Overview of Chemical Vapour Deposition

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Overview of Chemical Vapour Deposition - page 1
CHAPTER 1 Overview of Chemical Vapour Deposition ANTHONY C. JONES a AND MICHAEL L. HITCHMAN b a Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK; b Thin Film Innovations Ltd., Block 7, Kelvin Campus, West of Scotland Science Park, Glasgow, G20 0TH, UK 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 dis- tinguished 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. 1–4 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 (s or p) (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-b-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 chemi- cally 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 Chemical Vapour Deposition: Precursors, Processes and Applications Edited by Anthony C. Jones and Michael L. Hitchman r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org 1
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Overview of Chemical Vapour Deposition - page 2
2 Chapter 1 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 devel- opment of CVD processes, 2,5,6 and the published literature from the earliest days to the mid-1960s is covered by a comprehensive review by Powell et al. 7 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. 8 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. 9 However, these proved too fragile and later patents describe CVD processes for the deposition of various metals to produce more robust lamp filaments. 10,11 One of the earliest examples of the CVD of metals is the deposition of tungsten, reported as early as 1855. Wohler used WCl 6 with hydrogen carrier gas to deposit tungsten metal. 12 Later ¨ in the century (1890), the famous Mond Process was developed. This describes the deposition of pure nickel from nickel tetracarbonyl, Ni(CO) 4 , 13,14 and was used for the refinement of nickel ore. 15 The first reports of the deposition of silicon by CVD by the hydrogen reduction of SiCl 4 appear as early as 1909 16 and 1927, 17 and the widespread use of thin silicon films in the electronics industry is anticipated by the CVD of Si-based photo cells 18 and rectifiers 19 just after World War II. During the late 1950s, triisobutylaluminium, [Bu t Al] began to be used extensively to catalyze the 3 polymerization of olefins by the Ziegler–Natta process. At around the same time, it was found that the pyrolysis of [Bu t Al] gave high purity Al metal (499 at.%). This led to its use in the early 1980s 3 as a CVD precursor to Al metal for very large scale integration (VLSI) applications. 20,21 In patent literature of the late 1960s, aluminium trihydride (AlH 3 , alane) was found to be useful for plating Al films from the vapour phase and by electroless deposition, 22–24 which led to the much later use of alane adducts such as [AlH 3 (NMe 3 )] as CVD precursors for high purity Al thin films. 25 The reader is referred to Chapter 7 (Section 7.3) for recent developments in Al CVD.
Overview of Chemical Vapour Deposition - page 3
Overview of Chemical Vapour Deposition 3 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. 26 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, [SnO 2 :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 [Me 4 Sn], [SnCl 4 ] 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 TiO 2 , 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 SiI 4 produced by heating solid Si in the presence of iodine. 27 Epitaxial Si films were later produced in the 1970s on a large commercial scale by the pyrolysis of monosilane (SiH 4 ) in H 2 . 28 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 1C of a mixture of [Me 3 In] and liquid [PH 3 ]. 29 Next, in 1962, Harrison and Tomkins produced InSb in a closed tube by heating a mixture of [Me 3 In] and [SbH 3 ] at 160 1C, and they also produced GaAs by heating a mixture of [Me 3 Ga] and [AsH 3 ] at 200 1C. 30 In 1961 and 1965 patent applications by the Monsanto Co. claimed methods of depositing III-V compounds ‘‘suitable for use in semiconductor devices’’. 31,32 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 [Et 3 Ga] and [AsH 3 ] in an open tube system using H 2 as the carrier gas. 33 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. 34 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. 35 Subsequently, a wide range of III-V compounds were deposited by MOCVD (or MOVPE), including AlGaAs, 36 InP, InAs, InGaAs,InAsP, 37,38 GaN and AlN, 39 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 40 that had a low residual carrier concentration of n ¼ 7 Â 10 13 cm À3 and high electron mobility (m 77K ¼ 120 000 cm 2 V À1 s À1 ) (Section 1.7.2.4). 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
Overview of Chemical Vapour Deposition - page 4
