Know About CVD/Synthetic Diamond

Diamond has always been an outstanding and desirable material. With the invention of synthetic growth techniques at high pressures and temperatures in the fifties, it became  technical material, especially for mechanical applications. However, it was the advent of low pressure deposition techniques that made accessible the excellent mechanical, thermal, optical and electronic properties. With these chemical vapour deposition (CVD) techniques diamond became available in the form of extended thin films and free-standing plates or windows. Doping during deposition could be realized, making diamond a p-type semiconductor. With CVD-diamond a wealth of new applications opened up.

 

The fundamental problem of diamond synthesis is the allotropic nature of carbon. Under ordinary conditions graphite, not diamond, is the thermodynamically stable crystalline phase of carbon. Hence, the main requirement of diamond CVD is to deposit carbon and simultaneously suppress the formation of graphitic sp²-bonds. This can be realized by establishing high concentrations of non-diamond carbon etchants such as atomic hydrogen. Usually, those conditions are achieved by admixing large amounts of hydrogen to the process gas and by activating the gas either thermally or by a plasma.  
 

Hence, a common feature of all diamond CVD techniques is a gas-phase nonequilibrium, i.e. a high supersaturation of atomic hydrogen and of various hydrocarbon radicals. Typical deposition conditions are: 1 % methane in hydrogen as source gas, 700-1000°C deposition temperature and gas pressures in the range 30-300 Torr.

 

The various diamond CVD techniques differ mainly in the way of gas phase activation and dissociation. The most common techniques are microwave plasma assisted CVD and thermally assisted CVD, usually realized by gas activation with a hot filament, Each of these techniques has its pros and cons. The distinguishing features are the deposition rate, the deposition area and the quality of the deposited diamond. The maximum growth rate reported so far amounts to almost 1 mm/h. However, those high growth rates are usually limited to very small deposition areas (« 1 cm²). In general there is an inverse relationship between film quality and growth rate. Optically transparent films with high thermal conductivities are usually deposited at rates not exceeding 10 µm/h, regardless of the deposition technique. The excellent optical properties of diamond have been known for a long time. However, optical applications require extended discs or thin coatings not provided by natural diamond crystals. With the development of CVD techniques the situation has changed completely.

 

 

Diamond has a unique combination of optical, thermal, mechanical and electronic properties that makes diamond an ideal material for extreme applications: Optical Properties
	The excellent optical properties of diamond have been known for a long time. However, optical applications require extended discs or thin coatings not provided by natural diamond crystals. With the development of CVD techniques the situation has changed completely.  Broadband transparency Diamond is transparent from the UV (230 nm) to the far infrared. Only minor absorption bands resulting from two phonon absorption exist between 2.5 and 6 µm. Hence diamond is an ideal material for multispectral optical applications.  Wide band gap No thermal generation of charge carriers at elevated temperatures, hence no "thermal run away" as in the case of Germanium under laser irradiation. Furthermore, diamond does not become nonlinear at high radiation intensities.  High thermal conductivity> Absorbed energy is quickly dissipated to the edges of a diamond window where it can be removed by appropriate heat sinks and cooling techniques.  Mechanical and chemical stability Diamond is extremely hard, wear resistant and chemically inert.  It is an ideal material for hostile, highly erosive atmospheres.
	Thermal Properties
	One of many remarkable properties of diamond is its uncompeted thermal conductivity. In contrast to metals, where conduction electrons are responsible for the high thermal conductivity, heat is conducted in electrical insulators by lattice vibrations. With a sound velocity of 17500 m/s, diamond is the material with the highest Debye temperature (2220 K), exceeding that of most other insulating materials by an order of magnitude and leading to the highest thermal conductivity of any material at room temperature (20-25W/cmK), exceeding that of copper by a factor of five.    Today, CVD diamond is used for various thermal management applications such as submounts for integrated circuits and heat spreaders for high power laser diodes. Mechanical Properties Diamond is known for its extreme hardness. It exhibits an exceptional wear resistance and a low coefficient of friction. These properties make CVD diamond an ideal choice for highly demanding applications such as cutting tools for non-ferrous materials, surgical knives and wear resistant coatings.
	Dielectric Properties CVD diamond exhibits remarkable dielectric properties including a low dielectric constant of 5.7, a loss tangent below 0.00005 at 145GHz and a high dielectric strength of 1 000 000 V/cm. In combination with the extremely high thermal conductivity, low thermal expansion coefficient and high mechanical strength CVD diamond is an ideal dielectric window material. In particular for high-power microwave tubes (Gyrotron) with power levels exceedings 1 MW edge cooled diamond windows have found tremendous interest.

 

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