Chapter 1 Introduction
1.2 Motivation
Recently, substantial attention has been paid to the development of diamond films on non-diamond substrates in particular using methane and hydrogen gases in the CVD plasma. Although the diamond film synthesized to an extent, some problems remain unclear and only their solution can result in wide-scale commercialization, especially as the synthesis of diamond at low temperature and pressure for the non thermal stable substrates. The most important problem is the synthesis of diamond at low temperature and pressure without sacrificing their quality, purity, yield, and so on.
In general, the good quality of diamond film is synthesized at 1% methane in hydrogen as source gas, 700-1000°C deposition temperature and gas pressures in the range 20-300 Torr [23]. However, the high substrate temperature limits the use of
diamond films in many industrially important systems such as electronics substrate.
Deposition of diamond at low temperature is desired for technological applications to allow incorporation of the film into electronic and mechanical devices without damage. There have been several reported works for diamond deposition at low temperature and pressure but they are unsatisfactory at the present stage. Therefore, it would be desirable from both scientific and technological points of view to control not only temperature and pressure but also their quality of diamond. This will permit the use of a much wider range of substrate materials of industrial importance such as Al, polyimide, Si, SiC, GaN, GaAs, steel, Ni, and so on in different mechanical, electrical, optical and electronic applications. Lowering of substrate temperature during diamond synthesis could be an important step for deposition on low melting materials as well.
It is well known that for diamond deposition on non-diamond substrates is performed via two major processing steps. The first one, (i) nucleation, corresponds to the formation of diamond nuclei at the substrate surface. Nucleation procedures have been developed by performing either ex situ treatments on the substrate such as scratching, seeding, chemical treatments, or in situ methods before CVD (chemical vapor deposition) growth such as bias-enhanced nucleation (BEN) process. While the first technique leads to non-uniformity while the bias is limited by the conductivity of the substrate. A uniform nucleation throughout the substrate surface along with its density affects the subsequent growth stage. Therefore, we need to find some new and commercially available precursor for the synthesis of diamond at low temperature and pressure. It is also essential to improve the yield purity, and quality of diamond at low temperature and pressure. Chemical precursor is one of the best ways to overcome this problem. It can improve the quality and enhance the diamond yield without damaging the surface. Moreover, an appropriate synthetic route ultimately determines
applications of materials are heavily dependent upon their synthetic method. As a result, there have been tremendous efforts toward the development of new synthetic methodologies for several decades.
In this study, we have proposed very simple and new methods for pretreatment of the Si surface. We used non toxic chemical precursor such as adamantane (C10H16) for the synthesis of diamond film by microwave plasma chemical vapor deposition (MPCVD) at low temperature and pressure. Prior to the pretreatment, the substrates were not mechanically abraded/scratched. Adamantane was coated on silicon surface by hotplate method and then external methane and hydrogen gas used for diamond growth. The experimental results have shown that the synthesized diamond films are in high yield along with good quality at low temperature. Moreover, noble materials adsorb the hydrocarbon. Therefore, using platinum thin interlayer can enhance the diamond density.
Chapter 2 Literature Review
Carbon is an important element in nature and its allotropes include the hardest naturally occurring substance are the tetragonal bonded (sp3) diamond and also one of the softest substances are the trigonally bonded (sp2) graphite graphite trigonally bonded (sp2). With recent advancement in nanotechnology, new structure of carbon and synthetic production of diamond [24], fullerenes (C60)[25], carbon nanotubes[26], carbon fibers[27], diamond-like carbon[28], diamondoid hydrocarbons[29], as well as the development of graphene[30] are few examples of this continuously expanding research area.
This thesis concentrates specifically on the lower diamondoid hydrocarbons (particularly on adamantane) to the diamond growth. The following chapter and contents is a brief introduction to the study of diamond and related materials, particularly focusing on the topics mentioned above. As we brief discussed above about the various structure of carbon, however, now we are going to discuss about graphite and diamond.
2.1 Graphite
The mineral graphite is one allotropes of carbon. Graphite has a sheet like structure where the atoms all lie in a plane and are only weakly bonded to the graphite sheets above and below, as shown in Figure 1(a). While, diamond has a framework carbon structure where the carbon atoms are bonded to other carbon atoms in three dimensions (3D). In the diamond lattice structure, hybrid sp3 orbital forms strong covalent bonds with four neighboring carbon atoms, tetrahedrally arranged with equal angles of 109° 28’ to each other. This three-dimensional network of covalent bonds gives diamond its unique hardness and resistance to wear. Unlike the tetrahedral arrangement of atoms in diamond, the carbon atoms in graphite are arranged in the form of hexagonal rings in layers. In the graphite crystal structure, each carbon atom combines with its three neighbors by sp2 covalent bond with a bond order of 1.5.