4 Chapter 1 transistors (FETs) were soon produced, 41 as well as complex multilayer structures such as AlGaAs/ GaAs/AlGaAs double heterojunction lasers. 42 A particularly significant advance in III-V technology was the discovery of how to p-dope GaN-based semiconductors grown by MOVPE 43 (Chapter 6, Section 6.6.4.1) 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-T c superconducting oxides in the mid-1980s led to intense efforts to pre- pare these materials as thin films. This led to the development, beginning in the late 1980s, 44 of MOCVD techniques for the deposition of oxides such as YBa 2 Cu 3 O 7Àd , and other superconducting oxides. 45 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)O 3 and SrBi 2 Ta 2 O 9 , and early reports date back to the early 1990s. 46 More recent advances in the MOCVD of a range of ferroelectric oxide materials are given elsewhere 47 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-k) oxide insulating materials to replace the conventional SiO 2 insulating or capacitor layers. PVD techniques can not give the desired deposition control of the very thin films required, or the necessary step cov- erage 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 Al 2 O 3 , ZrO 2 , HfO 2, 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, 48,49 and was initially used for the manufacture of luminescent and dielectric films required in electroluminescent displays (Chapter 4, Section 4.5.1). 50,51 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 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
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Overview of Chemical Vapour Deposition 500 700 ° C [Me 3 Ga] (g) + [AsH 3 ] (g)  GaAs (s) + 3CH 4(g) 500 800 ° C [SiH 4 ] (g)  Si (s) + 2H 2(g) 350 475 ° C [SiH 4 ] (g) + O 2(g)  SiO 2(s) + 2H 2(g) 5 350 450 ° C 2[Ta(OEt) 5 ] (v) + 5H 2 O (v)  Ta 2 O 5(s) + 10EtOH (v) 960 ° C [NbCl 5 ] (v) + 5/2H 2(g) + ½xN 2(g)  NbN x(s) + 5HCl (v) Scheme 1.1 Overall reaction schemes for a variety of CVD processes. Partly pyrolysed precursor molecules in the gas phase Stagnant boundary layer Surface reactions Substrate Figure 1.1 Simple schematic representation of the MOCVD process. (After ref. 52, Copyright John Wiley & Sons Limited, 1992. Reproduced with permission.) the simple schematic 52 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 illu- strated in Figure 1.2, which indicates several key steps 4 : 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 [Me 3 Ga] and [AsH 3 ] 53,54 (Figure 1.3).
Overview of Chemical Vapour Deposition - page 6
6 Chapter 1 Figure 1.2 Precursor transport and reaction processes in CVD. (After ref. 4, p. 31, Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 1997. Reproduced with permission.) Figure 1.3 Plot of the normalized MOCVD growth rate of GaAs as a function of growth temperature. (After ref. 53, Copyright John Wiley & Sons Limited, 1987. Reproduced with permission.)
Overview of Chemical Vapour Deposition - page 7
Overview of Chemical Vapour Deposition 7 In this plot, of log of growth rate vs. reciprocal thermodynamic temperature, three regions are apparent. At lower growth temperatures the growth rate is controlled by the kinetics of chemical reactions occurring either in the gas-phase or on the substrate surface. This region is generally termed the region of kinetic growth control and the film growth rate increases exponentially with substrate temperature according to the Arrhenius equation: Growth rate / expðE A =RTÞ ð1:1Þ where E A is the apparent activation energy, R is the gas constant and T is the temperature. As the film growth rate is controlled by chemical kinetics, uniform film thickness can be achieved by minimizing temperature variations over the substrate surface, and this is the region utilized in hot- wall batch reactors used for the commercial production of Si epitaxial wafers by low pressure CVD. As the temperature increases, the growth rate becomes nearly independent of temperature and is controlled by the mass transport of reagents through the boundary layer to the growth surface, and this is termed the region of mass transport or diffusion-controlled growth. At even higher temperatures, the growth rate tends to decrease, due to an increased rate of desorption of film precursors or matrix elements from the growth surface and/or depletion of reagents on the reactor walls due to parasitic gas-phase side reactions. Gas-phase reactions become increasingly important with increasing temperature and higher partial pressures of the reactants. Notably, Figure 1.3 is rather misleading in that it suggests there are sharp changeovers between the various regions. This is because of the nature of the plot where the log of the growth rate is used. If the growth rate itself were plotted then a much more gradual transition from one rate controlling step to the next would be seen. 2 An Arrhenius type plot as in Figure 1.3 has to be used with caution to obtain an activation energy for a kinetic process since there will be a contribution from mass transport. The slope of the ‘‘kinetic region’’ will not give a true value of the activation energy, but a lower value. Also, the ordinate contains the growth rate that is precursor concentration dependent and this may vary with temperature so, again, a true value of the activation energy will not be given from the slope of the ‘‘kinetic region’’. A crucial factor that determines the relative importance of each regime is the pressure of the CVD reactor. From atmospheric pressure (760 Torr) to intermediate pressures (e.g. 10 Torr) gas phase reactions are important and, in addition, a significant boundary layer is present. Kinetics and mass transport can both play a significant role in deposition. As the pressure falls gas phase reactions tend to become less important, and particularly at pressures below 1 Torr layer growth is often controlled by surface reactions. At very low pressures (e.g.o10 À4 Torr) mass transport is completely absent and layer growth is primarily controlled by the gas and substrate temperature and by desorption of precursor fragments and matrix elements from the growth surface. A much more detailed discussion of the modelling of CVD processes is given in Chapter 3, and in Chapter 6 there is a detailed description of the chemistry (Section 6.3.2), thermodynamics, kinetics and hydrodynamics (Section 6.3.3) involved in III-V MOCVD technology. 