Thin Fllms-A HIstorical Perspective in the transmission electron microscope. Presently, gold leaf is used to deco- rate such diverse structures and objects as statues, churches, public building tombstones, furniture, hand-tooled leather, picture frames and, of course, illuminated manuscripts Thin-film technologies related to gold beating, but probably not as old, are nercury and fire gilding. Used to decorate copper or bronze statuary, the cold mercury process involved carefully smoothing and polishing the metal surface after which mercury was rubbed into it. Some copper dissolved in the mercury,forming a very thin amalgam film that left the surface shiny and smooth as a mirror. Gold leaf was then pressed onto the surface cold and bonded to the mercury-rich adhesive. Alternately, gold was directly amalga- mated with mercury, applied, and the excess mercury was then driven off by heating, leaving a film of gold behind. Fire gilding was practiced well into the nineteenth century despite the grave health risk due to mercury vapor. The hazard to workers finally became intolerable and provided the incentive to develop alternative processes, such as electroplating The history of gold beating and gilding is replete with experimentation and process development in diverse parts of the ancient world. Practitioners were concerned with the purity and cost of the gold, surface preparation, the uniformity of the applied films, adhesion to the substrate, reactions between and among the gold, mercury, copper, bronze(copper-tin), etc. prod safety, color, optical appearance, durability of the final coating, and compe tive coating technologies. As we shall see in the ensuing pages, modern thin-film technology addresses these same generic issues, albeit with a great compression of time. And although science is now in the ascendancy, there is still much room for art REFERENCES 1. L. B. Hunt, Gold Bull.9, 24(1976) 2. O. Vittori, Gold Bull. 12, 35(1979) 3. E. D. Nicholson, Gold Bull. 12, 161(1979)
xx Thin Films -A Historical Perspective in the transmission electron microscope. Presently, gold leaf is used to decorate such diverse structures and objects as statues, churches, public buildings, tombstones, furniture, hand-tooled leather, picture frames and, of course, illuminated manuscripts. Thin-film technologies related to gold beating, but probably not as old, are mercury and fire gilding. Used to decorate copper or bronze statuary, the cold mercury process involved carefully smoothing and polishing the metal surface, after which mercury was rubbed into it. Some copper dissolved in the mercury, forming a very thin amalgam film that left the surface shiny and smooth as a mirror. Gold leaf was then pressed onto the surface cold and bonded to the mercury-rich adhesive. Alternately, gold was directly amalgamated with mercury, applied, and the excess mercury was then driven off by heating, leaving a film of gold behind. Fire gilding was practiced well into the nineteenth century despite the grave health risk due to mercury vapor. The hazard to workers finally became intolerable and provided the incentive to develop alternative processes, such as electroplating. The history of gold beating and gilding is replete with experimentation and process development in diverse parts of the ancient world. Practitioners were concerned with the purity and cost of the gold, surface preparation, the uniformity of the applied films, adhesion to the substrate, reactions between and among the gold, mercury, copper, bronze (copper-tin), etc., process safety, color, optical appearance, durability of the final coating, and competitive coating technologies. As we shall see in the ensuing pages, modem thin-film technology addresses these same generic issues, albeit with a great compression of time. And although science is now in the ascendancy, there is still much room for art. REFERENCES 1. L. B. Hunt, Gold Bull. 9, 24 (1976). 2. 0. Vittori, Gold Bull. 12, 35 (1979). 3. E. D. Nicholson, Gold Bull. 12, 161 (1979)
Chapter A Review of Materials Science 1.1. INTRODUCTION A cursory consideration of the vast body of solid substances reveals what outwardly appears to be an endless multitude of external forms and structures possessing a bewildering variety of properties. The branch of study known as materials science and engineering evolved in part to classify those features that are common among the structure and properties of different materials in a manner somewhat reminiscent of chemical or biological classification schemes This dramatically reduces the apparent variety. From this perspective, it turns out that solids can be classified as typically belonging to one of only four categories(metallic, ionic, covalent, or van der Waals), depending on the nature of the electronic structure and resulting interatomic bonding forces Similar divisions occur with respect to the structure of solids. Solids are either internally crystalline or noncrystalline. Those that are crystalline can be further subdivided according to one of 14 different geometric arrays or lattices epending on the placement of the atoms. When properties are considered there are similar simplifying categorizations. Thus, materials are either good intermediate, or poor conductors of electricity, and they are either mechani cally brittle or can easily be stretched without fracture, and they are either
