It is worthwhile to briefly elaborate on this book's title and the connection between thin films and the broader discipline of materials science and engineer- ing. A dramatic increase in our understanding of the fundamental nature of materials throughout much of the twentieth century has led to the development of materials science and engineering. This period witnessed the emergence of polymeric, nuclear, and electronic materials, new roles for metals and ceram ics, and the development of reliable methods to process these materials in bulk and thin-film form. Traditional educational approaches to the study of materi als have stressed structure-property relationships in bulk solids, typically utilizing metals, semiconductors, ceramics; and polymers, taken singly or collectively as illustrative vehicles to convey principles. The same spirit is adopted in this book except that thin solid films are the vehicle. In addition the basic theme has been expanded to include the multifaceted processing structure-properties-performance interactions. Thus the original science core is preserved but enveloped by the engineering concerns of processing and performance. Within this context, I have attempted to weave threads of commonality among seemingly different materials and properties, as well as to draw distinctions between materials that exhibit outwardly similiar behavior. In particular, parallels and contrasts between films and bulk materials are recur ring themes An optional introductory review chapter on standard topics in materials establishes a foundation for subsequent chapters. Following a second chapter on vacuum science and technology, the remaining text is broadly organized into three categories. Chapters 3 and 4 deal with the principles and practices of film deposition from the vapor phase. Chapters 5-9 deal with the processes and phenomena that influence the structural, chemical, and physical attributes of films, and how to characterize them. Topics discussed include nucleation growth, crystal perfection, epitaxy, mass transport effects, and the role of stress. These are the common thin-film concerns irrespective of application The final portion of the book( Chapters 10-14) is largely devoted to specific film properties(electrical, magnetic, optical, mechanical) and applications well as to emerging materials and processes. Although the first nine chapters may be viewed as core subject matter, the last five chapters offer elective topics intended to address individual interests. It is my hope that instructors using this book will find this division of topics a useful one Much of the book reflects what is of current interest to the thin-film research nd development communities. Examples include chapters on chemical vapor deposition, epitaxy, interdiffusion and reactions, metallurgical and protective coatings, and surface modification. The field is evolving so rapidly that even the classics of yesteryear, e. g, Maissel and Glang, Handbook of Thin Film
xiv Preface It is worthwhile to briefly elaborate on this book’s title and the connection between thin films and the broader discipline of materials science and engineering. A dramatic increase in our understanding of the fundamental nature of materials throughout much of the twentieth century has led to the development of materials science and engineering. This period witnessed the emergence of polymeric, nuclear, and electronic materials, new roles for metals and ceramics, and the development of reliable methods to process these materials in bulk and thin-film form. Traditional educational approaches to the study of materials have stressed structure-property relationships in bulk solids, typically utilizing metals, semiconductors, ceramics; and polymers, taken singly or collectively as illustrative vehicles to convey principles. The same spirit is adopted in this book except that thin solid films are the vehicle. In addition, the basic theme has been expanded to include the multifaceted processingstructure-properties-performance interactions. Thus the original science core is preserved but enveloped by the engineering concerns of processing and performance. Within this context, I have attempted to weave threads of commonality among seemingly different materials and properties, as well as to draw distinctions between materials that exhibit outwardly similiar behavior. In particular, parallels and contrasts between films and bulk materials are recurring themes. An optional introductory review chapter on standard topics in materials establishes a foundation for subsequent chapters. Following a second chapter on vacuum science and technology, the remaining text is broadly organized into three categories. Chapters 3 and 4 deal with the principles and practices of film deposition