- Email: [email protected]
XV International Conference on ion–surface interactions
Vacuum 66 (2002) 95–98
Editorial
XV International Conference on ion–surface interactions The XV International Conference ‘‘Ion–Surface Interactions’’ (ISI-2001) took place from 27 to 31 August 2001 at the ‘‘Zvenigorodskii’’ conference center near Moscow. The conference was organized by Moscow Aviation Institute (MAI), Moscow State University (MSU), and Moscow Engineering Physics Institute (MEPhI). The Program Committee comprised scientists from Russia and foreign countries. Before the beginning of the conference, two volumes of extended abstracts of reports (970 pp.) were published. About 200 scientists from 15 countries: Austria, Belgium, Byelorussia, Denmark, England, France, Germany, Holland, Italy, Japan, Poland, Russia, Ukraine, USA and Uzbekistan participated in the conference. There were six main sections which were held in sequence: (1) Sputtering of the surface and modification of its structure under the action of ion bombardment; (2) Scattering of ions; transmission of ions through solids; (3) Secondary ion emission; (4) Emission of electrons, photons, and X-rays under ion bombardment of solids; desorption; (5) Physical principles of ion implantation and modification of surface properties; thin films; (6) Interaction of plasmas with solid surfaces. 252 reports were presented; 40 of these being invited reports. The primary emphasis was placed on the fundamental problems of interaction of ions with a surface but, in a number of reports, the applied problems were also considered. The hallmark of the past conference was the participation of world-famous scientists (among them, 26 were from Western Europe, USA, and Japan) and a very high level of presented reports. At the beginning of the conference, an address was given by Professor Sir Michael Thompson (Eng-
land), who was one of the founders of the study of ion–surface interactions. His review report was devoted to the atomic collision cascades in solids. In this report he outlined the history of the problem, showed the possibility of using the secondary-particle emission for studying the nature of radiation damage in targets, and noted the achievements of scientists in investigating special features of sputtering of single crystals. In the problems related to the processes of sputtering and ion scattering, much attention was given to the influence of first- and second-order phase transitions on the ejection of atoms under ion bombardment and to the regularities of the secondary emission of multi-atomic formations (clusters). Using the currently available methods of investigation, the structural phase transition predicted theoretically was studied on silver single crystals at TB600 K, and anharmonic thermal vibrations were established at a higher temperature. Interesting results were reported on the sputtering of the same matter in solid and liquid states (with Indium used as an example). A decrease in sputtering of a molten target was unexpectedly discovered that is most likely associated with annealing of defects (both radiation induced and intrinsic) and with the corresponding increase in the binding energy of surface atoms. An original manifestation of the ion-induced firstorder phase transition is also the formation of a wave-like relief under certain conditions of ion bombardment of a surface. The perception of the nature of this phenomenon is of importance not only for the fundamental science, but also in a number of applications, for example, in developing quantitative secondary ion mass spectrometry (SIMS). In a number of studies, the influence of
0042-207X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 7 4 - 4
96
Editorial / Vacuum 66 (2002) 95–98
the first-order phase transition associated with strain of a target on the secondary ion emission (SIE) was reported. It was shown that the mechanical stresses in a solid induce an increase in SIE. Some authors proposed that this phenomenon is caused by a modification of the band structure under target strain, which can significantly (up to 50%) increase the ionization probability of sputtered atoms as theoretical estimates confirm. According to other authors, this phenomenon is caused by a significant increase in the density of dislocations and by a decrease in the binding energy of surface atoms. A series of reports was devoted to the emission of secondary particles from materials near to their magnetic phase transition temperature (the second-order transition). In particular, it was noted that the difference in sputtering of Ni in ferromagnetic and paramagnetic states increased for oblique angles of ion incidence where the number of collisions resulting in emission of atoms from the surface increased. It should be noted that the dependence of sputtering on orientations of spins in the target, i.e., the ‘‘quantum effect in sputtering’’, was experimentally discovered as early as 1975, but many details of this phenomenon were not explained until now. For example, of concern is the abrupt increase in the atomic emission (up to a factor of 2–3) at the Curie point. Evidence of progress in this problem is the reported theoretical results, which explain the regularity observed by the simultaneous action of critical fluctuations of the magnetic subsystem and a nonequilibrium evaporation of weakly bonded surface atoms near the Curie temperature. The participants of the conference considered also the processes of formation of charged and excited states for secondary particles and the mechanism of emission for secondary neutral and charged clusters under bombardment by monatomic and multi-atomic particles. Solving these problems presents severe difficulties, but the timely development of experimental and computer equipment has led to distinct results. The use of highly sensitive methods made it possible to discover the sputtering of large-size neutral metal clusters