1.3.2 Variants of CVD As the brief historical overview of CVD given in Section 1.2 shows, CVD has gone through wide- ranging developments over the years. Since most CVD reactions are thermodynamically endo- thermic and they also have a kinetic energy of activation then energy has to be supplied to the reacting system. Traditionally, CVD processes have been initiated by a thermal energy input, which
Overview of Chemical Vapour Deposition - page 8
8 can be inputted by several methods, e.g.:     direct resistance heating of the substrate or substrate holder; rf induction of the substrate holder or susceptor; thermal radiation heating; photoradiation heating. Chapter 1 The use of thermal CVD can, however, be disadvantageous. For example, heat input can result in damage to temperature-sensitive substrates and so alternative forms of energy input have been developed which allow deposition at lower temperatures. One way of reducing growth temperatures is to use plasma-assisted or plasma-enhanced CVD (PECVD). 55 With this technique deposition can occur at very low temperatures, even close to ambient, since electrical energy rather than thermal energy is used to initiate homogeneous reac- tions for the production of chemically active ions and radicals that can participate in hetero- geneous reactions, which, in turn, lead to layer formation on the substrate. In non-thermal plasmas, which are typically generated by electrical discharges in the gas phase, the electron temperature is much higher than the gas temperature and inelastic collisions of the electrons with precursor molecules form the chemically active species. In addition, surfaces in the plasma can be bombarded with ions, electrons and photons, leading to changes in surface chemistry. Although PECVD usually allows lower temperature deposition than thermal CVD, the plasma bombardment of a surface often causes some substrate heating. PECVD processes have been widely used for the deposition of a large range of materials with standard and novel properties; both inorganic and organic materials, as well as polymers, have been prepared by the technique. There are compli- cations, though, with PECVD, including plasma damage of the substrate and the growing film, and a strong process dependency on several parameters such as rf power and frequency, gas pressure, reagent flow rate, reactor geometry, etc. Chapter 12 reviews PECVD technology and its applications. Another method of inputting energy to a CVD process is to use high energy photons. The process of photo-assisted CVD 56 involves interaction of light radiation with precursor molecules either in the gas phase or on the growth surface. Precursor molecules must absorb energy, and since tra- ditionally simple inorganic precursors have been employed this necessitates the use of UV radia- tion. If more complex molecules are used as precursors, then photosensitizing agents may need to be added. The use of organometallic precursors (with p- and s-bonded moieties) opens up the possibilities for a wider range of wavelengths, but this can lead to an increased potential for carbon incorporation. Photo-assisted CVD has similar potential advantages to those of PECVD; namely, low temperature deposition, modifications of properties of grown layers, e.g. dopant incorporation, and independent control of substrate temperature and dissociation of precursor. In addition, though, with masking or laser activation it is possible to achieve selected area growth. Chapter 11 considers photo-assisted CVD processes in detail. A rather different variation of CVD is atomic layer deposition (ALD) and the specialist version atomic layer epitaxy (ALE). 57 In this modification of CVD, gaseous precursors are introduced alternately to the reaction chamber, where they reach a saturated adsorption level on the substrate surface. Introduction of the precursors is separated other by an inert gas purge, which removes any excess precursor molecules and volatile by-products from the reaction chamber, thus preventing unwanted gas phase reactions. In marked contrast to traditional thermal CVD, which involves pyrolysis of precursor molecules, ALD proceeds through surface exchange reactions, such as hydrolysis, between chemisorbed metal-containing precursor fragments and adsorbed nucleophilic reactant molecules. A typical growth cycle in the ALD of a lanthanide oxide from a precursor [LnL 3 ] (e.g. L ¼ Cp, OR) occurs in the sequence H 2 O pulse/[LnL 3 ] pulse/N 2 purge/H 2 O pulse/N 2 purge (Figure 1.4).