I Chapter I A Review of Materials Science 1 .I. INTRODUCTION A cursory consideration of the vast body of solid substances reveals what outwardly appears to be an endless multitude of external forms and structures possessing a bewildering variety of properties. The branch of study known as materials science and engineering evolved in part to classify those features that are common among the structure and properties of different materials in a manner somewhat reminiscent of chemical or biological classification schemes. This dramatically reduces the apparent variety. From this perspective, it turns out that solids can be classified as typically belonging to one of only four categories (metallic, ionic, covalent, or van der Waals), depending on the nature of the electronic structure and resulting interatomic bonding forces. Similar divisions occur with respect to the structure of solids. Solids are either internally crystalline or noncrystalline. Those that are crystalline can be further subdivided according to one of 14 different geometric arrays or lattices, depending on the placement of the atoms. When properties are considered, there are similar simplifying categorizations. Thus, materials are either good, intermediate, or poor conductors of electricity, and they are either mechanically brittle or can easily be stretched without fracture, and they are either 1
Review of Materials Science optically reflective or transparent, etc. It is, of course, easier to recognize that property differences exist than to understand why they exist. Nevertheless much progress has been made in this subject as a result of the research of the past 50 years. Basically, the richness in the diversity of materials properties occurs because countless combinations of the admixture of chemical co tions, bonding types, crystal structures, and morphologies are available natu rally or can be synthesized In this chapter various aspects of structure and bonding in solids are reviewed for the purpose of providing the background to better understand the remainder of the book. In addition, several topics dealing with thermodynam ics and kinetics of atomic motion in materials are also included. These will later have relevance to aspects of the stability, formation, and solid-state reactions in thin films. Much of this chapter is a condensed adaptation of standard treatments of bulk materials, but it is equally applicable to thin films. Nevertheless, many distinctions between bulk materials and films exist, and they will be stressed where possible. Readers already familiar with concepts of broader coverage should consu. p this chapter; those who seek deeper and materials science may wish to ski he bibliography for recommended texts on this subject 1. 2. STRUCTURE 1.2.1. Crystalline Solids Many solid materials p an ordered intermal crystal stru xternal appearances that are not what we associate with the crystal ine-i.e, clear, transparent, faceted, etc. Actual crystal structures can be imagined to arise from a three-dimensional array of points geometrically and repetitively distributed in space such that each point has identical surroundings There are only 14 ways to arrange points in space having this property, and the resulting point arrays are known as Bravais lattices. They are shown in Fig 1-1 with lines intentionally drawn in to emphasize the symmetry of the lattice Only a single cell for each lattice is reproduced here, and the point arra actually stretches in an endlessly repetitive fashion in all directions. If an atom or group of two or more atoms is now placed at each Bravais lattice point, a physically real crystal structure emerges. Thus, if individual copper atoms populated every point of a face-centered cubic(FCC) lattice whose cube edge dimension, or so-called lattice parameter, were 3,615 A, the material known as
2 A Review of Materials Science optically reflective or transparent, etc. It is, of course, easier to recognize that property differences exist than to understand why they exist. Nevertheless, much progress has been made in this subject as a result of the research of the past 50 years. Basically, the richness in the diversity of materials properties occurs because countless combinations of the admixture of chemical compositions, bonding types, crystal structures, and morphologies are available naturally or can be synthesized. In this chapter various aspects of structure and bonding in solids are reviewed for the purpose of providing the background to better understand the remainder of the book. In addition, several topics dealing with thermodynamics and kinetics of atomic motion in materials are also included. These will later have relevance to aspects of the stability, formation, and solid-state reactions in thin films. Much of this chapter is a condensed adaptation of standard treatments of bulk materials, but it is equally applicable to thin films. Nevertheless, many distinctions between bulk materials and films exist, and they will be stressed where possible. Readers already familiar with concepts of materials science may wish to skip this chapter; those who seek deeper and broader coverage should consult the bibliography for recommended texts on this subject. 