Chemical Vapour Deposition: Precursors, Processes and Applicationsby Anthony C Jones (Editor)
Chemical Vapour Deposition (CVD) involves the deposition of thin solid films from chemical precursors in the vapour phase, and encompasses a variety of deposition techniques, including a range of thermal processes, plasma enhanced CVD (PECVD), photon- initiated CVD, and atomic layer deposition (ALD). The development of CVD technology owes a great deal to collaboration between different scientific disciplines such as chemistry, physics, materials science, engineering and microelectronics, and the publication of this book will promote and stimulate continued dialogue between scientists from these different research areas. The book is one of the most comprehensive overviews ever written on the key aspects of chemical vapour deposition processes and it is more comprehensive, technically detailed and up-to-date than other books on CVD. The contributing authors are all practising CVD technologists and are leading international experts in the field of CVD. It presents a logical and progressive overview of the various aspects of CVD processes. Basic concepts, such as the various types of CVD processes, the design of CVD reactors, reaction modelling and CVD precursor chemistry are covered in the first few chapters. Then follows a detailed description of the use of a variety CVD techniques to deposit a wide range of materials, including semiconductors, metals, metal oxides and nitrides, protective coatings and functional coatings on glass. Finally and uniquely, for a technical volume, industrial and commercial aspects of CVD are also discussed together with possible future trends, which is an unusual, but very important aspect of the book. The book has been written with CVD practitioners in mind, such as the chemist who wishes to learn more about CVD processes, or the CVD technologist who wishes to gain an increased knowledge of precursor chemistry. The volume will prove particularly useful to those who have recently entered the field, and it will also make a valuable contribution to chemistry and materials science lecture courses at undergraduate and postgraduate level.
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Chemical Vapour Deposition
Precursors, Processes and Applications
By Anthony C. Jones, Michael L. Hitchman
The Royal Society of ChemistryCopyright © 2009 Royal Society of Chemistry
All rights reserved.
Overview of Chemical Vapour Deposition
ANTHONY C. JONES AND MICHAEL L. HITCHMAN
1.1 Basic Definitions
In the broadest sense chemical vapour deposition (CVD) involves the formation of a thin solid film on a substrate material by a chemical reaction of vapour-phase precursors. It can thus be distinguished from physical vapour deposition (PVD) processes, such as evaporation and reactive sputtering, which involve the adsorption of atomic or molecular species on the substrate. The chemical reactions of precursor species occur both in the gas phase and on the substrate. Reactions can be promoted or initiated by heat (thermal CVD), higher frequency radiation such as UV (photo-assisted CVD) or a plasma (plasma-enhanced CVD). There is a sometimes bewildering array of acronyms covered by the overall cachet of CVD and the interested reader is referred to several reviews. Some of the more commonly used acronyms are defined below.
Metal-organic chemical vapour deposition (MOCVD) is a specific type of CVD that utilizes metal-organic precursors. In the strictest sense a metal-organic (or organometallic) compound contains a direct metal-carbon bond (σ or π) (e.g. metal alkyls, metal carbonyls). However, the definition of MOCVD has broadened to include precursors containing metal-oxygen bonds (e.g. metal-alkoxides, metal-β-diketonates) or metal-nitrogen bonds (e.g. metal alkylamides), and even metal hydrides (e.g. trimethylamine alane).
Metal-organic vapour phase epitaxy (MOVPE) or organometallic vapour phase epitaxy (OMVPE) is an MOCVD process that produces single crystal (i.e. epitaxial) films on single crystal substrates from metal-organic precursors. In MOCVD and MOVPE gas-phase reactions can sometimes play a significant role in the deposition chemistry.
Plasma-assisted or plasma-enhanced CVD (PECVD) is a technique in which electrical energy rather than thermal energy is used to initiate homogeneous reactions for the production of chemically active ions and radicals that can participate in heterogeneous reactions, which, in turn, lead to layer formation on the substrate. A major advantage of PECVD over thermal CVD processes is that deposition can occur at very low temperatures, even close to ambient, which allows temperature-sensitive substrates to be used.
Atomic layer deposition (ALD), sometimes called atomic layer epitaxy (ALE), alternatively-pulsed CVD, or atomic layer chemical vapour deposition (ALCVD), is a modification of the CVD process in which gaseous precursors are introduced sequentially to the substrate surface and the reactor is purged with an inert gas, or evacuated, between the precursor pulses. The chemical reactions leading to film deposition in ALD occur exclusively on the substrate at temperatures below the thermal decomposition temperature of the metal-containing precursor and gas-phase reactions are unimportant.
