ABSTRACT
Simplified mechanisms are presented for the growth of DLC films. The role of hydrogen and oxygen in the deposition is mentioned and detailed reactions are shown for one of the existing models of amorphous hydrogenated carbon film growth (a-C:H). The subplantation process for hydrogen free amorphous carbon (a-C) is discussed. A simple application example for the growth of a-C:H film using an oxygen-acetylene torch is included.

INTRODUCTION
Several review papers portray the preparation and state of the art of DLC films [1-5]. There are basically two different types of DLC films: amorphous hydrogenated carbon (a-C:H), and amorphous carbon (a-C). They are differentiated on the number of sp3 (diamond like) vs. sp2 (graphite like) bonds, and the role that hydrogen (or OH) plays in the formation and structure of the film. A diamond like structure exists when sp3 bonds form between carbon atoms during the growing of a carbon film. The sp3 fraction is defined as: (Eq. 1)
As the sp3 fraction increases the film tend more towards diamond properties and behavior. Diamond is a crystal with sp3 fraction of 1.

The first kind of DLC, that we will refer to as Amorphous Hydrogenated Carbon (a-C:H), hydrogen plays a fundamental role. A diamond like structure will not form without the presence of hydrogen, which can have a concentration up to 50% (atomic) of the final film obtained. The sp3 fraction in a-C:H films is usually less than 50%. There are an assortment of techniques used to deposit a-C:H films [1], all essentially based on chemical vapor deposition (CVD). The more common ones are: hot filament CVD, microwave plasma CVD, plasma jet, arc jet, plasma torch, and even the simple oxyacetylene welding torch [6]. All these techniques work by activating the gas phase carbon-containing precursor. The precursor (say CH4) has to be diluted in an excess of hydrogen typically to about 1% per volume. The temperature of the substrate is usually higher than 700 oC. All these methods share a common mechanism that will be discussed in the next section. DLC films produced by CVD have been a mainstream area of research during the past 2 decades, and they are starting to be incorporated into commercial applications [3].

The second kind of DLC, we will call amorphous carbon (a-C) or hydrogen free DLC. As its name implies, hydrogen is neither present nor necessary in its fabrication. There is a growing interest in these films since the sp3 fraction of bonds can be higher than 85%, making the film properties even closer to crystalline diamond. A review of the more common techniques to deposit a-C films can be found in [4]: direct ion beam, arc discharge, filtered arc, laser ablation, sputtering, ion assisted deposition (IAD), and mass selected ion beam deposition (MSIBD). Basically all this techniques are based on producing energetic ions (~10eV - 1KeV), which implant under the surface of an initially formed carbon layer. This carbon atoms inserted forcibly in a host carbon matrix form preferentially SP3 bonds. The diamond like structure then grows from the inside out. Even though a-C can surpass a-C:H films performance in most properties of interest they have been less studied because historically a-C:H films could be deposited two orders of magnitude faster than a-C. Nowadays they have comparable deposition rates, and is expected that more attention will be given to a-C films.

THEORY
The mechanism for the formation of DLC films differs radically depending if H is present or not. We will present here a schematic view of both processes.

a-C:H
Figure 1 shows, schematically, the step-by-step incorporation of a carbon atom into the film with an sp3 bond. Notice that carbon atoms in the bulk of the film are sp3 bonded (homoepitaxial growth of diamond from methyl radicals), but near the surface the atoms have dangling bonds. If these dangling bonds were not terminated in some way, they would react with each other forming sp2 bonds and producing graphite in the surface of the film. If, on the other hand, these dangling bonds are stabilized with hydrogen, there is the chance that a carbon-containing radical will bond on its place (CH3• in Figure 1), eventually incorporating another carbon into the film. The second role that hydrogen plays is that atomic hydrogen is known to etch graphitic sp2 carbon many times faster than diamond-like sp3, therefore, any graphite formed will be etched away preferentially. This helps to form a diamond-like film instead of a segregated combination of diamond and graphite.