from the vapor phase. Chapters 5-9 deal with the processes and phenomena that influence the structural, chemical, and physical attributes of films, and how to characterize them. Topics discussed include nucleation, growth, crystal perfection, epitaxy, mass transport effects, and the role of stress. These are the common thin-film concerns irrespective of application. The final portion of the book (Chapters 10-14) is largely devoted to specific film properties (electrical, magnetic, optical, mechanical) and applications, as well as to emerging materials and processes. Although the first nine chapters may be viewed as core subject matter, the last five chapters offer elective topics intended to address individual interests. It is my hope that instructors using this book will find this division of topics a useful one. Much of the book reflects what is of current interest to the thin-film research and development communities. Examples include chapters on chemical vapor deposition, epitaxy, interdiffusion and reactions, metallurgical and protective coatings, and surface modification. The field is evolving so rapidly that even the classics of yesteryear, e.g., Maissel and Glang, Handbook of Thin Film
Preface Technology(1970)and Chopra, Thin Film Phenomena(1969), as well as more recent books on thin films, e. g, Pulker, Coatings on Glass(1984), and Eckertova, Physics of Thin Films(1986), make little or no mention of these now important subjects As every book must necessarily establish its boundaries, I would like to int out the following: (1) Except for coatings( Chapter 12)where thicknesses range from several to as much as hundreds of microns (I micron or I um=10 meter), the book is primarily concerned with films that are less than I um thick.(2)Only films and coatings formed from the gas phase by physical(PVD)or chemical vapor deposition( CVD) processes are considered Therefore spin and dip coating, flame and plasma spraying of powders electrolytic deposition, etc, will not be treated. (3)The topic of polymer films could easily justify a monograph of its own, and hence will not be discussed here.(4)Time and space simply do not allow for development of all topics from first principles. (Nevertheless, I have avoided using the unwelcome phrase * It can be shown that and have refrained from using other textbooks or the research literature to fill in missing steps of derivations. )(5)A single set of units(e. g, CGS, MKS, SI, etc. has been purposely avoided to better address the needs of a multifaceted and interdisciplinary audience Common usage, commercial terminology, the research literature and simple bias and convenience have all played a role in the ecumenical display of units Where necessary, conversions between different systems of units are provided At the end of each chapter are problems of varying difficulty, and I believe a deeper sense of the subject matter will be gained by considering them. Three ery elegant problems (i.e. 9-6,-7,-8) were developed by Professor W. D Nix, and I thank him for their use By emphasizing immutable concepts, I hope this book will be spared the specter of rapid obsolescence. However, if this book will in some small measure help spawn new technology rendering it obsolete, it will have served a useful function
Preface xv Technology (1970) and Chopra, Thin Film Phenomena (1969), as well as more recent books on thin films, e.g., Pulker, Coatings on Glass (1984), and Eckertova, Physics of Thin Films (1986), make little or no mention of these now important subjects. As every book must necessarily establish its boundaries, I would like to point out the following: (1) Except for coatings (Chapter 12) where thicknesses range from several to as much as hundreds of microns (1 micron or 1 pm = lop6 meter), the book is primarily concerned with films that are less than 1 pm thick. (2) Only films and coatings formed from the gas phase by physical (PVD) or chemical vapor deposition (CVD) processes are considered. Therefore spin and dip coating, flame and plasma spraying of powders, electrolytic deposition, etc., will not be treated. (3) The topic of polymer films could easily justify a monograph of its own, and hence will not be discussed here. (4) Time and space simply do not allow for development of all topics from first principles. (Nevertheless, I have avoided using the unwelcome phrase “It can be shown that . . . ,” and have refrained from using other textbooks or the research literature to fill in missing steps of derivations.) (5) A single set of units (e.g., CGS, MKS, SI, etc.) has been purposely avoided to better address the needs of a multifaceted and interdisciplinary audience. Common usage, commercial terminology, the research literature and simple bias and convenience have all played a role in the ecumenical display of units. Where necessary, conversions between different systems of units are provided. At the end of each chapter are problems of varying difficulty, and I believe a deeper sense of the subject matter will be gained by considering them. Three very elegant problems (Le. 9-6, -7, -8) were developed by Professor W. D. Nix, and I thank him for their use. By emphasizing immutable concepts, I hope this book will be spared the specter of rapid obsolescence. However, if this book will in some small measure help spawn new technology rendering it obsolete, it will have served a useful function. Milton Ohring
