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Introduction to the viewpoint set on multilayers
Scripta METALLURGICA et MATERIALIA
Vol. 27, pp. 649-650, 1992 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
VIEWPOINT SET No. 19
INTRODUCTION
TO THE VIEWPOINT MULTILAYERS
SET ON
Robb T h o m s o n National Institute for Standards and Technology Gaithersburg, M D 20899 (Received July 6, 1992) (Revised July IS, 1992) In December, 1991, NIST held a small workshop at its Boulder facility to explore some aspects of the interesting new developments taking place in the materials science of thin multilayers. Our emphasis was focussed mostly on the coherence breakdown at the interfaces, and secondarily on their synthesis and mechanical properties. One of the participants was John Hirth, the editor of Scripta Metallurgica, and he suggested the topic of the workshop would be an excellent one for a viewpoint series in the journal. Concurring with his idea, the participants have prepared the following papers. The synthesis and investigation of multilayers and other small structures is currently enjoying a great vogue, in part because of the advance of synthetic techniques, in part because of their important technological applications, and in part because of the perceived potential for "materials engineering" with these structures. Most of the attention has been focussed on the semiconductors, with about an order of magnitude less activity in the metallic systems, but both have had a doubling time for papers published of about two years![1] From the start, the coherence breakdown at the interface between the layers has been a central concern for semiconductors, and the magnetic and mechanical properties have been important for metals. But most of the interest in the mechanical properties of the metallic system has focussed on the existence a n d / o r magnitude of the super-modulus effect, and on deformation studies conducted in free standing films. There is almost no work on fracture. At NIST, there has been an ongoing effort in synthesis of metallic systems, and we have been stimulated to investigate the properties of dislocations and cracks in these systems. Out of this background grew the workshop, with the topics constituting the papers in this series. The first paper in the series, by Foecke and Lashmore, is a review of progress in the synthesis of metallic multilayers, with an emphasis on electrochemical techniques. As noted above, the success of this technique, in which people at NIST have played a prominent part, was one of the principal reasons for organizing the workshop. There are a variety of methods for producing thin multilayers, down to monolayer thickness; however, electrochemical techniques have certain advantages, such as great simplicity and relative perfection of the final product. Foecke and Lashmore also review briefly the present status of the "super-modulus" effect, and provide an excellent review and bibliography on deformation and fracture of multilayers. Finally, they also present some very recent results on fracture at the interface in a coherent Cu/Ni multilayer system. Following Foecke and Lashmore are five papers on dislocations in strained layers. The first two, (the second and third in the series) by Hull and Bean summarize the energetic and kinetic factors which govern stress relaxation by misfit dislocations in semiconductor multilayers, with detailed comparisons between current experimental findings and theoretical predictions. These papers give a remarkably comprehensive overview of the situation in the semiconductors, in a very short space. The fourth paper in the series, by Freund, addresses dislocation interactions in the diamond lattice in greater detail and depth, with an emphasis on the interactions between dislocations during the formation of a misfit boundary. This paper addresses the mechanisms by which additional dislocations are added to a partially formed misfit array. The fifth paper, by Jesser, takes up some of the same issues of the previous three papers in a somewhat different manner. Jesser's topic is the role of the dislocation friction stress in the formation of misfit structures. 649 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.
650
VIEWPOINT
SET ON MULTILAYERS
Vol.
27, No.
Frictional forces can arise from both Peierls barriers as well as chemical pinning barriers, but pinning can mascarade as a kind of Peierls barrier if the pinning agents diffuse with the moving dislocation. Jesser discusses the rather complex situation which results, and analyses the prospects for sorting out the observations and mechanisms involved. The sixth paper, by Hirth, pursues detailed dislocation interactions further by analyzing the situation of two orthogonal dislocations interacting on a single interface plane. He shows that the interactions make only a minor contribution to the energy of the final misfit structure, but that they do contribute important inter-dislocation forces which should retard the formation of the boundary. The final topic, addressed in the seventh paper by Anderson, Lin and Thomson, is fracture of multilayers. This is a subject about which almost nothing is known, either experimentally, or theoretically, except for a recent report by Lashmore (see the paper by Foecke and Lashmore in this series) of a crack and associated dislocations on an interface in the Cu/Ni system. Nevertheless, Anderson, et aI. project present understanding about interracial fracture to the new situation in multilayers, report some recent unpublished work, and point up where some useful directions for research may lie. The overall perspective one gains from these papers is that this is a subject with considerable maturity in certain areas such as the semiconductor misfit problem, in great contrast to others, such as mechanical strength and failure, where only spotty knowledge is available. Nevertheless, in spite of the vast literature in the semiconductor case, one cannot say that field is closed, or that we are even in a position, yet, to tailor what is needed from a technological point of view. On the other hand, it is certainly true that the maturity of the semiconductor area does provide one with some ability to "piggy-back" the misfit situation in the metals. At the other end of the scale, in spite of some intriguing early findings, we know next to nothing about the fracture of multilayers, and the interpretation of a relatively large body of experimental results on the deformation of multilayers is still in a rudimentary state. We hope this viewpoint set will be helpful in pointing out some of the more intriguing scientific problems and challenges to the interested reader. References
1. C. M. Falco, Physics, Fabrication, and Applications of Multilayered Structures, ed. P. Dhez and C. Weisbuch, page 3, Plenum, New York, (1987).
