Principle of the process

The thermal process (or “thermal CVD”) is the most common sub-technique belonging to the Chemical Vapor Deposition family. Generally speaking, CVD techniques consist of bringing a volatile compound containing the material to be deposited into contact with either another gas near the surface of the substrate or the surface of the part itself.

The compound then reacts with one or the other to form a solid product that forms the deposit on the surface of the substrate. It is important, however, that only one type of solid product is formed from the reaction to allow residual species that should not be part of the coating to be removed from the chamber in gaseous form.

CVD type processes, including the thermal process, can be carried out at different pressure levels ranging from atmospheric pressure to high vacuum. However, a certain amount of energy is required to activate the reactions, which can take different forms.

In the thermal process, it is the temperature brought to the substrate that provides the energy used to promote the chemical reaction and the diffusion of the species brought to the surface. Several means can be used to heat the part to be coated:

  • Heating by Joule effect caused by the passage of a current through the part to be coated (resistive property of the part).
  • Heating by thermal radiation (heating rod): heating of the entire chamber to ensure better homogeneity of the coating in terms of thickness (increased mobility of the elements on the surface of the part).
  • High-frequency induction heating (requiring a part that is both electrically and thermally conductive).

Due to the technical ease of implementation of this type of installation, thermal processes are frequently used in industry but require (due to the temperatures used) the restrictive use of refractory materials for the composition of the parts to be coated.

Principle of the process 

This type of epitaxial growth process belongs to the family of CVD deposits (obtained by chemical reaction with the surface of the substrate) and is better known under the acronym “OMCVD” (Organo-Metallic Chemical Vapor Deposition). This is a technique in which the reactive precursors are in the form of organometallic compounds or hydrides which are carried to the part to be coated in a carrier gas passed through a bubbler containing the precursors. This crystalline growth process is used quite extensively in industry because of its highly reproducible nature and the high layer growth rates that can be achieved.

The general principle of this type of process is as follows: a substrate subjected to high temperatures (ranging from 600 °C for the elements to be deposited belonging to families III to V of the elements of the periodic table to 800 °C for the nitrides of the family III of elements) is swept by a carrier gas charged with reactive species. The elements that will make up the growing layer are pyrolyzed on the surface of the substrate and then deposited. The residues of the reaction which are not used for the growth of the layer are eliminated by the carrier gas (usually hydrogen or nitrogen).

However, it should be noticed that so-called parasitic reactions may occur on the surface of the material, which produce impurities that can impair the properties of the growing layer.

Unlike other CVD techniques, this process cannot be used under high vacuum but only at low pressure in a controlled atmosphere. On the other hand, its lower processing temperatures (compared to thermal CVD) may favour its use in cases where excessively high temperatures would represent a limiting point in the coating process.

In order to be able to produce a large number of coatings of different natures, numerous precursors can be obtained to respond correctly to the targeted problem. However, it is important to be aware of the often toxic nature of these precursors, which must be handled with care and whose waste must be recovered and treated in specialized centers.

Principle of the process

This type of CVD sub-category is based on the same general principle as so-called thermal CVD. Only the energy source changes. In fact, the energy is supplied here by means of a continuous or pulsed laser beam which will irradiate the surface of the part to be coated very locally (localized thermal activation of the chemical reaction) in order to produce the coating in a very precise manner (microelectronics) and at low temperature (suitable for less refractory materials).

However, it is also possible to irradiate the vapour phase consisting of the carrier gas and the precursor in order to increase the reactivity of the species involved.

Principle of the process 

The PECVD (Plasma Enhanced Chemical Vapor Deposition) process is a technique that is halfway between the PVD and Thermal CVD processes.

It involves producing a thin layer on the surface of a part to be coated from a gaseous element by means of chemical reactions. Compared to other CVD techniques, however, these reactions only take place after a precursor gas has been introduced and after a plasma has been set up, which then provides the energy required to dissociate the precursor forming the deposit. This plasma is created by means of a discharge (RF, DC or pulsed DC) between two electrodes ionising the species of plasma gas present in the chamber.

This type of process, although mainly used under a vacuum of around 10-3 millibar, can also be carried out at atmospheric pressure (increasingly used in industry). One of the advantages of this technique is the ability to produce coatings at very low temperatures compared to other CVD techniques.
Indeed, thanks to the use of plasma, it is possible to maintain electrons at a high level of excitation and therefore temperature while ions and much heavier atoms can be maintained at temperatures close to room temperature. Thus the electrons, by transferring their energy by collision with the other species, can cause the dissociation of the precursor molecules without increasing the average temperature within the chamber, which would not have been possible under other conditions.

Another advantage of this technique lies in the interaction of the ions with the growing material. Indeed, the ions coming to bombard the layer allow an increase in its density, an elimination of impurities and species not sufficiently bonded. Moreover, it allows to obtain important growth rates of the layers while preserving the quality of the obtained films.

It is also possible with this technique to deposit a large number of materials, whether metallic or ceramic.