Chemical Vapor Deposition

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].