Figure 2.1: Structure of (a) graphite and (b) diamond.
Table 2.1: Comparison of the bond strengths, crystal structure, transparency, thermal conductivity, and electrical resistivity of diamond and graphite [31, 32].
Property Diamond Graphite
The carbon-carbon bonds in the graphite mineral are actually quite stronger than those in diamond (Table 2.1). Graphite and graphite powder are valued in industrial applications for its self-lubricating and dry lubricating properties. Graphite is a soft, opaque, lubricious material while diamond is hard, transparent and abrasive. This difference in properties of these materials is determined by the nature of the chemical bonds and structure. There is a common belief that graphite's lubricating properties are solely due to the loose interlamellar coupling between sheets in the structure. The properties of graphite are highly anisotropic. Graphite can conduct electricity due to the vast electron delocalization within the carbon layers. Graphite is able to conduct electricity due to the unpaired fourth electron in each carbon atom. The structure of diamond and graphite are shown in Figure 2.1 and some interesting properties of diamond and graphite are summarized in Table 1. This unpaired 4th electron forms delocalised planes above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers. However, no electronic pathway exists between adjacent graphene layers since the 3.35 Å spacing between graphene layer planes acts as an electrically insulative “vacuum”
to electron transfer. The thermal conductivity perpendicular to the planes is very low by comparison due to the weak van der Waals bonding, as shown in Table 2.1. For this reason, graphite is not conductive between layers (parallel with “c” crystallographic axis).
There are three principal types of natural graphite:—lump, crystalline, and amorphous.
Lump graphite occurs in veins and is believed to be hydrothermal in origin. It is typically massive, ranging in particle size from extremely fine to coarse, platy intergrowths of fibrous or acicular crystalline aggregates with the long axis parallel to the enclosing wall rock. Crystalline flake graphite consists of isolated, flat, plate-like particles with angular, rounded, or irregular edges. It is usually found in layers or pockets in metamorphic rocks.
In some deposits, the flake graphite occurs as massive accumulations in veins, lenses, or pods. And amorphous graphite is formed by the thermal metamorphism of coal. The designation amorphous is a misnomer. Its relatively low degree of crystalline order and very fine particle size make it appear amorphous. It is usually of lower purity than the crystalline flake graphite and, therefore, commands a lower price than its more ordered counterpart [33].
Figure 2.2: Photographs of different kind of graphite [http://en.wikipedia.org/wiki/Graphite]
.
2.2 Diamond
Diamonds were first mined in India over 4000 years ago. For centuries diamonds have captured the hearts and minds of millions, including scientists. For most, the word diamond immediately relates to a brilliant gem, wealth, status and/or prosperity. For scientists, diamond is known as one of the strongest and most chemically inert material. Diamond is something superb, the peerless "king of gems" that glitters, dazzles, and symbolizes purity and strength. The structure of diamond is shown in Figure 2.1(b).
It can be regarded as a 3D network of carbon atoms tetrahedrally bonded by sp3
Figure 2.3: Theoretically predicted phase diagram of carbon [http://en.wikipedia.org/wiki/Diamond].
hybridized bonds. Since each carbon atom in the diamond lattice is firmly “supported” by four neighboring atoms, this structure makes the extreme properties of diamond. The extreme properties of diamond such as high mechanical strength, exceptional chemical inertness, outstanding thermal stability, and many other excellent properties made it as a supreme material for diverse applications [34, 35]. Despite being harder and denser than graphite, under ambient conditions, diamond is less stable by 2.9 kJ/ mole. Fortunately, a large activation barrier prevents an appreciable rate for interconversion from diamond to graphite. Since diamond is only metastable at standard temperature and pressures, the production of diamond from other forms of carbon is difficult. Before progressing further let us discuss how diamonds are formed in some more detail.
2.2.1 Natural production
Figure 2.1 (b) shows the structure of diamond. Diamond is the stable solid form of carbon at high pressure. Diamonds are formed deep inside the earths interior where crushing pressure and blistering heat work together for a long period of time to create the diamond lattice. At depths below 150-200 km, the pressure and temperature are such that diamond becomes the most thermodynamically stable form of carbon. Scientists believe that, narrow volcanic pipes running down into the earths interior allows diamonds to be transported via violent explosions to the earth’s surface. It is believed the time taken to form a natural diamond is approximately billions of years.