containing more than 200 atoms. It was shown that an increase in mass of bombarding ions induced no marked rise in the self-sputtering of multi-atomic clusters, which was reduced by a power law with the number of atoms in the cluster, but did lead to their significant excitation (‘‘hot clusters’’). The results obtained point to the important role of strong collisions in this process and favour the thermodynamic mechanism and the shock-wave model for explaining the regularities observed. However, no adequate theory describing the entire totality of experimental data of the cluster emission has been developed until now. Another investigation trend associated with the clusters is the possibility of their implantation into a target; this problem is of exceptional interest in nanoelectronics. In experiments and computer simulations, threshold energies (of the order of keV) and a cluster-substrate implantation depth (attaining 10 monolayers for a 10-atom Ag cluster) were found. To study the same set of problems, we refer to the development of modern computation codes and the technology of production of thinfilm coatings with participation of the sputtering process. The results obtained may be summarized as follows. The nanostructure of deposited films was investigated by computer simulation, and guidelines for predicting their mechanical and thermodynamic properties were found. An optimal energy (B20 eV) was found for depositing particles. This energy is sufficient for their adhesion to the surface, leads to no diffusion of these particles, and introduces no damage to the substrate. Ion bombardment was used for production of finegrained films with a record hardness and flexibility and also metastable alloys. When the surfaces of a number of new materials (such as ‘‘low-temperature’’ gallium arsenide, high-temperature superconductors, nitrides, and fullerenes) were irradiated by ion beams, very thin MOS structures with quasi-two-dimensional metal clusters and unique properties were obtained. In this case, the effect of self-organized formation of MOS structures occurred. Sputtering of the self-organized monolayers and adsorbed molecules of organic
Editorial / Vacuum 66 (2002) 95–98
materials on metal substrates were discussed. This new trend of investigations is of importance for understanding the mechanism of formation and rupture of chemical bonds between atoms in molecules and their adhesion with the substrate. Furthermore, it offers promise for the further development of new process methods in nanoelectronics and lithography. The experiments carried out showed that the bombardment of such layers by perfect-gas ions of keV energies led to the emission of very slow molecules and their fragments (with velocities of the order of thermal ones). This phenomenon can be associated with a two-step process of emission of molecules: first, the rupture of certain bonds takes place in a molecule owing to the action of primary ions, secondary electrons, and chemical reactions, then fragments move along the surface and evaporate. The possibility of such a process is confirmed by computer calculations with the Brenner multiparticle potential. In this area of study, also noteworthy is the investigation of the character of bonds in a molecule and in an adsorbate. The molecular-dynamic calculations for strong bonds and also the quantum-mechanical considerations from first principles have made it possible to reliably establish the covalent character of bonds between fullerene (C60) and dimers of a silicon substrate and to investigate their rupture under ion bombardment. Further we would like to outline briefly some other results, which aroused interest of participants of the conference. The sub-threshold kinetic electron emission from a pure metal surface and the excitation of plasmons in a target due to the potential energy of bombarding ions were observed. The quantum-dimensional effect was found for the resonance electron charge exchange between an ion and thin metal films. The influence of electronic structure of the surface on the character of its interaction with multi-charged ions was shown. The theory of electron stopping was developed for heavy and light ions and the special features of transmission of molecular ions through thin films were established. The resonance coherent excitation of atoms in glancing scattering
97