Chemical beam epitaxy (CBE) is high vacuum CVD technique that uses volatile metal-organic precursors and gaseous co-precursors. The closely related technique of metal-organic molecular beam epitaxy (MOMBE) uses volatile metal-organic precursors and co-precursor vapour derived from the solid element. In CBE and MOMBE the chemical reactions occur only on the substrate, leading to single crystal films and so gas-phase reactions play no significant role in film growth. Section 1.3 gives a more detailed description of these processes.
1.2 Historical Perspective
In common with many technologies, developments in CVD have largely arisen out of the requirements of society. These developments have been most rapid when other thin film deposition technologies have proved problematic or inadequate, for instance in the production of multiple thin films, as in modern semiconductor devices, or when the coating of large surface areas is required, as in large-scale functional coatings on glass. Several excellent reviews describe the historical development of CVD processes, and the published literature from the earliest days to the mid-1960s is covered by a comprehensive review by Powell et al. Therefore, this section gives only a brief description, highlighting some key advances.
Probably the earliest patent describing a CVD process was taken out by a certain John Howarth, for the production of "carbon black" for use as a pigment. Unfortunately, due to rather lax health and safety standards, the process only succeeded in burning down the wooden plant. The early electric lamp industry provided another early impetus for CVD, and a patent issued in 1880 to Sawyer and Mann describes a process for the improvement of carbon fibre filaments. However, these proved too fragile and later patents describe CVD processes for the deposition of various metals to produce more robust lamp filaments.
One of the earliest examples of the CVD of metals is the deposition of tungsten, reported as early as 1855. Wohler used WCl6 with hydrogen carrier gas to deposit tungsten metal. Later in the century (1890), the famous Mond Process was developed. This describes the deposition of pure nickel from nickel tetracarbonyl, Ni(CO)4, and was used for the refinement of nickel ore.
The first reports of the deposition of silicon by CVD by the hydrogen reduction of SiCl4 appear as early as 1909 and 1927, and the widespread use of thin silicon films in the electronics industry is anticipated by the CVD of Si-based photo cells and rectifiers just after World War II.
During the late 1950s, triisobutylaluminium, [But3Al] began to be used extensively to catalyze the polymerization of olefins by the Ziegler–Natta process. At around the same time, it was found that the pyrolysis of [But3Al] gave high purity Al metal (>99 at.%). This led to its use in the early 1980s as a CVD precursor to Al metal for very large scale integration (VLSI) applications. In patent literature of the late 1960s, aluminium trihydride (AlH3, alane) was found to be useful for plating Al films from the vapour phase and by electroless deposition, which led to the much later use of alane adducts such as [AlH3(NMe3)] as CVD precursors for high purity Al thin films. The reader is referred to Chapter 7 (Section 7.3) for recent developments in Al CVD.
Another important development in the history of CVD was the introduction of "on-line" CVD architectural coatings by Pilkington (now NSG Group). These coatings are deposited on a very large scale by atmospheric pressure CVD on a float glass production line. By applying the coating directly to the float glass manufacturing line, economies of scale and production are achievable that are not possible with "off-line" deposition processes such as sputtering. Perhaps the most notable of these is fluorine-doped tin oxide, [SnO2:F] developed by Pilkington in the mid-1980s ("Pilkington K-Glass"). This is a low thermal-emissivity (low-E) coating on windows, which prevents heat loss from the home and is essential to modern ecological energy saving efforts (Chapter 10, Section 10.1.1). It can be deposited using precursors such as [Me4Sn], [SnCl4] with halo-fluorocarbons or HF (Chapter 10, Section 10.2.2). A much more recent commercial product of Pilkington is "self-cleaning" glass. This has been coated on-line with a thin transparent film of TiO2, and this chemically breaks down dirt by photocatalysis in sunlight (Chapter 10, Section 10.6).
Despite the various developments in CVD described above, the major impetus to the technology has undoubtedly been provided by the rapid development of the microelectronics industry since the mid-1970s. This has led to a requirement for very thin high purity films with precise control of uniformity, composition and doping.
Thin epitaxial films of n- or p-doped Si are the basic requirement for all Si integrated circuit technology. One of the earliest reports of silicon epitaxy was the closed tube transport of SiI4 produced by heating solid Si in the presence of iodine. Epitaxial Si films were later produced in the 1970s on a large commercial scale by the pyrolysis of monosilane (SiH4) in H2.