2.2.2 Synthetic production
Apart from being the hardest known naturally occurring material till presently, diamond also enjoys several other superior properties. Diamond has very high wear resistance and a low coefficient of friction. Diamond is also inert to most chemical environments and hence diamond coatings can be used for corrosive and/or corrosion-erosion applications as well. It is also a bio-compatible material. The high carrier mobility [36], wide band gap and thermal stability [37] of diamond means that electronic devices would be able to operate faster and under much more extreme conditions than silicon based devices. It has very low thermal expansion coefficient at room temperature. Diamond also has low or negative electron affinity [37]. By virtue of these commendable properties, diamonds have a potential for wide ranging applications for wear and corrosion resistant applications, field emission, heat sinks, optical windows etc [37]. However the high costs of natural diamonds severely restricts their application potential. Scarcity and cost of the natural diamond led scientists to find means of making diamond in the laboratory. Although there have been several attempts to synthesize diamond from various sources of carbon since it was discovered to be an allotrope of carbon, two main techniques were invented in 1950; (i) High pressure and high temperature (HPHT) synthesis and (ii) chemical vapor deposition (CVD) synthesis.
2.2.2.1 High-Pressure High-Temperature Synthesis
With the realization in the nineteenth century that natural diamonds are produced deep under the earths crust under conditions of high temperature and high pressure, scientists started experiments to imitate this condition in laboratories. The first success in synthesising diamond particles by HPHT probably belongs to Swedish in about 1953 and in the Unites States by General Electric in 1955 [24]. They used pressures of 10 GPa and temperatures in excess of 2300 K to produce diamond crystals with an edge length of over 1 mm. HPHT, a process which mimics the way in which natural diamonds are formed by geophysical processes deep in the earth, has allowed synthetic diamond to become a key material in the cutting and abrasive business, being widely used in the machine tool industry, oil and gas drilling, mining, quarrying, and construction. Over the years the process has been refined, and it is now possible to produce different types of diamonds of several carats in mass [38]. It can be seen from the carbon phase diagram (Figures 2.3 and 2.4) that the diamond structure is stable at very high temperatures and pressures. Today, the diamond synthesis by HPHT process remains the dominant manufacturing process for industrial diamond, as shown in Figure 2.5.
Figure 2.4: Carbon phase diagram with temperature and pressure ranges corresponding to various diamond synthesis process [39].
Figure 2.5: Photograph of commercial HPHT diamond (ball point pen for scale).
2.2.2.2 Shock-wave Synthesis
Figure 2.6 shows the schematic diagram of Shock-wave for diamond synthesis.
Shock-wave synthesis methods are better than traditional HPHT methods. These methods only produce small grain size (nano) diamond powders. In 1961, Decarli et al. had observed diamond from graphite after explosive shocks at 300,000-atmosphere and Greiner et al also noticed diamond crystals with soot after the detonation of carbon-containing molecules at high explosive in an inert atmosphere [40, 41]. The nanocrystals of diamond are formed by shock-wave synthesis due to the incredibly short reaction time [42].
Figure 2.6: (a) Schematic diagram of the shock-wave for diamond synthesis (b) The
2.2.2.3 Chemical Vapor Deposition (CVD)
Synthesis of diamond by Chemical Vapor Deposition (CVD) uses a low pressure process. This technology opens the possibility of making new shapes, coatings, films and qualities that can exploit diamond’s unique properties in a breath-taking array of industries. The possible growth of diamond by CVD was first shown by Angus et al. in 1968 [44]. It is a process which includes both gas phase reactions and gas-solid surface reactions. The process relies on decomposing carbon-containing gas molecules, such as methane, acetylene or carbon dioxide at sub-atmospheric pressure and depositing diamond as a film on a substrate. Figure 2.7 shows the appropriate ratio of each elements such as C, H, and O for the diamond growth [45]. The importance of CVD methods is that they do not require the huge pressures required for HPHT synthesis and can create diamond that can be tailored for a wide range advanced engineering applications.
Figure 2.7: The Bachmann Diagram [45]. This diagram shows the relative proportions of C, H, and O required in the gas phase for CVD diamond growth to occur.
Recently, there are four main CVD methods used to create diamond films. The CVD techniques, classified by means of how the energy is coupled into the system (called gas activation), which will be briefly introduced in the following sections.
2.2.2.3.1 Hot filament
Figure 2.8: (a) Schematic and (b) Photograph of hot filament CVD reactor[http://www.chm.bris.ac.uk/pt/diamond/end.htm].
In 1976, Soviet scientists have successfully diamond synthesized on non-diamond substrates by hot filament CVD (Hot Filament Chemical Vapor Deposition, HFCVD).