from the surface and the backscattering of light slow ions from the adsorbed layers were studied. The regularities of kinetic electron emission under bombardment of a target by molecular ions were determined. The mechanism of formation of sputtered clusters in the thermal-spike model was proposed, and molecular effects were established, which occur during implantation. The interaction of ions on astronomical bodies was considered. A modification in properties of targets under ion bombardment was investigated, and the special features of the interaction between the plasma and International Tokomak Experimental Reactor (ITER) materials were shown. In closing the conference, Professor Raul Baragiola (USA) made an analysis of the results achieved in investigating the interaction of ions with a surface and noted the most important problems, which remain to be solved in the future. First of all, these are the acquisition of fundamental knowledge on the differential characteristics of the flows of scattered, sputtered, and desorbed particles under the ion bombardment of single- and multi-component materials including those offering promise for various practical applications (in device engineering, microelectronics, space technology); the improvement of theoretical descriptions and the simulation of effects resulting from the bombardment of solids by ions and electrons; the development of new methods of modification and diagnostics of the surface and; the widening of the circle of application of scientific achievements for new technology. The participants of the conference expressed their gratitude to the Ministry of Atomic Power of Russia, the Russian Foundation for Basic Research, the Ministry of Industry, Science, and Technology of Russia, and also to the publishing house Elsevier and the company Caburn-MDC (England) for the financial support of the conference. The great activity of the scientists from MAI, MSU, and MEPhI and the members of the programme committee in organizing and performing the conference was noted. Attention was drawn to the increasing interest in the conference from the leading foreign specialists, whose active
98
Editorial / Vacuum 66 (2002) 95–98
participation in this work gave the possibility for scientists from Russia and from former Soviet Union countries to discuss many science problems and come to agreement on new cooperative investigations. It was decided that the next, XVI International Conference on ion-surface interactions will be held in Zvenigorod in 2003 and its organization again entrusted to MAI, MSU, and MEPhI.
Vera E. Yurasova Faculty of Physics, Moscow State University, Moscow, Russia John S. Colligon Department of Chemistry and Materials, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK E-mail address: [email protected]
Editorial
XV International Conference on ion–surface interactions The XV International Conference ‘‘Ion–Surface Interactions’’ (ISI-2001) took place from 27 to 31 August 2001 at the ‘‘Zvenigorodskii’’ conference center near Moscow. The conference was organized by Moscow Aviation Institute (MAI), Moscow State University (MSU), and Moscow Engineering Physics Institute (MEPhI). The Program Committee comprised scientists from Russia and foreign countries. Before the beginning of the conference, two volumes of extended abstracts of reports (970 pp.) were published. About 200 scientists from 15 countries: Austria, Belgium, Byelorussia, Denmark, England, France, Germany, Holland, Italy, Japan, Poland, Russia, Ukraine, USA and Uzbekistan participated in the conference. There were six main sections which were held in sequence: (1) Sputtering of the surface and modification of its structure under the action of ion bombardment; (2) Scattering of ions; transmission of ions through solids; (3) Secondary ion emission; (4) Emission of electrons, photons, and X-rays under ion bombardment of solids; desorption; (5) Physical principles of ion implantation and modification of surface properties; thin films; (6) Interaction of plasmas with solid surfaces. 252 reports were presented; 40 of these being invited reports. The primary emphasis was placed on the fundamental problems of interaction of ions with a surface but, in a number of reports, the applied problems were also considered. The hallmark of the past conference was the participation of world-famous scientists (among them, 26 were from Western Europe, USA, and Japan) and a very high level of presented reports. At the beginning of the conference, an address was given by Professor Sir Michael Thompson (Eng-
land), who was one of the founders of the study of ion–surface interactions. His review report was devoted to the atomic collision cascades in solids. In this report he outlined the history of the problem, showed the possibility of using the secondary-particle emission for studying the nature of radiation damage in targets, and noted the achievements of scientists in investigating special features of sputtering of single crystals. In the problems related to the processes of sputtering and ion scattering, much attention was given to the influence of first- and second-order phase transitions on the ejection of atoms under ion bombardment and to the regularities of the secondary emission of multi-atomic formations (clusters). Using the currently available methods of investigation, the