Interest in the use of metal-organic compounds for CVD applications began in the early 1960s. The first reported preparation of a III-V material from a Group III metal-organic and a Group V hydride was by Didchenko et al. in 1960, who prepared InP in a closed tube by the thermal decomposition at 275-300 °C of a mixture of [Me3In] and liquid [PH3]. Next, in 1962, Harrison and Tomkins produced InSb in a closed tube by heating a mixture of [Me3In] and [SbH3] at 160 °C, and they also produced GaAs by heating a mixture of [Me3Ga] and [AsH3] at 200 °C. In 1961 and 1965 patent applications by the Monsanto Co. claimed methods of depositing III-V compounds "suitable for use in semiconductor devices". The processes involved the pyrolysis of volatile Group III and Group V compounds in an open tube system on a cubic crystal substrate to produce epitaxial films.
However, the Monsanto applications were of a rather general nature, listing a large range of volatile Group III compounds, and the few specific process examples given mainly involved Group III trihalides. In 1968, Manasevit and co-workers at the Rockwell Corporation gave the first clear description of the use of metal-organic compounds for the chemical vapour deposition of III-V materials. The first publication describes the deposition of GaAs by pyrolysis of a gas phase mixture of [Et3Ga] and [AsH3] in an open tube system using H2 as the carrier gas. Manasevit named the technique metal-organic chemical vapour deposition (MOCVD) and a patent was later filed for the MOCVD of a range of III-V materials and wide band-gap compound semiconductors.
The emphasis in Manasevit's early work was on growth of non-epitaxial films on insulating substrates such as sapphire and spinel. However, in 1969 the growth of epitaxial GaAs on a GaAs substrate by metal-organic vapour phase epitaxy (MOVPE) was demonstrated. Subsequently, a wide range of III-V compounds were deposited by MOCVD (or MOVPE), including AlGaAs, InP, InAs, InGaAs,InAsP, GaN and AlN, although semiconductor device quality III-V materials still had not been produced. This was due largely to low purity precursors (often obtained from commercial suppliers of metal-organics for catalysis applications) and non-optimized MOCVD reactors and processes. In 1975, however, high-purity device quality GaAs films were grown that had a low residual carrier concentration of n = 7 x 1013cm-3 and high electron mobility (μ77K = 120000 cm2V-1 s-1) (Section 126.96.36.199). Conventional techniques for the deposition of III-V materials such as liquid phase epitaxy (a combined melt of the components) proved incapable of producing the very thin multilayer structures required for efficient III-V devices and so MOVPE technology developed with ever increasing pace, and state-of-the-art GaAs photocathodes and field effect transistors (FETs) were soon produced, as well as complex multilayer structures such as AlGaAs/ GaAs/AlGaAs double heterojunction lasers. A particularly significant advance in III-V technology was the discovery of how to p-dope GaN-based semiconductors grown by MOVPE (Chapter 6, Section 188.8.131.52) as this spawned the growth of a large industry in full-colour high-brightness light emitting diodes for energy efficient displays (Chapter 13, Section 13.7).
The reader is referred to Chapter 6 for a detailed description of the MOCVD of III-V compound semiconductor materials, including (Section 6.3.1) further details on the historical development of the technology. In many ways this is now a mature technology, more the province of salesmen and chemical buyers than development scientists, and commercial developments in this technology are described in Chapter 13.
The discovery of high-Tc superconducting oxides in the mid-1980s led to intense efforts to prepare these materials as thin films. This led to the development, beginning in the late 1980s, of MOCVD techniques for the deposition of oxides such as YBa2Cu3O7-δ, and other superconducting oxides. The difficulty in transporting low volatility metal precursors was largely responsible for the introduction of liquid injection MOCVD (Section 1.5 Figure 1.15). There has also been a great deal of effort devoted to the MOCVD of ferroelectric oxides such as Pb(Zr,Ti)O3 and SrBi2Ta2O9, and early reports date back to the early 1990s. More recent advances in the MOCVD of a range of ferroelectric oxide materials are given elsewhere and in Chapter 8 (Section 8.4).
The rapid recent advances in Si integrated circuit technology have largely been achieved by aggressive shrinking or "scaling" of devices such as metal oxide semiconductor field effect transistors (MOSFETS) and dynamic random access memories (DRAMs) (Chapter 8, Section 8.3, and Chapter 9, Section 9.2.2 and Figure 9.2). This has led to a requirement for new high-permittivity (or high-κ) oxide insulating materials to replace the conventional SiO2 insulating or capacitor layers. PVD techniques can not give the desired deposition control of the very thin films required, or the necessary step coverage in high aspect-ratio device structures such as trench- and stack-structured DRAMs. Therefore, over the last few years there has been an intense effort to develop CVD processes for the deposition of high-permittivity metal oxides, such as Al2O3, ZrO2, HfO2, Zr- and Hf- silicate and the lanthanide oxides, and many CVD developments in this area are also detailed in Chapter 8 (Section 8.3).
