What is chemical vapour deposition (CVD)?

Chemical vapour deposition (CVD) is the technique used to grow the diamonds for our diamond vacuum viewports and our x‑ray anode diamond tips.

Diamond is a highly valued material in the vacuum technology industry due to its exceptional qualities surrounding thermal conductivity and broadband optical transparency. Being able to manufacture diamonds in a more sustainable and environmentally responsible manner is a huge positive step forward. Starting from back in 1955, the approach of synthetically producing diamonds has paved the way for techniques such as CVD to arise.

Atomic force microscopy reveals the fine-grained structure on this unpolished nucleation face of CVD diamond. The individual grains in the image have different brightnesses, indicating subtle differences in height.

How is CVD diamond made?

The CVD process starts with a diamond or non‑diamond substrate, which is placed within a vacuum chamber. This chamber is filled with carbonaceous (carbon‑containing) gas, such as methane (CH₄), as well as an excess of hydrogen gas.

An energy source, from either a flame, a hot filament or a plasma, allows the chamber to reach over 800°C and this heat breaks the bonds within the gases. This breaks the gases down into their core components and creates “radicals”. These are atoms or molecules with unpaired electrons making them highly reactive and keen to form bonds.

Building diamond atom by atom

The carbon from the methane attaches to the substrate, atom by atom, which is a lengthy process of building up the carbon with sp³ bonds. These bonds form a tetrahedral shape and this is the structure of diamond.

However, this leaves behind the hydrogen from both the hydrogen gas and the methane. This hydrogen is extremely useful in stopping the formation of graphite on the substrate and for acting as a catalyst for the diamond to form.

Atomic force microscopy reveals much larger grains on the polished growth face of CVD diamond. During the CVD growth process the grains grow upward and outward and those that grow fastest dominate and prevent further growth of the smaller grains. Identification of the growth face means that the diamond can be oriented optimally as a heat spreader.

Diamond vs graphite structure

Both diamond and graphite are covalently bonded allotropes of carbon, however, they are constructed differently.

As previously mentioned, carbon atoms within diamonds have tetrahedral bonds, while graphite is formed of layers of carbon. This difference means that diamond is extremely rigid while the layers of graphite can slide over themselves.

Preventing graphite formation

To have a more effective and pure diamond we want to either avoid the formation of graphite or remove as much graphite from the substrate as possible. The structure is the very thing that allows the hydrogen to stop graphite from creating an impurity. Diamond is built from sp³ bonds, while graphite forms sp² bonds on the surface.

Hydrogen can etch graphite, by breaking weak carbon-to-carbon bonds and this returns the buildup of graphite into gas. This process also changes the free energies of the carbon within the graphite, providing a lower energy pathway for the structure to convert into diamond.

Additionally, hydrogen can catch the “dangling bonds” left at the surface of CVD diamond and this removes the chance for graphite to start forming. Hydrogen also etches the diamond while it is forming; however, this is at a slower rate and does not affect the overall growth of the diamond.

A brief history of synthetic diamonds

Synthetic diamonds have been created since 1955 when a method called high pressure, high temperature (HPHT) was used. Since then, we have moved from this method and now CVD is the most widely used technique for growing synthetic diamonds.