1.2. STRUCTURE 1.2.1. Crystalline Solids Many solid materials possess an ordered internal crystal structure despite external appearances that are not what we associate with the term crystalline-Le., clear, transparent, faceted, etc. Actual crystal structures can be imagined to arise from a three-dimensional array of points geometrically and repetitively distributed in space such that each point has identical surroundings. There are only 14 ways to arrange points in space having this property, and the resulting point arrays are known as Bravais lattices. They are shown in Fig. 1-1 with lines intentionally drawn in to emphasize the symmetry of the lattice. Only a single cell for each lattice is reproduced here, and the point array actually stretches in an endlessly repetitive fashion in all directions. If an atom or group of two or more atoms is now placed at each Bravais lattice point, a physically real crystal structure emerges. Thus, if individual copper atoms populated every point of a face-centered cubic (FCC) lattice whose cube edge dimension, or so-called lattice parameter, were 3.615 A, the material known as 0
ructure CUBIC TRICLINIC MONOCLINIC MONOCLINIC (a+B+y+ 90, a+b+e)SIMPLE BASE CENTERED ORTHORHOMBIC HEXAGONAL RHOMBOHEDRAL TETRAGONAL Figure 1-1. The 14 Bravais space lattices. metallic copper would be generated; and similarly for other types of lattices and atoms The reader should realize that just as there are no lines in actual crystals there are no spheres. Each sphere in the Cu crystal structure represents the atomic nucleus surrounded by a complement of 28 core electrons [i.e, (1s)2
1.2. Structure 3 CUBIC TRICLINIC MONOCLINIC, MONOCLINIC, + p + y* 90, at b 4 e) SIMPLE BASE CENTERED ORTHORHOMBIC HEXAGONAL RHOMBOHEDRAL TETRAGONAL Figure 1-1. The 14 Bravais space lattices. metallic copper would be generated; and similarly for other types of lattices and atoms. The reader should realize that just as there are no lines in actual crystals, there are no spheres. Each sphere in the Cu crystal structure represents the atomic nucleus surrounded by a complement of 28 core electrons [i.e., (1s)
A Review of Materlals Sclence (2s)2(2p)(3s)2(3p)(3d) 0] and a portion of the free-electron gas contributed by 4s electrons. Furthermore, these spheres must be imagined to touch in certain crystallographic directions, and their packing is rather dense. In FCC structures the atom spheres touch along the direction of the face diagonals i. e, [110], but not along the face edge directions, i.e., [100]. This means that the planes containing the three face diagonals shown in Fig. 1-2a, i.e., the (111)plane, are close-packed. On this plane the atoms touch each other in much the same way as a racked set of billiard balls on a pool table. All other planes in the FCC structure are less densely packed and thus contain fewer atoms per unit area Placement of two identical silicon atoms at each FCC point would result in the formation of the diamond cubic silicon structure(Fig. 1-2c), whereas the rock-salt structure(Fig. 1-2b)is generated if sodium-chlorine groups were substituted for each lattice point. In both cases the positions and orientation of the two atoms in question must be preserved from point to point In order to quantitatively identify atomic positions as well as planes and directions in crystals, simple concepts of coordinate geometry are utilized First, orthogonal axes are arbitrarily positioned with respect to a cubic lattice (e.g. FCC) such that each point can now be identified by three coordinates 12°个 ● FIgure 1-2. (a)(111)plane in FCC lattice; (b)rock-salt structure, e.g., Nacl; Na . CI .;(c)diamond cubic structure, e.g., Si, Ge;(d)zinc blende structure, e.g., GaAs
4 A Review of Materials Science (2s)* (2~)~ (3~)~ (3~)~ (3d)'OI and a portion of the free-electron gas contributed by 4s electrons. Furthermore, these spheres must be imagined to touch in certain crystallographic directions, and their packing is rather dense. In FCC structures the atom spheres touch along the direction of the face diagonals, i.e., [110], but not along the face edge directions, i.e., [lOO]. This means that the planes containing the three face diagonals shown in Fig. 1-2a, i.e., the (111) plane, are close-packed. On this plane the atoms touch each other in much the same way as a racked set of billiard balls on a pool table. All other planes in the FCC structure are less densely packed and thus contain fewer atoms per unit area. Placement of two identical silicon atoms at each FCC point would result in the formation of the diamond cubic silicon structure (Fig. 1-2c), whereas the rock-salt structure (Fig. 1-2b) is generated if sodium-chlorine groups were substituted for each lattice point. In both cases the positions and orientation of the two atoms in question must be preserved from point to point. In order to quantitatively identify atomic positions as well as planes and directions in crystals, simple concepts of coordinate geometry are utilized. First, orthogonal axes are arbitrarily positioned with respect to a cubic lattice (e.g., FCC) such that each point can now be identified by three coordinates a. b. W C. d Figure 1-2. (a) (11 1) plane in FCC lattice; (b) rock-salt structure, e.g., NaC1; Na 0, C1 0; (c) diamond cubic structure, e.g., Si, Ge; (d) zinc blende structure, e.g., GaAs