Shrinking device dimensions also make it necessary to modify existing multilevel metallization technologies. This has led to recent efforts to deposit Al and Cu by MOCVD (Chapter 7, Sections 7.3 and 7.4), as well as stimulating research into the MOCVD of diffusion barriers such as TiN and TaN (Chapter 9, Sections 9.3.1 and 9.3.2).
Atomic layer deposition (ALD) was first introduced by T. Suntola and co-workers in the early 1970s, and was initially used for the manufacture of luminescent and dielectric films required in electroluminescent displays (Chapter 4, Section 4.5.1). More recently, ALD has been used to deposit the very thin conformal oxide films required as gate insulators in CMOS technology and in DRAM capacitor layers; see Chapters 4 (Section 4.5.3) and 8 (Section 8.3).
It is impossible to do justice here to the huge volume of research and development carried out on CVD over the past 100 years or so, but hopefully this brief survey gives a flavour of the great advances made.
1.3.1 Chemical Vapour Deposition Processes
1.3.1 Conventional CVD Processes
CVD processes are extremely complex and involve a series of gas-phase and surface reactions. They are often summarized, though, by overall reaction schemes, as illustrated in Scheme 1.1.
An overall reaction scheme tells us little about the physicochemical processes and the gas-phase and surface reactions involved. A more informative illustration of a CVD process is illustrated by the simple schematic for an MOCVD reaction carried out at moderate pressures (e.g. 10–760 Torr) shown in Figure 1.1. A significant feature of the process is the presence of a hot layer of gas immediately above the substrate, termed the "boundary layer", and at these pressures gas-phase pyrolysis reactions occurring in the layer play a significant role in the MOCVD deposition process.
A more detailed picture of the basic physicochemical steps in an overall CVD reaction is illustrated in Figure 1.2, which indicates several key steps:
1. Evaporation and transport of reagents (i.e. precursors) in the bulk gas flow region into the reactor;
2. Gas phase reactions of precursors in the reaction zone to produce reactive intermediates and gaseous by-products;
3. Mass transport of reactants to the substrate surface;
4. Adsorption of the reactants on the substrate surface;
5. Surface diffusion to growth sites, nucleation and surface chemical reactions leading to film formation;
6. Desorption and mass transport of remaining fragments of the decomposition away from the reaction zone.
In traditional thermal CVD, the film growth rate is determined by several parameters, the primary ones being the temperature of the substrate, the operating pressure of the reactor and the composition and chemistry of the gas-phase. The dependence of film growth rate on substrate temperature is typified by the growth of GaAs by MOCVD using [Me3Ga] and [AsH3] (Figure 1.3).
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Meet the Author
Anthony C Jones is a Professor in the Department of Chemistry in the University of Liverpool. He has over twenty five years experience in the development of precursors for use in metalorganic chemical vapour deposition (MOCVD) and atomic layer deposition (ALD), and in the chemistry of CVD processes. He has introduced a number of novel synthesis and purification routes for precursors used in the MOCVD of III-V and II-VI compound semiconductors (e.g. adduct purification), and a number of new improved precursors were developed. He was one of the founder members of Epichem Ltd. (now SAFC Hitech, Bromborough, UK) and he has published over one hundred and fifty papers in refereed journals and has filed twenty patents. His research has been recognised by the award in 1996 of the Michael A. Lunn Outstanding Contributor Award for research on precursors for indium phosphide and related compounds, as well as having numerous invitations to present his work at conferences in the UK and abroad. He is a Fellow of the Royal Society of Chemistry and Associate Scientific Editor of the Journal Chemical Vapor Deposition. Professor Jones' recent research interests include the development of precursors for the MOCVD and ALD of dielectric and ferroelectric oxide films for microelectronics applications. Michael Hitchman has been working in the area of CVD for many years, starting some 35 years ago in the RCA Laboratories in Zurich on the electrochemistry of electrochromic displays where he found alternative research interests. He turned his attention to another system involving homogeneous chemistry, heterogeneous process, and mass transport, namely, CVD, and found that rotating disks were just as useful and powerful for CVD as for electrochemistry. Since that time he has studied, and published extensively on, a wide range of CVD systems and materials. He is author of over 100 papers in refereed journals and of six patents. In 1993 the edited volume (with Klavs Jensen) on Chemical Vapor Deposition - Principles and Applications appeared and he was awarded the British Vacuum Council Medal and Prize for his work on CVD. He is a Fellow of the Royal Society of Chemistry and was elected a Fellow of the Royal Society of Edinburgh in 1995. For the last 13 years he has been Editor of the Chemical Vapor Deposition journal. Recently he has retired from university life and has founded two companies, one of which - Thin Film Innovations Ltd - is seeking to capitalize on his knowledge of materials science using CVD, electrochemistry and a variety of other deposition techniques.
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