Frequently Asked Questions about CVD Diamond
CVD is an acronym for chemical vapor deposition. This means that a material is deposited from a gas onto a substrate and that chemical reactions are involved.
Synthetic diamond prepared by CVD techniques.
Diamond consists – like coal or graphite - of carbon. The main difference is the arrangement of the carbon atoms in the material (i.e. in the crystal lattice). Unfortunately graphite is the more stable carbon allotype (form) and therefore diamond is very rare in nature. To convert graphite into diamond high pressure and high temperatures (HPHT) must be applied. Under those conditions diamond becomes the more stable carbon allotype. This is the basis of the HPHT growth technique developed in the 1950ties. The possibility to deposit diamond from the gas phase (CVD) has been discovered later in the 1970/1980ties. For diamond CVD a carbon containing gas is decomposed and the carbon atoms are deposited on a surface. By proper conditions the growth of diamond crystals can be enhanced and the growth of graphite is suppressed.
For diamond CVD atomic hydrogen plays a key role. It is obtained by dissociating hydrogen molecules (H2). So, what we need is a process gas that consists mainly of hydrogen (>90 %) and a gas activation, e.g. an intense plasma or a hot filament, to break up the hydrogen molecules. Atomic hydrogen is known to selectively etch graphite and to break up double bonds thus converting graphitic bonds into diamond bonds.
Usually a mixture of hydrogen and methane
700-900°C. Lower temperatures are possible but at strongly reduced growth rates
On large areas (>100 cm2) diamond is usually deposited at growth rates between 0.1 and 10 micron per hour. Hence it is a very slow process. For small areas (<1 cm2) much higher growth rates (>100 micron per hour) have been demonstrated.
Diamond can be deposited on various materials like diamond, silicon, tungsten, molybdenum, silicon carbide, silicon nitride, quartz glass, cemented carbide etc. The main requirements are: the material must be able to withstand high temperatures, it must not be attacked by the activated process gas and it must not dissolve carbon.
By depositing diamond on a diamond crystal (seed crystal) the size of the crystal can be increased. In this case new carbon atoms are added to the existing diamond lattice. This is called homoepitaxy. On non-diamond substrates a pretreatment of the surface is necessary to allow diamond formation. E.g. by polishing a silicon substrate with diamond powder tiny diamond particles remain on the surface that act as seeds for the growth of small diamond crystals. During deposition the size of these crystals increases until they form a continuous compact layer of small diamond crystals (grains) – i.e. polycrystalline diamond. Hence only polycrystalline diamond is available as large area disks or coatings.
Thin diamond films can be prepared on areas as large as 0.5 m2 using an array of hot filaments for gas activation. Diamond coated silicon wafers are usually prepared by microwave plasma deposition. Here the maximum wafer diameter is between 4” (2.45 GHz excitation) and 8” (915 MHz excitation). Diamond disks are obtained by growing a thick diamond layer on a substrate and by removing the substrate thereafter. The typical size of these disks is 1-12 cm in diameter. Finally, the size of diamond single crystals depends mainly on the size of the seed crystal used. Unfortunately the availability of large area seed crystals is very limited.
Free-standing diamond membranes mounted on a silicon support have been demonstrated with thicknesses as low as 30 nm. On the other end of the scale diamond disks with more than 2 mm thickness are commercially available.
Usually the grain size is in the sub-micron range at the beginning of diamond growth. With increasing thickness the grains grow larger. Usually the grain size at the growing surface of a diamond film is about 10 % of the film thickness.
By enforced re-nucleation during diamond growth the diamond grains can be kept very small. This material is called nanocrystalline or ultra-nanocrystalline diamond (UNCD). This material does not have all the unique properties of diamond (e.g. it is not transparent and the thermal conductivity is low). But it exhibits a pretty smooth surface and there are interesting applications in tribology, micro-mechanics and bio-chemical sensing.
The surface of an as grown polycrystalline diamond film is pretty rough (like sandpaper). Hence polishing is a very important processing step for many applications. The surface roughness of polished diamond is usually a few nanometers.
DLC is an acronym for diamond-like carbon. Other denotations are a-C:H or a-C. In this material the carbon atoms do not form a crystal lattice but a random (amorphous) network. It is deposited by bombarding a surface with energetic (hydro-)carbon particles. This can be done at room temperature. DLC films exhibit quite a lot of compressive stress which limits the maximum thickness. It is somehow “diamond-like” as it is chemically stable and hard, however, it is clearly no diamond.
Natural diamonds show a widespread range of properties. A sophisticated system for the categorization of natural diamond stones with respect to color, clarity, defects and so on has been established. High-quality polycrystalline CVD diamond does have similar properties like perfect natural stones. For mono-crystalline CVD diamond properties have been demonstrated that even surpasses those of the best natural stones.
The answer is yes and no. Applications needing small amounts of diamond such as thin diamond films or membranes or small area diamond parts are rather cheap. However, thick, large area diamond disks of e.g. more than 1 mm thickness are rather expensive. This is mainly because the deposition process requires quite a lot of energy, the growth rates are low and the equipment expensive. So usually diamond is only used if necessary. On the other hand the benefit from a diamond product often exceeds the costs by far. In particular when diamond is an enabling key component i.e. in the case of extreme applications where no other material meets the requirements.
The anwer is no. Most of the CVD diamond products are for technical applications serving small niche markets. CVD diamond gem stones for jewelry applications may become important in the future as natural diamond sources are expected to dwindle some day. If “man made” diamond will be accepted by customers remains to be seen.
You may have a look at the “CVD diamond booklet” which can be downloaded at www.diamond-materials.de
There are various companies that provide CVD diamond either as raw material, as polished diamond discs or as ready-to-be-used products. Some of them also offer specific coating services.