6
Vol. 27, pp. 649-650, 1992 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
VIEWPOINT SET No. 19
INTRODUCTION
TO THE VIEWPOINT MULTILAYERS
SET ON
Robb T h o m s o n National Institute for Standards and Technology Gaithersburg, M D 20899 (Received July 6, 1992) (Revised July IS, 1992) In December, 1991, NIST held a small workshop at its Boulder facility to explore some aspects of the interesting new developments taking place in the materials science of thin multilayers. Our emphasis was focussed mostly on the coherence breakdown at the interfaces, and secondarily on their synthesis and mechanical properties. One of the participants was John Hirth, the editor of Scripta Metallurgica, and he suggested the topic of the workshop would be an excellent one for a viewpoint series in the journal. Concurring with his idea, the participants have prepared the following papers. The synthesis and investigation of multilayers and other small structures is currently enjoying a great vogue, in part because of the advance of synthetic techniques, in part because of their important technological applications, and in part because of the perceived potential for "materials engineering" with these structures. Most of the attention has been focussed on the semiconductors, with about an order of magnitude less activity in the metallic systems, but both have had a doubling time for papers published of about two years![1] From the start, the coherence breakdown at the interface between the layers has been a central concern for semiconductors, and the magnetic and mechanical properties have been important for metals. But most of the interest in the mechanical properties of the metallic system has focussed on the existence a n d / o r magnitude of the super-modulus effect, and on deformation studies conducted in free standing films. There is almost no work on fracture. At NIST, there has been an ongoing effort in synthesis of metallic systems, and we have been stimulated to investigate the properties of dislocations and cracks in these systems. Out of this background grew the workshop, with the topics constituting the papers in this series. The first paper in the series, by Foecke and Lashmore, is a review of progress in the synthesis of metallic multilayers, with an emphasis on electrochemical techniques. As noted above, the success of this technique, in which people at NIST have played a prominent part, was one of the principal reasons for organizing the workshop. There are a variety of methods for producing thin multilayers, down to monolayer thickness; however, electrochemical techniques have certain advantages, such as great simplicity and relative perfection of the final product. Foecke and Lashmore also review briefly the present status of the "super-modulus" effect, and provide an excellent review and bibliography on deformation and fracture of multilayers. Finally, they also present some very recent results on fracture at the interface in a coherent Cu/Ni multilayer system. Following Foecke and Lashmore are five papers on dislocations in strained layers. The first two, (the second and third in the series) by Hull and Bean summarize the energetic and kinetic factors which govern stress relaxation by misfit dislocations in semiconductor multilayers, with detailed comparisons between current experimental findings and theoretical predictions. These papers give a remarkably comprehensive overview of the situation in the semiconductors, in a very short space. The fourth paper in the series, by Freund, addresses dislocation interactions in the diamond lattice in greater detail and depth, with an emphasis on the interactions between dislocations during the formation of a misfit boundary. This paper addresses the mechanisms by which additional dislocations are added to a partially formed misfit array. The fifth paper, by Jesser, takes up some of the same issues of the previous three papers in a somewhat different manner. Jesser's topic is the role of the dislocation friction stress in the formation of misfit structures. 649 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.
650
VIEWPOINT
SET ON MULTILAYERS
Vol.
27, No.
Frictional forces can arise from both Peierls barriers as well as chemical pinning barriers, but pinning can mascarade as a kind of Peierls barrier if the pinning agents diffuse with the moving dislocation. Jesser discusses the rather complex situation which results, and analyses the prospects for sorting out the observations and mechanisms involved. The sixth paper, by Hirth, pursues detailed dislocation interactions further by analyzing the situation of two orthogonal dislocations interacting on a single interface plane. He shows that the interactions make only a minor contribution to the energy of the final misfit structure, but that they do contribute important inter-dislocation forces which should retard the formation of the boundary. The final topic, addressed in the seventh paper by Anderson, Lin and Thomson, is fracture of multilayers. This is a subject about which almost nothing is known, either experimentally, or theoretically, except for a recent report by Lashmore (see the paper by Foecke and Lashmore in this series) of a crack and associated dislocations on an interface in the Cu/Ni system. Nevertheless, Anderson, et aI. project present understanding about interracial fracture to the new situation in multilayers, report some recent unpublished work, and point up where some useful directions for research may lie. The overall perspective one gains from these papers is that this is a subject with considerable maturity in certain areas such as the semiconductor misfit problem, in great contrast to others, such as mechanical strength and failure, where only spotty knowledge is available. Nevertheless, in spite of the vast literature in the semiconductor case, one cannot say that field is closed, or that we are even in a position, yet, to tailor what is needed from a technological point of view. On the other hand, it is certainly true that the maturity of the semiconductor area does provide one with some ability to "piggy-back" the misfit situation in the metals. At the other end of the scale, in spite of some intriguing early findings, we know next to nothing about the fracture of multilayers, and the interpretation of a relatively large body of experimental results on the deformation of multilayers is still in a rudimentary state. We hope this viewpoint set will be helpful in pointing out some of the more intriguing scientific problems and challenges to the interested reader. References
1. C. M. Falco, Physics, Fabrication, and Applications of Multilayered Structures, ed. P. Dhez and C. Weisbuch, page 3, Plenum, New York, (1987).
6