Hydrogen also reacts with the neutral species (say CH4) forming the radicals that will bind into the surface. When using more complex hydrocarbon molecules, hydrogen works by breaking them into smaller pieces and preventing the formation of polymers in the gas phase or in the surface. Hydrogen is therefore fundamental for the formation of a-C:H films and this explains why usually H2 is up to 99% of the gas used in CVD. It is important to emphasize that Figure 1 is a simplified representation of the mechanism for a-C:H films growth. The actual model consists of 12 reversible chemical reactions (a subset of those shown in Figure 2) [7].

In oxygen-containing systems, OH ions are the active species preferentially of H. OH ions fulfill about the same roles as H ions. In this case, OH ions have a competing effect on the system. They increase the film growth because they are more efficient for the creation of new radicals than H ions. On the other hand, they can pair with H radicals, and stabilize the bond impeding the incorporation of new carbon atoms. Which process dominates depends on the concentration of oxygen in the system. Up to about 0.5% of O2, the net effect is an increase in film growth [8]. OH ions are also more effective in removing graphitic carbon, so better quality films are obtained when using oxygen. Figure 3 shows the steps where OH ions produce radicals (steps 1, 3, and 5 of Figure 1), and where it destroys hydrogen radicals (step 2, Figure 1). The addition of oxygen also complicates the model adding 6 new reversible reactions (12 reactions) involving OH. The model has then 18 reversible reactions. Some reactions however can be considered non-reversible, and others can be ignored. The total number (forward and backwards) of reactions that Figures 1 and 3 are representing is 24. Figure 2 shows the detailed reactions including known and estimated rate coefficients (taken from [8]).

a-C
In hydrogen-free DLC films, the mechanism of growth is completely different. In these systems there are not passivating agents and so diamond can’t grow from the surface. Interestingly, carbon particles with enough energy (~10eV-1KeV) can be inserted below the surface of a preformed carbon film. At the local atomic scale this process is not very different from the high-temperature high-pressure method to form diamond. The boundary between diamond and graphite in the phase diagram from carbon [9] is given by: (Eq. 2)
Equation 2 is a thermodynamic equilibrium, indicating that above that P-T line sp3 is the natural occurring configuration. In the subplantation process the deformation induced in the lattice from the forcible insertion of a carbon atom is the condition that drives the formation of sp3 bonds. Locally therefore, it is considered that conditions similar to those of very high pressure and/or temperature occurs to the point where sp3 bonding formation is the favored process (albeit, not at thermodynamic equilibrium). The process for the incorporation of a carbon atom consists then of the following stages:
1. Collisional Stage: Stopping via atomic collisions, ionization and phonon excitations (~10-13 s).
2. Thermalization Stage: Dissipation of excess energy in the film (~10-12 s).
3. Relaxation Stage: diffusion, chemical reactions, and phase transformations (~10-10 up to seconds).

a-C films can only grow after a first layer of carbon material has been formed. This layer is formed also by ion implantation. Carbon atoms are implanted, until they form a layer under the surface to be coated, eventually the material on top (original substrate) is sputtered away or diluted until a pure carbon film evolves. Further growth of the film proceeds with the aforementioned stages. The three more important factors for the formation of a-C films are the ion energy, substrate temperature and deposition rate.

Ion energy is required to be above a certain threshold value so that carbon can penetrate the subsurface layer of amorphous sp2 films. It shouldn’t be too high; otherwise the energetic ions will destroy the sp3 bonds formed via atomic displacements and secondary collisions (radiation damage). The substrate temperature should be kept low (~150 oC) since the new carbon atoms recently subplanted are usually very close to the surface and thermal migration could allow them to reach the surface and relax without forming sp3 bonds. Finally, the deposition rate has to be controlled to keep the C flux to the surface smaller than the flux of penetrating and incorporating C species (to avoid growing too much graphite in the surface), and also to avoid raising the target temperature, as mentioned before.

