What is CVD?

CVD stands for chemical vapor deposition.

What is CVD Diamond?

CVD Diamond is one type of synthetic diamond which prepared by CVD technique in lab atmosphere.

Why it is difficult to make CVD Diamond?

Diamonds are like coal or graphite of carbon. The main difference is the design of the carbon atoms in the material (i.e., in the crystal lattice). Unfortunately, graphite is the more stable carbon form, and therefore diamond is costly. To transform graphite into diamond high pressure and high temperatures (HPHT) must be implemented. Under those conditions, the diamond becomes the more durable carbon form. This is the basis of the HPHT growth technique stated in the 1950s. The possibility to deposit diamond from the gas phase (CVD) has been found later in the 1970/1980s. For CVD diamond, a carbon-containing gas is decayed, and the carbon atoms are placed on a surface. By proper conditions, the growth of diamond crystals can be improved, and the growth of graphite is overcome.

How does it work?

For CVD diamond atomic hydrogen plays a vital role. It is taken by dividing hydrogen molecules (H2). So, what we require is a process gas that has mainly of hydrogen (>90 %) and a gas activation, e.g., an extreme plasma or a hot fiber, to split up the hydrogen molecules. Atomic hydrogen is known to etch graphite selectively and to split up double bonds, thus turning graphitic bonds into diamond bonds.

What gases are commnly used for CVD Diamond?

Mostly a mixture of methane and hydrogen.

What temperature is required?

It require above 700-900°C. lower temperature are also possible but it strongly slow down the growth rates.

How fast CVD Diamond grow?

On vast areas (>100 cm2), diamond is usually placed at growth rates within 0.1 and 10 microns per hour. Hence it is a prolonged process. For small areas (<1 cm2), much bigger growth rates (>100 microns per hour) have been confirmed.

What substrate material can be used?

Diamond can be placed on various materials similar to diamond, silicon, tungsten, molybdenum, silicon carbide, silicon nitride, quartz glass, bonded carbide, etc. The main elements are: the material must be able to confront high temperatures, the stimulated process gas must not attack it, and it must not terminate carbon.

What is the difference between single- and polycrystalline diamond?

We can increase the size of the crystal by depositing diamond on the diamond crystal. After this process, new carbon atoms are added to the old diamond lattice, which is called homoepitaxy. On non-diamond substrates, pretreatment of the surface is required to allow diamond formation. E.g., by polishing a silicon substrate with diamond powder, tiny diamond particles remain on the surface that works as seeds for the growth of small diamond crystals. During the deposition, the size of these crystals grows until they form a continuous compact layer of tiny diamond crystals (grains) – i.e., poly crystalline diamond.

What areas are available?

Thin diamond films can be prepared on areas as large as 0.5 m2 using an array of hot threads for gas activation. Diamond-coated silicon wafers are generally made by microwave plasma displacement. Here the maximum wafer diameter is within 4” (2.45 GHz excitation) and 8” (915 MHz excitation). Diamond disks are gained by growing a thick diamond zone on a substrate and by separating the substrate after that. The standard size of these disks is 1-12 cm in diameter. Finally, the size of single diamond crystals depends mostly on the size of the seed crystal used. Sadly, the availability of wide-area seed crystals is minimal.

What thicknesses are possible?

Free-standing diamond layers mounted on silicon support have been demonstrated with depths as low as 30 nm. On another end of the scale, diamond disks with more than 2 mm depth are commercially available.

What are the standard grain sizes of a polycrystalline diamond?

Usually, the grain size is in the sub-micron range at the start of diamond growth. With increasing thickness, the grains tend larger. Often, the grain size at the growing surface of a diamond film is about 10 % of the film thickness.

How does CVD diamond compare to DLC?

Usually, the grain size is in the sub-micron range at the start of diamond growth. With increasing thickness, the grains tend larger. Often, the grain size at the growing surface of a diamond film is about 10 % of the film thickness.