structural phase transition predicted theoretically was studied on silver single crystals at TB600 K, and anharmonic thermal vibrations were established at a higher temperature. Interesting results were reported on the sputtering of the same matter in solid and liquid states (with Indium used as an example). A decrease in sputtering of a molten target was unexpectedly discovered that is most likely associated with annealing of defects (both radiation induced and intrinsic) and with the corresponding increase in the binding energy of surface atoms. An original manifestation of the ion-induced firstorder phase transition is also the formation of a wave-like relief under certain conditions of ion bombardment of a surface. The perception of the nature of this phenomenon is of importance not only for the fundamental science, but also in a number of applications, for example, in developing quantitative secondary ion mass spectrometry (SIMS). In a number of studies, the influence of
0042-207X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 7 4 - 4
96
Editorial / Vacuum 66 (2002) 95–98
the first-order phase transition associated with strain of a target on the secondary ion emission (SIE) was reported. It was shown that the mechanical stresses in a solid induce an increase in SIE. Some authors proposed that this phenomenon is caused by a modification of the band structure under target strain, which can significantly (up to 50%) increase the ionization probability of sputtered atoms as theoretical estimates confirm. According to other authors, this phenomenon is caused by a significant increase in the density of dislocations and by a decrease in the binding energy of surface atoms. A series of reports was devoted to the emission of secondary particles from materials near to their magnetic phase transition temperature (the second-order transition). In particular, it was noted that the difference in sputtering of Ni in ferromagnetic and paramagnetic states increased for oblique angles of ion incidence where the number of collisions resulting in emission of atoms from the surface increased. It should be noted that the dependence of sputtering on orientations of spins in the target, i.e., the ‘‘quantum effect in sputtering’’, was experimentally discovered as early as 1975, but many details of this phenomenon were not explained until now. For example, of concern is the abrupt increase in the atomic emission (up to a factor of 2–3) at the Curie point. Evidence of progress in this problem is the reported theoretical results, which explain the regularity observed by the simultaneous action of critical fluctuations of the magnetic subsystem and a nonequilibrium evaporation of weakly bonded surface atoms near the Curie temperature. The participants of the conference considered also the processes of formation of charged and excited states for secondary particles and the mechanism of emission for secondary neutral and charged clusters under bombardment by monatomic and multi-atomic particles. Solving these problems presents severe difficulties, but the timely development of experimental and computer equipment has led to distinct results. The use of highly sensitive methods made it possible to discover the sputtering of large-size neutral metal clusters
containing more than 200 atoms. It was shown that an increase in mass of bombarding ions induced no marked rise in the self-sputtering of multi-atomic clusters, which was reduced by a power law with the number of atoms in the cluster, but did lead to their significant excitation (‘‘hot clusters’’). The results obtained point to the important role of strong collisions in this process and favour the thermodynamic mechanism and the shock-wave model for explaining the regularities observed. However, no adequate theory describing the entire totality of experimental data of the cluster emission has been developed until now. Another investigation trend associated with the clusters is the possibility of their implantation into a target; this problem is of exceptional interest in nanoelectronics. In experiments and computer simulations, threshold energies (of the order of keV) and a cluster-substrate implantation depth (attaining 10 monolayers for a 10-atom Ag cluster) were found. To study the same set of problems, we refer to the development of modern computation codes and the technology of production of thinfilm coatings with participation of the sputtering process. The results obtained may be summarized as follows. The nanostructure of deposited films was investigated by computer simulation, and guidelines for predicting their mechanical and thermodynamic properties were found. An optimal energy (B20 eV) was found for depositing particles. This energy is sufficient for their adhesion to the surface, leads to no diffusion of these particles, and introduces no damage to the substrate. Ion bombardment was used for production of finegrained films with a record hardness and flexibility and also metastable alloys. When the surfaces of a number of new materials (such as ‘‘low-temperature’’ gallium arsenide, high-temperature superconductors, nitrides, and fullerenes) were irradiated by ion beams, very thin MOS structures with quasi-two-dimensional metal clusters and unique properties were obtained. In this case, the effect of self-organized formation of MOS structures occurred. Sputtering of the self-organized monolayers and adsorbed molecules of organic