Acknowledgments t the top of my list of acknowledgments I would like to thank John Vossen for his advice and steadfast encouragement over a number of years. This book would not have been possible without the wonderfully extensive intellectual and physical resources of AT& T Bell Laboratories, Murray Hill, NJ, and the careful execution of the text and figures at Stevens Institute. In particular Bell abs library was indispensable and I am indebted to at& T for allowing to use it. My long association with Bell Labs is largely due to my dear friend L C. Kimerling(Kim), and I thank him and At T for supporting my efforts there. I am grateful to the many Bell Labs colleagues and students in the Stevens Institute of Technology/Bell Labs"On Premises Approved Program (OPAP), who planted the seed for a textbook on thin films. In this regard D. C. Jacobson should be singled out for his continuous help with many aspects of this work. The following Bell Labs staff members contributed this book through helpful comments and discussions, and by contributing fig. ures, problems, and research papers: J. C Bean, J. L. Benton, W.L. Brown, F Capasso, G. K. Celler, A.Y. Cho, J. M. Gibson, H J. Gossmann, R Hull R. w.Knoell, R F. Kopf, Y Kuk, H S. Luftman, S Nakahara, M. B Panish, J, M. Poate, S. M. Sze, K. L. Tai, w.w. Tai, H. Temkin, L. F. Thompson, L. E. Trimble, M. J. Vasile, and R. Wolfe. I appreciate their time and effort spent on my behalf. Some very special people at Stevens enabled the book to reach fruition They include Pat Downes for expertly typing a few versions of the complete
Acknowledgments At the top of my list of acknowledgments I would like to thank John Vossen for his advice and steadfast encouragement over a number of years. This book would not have been possible without the wonderfully extensive intellectual and physical resources of AT & T Bell Laboratories, Murray Hill, NJ, and the careful execution of the text and figures at Stevens Institute. In particular Bell Labs library was indispensable and I am indebted to AT & T for allowing me to use it. My long association with Bell Labs is largely due to my dear friend L. C. Kimerling (Kim), and I thank him and AT&T for supporting my efforts there. I am grateful to the many Bell Labs colleagues and students in the Stevens Institute of Technology/Bell Labs “On Premises Approved Program (OPAP),” who planted the seed for a textbook on thin films. In this regard D. C. Jacobson should be singled out for his continuous help with many aspects of this work. The following Bell Labs staff members contributed to this book through helpful comments and discussions, and by contributing figures, problems, and research papers: J. C. Bean, J. L. Benton, W. L. Brown, F. Capasso, G. K. Celler, A. Y. Cho, J. M. Gibson, H. J. Gossmann, R. Hull, R. W. Knoell, R. F. Kopf, Y. Kuk, H. S. Luftman, S. Nakahara, M. B. Panish, J. M. Poate, S. M. Sze, K. L. Tai, W. W. Tai, H. Temkin, L. F. Thompson, L. E. Trimble, M. J. Vasile, and R. Wolfe. I appreciate their time and effort spent on my behalf. Some very special people at Stevens enabled the book to reach fruition. They include Pat Downes for expertly typing a few versions of the complete xvii
text during evenings that she could have spent more pleasantly: Eleanor Gehler, for kindly undertaking much additional typing: Kamlesh Patel for his professional computerized drafting of the bulk of the figures: Chris Rywalt Manoj Thomas, and Tao Jen for carefully rendering the remainder of the figures; Mehboob Alam and Warren Moberly for their computer help in compiling the index, and drafting the cover, respectively: Dick Widdicombe, Bob Ehrlich, Dan Schwarcz, Lauren Snyder, and Noemia Carvalho for many favors; Professor R. Weil for helpful comments; Profs. W. Carr, H. Salwen and T. Hart for their expert and generous assistance on several occasions; Professor B Gallois and G. M. Rothberg for support; and those at Stevens responsible for granting my sabbatical leave in 1988. My sincere thanks to al of ye Lastly, I am grateful to several anonymous reviewers for many pertinent comments and for uncovering textual errors. They are absolved of all responsi bility for any shortcomings that remain. This book is lovingly dedicated to Ahrona, Avi, Noam, and Feigel, who in varying degrees had to contend with a less that a full-time husband and father for too many years
