Making Diamonds from Methane? V2.0

Key words:Diamond, Chemical vapour deposition, physical properties, industrial applications

The word diamond conjures up thoughts of gem stones, wealth and special occasions. But to the scientist, diamond is impressive because of its wide range of extreme properties. As Table 1 shows, by most measures diamond is 'the biggest and best'; it is the hardest known material, has the lowest coefficient of thermal expansion, is chemically inert and wear resistant, offers low friction, has high thermal conductivity, and is electrically insulating and optically transparent from the ultra-violet (UV) to the far infrared (IR). Therefore, it is no surprise to learn that as well as being a gemstone used in jewellery, diamond already finds use in many diverse applications such as a heat sink, as an abrasive, as inserts and/or wear-resistant coatings for cutting tools and within the optics and electronics industries.

World interest in diamond has been further increased by the recent discovery that it is possible to produce polycrystalline diamond films, or coatings, by a wide variety of chemical vapour deposition (CVD) techniques using a hydrocarbon gas (typically methane) in an excess of hydrogen. This CVD diamond can show mechanical and electronic properties comparable to those of natural diamond.

Table 1: Some of the outstanding properties of diamond
Extreme mechanical hardness: ~90 GPa(i.e.harder than steel)
Strongest known material, highest bulk modulus: 1.2 × 1012 N/m2, lowest compressibility 8.3 × 10-13 m2/ N.
Highest known thermal conductivity at room temperature:2 × 103 W / m / K.(5x that of copper)
Thermal expansion coefficient at room temperature:0.8 × 10-6 K (comparable with that of invar ie does not expand much at all)).
Broad optical transparency: from the deep UV to the far IR region of the electromagnetic spectrum.
Good electrical insulator (room temperature resistivity: ~1016 Ω cm
Resistivity. Diamond can be doped to change its resistivity over the range 10-106 Ω cm, so becoming a semiconductor with a wide bad gap of 5.4 eV.
Very resistant to chemical corrosion.
Biologically compatible.
Electron affinity:low or 'negative'

The CVD Process

Chemical vapour deposition, as its name implies, involves a gas-phase chemical reaction occurring above a solid surface, which, here, causes carbon (hopefully in the form of diamond rather than soot!) deposition onto that surface. All CVD techniques for producing diamond films require a means of activating gas-phase carbon-containing precursor molecules. This generally involves thermal (e.g. hot filament) or plasma (Including microwave) activation, or use of combustion flame (oxyacetylene or plasma torches).

Whilst each method differs in detail, all methods share features in common. For example, growth of diamond rather than deposition of other, less well-defined, forms of carbon, normally requires that the substrate be maintained at a temperature in the range 1000-1400 K, and that the precursor gas be diluted in an excess of hydrogen.

Thermodynamically, graphite, not diamond, is the stable form of solid carbon at ambient pressures and temperatures. The fact that diamond films can be formed by CVD techniques is inextricably linked to the presence of hydrogen atoms in the mixture.

Future Research

One of the great challenges facing researchers in CVD diamond technology is to increase the growth rates to economically viable rates, (hundreds of μm/h), or even mm/hr) without compromising film quality. Progress is being made using microwave deposition reactors, since the deposition rate has been found to scale approximately linearly with applied microwave power.Some of the more obvious applications of this research, such as cutting tools and heat sinks, have reached the market-place and it should not be too long before this fledgling technology begins to make a significant impact in many areas of modern life.However, several issues need to be addressed before this can happen:

  • Growth rates need to be increased (by one or more orders of magnitude) without loss of film quality.
  • Deposition temperatures need to be severely reduced, allowing low-melting-point materials to be coated, and to increase the number of substrates onto which adherent diamond films can be deposited.
  • Substrate areas need to be scaled up, again without loss of uniformity or film quality, from the few square cm at present to 6” or 8”-diameter wafers that are standard in the semiconductor industry.
  • For electronic applications, single-crystal diamond films are desperately needed, along with reliable techniques for patterning and controlled doping.

Further reading

For those interested in more detail please see:

Acknowledgements

This piece is taken from an article which appeared in Endeavour Magazine 19(3), (1995) pp101-106. (© copyright Elsevier, 1995)

Paul May is aProfessor of physical chemistry at the University of Bristol. Paul’s current research includes CVD deposition and he heads up the CVD Diamond Group at Bristol.

Making Diamonds from Methane?

Questions

  1. In CVD what do you think must happen to methane to convert it to carbon? (1 mark)
  2. What is the main property of diamond that makes it suitable for a drill bit? (1 mark)
  3. The data in the table is in temperature units given the symbol ‘K’. (a) what does K stand for and (b) how do you convert measurements in ‘K’ to oC? (2 marks)
  4. If you held a microscope slide slab of diamond and a glass microscope slide into a beaker of ice at the same time, one in each hand, what would you expect to observe? (1 mark)
  5. Which property of diamond makes it suitable for use in infra-red spectroscopy? (1 mark)
  6. How many single covalent bonds does each carbon atom have in the diamond structure? (1 mark)
  7. In CVD produced diamonds which element do you think bonds with the outermost carbons of the structure? Hint: carbon atoms always have 4 bonds; in a surface carbon, three bonds will point downwards into the bulk and join to another carbon, leaving one bond to stick out from the surface…what element attaches here. Another hint: the element must be one that only has one bond.
  8. Draw a representation of the structure of (a) diamond and (b) graphite. (4 marks)

Making Diamonds from Methane?

Questions

  1. In CVD what do you think must happen to methane to convert it to carbon? (1 mark)
  1. What is the main property of diamond that makes it suitable for a drill bit? (1 mark)
  1. The data in the table is in temperature units given the symbol ‘K’. (a) what does K stand for and (b) how do you convert measurements in ‘K’ to oC? (2 marks)
  1. If you held a microscope slide slab of diamond and a glass microscope slide into a beaker of ice at the same time, one in each hand, what would you expect to observe? (1 mark)
  1. Which property of diamond makes it suitable for use in infra-red spectroscopy? (1 mark)
  1. How many single covalent bonds does each carbon atom have in the diamond structure? (1 mark)
  1. In CVD produced diamonds which element do you think bonds with the outer most carbons of the structure?
  1. Draw a representation of the structure of (a) diamond and (b) graphite. (4 marks)