Editorial / Vacuum 66 (2002) 95–98
materials on metal substrates were discussed. This new trend of investigations is of importance for understanding the mechanism of formation and rupture of chemical bonds between atoms in molecules and their adhesion with the substrate. Furthermore, it offers promise for the further development of new process methods in nanoelectronics and lithography. The experiments carried out showed that the bombardment of such layers by perfect-gas ions of keV energies led to the emission of very slow molecules and their fragments (with velocities of the order of thermal ones). This phenomenon can be associated with a two-step process of emission of molecules: first, the rupture of certain bonds takes place in a molecule owing to the action of primary ions, secondary electrons, and chemical reactions, then fragments move along the surface and evaporate. The possibility of such a process is confirmed by computer calculations with the Brenner multiparticle potential. In this area of study, also noteworthy is the investigation of the character of bonds in a molecule and in an adsorbate. The molecular-dynamic calculations for strong bonds and also the quantum-mechanical considerations from first principles have made it possible to reliably establish the covalent character of bonds between fullerene (C60) and dimers of a silicon substrate and to investigate their rupture under ion bombardment. Further we would like to outline briefly some other results, which aroused interest of participants of the conference. The sub-threshold kinetic electron emission from a pure metal surface and the excitation of plasmons in a target due to the potential energy of bombarding ions were observed. The quantum-dimensional effect was found for the resonance electron charge exchange between an ion and thin metal films. The influence of electronic structure of the surface on the character of its interaction with multi-charged ions was shown. The theory of electron stopping was developed for heavy and light ions and the special features of transmission of molecular ions through thin films were established. The resonance coherent excitation of atoms in glancing scattering
97
from the surface and the backscattering of light slow ions from the adsorbed layers were studied. The regularities of kinetic electron emission under bombardment of a target by molecular ions were determined. The mechanism of formation of sputtered clusters in the thermal-spike model was proposed, and molecular effects were established, which occur during implantation. The interaction of ions on astronomical bodies was considered. A modification in properties of targets under ion bombardment was investigated, and the special features of the interaction between the plasma and International Tokomak Experimental Reactor (ITER) materials were shown. In closing the conference, Professor Raul Baragiola (USA) made an analysis of the results achieved in investigating the interaction of ions with a surface and noted the most important problems, which remain to be solved in the future. First of all, these are the acquisition of fundamental knowledge on the differential characteristics of the flows of scattered, sputtered, and desorbed particles under the ion bombardment of single- and multi-component materials including those offering promise for various practical applications (in device engineering, microelectronics, space technology); the improvement of theoretical descriptions and the simulation of effects resulting from the bombardment of solids by ions and electrons; the development of new methods of modification and diagnostics of the surface and; the widening of the circle of application of scientific achievements for new technology. The participants of the conference expressed their gratitude to the Ministry of Atomic Power of Russia, the Russian Foundation for Basic Research, the Ministry of Industry, Science, and Technology of Russia, and also to the publishing house Elsevier and the company Caburn-MDC (England) for the financial support of the conference. The great activity of the scientists from MAI, MSU, and MEPhI and the members of the programme committee in organizing and performing the conference was noted. Attention was drawn to the increasing interest in the conference from the leading foreign specialists, whose active
98
Editorial / Vacuum 66 (2002) 95–98
participation in this work gave the possibility for scientists from Russia and from former Soviet Union countries to discuss many science problems and come to agreement on new cooperative investigations. It was decided that the next, XVI International Conference on ion-surface interactions will be held in Zvenigorod in 2003 and its organization again entrusted to MAI, MSU, and MEPhI.
Vera E. Yurasova Faculty of Physics, Moscow State University, Moscow, Russia John S. Colligon Department of Chemistry and Materials, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK E-mail address: [email protected]