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Figure 1. Schematic sketches of the various approaches being used for the synthesis of diamond films, (a) DC plasma technique, (B) Microwave assisted plasma generation and (c) Hot filament technique (d) RF excitation.

of several microns per hour), the ability to deposit diamond films on a variety of materials including insulators like quartz and the relative ease of scaling the technology to permit diamond deposition over large areas and, perhaps, the continuous processing of materials to deposit diamond on materials and sub assemblies of interest. Some of the disadvantages of this technique, as presently understood, include the difficulty of nucleating diamond films on substrates without preparing the substrates In one of several ways. It has been found that mechanical abrasion of the substrate is required in order to promote the nucleation of diamond films. The growth of thin films of diamond which are continuous and pinhole free is a difficult technology with the use of microwave plasmas.

The DC technique offers several advantages including the relative ease of nucleation. The charge on the substrate is thought to promote diamond nucleation more easily than in the case of uncharged substrates in the microwave process. The DC technique has been shown to yield the highest growth rate of all the techniques to date with reported growth rates of 180 p.M /hour. This approach utilizes an arc jet and requires very high power levels to achieve the high growth rates. The DC technique has been scaled to achieve uniform deposits of diamond films over substrate areas of over 100 cm2. Some of the disadvantages of the DC technique include the requirement that the material to be coated with diamond be an electrical conductor since the substrate forms a part of the electrical circuit. In addition, the growth rates normally achieved with the DC approach, in the absence of approaches such as the arc jet technique or the use of excessively high and thus potentially uneconomic levels of energy input, are fairly low. Scaling the DC technique will call for a different set of equipment considerations than the microwave approach.

The hot filament technique presents a different set of opportunities and problems. It is not yet clear what the operative mechanisms are in this approach to diamond synthesis. Growth rates in excess of one micrometer per hour have been demonstrated with this approach and it is a technique characterized by the relative ease of implementation for experimental purposes. The advantages include the relative simplicity of the experimental approach and the reasonably high growth rates achievable. However several disadvantages make this the most difficult approach for scaling. The lifetime of the filament can be short since it is prone to reacting with the plasma environment, most commonly by forming carbides which eventually leads to failure of the filament due to changes in its electrical and mechanical properties. However the judicious choice of materials may allow this fundamental limitation to be overcome. Some approaches suggested include the use of carbon filaments and the use of thoriated tungsten filaments. Scaling presents unique problems with this approach.

The RF approach has not received much attention to date. In many respects the RF approach should be similar to the microwave approach. The technique should be more amenable to scaling since RF approaches to plasma processing are the most commonly used techniques in the semiconductor industry for processes such as the plasma enhanced deposition of oxides and nitrides of silicon. However to date there has not been much reported work on the use of RF excitation for diamond deposition.

At Crystallume the deposition of thin diamond films over large areas (4 in. diameter substrates) is achieved routinely with thickness uniformities across 4 in diameter substrates of ±10 %. The films, grown on a variety of materials, are typically polycrystalline with grain sizes ranging from tens of nanometers to several micrometers. Figure 4 shows typical examples of the surface structure of the films as observed in a scanning electron microscope.

As discussed above numerous processes and configurations have been used to achieve diamond film growth, with a wide range of growth rates and perfection of deposited films. (See for example, references 1-13 for recent work.) The mechanism of growth, however, is not well understood at present. In particular, it is not clear why the metastable diamond form deposits in preference to the graphite modification. Most proposed mechanisms suggest that the enhancement mode produces non-equilibrium carbon containing species with the diamond sp3 bonding, along with atomic hydrogen. The carbon containing species thus prefer to grow in the diamond sp3 bonded form of carbon in preference to the sp2 bonded graphite form. The presence of atomic hydrogen may: (a) hydrogenate any unpaired sp2 carbon bonds; (b) influence the thermochemistry of the deposition reaction; and (c) may selectively vaporize any graphitic or amorphous forms of carbon that may form.

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Raman spectra from natural diamond, graphite, diamond like carbon (DLC) and thin film diamond formed by the plasma enhanced chemical vapor deposition process. From these spectra it is clear that identification of the various allotropes of carbon is readily achieved with this technique and the thin film plasma synthesized form of diamond is very similar to natural diamond crystals in the overall shape and location of the main peaks in the Raman spectrum. However the fine structure of the Raman spectra from various forms of carbon deposited with the use of different techniques has been a significant focus of llnitluni s ciinnncitniini I on inniouo.uni cnnriiiir

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Figure 2. Typical Raman spectra of natural diamond ( left) and graphite ( right). [ Spectra courtesy of Intel Corporation.]

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