xviii Acknowledgments text during evenings that she could have spent more pleasantly; Eleanor Gehler, for kindly undertaking much additional typing; Kamlesh Pate1 for his professional computerized drafting of the bulk of the figures; Chris Rywalt, Manoj Thomas, and Tao Jen for carefully rendering the remainder of the figures; Mehboob Alam and Warren Moberly for their computer help in compiling the index, and drafting the cover, respectively; Dick Widdicombe, Bob Ehrlich, Dan Schwarcz, Lauren Snyder, and Noemia Carvalho for many favors; Professor R. Weil for helpful comments; Profs. W. Carr, H. Salwen, and T. Hart for their expert and generous assistance on several occasions; Professor B. Gallois and G. M. Rothberg for support; and those at Stevens responsible for granting my sabbatical leave in 1988. My sincere thanks to all Lastly, I am grateful to several anonymous reviewers for many pertinent comments and for uncovering textual errors. They are absolved of all responsibility for any shortcomings that remain. This book is lovingly dedicated to Ahrona, Avi, Noam, and Feigel, who in varying degrees had to contend with a less that a full-time husband and father for too many years. of you
Thin Films -A Historical Perspective Thin-film technology is simultaneously one of the oldest arts and one of the newest sciences. Involvement with thin films dates to the metal antiquity. Consider the ancient craft of gold beating, which has been practiced continuously for at least four millenia. Gold's great malleability enables it to be hammered into leaf of extraordinary thinness while its beauty and resistance to chemical degradation have earmarked its use for durable ornamentation and protection purposes. The Egyptians appear to have been the earliest practition- ers of the art of gold beating and gilding. Many magnificent examples of statuary,royal crowns, and coffin cases which have survived intact attest to the level of skill achieved. The process involves initial mechanical rolling followed by many stages of beating and sectioning composite structures consisting of gold sandwiched between layers of vellum, parchment, and assorted animal skins. Leaf samples from Luxor dating to the Eighteenth Dynasty(1567-1320 B. C )measured 0.3 microns in thickness. As a frame of reference for the reader, the human hair is about 75 microns in diameter. Such leaf was carefully applied and bonded to smoothed wax or resin-coated wood surfaces in a mechanical (cold) gilding process. From Egypt the art spread as indicated by numerous accounts of the use of gold leaf in antiqu Today, gold leaf can be machine-beaten to 0 I micron and to 0.05 micron when beaten by a skilled craftsman. In this form it is invisible sideways and quite readily absorbed by the skin. It is no wonder then that British gold beaters were called upon to provide the first metal specimens to be observed XIX
Thin Films - A Historical Perspective Thin-film technology is simultaneously one of the oldest arts and one of the newest sciences. Involvement with thin films dates to the metal ages of antiquity. Consider the ancient craft of gold beating, which has been practiced continuously for at least four millenia. Gold’s great malleability enables it to be hammered into leaf of extraordinary thinness while its beauty and resistance to chemical degradation have earmarked its use for durable ornamentation and protection purposes. The Egyptians appear to have been the earliest practitioners of the art of gold beating and gilding. Many magnificent examples of statuary, royal crowns, and coffin cases which have survived intact attest to the level of skill achieved. The process involves initial mechanical rolling followed by many stages of beating and sectioning composite structures consisting of gold sandwiched between layers of vellum, parchment, and assorted animal skins. Leaf samples from Luxor dating to the Eighteenth Dynasty (1567-1320 B.C.) measured 0.3 microns in thickness. As a frame of reference for the reader, the human hair is about 75 microns in diameter. Such leaf was carefully applied and bonded to smoothed wax or resin-coated wood surfaces in a mechanical (cold) gilding process. From Egypt the art spread as indicated by numerous accounts of the use of gold leaf in antiquity. Today, gold leaf can be machine-beaten to 0.1 micron and to 0.05 micron when beaten by a skilled craftsman. In this form it is invisible sideways and quite readily absorbed by the skin. It is no wonder then that British gold beaters were called upon to provide the first metal specimens to be observed xix