The schematic diagram of HFCVD is shown in Figure 2.8. It uses a metal coil, resistively heated to around 2000~2500 K to activate the gas phase reactions. In the HFCVD, normally researchers are using the mixture of methane and hydrogen. The filament works as a power source and catalyst at the same time to help dissociate the H2. The resulting H atoms then initiate most gas phase reactions with the hydrocarbon and finally lead to diamond deposition on the Si substrate, which is heated separately by an electric heater to 1000~1200 K. Therefore, the properties of the filament are very important for HFCVD.
The commonly used filament material is a kind of chemically-inert metal, e.g. tungsten or tantalum. However, under the high temperature, it will inevitably react with carbon-containing species and gradually degrade, and finally become more brittle and resistive.
This then will influence both the power coupling efficiency and the catalysis activity.
contain oxidizing or corrosive gases. Even so, the contamination from the filament material is still difficult to avoid. Thus, usually, HFCVD-grown diamond has low quality and is suitable for mechanical, but not for electronic applications.
2.2.2.3.2 Arcjet plasma
Figure 2.9: (a) Schematic and (b) Photograph of a DC arcjet reactor[http://www.chm.bris.ac.uk/admin/tpw.htm].
The DC arcjet is another CVD method to produce diamond. The schematic diagram of Arcjet plasma is shown in Figure 2.9 In this system, an anode and a cathode are connected by a DC power supply. Between the two electrodes a discharge region is formed. When the mixture of gases flow through this region, ionization occurs and a jet of plasma is generated and accelerated by a pressure drop towards the substrate, where the diamond film is deposited. The advantage of this technique is its high growth rate, which is usually unobtainable by other methods. The maximum can be 1mm/hr [47].
However, this method cannot grow diamond over large areas and, again, metal contamination (from the cathode) tends to impair the diamond purity and quality.
2.2.2.3.3 Microwave plasma
Microwave plasma is now the most popular way to produce high quality diamond film. The two most common types of MWCVD reactor are shown in Figure 2. 10. The first microwave plasma CVD reactor was designed at NIRIM [32] using quartz tube of 45-55 mm in diameter that perpendicular penetrates the waveguide for 2.45 GHz, as schematically in Figure 2.10 (a). By this reactor a diamond film coating is possible on a 1-inch Si wafer at maximum, but in most cases, a piece of Si that is only less than 1 cm2 is used as the substrate. In contrast, uniform diamond film coating on large area (2-inch square) is possible using an ASTeX-type reactor, which is shown in Figure 2.10 (b). In a microwave reactor, the gases mixture is introduced. The microwave power is coupled into the chamber through a quartz window. Firstly, electrons will pick up energy from the electromagnetic field. Then, through their collisions, the energy is transferred to the heavy species, making them dissociated, excited or ionized. The “active” species so produced then react on the substrate surface and form the diamond film. The advantage of this method is that there is no electrode or filament in the reactor. This provides a clean environment for diamond growth. Also, the diamond growth rate is relatively fast due to high input power and the immersion of the substrate into the plasma. The main drawback is that such systems are usually expensive. Therefore, we are now entering a new phase for CVD diamond where companies are exploring a raft of new applications.
Improvements in plasma-type CVD processes are allowing the growth of polycrystalline and single crystal CVD diamond films with fewer defects and with consistent characteristics.
Figure 2.10: Schematic diagram of (a) NIRIM type [48] and (b) ASTEX type microwave reactor [http://www.chm.bris.ac.uk/pt/diamond/stuthesis/chapter1.htm].
2.3 Properties and applications of diamond
The synthetic diamonds are manufactured in a laboratory by various processes (as described above) for more than half a century. However, in recent years it has become possible to produce gem quality synthetic diamonds of significant size. The large size, high thermal conductivity, low thermal expansion, and low dielectric constant of diamonds makes them excellent as heat sinks [37]. As stated above, they are also the substrate of choice for homoepitaxial diamond CVD. Microcrystalline CVD diamond films are the most versatile form of synthetic diamond. They can be used as protective coatings, for heat dissipation, and even as windows in fusion reactors [37, 49, 50]. The high surface roughness of microcrystalline diamond (MCD) is not well suited for optical, wear and thermal management applications. However, nanocrystalline CVD diamond
The synthetic diamonds are manufactured in a laboratory by various processes (as described above) for more than half a century. However, in recent years it has become possible to produce gem quality synthetic diamonds of significant size. The large size, high thermal conductivity, low thermal expansion, and low dielectric constant of diamonds makes them excellent as heat sinks [37]. As stated above, they are also the substrate of choice for homoepitaxial diamond CVD. Microcrystalline CVD diamond films are the most versatile form of synthetic diamond. They can be used as protective coatings, for heat dissipation, and even as windows in fusion reactors [37, 49, 50]. The high surface roughness of microcrystalline diamond (MCD) is not well suited for optical, wear and thermal management applications. However, nanocrystalline CVD diamond