phys. D. 20 Ž1987. 838. M.B. Hopkins, M. Bacal, W.G. Graham, J. Appl. Phys. 70 Ž1991. 2009. T.H. Ahn, K. Nakamura, H. Sugai, Jpn. J. Appl. Phys. 34 Ž1995. L1405. K.W. Ehlers, K.N. Leung, Rev. Sci. Instrum. 51 Ž1980. 721. M.L. Yu, Phys. Rev. Lett. 40 Ž1978. 574. P.J. Schneider, K.H. Berkner, W.G. Graham, R.V. Pyle, J.W. Stearns, Phys. Rev. B23 Ž1981. 941. J.R. Hiskes, P.J. Schneider, Phys. Rev. B23 Ž1981. 949. M. Seidel, A. Pargellis, Phys. Rev. B26 Ž1982. 1. M. Wada, R.V. Pyle, J.W. Stearns, J. Vac. Sci. Technol. A1 Ž1983. 981. M. Wada, R.V. Pyle, J.W. Stearns, J. Appl. Phys. 67 Ž1990. 6334. Y. Okabe, M. Sasao, H. Yamaoka, M. Wada, J. Fujita, Jpn. J. Appl. Phys. 30 Ž1991. 1307. G.D. Alton, Y. Mori, A. Takagi, A. Ueno, S. Fukumoto, Nucl. Instrum. Methods A270 Ž1988. 194. G.D. Alton, Y. Mori, A. Takagi, A. Ueno, S. Fukumoto, Nucl. Instrum. Methods B40r41 Ž1989. 1008. S.R. Walther, K.N. Leung, W.B. Kunkel, J. Appl. Phys. 64 Ž1988. 3424. S.R. Walther, K.N. Leung, W.B. Kunkel, Appl. Phys. Lett. 54 Ž1989. 210. K. N Leung, S.R. Walther, W.B. Kunkel, Phys. Rev. Lett. 62 Ž1989. 764. Y. Mori, T. Okuyama, A. Takagi, D. Yuan, Nucl. Instrum. Methods A301 Ž1991. 1. T. Inoue, M. Hanada, M. Mizuno, Y. Ohara, Y. Okumura, Y. Suzuki, M. Tanaka, K. Watanabe, J. Allesi ŽEds..rA. Hershkovitch ŽVol. Ed.., Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 287, 1990, p. 316. K. Shinto, Y. Okumura, T. Ando, et al., Jpn. J. Appl. Phys. 35 Ž1996. 1894. W.G. Graham, Phys. Lett. 73A Ž1979. 186. T. Okuyama, Y. Mori, Rev. Sci. Instrum. 63 Ž1992. 2711. B.S. Lee, M. Seidl, Appl. Phys. Lett. 61 Ž1992. 2857. J.D. Isenberg, H.J. Kwon, M. Seidl, Rev. Sci. Instrum. 63 Ž1992. 5289. H.L. Cui, J. Vac. Sci. Technol. A9 Ž1991. 1823. M. Wada, H. Tsuda, Production and Neutralization of Negative Ions and Beams, AIP, Conf. Proc. no. 287, 1990, p. 746.