APPLICATION EXAMPLE
We are interested in making coatings that help to prevent breakdown (sparks) in RF antennas used to inject energy into fusion devices (tokamaks). These antennas have complex shapes as can be seen in Figure 4. One of the factors that can help to prevent breakdown in antennas are coatings with materials that has a low secondary electron coefficient (). Values of  lower than 1.0 are highly desirable. It have been shown that some carbonized coating and surface treatment combinations gives =0.88 [10]. Diamond is a very flexible material and further research is required to determine if it is a good material for the purpose at hand, since B-doped diamond have very high  [11].

Oxygen-Acetylene Torch:
Deposition of films using a simple oxygen-acetylene (C2H2) torch was reported in [6]. The film growth rate using this technique is high (~100 m/h). The main parameters to control are the substrate temperature (the torch flame temperature is too high per se, see conditions below), and ratio (R) of O2 to C2H2. A schematic representation of the experimental setup is shown in Figure 5. The DLC film will form under the appropriate conditions (0.9 < R < 1.2, 650 < T < 1050 oC) on the feather (section B of the flame). The setup is basic, and the only equipment required to add to that shown in Figure 5 is a temperature sensor and flow meters to control the gas ratio. It is reported to work on several materials like Si, BN, Mo, Nb, Ta, and TiC. The anisotropy of the film obtained (because of the circular geometry of the torch) shouldn’t be much of a problem since I would need to coat mainly the points of high field in my antenna (sharp edges and corners).

WORD COUNT: 1825.

REFERENCES
[1] P. W. May. Phil. Trans. R. Soc. Lond. A 358, 473 (2000).
[2] S. T. Lee, et al. Materials Science and Engineering, 25, 123 (1999).
[3] A. Grill. Diamond and Related Materials 8, 428 (1999).
[4] Y. Lifshitz. Diamond and Related Materials 8, 1659 (1999).
[5] J. Robertson. Surface and Coatings Technology 50, 185 (1992).
[6] L. M. Hanssen, et al. Materials Letters 7, 289 (1988).
[7] S. H. Harris. Appl. Phys. Lett. 56, 2298 (1990).
[8] T-H. Kim and T. Kobayashi. Jpn. Appl. Phys. 33, L459 (1994).
[9] H. Kanda and T. Sekine, in Properties, Growth, and Applications of Diamond, edited by M. H. Nazare and A. J. Neves. (Inspec 2001), B1.1. P 247.
[10] D. Ruzic, et al. J. Vac. Technol. 20, 1313 (1982).
[11] A. Shih and J.E. Yater, in Properties, Growth, and Applications of Diamond, edited by M. H. Nazare and A. J. Neves. (Inspec 2001), A3.3. P 82.
[12] D. W. Swain. PowerPoint Presentation. ICRF Antenna R&D Summary. DWS-ICRF Workshop. 2001.

FIGURES

Figure 1. Simplified process occurring at the diamond surface leading to DLC film growth with hydrogen as pasivating agent. Notice that without hydrogen the dangling bonds would reconstruct the surface and relax to sp2 bonds (graphite). Taken without permission from [1]. Figure 2. Reactions in the model for homoepitaxial growth of diamond from methyl radicals on a hydrogenated, electrically neutral (100) surface. Taken without permission from [8]. Lowercase reactions refer to hydrogen-only systems, uppercase reaction occur when oxygen is also present. Figure 3. OH induced processes in oxygen containing systems. OH simultaneously promotes growth via more efficient creation of radicals, and decrease ions via H* termination. Which processes dominate depends on how much oxygen is present. Increase net growth occurs up to 0.5% O2 concentration [8].

Figure 4. Folded Waveguide LHD in OakRidge National Lab. F=27MHz, P2 MW. Taken without permission from [12].

Figure 5. Oxygen Acetylene Torch Experimental Setup. Flame zones: (A) the inner cone, (B) acetylene feather, (C) outer flame. Growth of the diamond film occurs in zone B, for appropriate substrate temperature, materials, and gas ratios.