Overview of Chemical Vapour Deposition - page 9
Overview of Chemical Vapour Deposition 9 Figure 1.4 Schematic of an ALD growth cycle for deposition of a lanthanide oxide thin film from a lanthanide precursor [LnL 3 ] (e.g. L ¼ Cp, OR) and H 2 O. In the first step a pulse of H 2 O gives a reactive [OH]-terminated surface. A pulse of the LnL 3 precursor then leads to a chemisorbed [(L) 2 Ln–O–] or [(L)Ln–O 2 –] surface species, and the liber- ated LH species are removed by a N 2 purge. The surface is then effectively terminated with un- reactive L groups and growth self-limits. The next H 2 O pulse removes the remaining L groups and regenerates a reactive [OH]-terminated surface. Under optimum conditions, film growth proceeds through self-limiting surface reactions of a saturated adsorbent in which one ALD cycle produces one monolayer of material. However, due to steric hindrance or lack of reactive surface sites, the growth rate per cycle is often consider- ably less than one monolayer. This is illustrated in Figure 1.4 where, for steric reasons, the large LnL 3 molecules are unable to react with all the surface-bound OH groups. Nevertheless, the growth rate per cycle is constant, so the thickness of the thin film can be controlled simply and accu- rately by varying the number of deposition cycles. Because ALD reactions occur exclusively on the substrate surface, the process can give superior step-coverage (or improved conformality) to traditional CVD, and so ALD has become the technique of choice for film deposition on very high-aspect ratio substrates. In ALD it is important that surface reactions predominate and that thermal decomposition of the precursor is minimized or avoided altogether, otherwise self- limiting growth will break down. ALD processes are, therefore, generally carried out at substrate temperatures in the region 200–350 1C, which is below the thermal decomposition temperature of most precursors. Chapter 4 (Section 4.2.1) gives a much more detailed discussion about ALD processes. Chemical beam epitaxy (CBE) is a rather specialized CVD technique. This is a high vacuum process that uses volatile metal-organic precursors (e.g. a Group III metal alkyl) and gaseous co- precursors (e.g. AsH 3 or PH 3 ). The closely related technique of metal-organic molecular beam epitaxy (MOMBE) uses volatile metal-organic precursors and a co-precursor vapour derived from the solid element (e.g. As, P). 58,59 In CBE and MOMBE the chemical reactions occur only on the substrate surface and so gas-phase reactions play no significant role in the growth process. The use of CBE has perhaps declined somewhat in recent years, but its aims are to combine the advantages
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10 Chapter 1 of metal-organic vapour phase epitaxy (MOVPE) with those of molecular beam epitaxy (MBE). Since CBE is an ultrahigh vacuum (UHV) technique a potential advantage over conventional CVD is the ability to use vacuum in situ diagnostic techniques (e.g. RHEED, AES, MBMS) that provide real time analytical information on the growth process. The growth kinetics of the CBE growth process are shown schematically in Figure 1.5, where it is compared to MOVPE and MBE processes. 60 Since CBE is an UHV technique, precursor deso- rption from the surface limits CBE growth processes to less than about 700 1C. It is therefore necessary to pre-pyrolyse thermally stable precursors. For example, in GaAs growth, thermally stable [AsH 3 ], has to be pre-decomposed to form the active surface species, As 2 . A consequence of this is that there is a lack of active [AsH x ] species, which remove carbon-containing fragments in MOCVD, 61 and this leads to problems of carbon contamination in CBE. There are still many other variants of CVD, but whatever the variant it is very apparent that, for a process to occur, starting materials, or precursors, are required, as is some form of reactor. The next two sections give a brief tour of these topics. Figure 1.5 Schematic representation of the growth kinetics involved in MOVPE, CBE and MBE (molecular beam epitaxy, a PVD technique). (After ref. 60, Copyright John Wiley & Sons Limited, 1988. Reproduced with permission.)
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